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ANNUAL REVIEWMEETING
October 15 - 16, 2012
UNIVERSITY OF MIAMI
ROSENSTIELSCHOOL of MARINE &ATMOSPHERIC SCIENCE
CENTER FOR CARBONATE RESEARCHCSL
CENTER FOR CARBONATE RESEARCH
INDUSTRIAL ASSOCIATES
CSL
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CSL – CENTER FOR CARBONATE RESEARCHANNUAL REVIEW MEETING 2012
TABLE OF CONTENTS
Meeting Program ................................................................................................... iii
Meeting Participants .............................................................................................. viiMembers & Associates ............................................................................................. xSea-Level Oscillations during the Sea-Level Highstands: Evidence from the Basinal Section off Great Bahama Bank
Gregor P. Eberli .......................................................................................................................................... 1
Evidence and Amplitude of Sea-Level Oscillations during the Last Interglacial Highstand (MIS 5e) from the Bahamas
Kelly L. Jackson, Gregor P. Eberli, Samuel B. Reid, Donald F. McNeill, and Paul M. Harris ............. 5
Late Quaternary Growth History of Glover’s Reef, Belize: Insights into Reef Distribution and Quaternary Sea Level
Noelle J. Van Ee, Gregor P. Eberli, Flavio S. Anselmetti, Peter K. Swart, and Gischler .................................................................................................................................... 11
Influence of Depositional and Diagenetic Heterogeneities on Hydraulic Conductivity in Reefal Carbonates: Preliminary Packer Injection Tests in the Southern Dominican Republic
Viviana Díaz, James S. Klaus, Donald F. McNeill, and Peter K. Swart ............................................... 17
Origin and Diagenesis of Microbialites on the Uplifted Atoll of Maré, New Caledonia
Chelsea L. Pederson, Donald F. McNeill, James S. Klaus, and Peter K. Swart .................................... 21
Microbial Community Characterization and Functional Gene Diversity of Oolitic Grains from Great Bahama Bank
Mara R. Diaz, Alan M. Piggot, Gregor P. Eberli, and James S. Klaus ................................................. 25
Pore Structure and Petrophysical Characterization of Microbialites Gregor P. Eberli, Ralf J. Weger, Jan Norbisrath, and Giovanna della Porta .................... ............... 29
Rock Fluid Interaction: How Dissolution Induced Changes in Pore Structure Affect Acoustic Velocity
Ralf J. Weger, Peter K. Swart, Gregor P. Eberli, and Mark Knackstedt .............................................. 33
New Insights into Slope Processes from the Bahamas and West Florida Gregor P. Eberli, Donald F. McNeill, Thierry Mulder, Emanuelle Ducassou, Dierk Hebblen, Claudia Wienberg, and Paul Wintersteller ..................................... ..................................................... 37
Variability of Slope and Basin Floor Morphology along Southwestern Great Bahama Bank
Andrew Jo, Gregor P. Eberli, and Mark Grasmueck ............................................................................. 43
Composition of Cold-Water Coral Mound “Matterhorn” and its Surrounding Sediments in The Straits of Florida
Rani Sianipar, Gregor P. Eberli, and Emmanuelle Ducasou .............................................................. 49
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Petrophysical Perspective of Cretaceous-Tertiary Re-deposited Carbonates From the Apennines and the Adriatic Sea, Italy
Irena A. Maura, Gregor P. Eberli, and Daniel Bernoulli ............................... ...................................... 57
Sedimentology, Geometries and Link to the Subsurface from a Field-Scale Analog: The Sierra de la Vaca Muerta
Michael Zeller, Samuel B. Reid, David L. Giunta, Ralf J. Weger, Gregor P. Eberli, and Luis Massaferro ....... .......................................................................................................................63
Decoupled Inorganic and Organic Carbon Isotope Records: A Global Signal Unrelated to Global Carbon Cycling?
Amanda M. Oehlert and Peter K. Swart ................................................................................................. 73
Application of Cavity Ringdown Spectroscopy to Stable Isotopic Monitoring of CO2 Sequestration during Enhanced Oil Recovery
Benjamin T. Galfond, Daniel D. Riemer, and Peter K. Swart ............................................................... 79
Sub-Micron Digital Image Analysis (BIBSEM-DIA), Pore Geometries and Electrical Resistivity in Carbonate rocks
Jan H. Norbisrath, Gregor P. Eberli, Ralf J. Weger, Klaas Verwer, Janos Urai, Desbois, and Ben Laurich ....................................................................................... ............. 83
Using Clumped Isotopes to Understand Early Diagenesis Peter K. Swart, Monica M. Arienzo, Sean T. Murray, Yula Hernawati, James S. Klaus, and Donald F. McNeill .................................................................................................................................... 91
New Insights into Dolomitization Using Clumped Isotopes Sean T. Murray, Monica M. Arienzo, and Peter K. Swart .................................................................... 95
Speleothems: A Model System for the Study of Fluid Inclusions and Clumped Isotopes
Monica M. Arienzo, Sean T. Murray, Hubert B. Vonhof, and Peter K. Swart ................................... 101
Seismic and GPR Imaging of Fractures in Carbonate Reservoirs Using 3D Diffraction Responses Caused by Fracture Intersections
Mark Grasmueck, Tijmen Jan Moser, and Michael A. Pelissier ................... .................................... 107
4D GPR for Characterization of Fluid Flow in Carbonates: Insights from Structural- vs. Stratigraphic-Controlled Domains and Comparison with Eclipse Dynamic Modeling
Pierpaolo Marchesini, Mark Grasmueck, Gregor P. Eberli, and Ralf J. Weger ................................ 113
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MEETING PROGRAM
2012 Annual Review Meeting
MONDAY OCTOBER 15, 2012 MORNING
08:30 Coffee 09:00 Welcome and Introduction
Gregor P. Eberli
09:10 Sea-Level Oscillations during the Sea-Level Highstands: Evidence from the Basinal Section off Great Bahama Bank
Gregor P. Eberli ........................................................................................................................ 1
09:30 Evidence and Amplitude of Sea-Level Oscillations during the Last Interglacial Highstand (MIS 5e) from the Bahamas
Kelly L. Jackson, Gregor P. Eberli, Samuel B. Reid, Donald F. McNeill, and Paul (Mitch) Harris .................................................................................................................. 5
09:50 Late Quaternary Growth History of Glover’s Reef, Belize: Insights into Reef Distribution and Quaternary Sea Level
Noelle J. Van Ee, Gregor P. Eberli, Eberhard Gischler, and Flavio Anselmetti ................. 11
10:10 Coffee Break
10:40 Influence of Depositional and Diagenetic Heterogeneities
on Hydraulic Conductivity in Reefal Carbonates: Preliminary Packer Injection Tests in the Southern Dominican Republic
Viviana Diaz, James S. Klaus, Donald F. McNeill, and Peter K. Swart ............................. 17
10:55 Origin and Diagenesis of Microbialites on the Uplifted Atoll of Maré, New Caledonia
Chelsea Pederson, Donald F. McNeill, James S. Klaus, and Peter K. Swart ...................... 21
11:15 Bacterial Community Characterization and Functional Gene Diversity of Oolitic Grains from Great Bahama Bank
Mara R. Diaz, Alan M. Piggot, Gregor P. Eberli, and James S. Klaus ............................... 25
11:30 Pore Structure and Petrophysical Characterization of Microbialites
Gregor P. Eberli, Ralf J. Weger, and Giovanna della Porta .............................................. 29
11:45 Rock Fluid Interaction: How Dissolution Induced Changes in Pore Structure Affect Acoustic Velocity
Ralf J. Weger, Peter K. Swart, Gregor P. Eberli, and Mark Knackstedt .................... 33
12:00 Lunch
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MONDAY OCTOBER 15, 2012 AFTERNOON
13:15 New Insights into Slope Processes from the Bahamas and West Florida
Gregor P. Eberli, Donald F. McNeill, Thierry Mulder, Emanuelle Ducassou, Dierk Hebblen, Claudia Wienberg, and Paul Wintersteller ............................................... 37
13:45 Variability of Slope and Basin Floor Morphology along Southwestern Great Bahama Bank
Andrew Jo, Gregor P. Eberli, and Mark Grasmueck ........................................................... 43
14:05 Composition of the Cold-Water Coral Mound “Matterhorn” and It’s Surrounding Sediments in the Straits of Florida
Rani Sianipar, Gregor P. Eberli, and Emanuelle Ducassou............................................... 49
14:20 Petrophysical Perspective of Cretaceous-Tertiary Re-deposited Carbonates from The Apenninnes and The Adriatic Sea, Italy
Irena A. Maura, Gregor P. Eberli, and Daniel Bernoulli .................................................... 57
14:40 Coffee Break
15:15 Sedimentology, Geometries and Link to the Subsurface
from a Field-Scale Analog - The Sierra de la Vaca Muerta Michael Zeller, Samuel B. Reid, David L. Giunta, Ralf J. Weger, Gregor P. Eberli, and Jose Luis Massaferro ........................................................................63
15:45 Decoupled Inorganic and Organic Carbon Isotope Records: A Global Signal Unrelated to Global Carbon Cycling?
Amanda Oehlert and Peter K. Swart .................................................................................... 73
16:05 Application of Cavity Ringdown Spectroscopy to Stable Isotopic Monitoring of CO2 Sequestration during Enhanced Oil Recovery
Benjamin T. Galfond, Daniel D. Riemer, and Peter K. Swart ............................................. 79
16:20 Sub-Micron Digital Image Analysis (BIBSEM-DIA), Pore Geometries and Electrical Resistivity in Carbonates
Jan H. Norbisrath, Gregor P. Eberli, Ralf J. Weger, Klaas Verwer, Janos Urai, Guillaume Desbois, and Ben Laurich ................................................................................... 83
16:45 Reception in Commons
18:15 Return to Hotel
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TUESDAY OCTOBER 16, 2012 MORNING
08:00 Coffee 08:15 Using Clumped Isotopes to Understand Early Diagenesis
Peter K. Swart, Monica M. Arienzo, Sean T. Murray, Yula Hernawati, James S. Klaus, and Donald F. McNeill ................................................................................ 91
08:35 New Insight into Dolomitization using Clumped Isotopes Sean T. Murray, Monica M. Arienzo, and Peter K. Swart .................................................. 95
08:50 Speleothems: A Model System for the Study of Fluid Inclusions and Clumped Isotopes
Monica M. Arienzo, Sean T. Murray, Hubert B. Vonhof, and Peter K. Swart ................ 101
09:10 Coffee Break
09:50 Seismic and GPR Imaging of Fractures in Carbonate
Reservoirs using 3D Diffraction Responses Caused by Fracture Intersections
Mark Grasmueck, Tijmen Jan Moser, and Michael A. Pelissier ....................................... 107
10:10 4D GPR for Characterization of Fluid Flow in Carbonates: Insights from Structural- versus Stratigraphic-controlled Domains and Comparison with Eclipse Dynamic Modeling
Pierpaolo Marchesini, Mark Grasmueck, Gregor P. Eberli, and Ralf J. Weger .............. 113
10:30 Future Projects & Discussion
12:15 Lunch
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TUESDAY OCTOBER 16, 2012 AFTERNOON
13:15 Introduction to Fieldtrip 13:45 Core Workshop: Reef Successions in the Dominican
Republic and Pleistocene Oolitic Grainstones 15:00 Fieldtrip departs
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INDUSTRIAL ASSOCIATES ANNUAL REVIEW MEETINGCSL - CENTER FOR CARBONATE RESEARCH
MEETING PARTICIPANTS
Miami, October 15 - 16, 2012
AbdulJaleel AbuBshait Saudi Aramco EXPEC Advanced Research Center Expec Annex Bldg. 137, Rm. 2660 Dhahran 31311, Saudi Arabia [email protected] Frederico Andrade Petrobras S.A. UO-BC/ATP-NE/RES Edifício geofísico Caseli, 1 andar Avenida Elias Agostinho, 665 ZIP 27913- 350 - Imbetiba Macaé, RJ, Brazil [email protected] Jiro Asada Inpex Corporation Akasada Biz Tower, 5-3-1-Akasaka Minato-ku, Tokyo 107-6332 Japan [email protected] Gregor Bächle Shell International Exploration and Production Inc. 200 North Dairy Ashford, WCK5328 Houston, TX 77079, USA [email protected] Alexandre Berner Petrobras S.A. UN-RIO/ATP-BRC/RES Avenida Presidente Vargas 3131/1402 Rio de Janeiro, RJ, Brazil Zip Code 20210-911 [email protected]
Marcelo Blauth PETROBRAS E&P/ENGP/PR/GR Av. Republica do Chile, 330 9 Andar – Centro 20031-170 Rio de Janeiro, RJ, Brazil [email protected] Ornella Borromeo ENI S.p.A. Exploration & Production Division Via Emilia 1 20097 S. Donato Milanese, Italy [email protected] Vivek Chitale BP America Inc. 501 Westlake Park Blvd. Houston, TX 77079 [email protected] Anita Csoma ConocoPhillips 600 N. Dairy Ashford, PR 3064 Houston, TX 77079 [email protected] Alfredo Grell Petrobras S.A. E&P/UO-BC / ATP-C-S / RES Av. Elias Agostinho, 665 - Imbetiba Zip code:27913 350 Macaé, RJ, Brazil [email protected]
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Paul Gucwa Bahamas Petroleum Company Montague Sterling Center, 2nd Floor East Bay Street P.O. Bxo SS-6276 Nassau, Bahamas [email protected] Mitch Harris Chevron Energy Technology Company 6001 Bollinger Canyon Rd., D-1212 San Ramon, CA 94583-2324 [email protected] Claude-Alain Hasler Shell Global Solutions Kessler Park 1 2288 GS Rijswijk The Netherlands [email protected] Karl Henck BP – Brazil North Campos Exploration TL 501 WestLake Park Blvd. Houston, TX 77079 [email protected] Peter Holterhoff Hess Corporation Hess Tower 1501 McKinney Houston, TX 77010 [email protected] Iulian Hulea Shell Global Solutions Postbus 60 2280 AB Rijswijk The Netherlands [email protected]
Jeroen Kenter Statoil ASA Sandsliveien 90 5254 Sandsli, Bergen, Norway [email protected] Jesse Koch BP America 501 Westlake Park Blvd. Houston, TX 77079 [email protected] Jose Luis Massaferro YPF S.A. Macacha Güemes 515 (C1106BKK), Puerto Madero Buenos Aires, Argentina [email protected] Paul Milroy BG Group Exploration & Production Thames Valley Park Reading RG6 1PT United Kingdom [email protected] Elena Morettini YPF S.A. Piso 23 Macacha Güemes 515 (C1106BKK), Buenos Aires, Argentina [email protected] Leo Piccoli BP Exploration & Production Inc. 580 Westlake Park Blvd. Houston, TX 77079 [email protected]
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Chris Piela BP America Inc. 501 WestLake Park Blvd. Houston, TX 77079 [email protected] Aimee Scheffer ConocoPhillips 600 N. Dairy Ashford Houston, TX 77079 [email protected] Clara Sotelo Ecopetrol S.A. Calle 37 No. 8-43 Edificio Colgás – Piso 8 Bogotá, Colombia [email protected] Alice Stagner ConocoPhillips 600 N. Dairy Ashford Houston, TX 77079 [email protected] Carl Steffensen BP America Inc. 200 WestLake Park Blvd. Houston, TX 77079 [email protected] Mark Steinhauff Saudi Aramco EXPEC Advanced Research Center Expec Annex Bldg. 137, Rm. 2670 Dhaharan 31311, Saudi Arabia [email protected]
Ingo Steinhoff BP America Inc. 200 Westlake Park Blvd Houston, TX 77079 [email protected] Mario Suárez Ecopetrol S.A. Calle 37 No. 8-43 Edificio Colgás – Piso 8 Bogotá, Colombia [email protected] Alana Tischuk Brazil Team Maersk Oil Houston 2500 Citywest Blvd., Suite 2500 Houston, TX 77042-3041 [email protected] Anette Uldall Maersk Oil & Gas Esplanaden 50 1263 Copenhagen K Denmark [email protected] Klaas Verwer Statoil ASA Sandsliveien 90 5254 Sandsli, Bergen, Norway [email protected]
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CSL - CENTER FOR CARBONATE RESEARCH
MEMBERS & ASSOCIATES
Miami, October 15 - 16, 2012
PRINCIPAL INVESTIGATORS Gregor P. Eberli Professor, Seismic Stratigraphy Mark Grasmueck Associate Professor, Subsurface Imaging James S. Klaus Assistant Professor, Paleontology Donald F. McNeill Scientist, Sedimentology Peter K. Swart Professor, Geochemistry
ASSOCIATE SCIENTISTS Mara R. Diaz Greta Mackenzie Ralf J. Weger
SCIENTIFIC COLLABORATORS Emmanuelle Ducassou University of Bordeaux, France Dierk Hebbeln University of Bremen, Germany Mark A. Knackstedt Australian National University, Australia Thierry Mulder University of Bordeaux, France Claudia Wienberg University of Bremen, Germany
VISITING RESEARCHER Marcelo Blauth Petrobras, S.A.
STUDENTS Monica Arienzo, Deniz Atasoy, Caitlin Augustin, Quinn Devlin, Viviana Diaz, Ben Galfond, Kelly Jackson, Andrew Jo, Deniz Kula, Pierpaolo Marchesini, Irena Maura, Sean Murray, Jan Norbisrath, Amanda Oehlert, Erica Parke, Al Piggot, Rani Sianipar, Hasan Calgar Usdun, Noelle Van Ee, Michael Zeller
RESEARCH ASSOCIATES Amel Saied
STAFF Karen Neher Office Manager Cory Schroeder Technical Specialist
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SEA-LEVEL OSCILLATIONS DURING THE SEA-LEVELHIGHSTANDS: EVIDENCE FROM THE BASINAL SECTION
OFF GREAT BAHAMA BANKGregor P. Eberli
KEY FINDINGS
Cores retrieved during ODP Leg 166 in the Straits of Florida recovered a thick succession of alternating meter-thick, marl-rich, pelagic/neritic limestone alternations (“cycles”) that are largely paced by the orbital precession.
Several intervals display, however, a sub-orbital frequency in the alternations:
o The interval between 12.7-13.6 myrs (802-910 mbsf) has 81 marl/limestone alternations, indicating a cycle duration of 11.1 kyrs.
o Spectral analysis of the gamma log of a Late Miocene interval shows a strong peak of 11 kyrs, in addition to the orbital frequencies.
o In the early Pliocene, the 18O record of the shallow-dwelling foraminifera G. sacculifer and the aragonite content are dominated by sub-Milankovitch variability.
Sub-orbital climate and sea-level changes occurred throughout the Neogene.
SIGNIFICANCE
Carbonate cyclostratigraphy generally assumes that the highest frequency of climate driven sea-level changes are produced by insulation changes produced by the orbital precession, i.e. 19-21 kyrs. In many modeling efforts of the carbonate system the timing and frequency of sea-level fluctuations are, thus, computed within the Milankovitch frequencies. In addition, spectral analyses of carbonate cycles regularly tune the time series to one of the Milankovith frequency prior to the statistical tests.
There is, however, increasing evidence that oscillations during sea-level highstands occur. If this is the case, a revisiting is necessary of the driving forces forming carbonate cycles that are often reservoir flow units. To prove the sub-orbital frequency of sea-level oscillations a precise age model is needed but precise dating is an inherent problem in shallow-water carbonates. The periplatform area of Great Bahama Bank offers the opportunity to assess the frequency of platform-derived material in well-dated sections.
SEDIMENTARY CYCLES IN THE STRAITS OF FLORIDA
The marl-limestone alternations (“ cycles”) in the Straits of Florida
On the slopes and in the basins surrounding the Great Bahama Bank, aragonite cycles and turbidite composition are indicators of high-frequency sea-level fluctuations (e.g., Schlager et al., 1994). Cores retrieved during ODP Leg 166 in the Straits of Florida recovered a thick succession of alternations (“cycles”) between meter thick layers with platform-derived material and thin layers with more pelagic sediments (Figure 1).
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Carbonate-rich intervals are interpreted to reflect periods of high sea level when the platform is flooded and aragonite mud is exported to the basins while the thin intervals correspond to times of increased pelagic and siliciclastic input during sea-level lowstands and early transgression before the platform is flooded again (Eberli et al., 1997; Rendle and Reijmer, 2002; Betzler et al., 1999; Isern and Anselmetti, 2002). The cycles can be recognized in cores and on logs (Figure 2).
Figure 1. (Left) Schematic drawing of the marl-limestone alternations in the Straits of Florida. The white limestone is composed of mostly platform derived-material with over 90% aragonite in the Pleistocene sections. The dark intervals are always thinner and are composed of pelagic foraminifers and nannofossils and various amounts of silt and clay (modified from Eberli et al., 1997). (Right) Core photograph across Miocene marl-limestone alternation with photomicrographs and SEM images documenting the composition and diagenesis. Neritic components are visible in the light limestone portion while pelagic foraminifers are seen on the photomicrographs and clay minerals in the SEM images of the dark intervals. Because the limestone intervals contain platform-derived material, they are interpreted to represent times of high sea level.
Based on coupled bio- and cyclostratigraphy Kroon et al. (2000) determined that these
cycles are tied to orbital forcing mechanisms. Using the Formation MicroScanner (FMS) logs to measure cycle thickness Williams et al. (2002) show that the sedimentary cycles are paced by the Earth’s climatic precession for the time interval of 9-12 myrs. In addition, the cycle thickness contains long-term cycles that repeat about every 400 kyrs, which they correlate to the 400 kyrs cycles in orbital eccentricity.
Evidence of sub-orbital cycles
Two studies found sub-orbital signals in the marl-limestone alternations. Bernet (2001) documents cycles of 11 kyrs duration in two intervals of along the Bahamas Transect. The first is between 802-910 m at ODP site 1006 and is dated 12.7-13.6 myrs. Within this 900 kyrs time interval are 81 marl-rich pelagic/neritic limestone alternations, indicating
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cycle duration of 11.1 kyrs. In an older section, spectral analysis of the gamma-ray logs within Upper Miocene marl/limestone alternations in Late Miocene strata at ODP site 1003 produced strong peaks in orbital frequencies of obliquity (40 kyrs) and precession (23 kyrs) but also a peak at 11kyrs that is stronger than the obliquity peak (Figure 3).
Figure 2. (Left) Core photograph of Miocene marl-limestone alternation in Site 1007, Core 1007 84R (modified from Bernet, 2001). (Right) Cyclic variations in the FMS (Formation MicroScanner) resistivity image log, FMS resistivity average, resistivity log (SFLU), and normal and high resolution porosity logs (APLC,HALC) of the Pliocene section in Site ODP 1006. Picked cycles, used for cycle counting, are indicated by horizontal lines (modified from Williams et al., 2002).
Reuning et al. (2006) analyzed magnetic susceptibility, aragonite content and 18O records from two different planktonic foraminifera species in an early Pliocene core interval from ODP site 1006. They found that the 18O record of the shallow-dwelling foraminifera G. sacculifer and the aragonite content are dominated by sub-Milankovitch variability. Because the magnetic susceptibility and the deeper-dwelling foraminifera G. menardii show precession frequency they interpret the sub-Milankovitch frequency as a climate rather than sea-level driven signal. The sub-orbital signal in the aragonite content is, however, indicating that sea-level changed with the climate signal. Many studies document that aragonite is produced on the platform during high sea level and exported to the basins during these times (Boardman and Neumann, 1984; Schlager et al., 1994; Rendle and Reijmer, 2002).
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IMPLICATIONS
The sub-orbital signal in the marl/limestone alternations in the cores of ODP Leg 166 indicates that sub-orbital sea-level oscillations occurred throughout the Neogene. If indeed sub-orbital oscillations are able to produce shallowing-upward carbonate cycles, the possibility of tracing the orbital forcing mechanisms past the Mes0zoic is diminished because large uncertainties are introduced in spectral analyses of these ancient deposits. Sub-orbital sea-level oscillations might explain the overabundance of cycles in some ancient successions, like the Latemar where roughly 600 cycles were deposited in 3 myrs.
Figure 3. Spectral analysis of the Late Miocene strata using the gamma ray log. In addition to the obliquity and the precession a peak at 11 kyrs indicates sub-orbital cyclicity (modified from Bernet, 2001).
REFERENCES
Bernet, K. H., 2001, The record of hierarchies of sea level fluctuations in cores, logs, and seismic data along the Great Bahama Bank Transect: University of Miami Ph.D. Dissertation, 210 pp.
Betzler, C., Reijmer J. J. G., Bernet, K., Eberli, G. P., and F. S. Anselmetti, 1999, Sedimentary patterns and geometries of the Bahamian outer carbonate ramp (Miocene and lower Pliocene, Great Bahama Bank): Sedimentology, v. 46, p. 1127-1145.
Boardman, M. R., and A. C. Neumann, 1984, Sources of periplatform carbonate: Northwest Providence Channel, Bahamas: Journal of Sedimentary Petrology, v. 50, p. 1121-1148.
Eberli, G. P., Swart, P. K., Malone, M., et al., 1997, Proceedings ODP, Init. Repts., 166: College Station, TX (Ocean Drilling Program), 850 pp.
Kroon, D., Williams, T., Pirmez, C., Spezzaferri, T., Sato, T., and J. D. Wright, 2000, Coupled early Pliocene- middle Miocene bio-cyclostratigraphy of Site 1006 reveals orbitally induced cyclicity patterns of Great Bahama Bank carbonate production: Proceeding of ODP, Scientific Results, v. 166, p. 155-166.
Rendle, R. H., and J. J. G. Reijmer, 2002, Quaternary slope development of the western, leeward margin of Great Bahama Bank: Marine Geology, v. 185, p. 143-164.
Reuning, L., Reijmer, J. J. G., Betzler, C., Timmermann, A., and S. Steph, 2006, Sub-Milankovitch cycles in periplatform carbonates from the early Pliocene Great Bahama Bank: Paleoceanography, v. 21, p. 1007-1021.
Schlager, W., Reijmer, J. J. G., and A. W. Droxler, 1994, Highstand shedding of carbonate platforms: Journal of Sedimentary Research, v. B64, p. 270-281.
Williams, T., Kroon, D., and S. Spezzaferri, 2002, Middle-Upper Miocene cyclostratigraphy of downhole logs and short to long term astronomical cycles in carbonate production of Great Bahama Bank: Marine Geology, v. 185, p. 75-93.
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EVIDENCE AND AMPLITUDE OF SEA-LEVEL OSCILLATIONS DURING THE LAST INTERGLACIAL HIGHSTAND (MIS 5E)
FROM THE BAHAMASKelly L. Jackson, Gregor P. Eberli, Samuel B. Reid, Donald F. McNeill,
and Paul M. Harris1
Chevron Energy Technology Company
KEY FINDINGS
Positions of reefs, beach, and eolian deposits provide evidence that sea level during the last interglacial highstand (MIS 5e) 115-125 ka was not a single rise and fall but fluctuated with a minimum oscillation of 10 m within a few thousand years.
During MIS 5e, sea-level first rose to 7 m above present, then dropped by ~ 2 m before the mid- MIS 5e sea-level drop of at least 10 m. Next, sea level rose to form the younger MIS 5e highstand and then dropped at the end of MIS 5e in downstepping pulses.
Highstand sea-level oscillations create complex patterns of carbonate sediment deposition and promote extensive lateral accretion.
IMPORTANCE
New evidence from New Providence Platform, Bahamas, indicates that sea-level oscillated a minimum of 10 m during MIS 5e, creating early and late substages within the 5e highstand. A 10 m + oscillation exposed Great Bahama Bank, creating two separate depositional cycles within 10,000 years. This highstand oscillation requires a suborbital forcing mechanism of much shorter duration than Milankovitch frequencies. This is important because it contradicts the preconceived notion that precession is the controlling factor of high-frequency sequences and the building blocks of carbonate cycles which we assume are reservoir flow units. In addition, this new evidence documents that rapid climate changes, which previously were thought to only take place during glacial times, can take place during warm interglacial periods.
Figure 1. Three criteria are used to identify sea level in core and outcrop in New Providence and the Exuma Cays, Bahamas: (1) The elevation of the transition from beach to eolian dunes, (2) The elevation of reefs plus an assumed water depth of 2-5 m, and (3) Exposure horizons (calcretes, caliche crusts) separating subtidal facies in core.
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SEDIMENTOLOGIC INDICATORS FOR SEA-LEVEL AMPLITUDE
Sea level was 6-7 m higher than present during the last interglacial highstand 115-125 ka (MIS 5e). Exposure horizons and lithologic changes in cores and outcrops combined with age dating in the Exuma Cays and New Providence, Bahamas, provide sedimentologic and stratigraphic evidence of sea-level oscillations during MIS 5e.
The position of sea level during MIS 5e is assessed using three criteria: 1) The elevation of the transition from beach to eolian dunes, in particular the first occurrence of keystone vugs in the beach sediments, 2) The elevation of reefs plus an assumed water depth of 2-5 m, and 3) Exposure horizons (calcretes, caliche crusts) separating subtidal facies in core document the sea-level drop below the former sediment surface (Figure 1).
1. Beaches and Eolian Dunes
Carbonate eolian dunes form adjacent to the modern beach and start cementing quickly after deposition. Unlike siliciclastic dunes, carbonate eolian dunes do not migrate therefore one ridge of eolian dunes implies one sea-level position in a carbonate coastal system. On New Providence, three low-relief beach ridges prograde toward the modern shoreline. Using the lowest occurrence of keystone vugs as a sea-level proxy, the beach ridges downstep from +7.6 to +7.0 to +5.1 m above present sea level over a lateral area of ~4 km (Garrett and Gould, 1984; Reid, 2010). In between the +7.0 m and +5.1 m beach ridges, there is a calcrete separating subtidal facies in core indicating a drop in sea level during MIS 5e. At two locations within the Exuma Cays Land and Sea Park, there are five parallel north-south trending eolian dune ridges that were deposited during MIS 5e. Beach facies in core are slightly inclined laminated grainstones with fenestral pores. Eolian facies in core feature high-angle cross-bedding, meniscus calcite cement and/or equant-bladed and dogtooth spar. Both facies have interparticle porosity and intraparticle-moldic porosity where the peloids and ooids started to dissolve away (Figure 2). The subsurface position of the beach to dune transition in cores WW1-3 and BI1-OB1 deepens from west to east, indicating downstepping toward Exuma Sound as a result of pulsed ice build-up towards the glacial period.
2. Reefs
The presence of depth-specific coral species (i.e., Acropora palmata, Acropora cervicornis, etc.) in core and outcrops are good indicators of past sea level. Pleistocene shallow-water reefs are present at +1.5 m above present sea level on Rocky Dundas (dominated by Acropora cervicornis) in the north-central Exumas as well as on Darby Island (mixed shallow-water species) in the southern Exumas indicating that sea level was higher than today when the reef was alive. At Bitter Guana Cay, the shallow-water reef facies are at approximately -1.8 to -2.8 m below present sea level. Halley et al. (1991) dated a +1.5 m reef terrace on the leeward side of Norman’s Pond Cay as 117 ± 5 to 119 ± 4 ka. Open system U-Th dating of Montastraea faviolata at -3.5 m depth dated the reef on Little Darby to 122 ± 1.32 ka old. Both dates confirm that these reefs were alive during MIS 5e. Thompson et al. (2011) documented stacked MIS 5e reefs in San Salvador, Bahamas, with an older MIS 5e reef at 123.5 ka and a younger MIS 5e reef at 119.5 ka, separated by an unconformity that they interpreted as an exposure related to the mid-MIS 5e sea-level drop. In the Little Darby core a younger reef is recovered above the dated reef. Unfortunately, the contact is not recovered in the core and the younger reef is diagentically too altered to allow U/Th dating.
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3. Exposure Horizons within Subtidal Deposits
The presence of an exposure horizon separating subtidal facies in core indicates a drop in sea level during MIS 5e. On New Providence, subtidal burrowed and bedded grainstones in short cores feature a cm-scale calcrete 20-30 cm within MIS 5e subtidal deposits, documenting an intermittent lowering of sea level during MIS 5e. Exposure horizons are also present in cores from the Exumas separating eolian and subtidal facies.
Figure 2. (A) Photomicrograph of peloidal oolitic grainstone featuring fenestral porosity typically found in beach to eolian dune environments. (B) Peloidal oolitic grainstone with intraparticle to moldic porosity and solution enhancement of the original fenestral pores. (C) Eolian peloidal oolitic grainstone featuring dogtooth and meniscus calcite cement, interparticle porosity, and intraparticle porosity. (D) Close-up of a partially dissolved ooid with equant dogtooth spar surrounding the grains.
Summary of the Sea-level Oscillations during MIS 5e
New evidence from New Providence and the Exuma Cays reveals that sea-level oscillated a minimum of 10 m and created early and late substages of MIS 5e. New Providence beach deposits at +7.6 m above present sea level represent the older peak of MIS 5e sea level. A down-stepping beach ridge indicates a subsequent sea-level position at +7.0 m. A calcrete in the subtidal deposits adjacent to the beach documents the mid-MIS 5e sea-level drop. In the Exumas, a calcrete associated with this fall separates subtidal facies at -2.0 m. Sea-level rises again to form the younger MIS 5e highstand; this rise is represented by a beach ridge at +5.1 m on New Providence Island and Exumas reefs up to +1.5 m above modern sea level. Parallel down-stepping beach to eolian dune transitions provide evidence for a pulsed down-stepping of sea level at the end of MIS 5e. The lowest occurrence of this transition is approximately -12 m below present sea level.
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Taking the lowest occurrence of calcretes that mark the mid-MIS 5e sea-level fall and the highest beach elevation into account, the MIS 5e sea-level oscillated a minimum of 10 m (9 m plus at least 1 m to expose the strata) (Figure 3). This sedimentologic evidence corroborates the 15 m estimate from Caribbean corals (Thompson and Goldstein, 2005; Thompson et al., 2011). In addition, the downstepping pattern within and at the end of MIS 5e documents pulsed changes of sea level during MIS 5e that likely coincides with pulsed ice buildup.
Figure 3. (A) MIS 5e sea level from Thompson and Goldstein (2005). (B) MIS 5e sea level interpreted from the facies successions in the Exumas and New Providence, Halley et al. (1991), Aurell et al. (1995), and Reid (2010).
COMPLEX CARBONATE SEDIMENT DEPOSITION AND ACCRETION
The Exuma Cays and surrounding carbonate sand bodies of Great Bahama Bank feature a stacked succession of carbonate grainstones that display complex patterns of facies juxtapositions and lateral accretion. This heterogeneity is the direct product of sea-level oscillations during Holocene and Pleistocene sea-level highstands. The recognition of these suborbital sea-level oscillations explains complicated facies patterns in shallow-water carbonate strata.
The numerous and varied islands of the Exuma Cays span 170 km NE-SW and 5-10 km W-E and are composed of Holocene (<6,000 ybp), MIS 5e (~125,000 ybp), and older Pleistocene strata. These stratigraphic units are laterally accreted forming the complex island stratigraphy of the Exumas. Field mapping and ground-truthing of satellite imagery of one key island (Hawksbill Cay) documents patterns of Holocene and Pleistocene facies accretion laterally; 38% of the island is Pleistocene at the surface and Holocene ridges were deposited around the Pleistocene topography in a complex fashion (Harris and Ellis, 2009). Holocene ridges have elevations from near sea level to +12 m
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while Pleistocene landforms have elevations from near sea level to +19 m. This complex pattern of carbonate sediment deposition and accretion extends along the entire Exumas windward margin. Antecedent Pleistocene topography directly affects the distribution of shoals and underwater environments along the margin.
The stratigraphic architecture resulting from sea-level oscillations produces heterogeneous grainstones with a wide range of grain sizes, sedimentary structures, and diagenetic features. Many cores, including HY1 (Figure 4) feature selective cementation or diagenetic partitioning of the oolitic peloidal grainstone. Common to most cores is the presence of moldic porosity (most commonly oomoldic porosity). This diagenesis creating the porosity complicates dating stratigraphic units as most of the original material has been diagenetically altered or dissolved away.
Figure 4. (A, B) Photomicrographs showing an example of differential cementation in eolianite ooid grainstones from Harvey Cays. (B) In some areas the grains are almost completely dissolved away showing intraparticle-moldic porosity while in other areas, the grains are micritized but remain intact.
IMPLICATIONS FOR CYCLOSTRATIGRAPHY AND RESERVOIR HETEROGENEITY
The recognition of suborbital sea-level oscillations has two major implications. First, sea-level falls within interglacials indicate that a mechanism exists that can produce sea-level fluctuations during times when the Earth is considered ice free, i.e., the greenhouse world. This might explain the cyclic nature of Cretaceous and Triassic strata. The short duration of these oscillations produces uncertainty regarding the commonly accepted notion of Milankovitch cyclostratigraphy in carbonates.
Second, the combined product of high-frequency orbital sea-level changes and suborbital oscillations is a complex lateral and vertical stratigraphic architecture that juxtaposes grainstones deposited in different environments. Antecedent topography from older sea-level oscillations and the repetition of sea-level oscillations during each subsequent highstand yields a complex stratigraphic architecture of lateral accretion and overstepping wedges. Suborbital sea-level oscillations have dramatic effects on carbonate depositional environments and occur on timescales of just a few thousand years. As illustrated by the prograding wedges on New Providence, the highstand oscillations enable the lateral accretion of shoal systems that one single sea-level rise would not achieve.
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REFERENCES
Aurell, M., McNeill, D. F., Guyomard, T., and P. Kindler, 1995, Pleistocene shallowing-upward sequences in New Providence, Bahamas: signature of high-frequency sea-level fluctuations in shallow carbonate platforms: Journal of Sedimentary Research, v. B65, no. 1, p. 170-182.
Garrett, P., and S. J. Gould, 1984, Geology of New Providence Island, Bahamas: Geological Society of America Bulletin, v. 95, p. 209-220.
Halley, R. B., Muhs, D. R., Shinn, E. A., Dill, R. F., and J. L. Kindinger, 1991, A +1.5-m reef terrace in the southern Exuma Islands, Bahamas: Geological Society of America Abstracts and Programs, v. 23, no. 1, p. 40.
Harris, P. M., and J. Ellis, 2009, Satellite imagery, visualization and geological interpretation of the Exumas, Great Bahama Bank: an analog for carbonate sand reservoirs: SEPM Short Course Notes No. 53, DVD.
Reid, S. B., 2010, The complex architecture of New Providence Island (Bahamas) built by multiple Pleistocene sea level highstands: University of Miami M.S. Thesis, Open Access Theses, Paper 77, http://scholarlyrepository.miami.edu/oa_theses/77.
Thompson, W. G., and S. L. Goldstein, 2005, Open-system coral ages reveal persistent suborbital sea-level cycles: Science, v. 308, p. 401-404.
Thompson, W. G., Curran, H. A., Wilson, M. A, and B. White, 2011, Sea-level oscillations during the last interglacial highstand recorded by Bahamas corals: Nature Geoscience, v. 4., p. 684-687.
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LATE QUATERNARY GROWTH HISTORY OF GLOVER’SREEF, BELIZE: INSIGHTS INTO REEF DISTRIBUTION AND
QUATERNARY SEA LEVELNoelle J. Van Ee, Gregor P. Eberli, Flavio S. Anselmetti1, Peter K. Swart,
and Eberhard Gischler2 1) EAWAG, Sedimentology Group, Dübendorf, Switzerland
2) Goethe University, Frankfurt am Main, Germany
KEY FINDINGS
Pleistocene reefs strongly influence modern reef distribution.
- 93% of seismically imaged modern patch reefs sit on antecedent topographic highs.
- Modern patch reefs are comprised mainly of in situ reef facies.
Unequal filling of accommodation space and asymmetric map facies typify Quaternary platform development.
- No indication in seismic data of tilting or faulting of Late Pleistocene succession.
- Modern and Pleistocene bathymetry relate to differential ability of facies and hydrodynamic regime to fill accommodation space.
- Satellite facies mapping and grain size analysis shows windward – leeward asymmetry on both a platform and patch reef scale.
Quaternary sea-level oscillations have left strong isotopic and petrographic signatures while forcing innovative attempts to date Late Pleistocene strata.
Figure 1. Study area with location of cores and seismic lines. Inset image modified from Google.
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SIGNIFICANCE
The location of Glover’s Reef proximate the North America – Caribbean plate boundary and along a known precipitation gradient make it an ideal candidate for studying possible controls on reef distribution and platform development. Despite separation from the mainland during the Quaternary, Glover’s Reef is a heterogeneous carbonate system. Understanding Glover’s Reef can provide insights for predicting occurrence, extent, and quality of carbonate reservoirs in reef mound – patch reef complex or isolated platform depositional systems.
RESULTS
Reef Distribution
A single-channel seismic survey of over 100 km of grid lines in conjunction with N-S and E-W rotary core transects (Figure 1) show no evidence of faults/folds controlling patch reef locations. Rather, 93% of surveyed Holocene patch reefs are located on Pleistocene topographic highs (Figure 2). In situ reef facies in patch reef cores indicate that these highs are growth-induced: reefs sit on reefs (Figure 3).
Figure 2. Seismic data showing influence of antecedent topography and patch reef core (see Figure 1 for location). Holocene reefs and modern sea bottom are traced in green. The Pleistocene top is traced with red. Not every Pleistocene topographic high results in a Holocene patch reef, but the vast majority (93%) of surveyed Holocene patch reefs are located on antecedent highs.
Platform Development
Although modern bathymetry deepens in the south of the platform, there is no evidence to support a tilted Pleistocene surface. Maps constructed of the Pleistocene topography from the seismic grid show a relatively flat surface except for locally beneath patch reefs (Figure 4). These grid maps also indicate that observed modern lagoon bathymetry is caused by a wedge of Holocene sediment that is thickest in the northeast corner of the lagoon and thins southward. The northeast corner is the most windward corner of the platform and the location of one of only several breaks in the reef-rimmed platform.
Inconsistencies in the location of Pleistocene horizons in the cores correlate to
antecedent topography and variable growth potential of facies. Patch reef cores sit on antecedent highs while windward cores contain facies with fast-growing Acropora species and early marine cementation, in contrast to leeward core G2 (Figure 3).
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Satellite-based facies mapping, and modern grain size analyses also provide evidence for platform development in response to a dominant northeast to southwest trade wind regime. Asymmetric facies belts are apparent on both a platform and patch reef scale (Figure 5). Grain size analyses of windward and leeward transects illustrate that median grain size decreases steadily with distance from the windward margin in contrast to a sharp jump at 800 m from the leeward margin. The poorest sorting is found closest to the margin on the windward side but furthest from the margin on the leeward side.
Figure 3. Rotary cores collapsed along leeward – windward transect shown with lithologies. OR = Oreaster Reef, AR = Aurelia Reef, SW = Southwest Caye, MC = Middle Caye, ER = East Rim, NR = North Rim. Note that cores GR3, OR, and AR are from lagoon patch reefs.
Sea-level History
Negative carbon and positive oxygen stable isotope excursions suggest three exposure events in the Pleistocene, including the Last Glacial Maximum. Multiple exposures to meteoric water has leached almost all original aragonite from the sediment and left a petrographic signature of vuggy dissolution and recrystallization to blocky calcite (Figure 6). Lack of appropriate U-Th dating material has led to exploration of new techniques of amino acid racemization applied to coral geochronology. The two youngest Pleistocene sequences (P1 & P2) are relatively close in age, perhaps even substages of Marine Isotope Stage 5e. Holocene U-Th and C-14 ages of coral and peat fall along existing sea-level curves for the western Caribbean and suggest an accumulation of organic material around 8000-7500 years before present (BP) and initial coral growth shortly thereafter at approximately 7000 years BP (Figure 7).
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Figure 4. Horizons traced on seismic lines were interpolated to create maps of modern bathymetry, depth to the top of the Pleistocene sequence, and the thickness of Holocene sediment. The thickest areas of sediment correspond to breaks in the platform rim.
Figure 5. Asymmetric distribution of modern facies and grain size distribution. (A) Asymmetric facies are apparent on both platform and patch reef scale. (B) Windward transect GR7 is very coarse and poorly sorted near the platform margin. (C) Leeward transect GR11 is most poorly sorted away from the platform margin.
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Figure 6. Quaternary sea-level history. (A) Negative carbon and positive oxygen stable isotope excursions are interpreted to represent exposure events in the Southwest Caye core. (B) Three values from two Pleistocene sequences cluster together on a DL Glutamic – Aspartic acid cross plot, suggesting the samples are close in age, perhaps even substages of MIS 5e. (C) Thin sections from the Southwest Caye core display common petrographic signatures of meteoric diagenesis: blocky recrystallization to calcite and vuggy dissolution.
INTERPRETATION AND IMPLICATIONS
Although previous work on the Belize margin has stressed that reef distribution is controlled by underlying fault/fold structures or karst processes, we find that on Glover’s Reef, Holocene reefs build on top of Pleistocene reefs. Many of the platform heterogeneities can be explained by the strong control of consistent wind direction on platform development. Additional vertical heterogeneities have been introduced by platform response to high frequency sea-level oscillations. This work implies that even isolated carbonate systems can build complicated architecture in relatively short time intervals.
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Figure 6. New U-Th and C-14 dates from Glover’s Reef compared to existing data from the Western Atlantic.
REFERENCES
Blanchon, P., Jones, B., and D. C. Ford, 2002, Discovery of a submerged relic reef and shoreline off Grand Cayman: further support for an early Holocene jump in sea level: Sedimentary Geology, v. 147, p. 253-270.
Gischler, E., and J. H. Hudson, 1998, Holocene development of three isolated carbonate platforms, Belize, Central America: Marine Geology, v. 144, p. 333-347.
Lightly, R.G., MacIntyre, I.G., and R. Stuckenrath, 1982, Acropora palmata reef framework: A reliable indicator of sea level in the Western Atlantic for the past 10,000 years: Coral Reefs, v. 1, p. 125-130.
Toscano, M.A., and I. G. MacIntyre, 2003, Corrected Western Atlantic sea-level curve for the last 11,000 years based on calibrated C14 dates from Acropora palmata framework and intertidal mangrove peat: Coral Reefs, v. 22, p. 257-270.
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INFLUENCE OF DEPOSITIONAL AND DIAGENETIC HETEROGENEITIES ON HYDRAULIC CONDUCTIVITY IN
REEFAL CARBONATES: PRELIMINARY PACKER INJECTIONTESTS IN THE SOUTHERN DOMINICAN REPUBLIC
Viviana Díaz, James S. Klaus, Donald F. McNeill, and Peter K. Swart
KEY FINDINGS
Stratal packer well injection tests reveal a general trend of decreasing hydraulic conductivity (K) measured over a 25 m interval that spans two Pleistocene high-stand reef sequences.
Decreasing (K) is related to depositional facies as well as diagenetic overprint associated with dissolution and cementation in the meteoric environment.
Hydraulic conductivity calculated from injection tests is, with exceptions, positively correlated to plug permeability measurements and inversely correlated to core recovery.
SIGNIFICANCE
Depositional and diagenetic heterogeneities within carbonate rocks can influence flow and transport parameters, and it is increasingly recognized that sedimentological and stratigraphic models can provide a method of incorporating geological variability into models of fluid flow in both sedimentary aquifers (Fraser and Davis, 1998) and petroleum reservoirs (Fogg and Lucia, 1990; Lucia, 2007). In carbonates, the porosity and permeability structure is dependent on both matrix porosity and the development of larger scale secondary porosity. The resulting complex porosity distribution in carbonates is reflected in the hydraulic-conductivity. If one views dissolution zones as discrete heterogeneities, the challenge of predicting transport in carbonate rocks is one of characterizing the hydraulic-conductivity distribution at a scale that captures the variability of these heterogeneities.
An ongoing challenge in assessing reservoir properties is the integration of multiple data sources (thin section, core data, well log, geological models, seismic data, well tests, simulation cells) and their variable scales (Figure 1). This study will integrate surface and subsurface stratigraphic, depositional, diagenetic and petrophysical data with hydrogeological data in order to characterize the factors that influence hydraulic properties of Plio-Pleistocene reefal limestones of the Dominican Republic (Figures 2-3). When completed, vertical profiles of hydraulic conductivity will be obtained from short-interval packer tests performed in a transect of six wells drilled perpendicular to the prograding depositional packages. Previous stratigraphic analyses of depositional and diagenetic facies, combined with detailed petrophysical characterizations will be combined with the short-interval (<1 m) packer tests. These packer tests provide for calculation of hydraulic conductivity data on both matrix and dissolution zones. The long-term goal of the project is to obtain hydrostratigraphic data from the Plio-Pleistocene carbonates that can ultimately be incorporated into a reservoir flow simulation model.
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Figure 1. A range of scales for data used in geological models and flow simulation models. The need for upscaling and downscaling of porosity and permeability data versus coarser-scale facies and seismic data.
Figure 2. Location of Southern Dominican Republic. Cores 1 through 5 were drilled in a transect from the oldest terrace at 50 m to the youngest terrace at 5 m above sea level.
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Figure 3. Cross-section of prograding Plio-Pleistocene reefs in Southern Dominican Republic. Figure shows core location and shallowing upward cycles with facies distribution.
RESULTS FROM THE FIELD TEST
Preliminary short interval (1 m) packer tests were performed in core 3 (30 m terrace) from the surface to a depth of ~24 m. This interval is characterized by three distinct highstand reef sequences separated by a subaerial exposure at ~12.5 m in the core. The lower sequence is characterized by reef front facies dominated by branching corals in a packstone matrix, the next sequence is characterized by a shallower reef crest facies with a mix of massive and branching corals in a packstone to grainstone matrix. The uppermost sequence is characterized by the back-reef lagoon with branching corals in a wackestone to packstone matrix. Carbon and oxygen isotopes through this interval reflect progressive diagenetic overprinting in the lower sequence. The light values of
Figure 4. Plot showing hydraulic conductivity with depth in core 3. Measurements span two highstand reef sequences separated by a subaerial exposure surface. Stable isotope data and permeability data from petrophysical cores is included for comparison.
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carbon at the top of the core and at 12.5 m reflect the two periods of subaerial exposure. Permeability values from petrophysical plugs ranged from 0.2-414.8 mD in the lower interval, and 119.7-2850 mD in the upper interval. Values of hydraulic conductivity from the 1m packer tests ranged between 0.007-0.013 cm/s in the lower unit and 0.011-0.024
cm/s in the upper unit. A general trend of decreasing hydraulic conductivity can be seen from the surface to the subaerial exposure at 12.5 m (Figure 3). Core recovery, used as a porosity proxy due to the absence of log data, appears inversely correlated to the hydraulic conductivity. While plug permeability and hydraulic conductivity are positively correlated below 5 m, they appeared inversely correlated in the upper 5 m of the core.
INTERPRETATION AND IMPLICATIONS
Changes in hydraulic conductivity down core correlate with secondary dissolution and cementation in the meteoric environment. In the upper reef crest sequence, cementation and recrystallization increase with core depth. Vadose diagenesis is more prominent in these sections where characteristic cements are matrix microspar and blocky calcite inside moldic pores. As a result, interparticle porosity preservation is more abundant. The lowest K values are in the reef front where thin-sections show fringe cements occluding interparticle pores in the phreatic zone. Consequently, there are fewer pathways for fluid flow. In addition, micritic matrix areas are also more common in the reef front and usually have isolated moldic porosity with low pore connectivity. Core recovery was used as a proxy for porosity. There is an inverse correlation with the hydraulic conductivity. In the more massive and sandier zones where cementation is more pe rvasive, core recovery is higher than the muddier and moldic branching coral section of the back-reef and reef front.
It has been suggested that petrophysical data obtained from small-scale plugs may not accurately reflect heterogeneities at a scale relevant to fluid flow within a reservoir. A preliminary correlation between the permeability values calculated from plugs (Ditya, 2012), and the hydraulic conductivity calculated from in situ packer tests suggests that in this example small-scale plug data can be reasonably scaled up for geomodels, however exceptions do exist. The lack of correlation in the upper 5 m may just be a reflection of the low resolution of permeability data or dissolution characteristics close to the subaerial exposure not represented at the plug scale.
When completed, the study will provide a facies/diagenesis/acoustic-based correlation of permeability (hydraulic conductivity) data after several stages of post-depositional stabilization. These correlations can then be used as formation properties, and input as model parameters as part of the upscaling for reservoir flow simulations.
REFERENCES
Ditya, A., 2012, Petrophysical characterization of Pliocene-Pleistocene reefal carbonates, southern Dominican Republic: University of Miami M.S. Thesis, Open Access Theses, Paper 358, http://scholarlyrepository.miami.edu/oa_theses/358.
Fogg G. E., and J. F. Lucia, 1990, Reservoir modeling of restricted platform carbonates: geologic/geostatistical characterization of interwell-scale reservoir heterogeneity, Dune Field, Crane County, Texas: University of Texas BEG Report Investigation 190, 65 p.
Fraser G. S., and J. M. Davis (Eds), 1998, Hydrogeologic models of sedimentary aquifers: SEPM Concepts Hydrogeological Environmental Geology, v. 1, 188 p.
Lucia, J. F., 2007, Carbonate reservoir characterization: An integrated approach: Springer, 336 p.
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ORIGIN AND DIAGENESIS OF MICROBIALITES ON THE UPLIFTED ATOLL OF MARÉ, NEW CALEDONIA
Chelsea L. Pederson, Donald F. McNeill, James S. Klaus, and Peter K. Swart
KEY FINDINGS
Maré microbialites consist of aragonitic/HMC and intraclast nuclei surrounded by a 0.1-3 mm microbial envelope
Microbialite formation is interpreted to have formed just prior to subaerial exposure, in shallow, agitated marine conditions
Despite subaerial exposure of nearly 3 my and pervasive meteoric cementation, the grain nuclei remain largely unaltered from their original structure and mineralogy
The constructive microbial envelope deters meteoric dissolution and formation of secondary moldic porosity
SIGNIFICANCE
The influence of microbes on carbonate textures is well known from studies of stromatolites that date back to the Precambrian. In the Phanerozoic, with the diversification of life forms, the dominance and influence of microbes in the marine realm has been largely relegated to niche environments. However, microbial influence on carbonate textures and recognition as distinct facies has been increasingly recognized, especially in the calibration of geological and reservoir models. This study contributes to the understanding of how microbial processes can influence rock textures with the characterization of proposed cyanobacterial formed grains associated with the final stages of marine deposition of Maré, New Caledonia (Figures 1 & 2) and the onset of uplift-driven subaerial exposure. By characterizing the chemical and physical attributes of these microbial carbonates we will provide a better understanding of their formation, environment of origin, and influence on early diagenesis.
Figure 1. Global Mapper topographic image of Maré, New Caledonia.
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Figure 2. Cross section of the La Roche faro, where the inter-bedded microbialites occur.
RESULTS
Physical Characterization
Reefal units representing the final stages of marine deposition of the Maré atoll, New Caledonia (Figure 1 & 2) are inter-bedded with horizontal units of large coated grains. The coated grains range in size from 1-10mm. Each grain contains a nuclear fragment surrounded by a microbial coating with irregular concentric layering (Figure 4a). Meteoric cements bind the coated grains and partially fill the interparticle pore space. The irregular concentric grains with diameters <10cm are therefore termed oncolites. The general composition of the oncolite beds consists of a minimum of three distinct components (Figure 4b). The innermost layer is the nuclei, made of aragonitic skeletal fragments and intraclasts. Aragonitic mollusk shell fragments, some with their original nacre (mother-of-pearl) are sometimes present. Nuclei thickness ranges from 0.60-4mm. The second layer is a microbial coating (rind) surrounding the nuclei. Rind thickness ranges from 1.5-6 mm, representing up to 50% of the total oncoid diameter. While most nuclei are not well rounded, the encircling rind generally displays a more circular shape. The microbial rinds consist of fairly equant euhedral 1-3 micron diameter crystals, tightly packed, with relatively low-permeability (Figure 4b & 4c). Point counting has determined the microbialite samples to have porosities of less than 5%, with some thin section samples having less than 1.5%. The third component of the oncolite grains consists of blocky meteoric cement, representing 25-35% of the total sample area. The coarse low-Mg calcite crystals encase the grains, yet the meteoric fluids appear to not have penetrated the dense microbial rind, allowing the preservation of the inner nuclei.
Geochemical Characterization
The average 18O signal for the nuclei, rind, and cement layers were -4.7, -4.9, and -5.6 respectively. The average 12C signal for the nuclei, rind, and cement layers were -6.7, -7.6, and -10.1. These stable isotopic results show a general trend toward lighter carbon and oxygen isotopes as you move from the nuclei to the cement (Figure 3). This trend records diagenetic alteration of the microbial rinds by freshwater.
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Figure 3. Stable isotopes of carbon and oxygen measured in ‰VPDB for the cement, rind, and nuclei layers of Maré oncolites.
INTERPRETATION AND IMPLICATIONS
One key feature of the microbial rind is its influence on meteoric diagenesis. Unlike subaerially exposed ooid grainstones, the microbial rind of the large oncolites inhibits nuclei dissolution and the formation of associated moldic porosity. These low-permeability rinds form an effective seal around the skeletal or composite nuclear grains. The original mineralogy of the rind is unknown, but it was likely low-Mg calcite based on crystal shape and absence of diagenetic features. Fibers in the rind, likely of microbial origin, indicate in situ biological formation (Figure 4d). A layer of blocky calcite spar cement, typical of meteoric conditions, encases the concentric oncolitic grains. The sharp boundary between the coated grain and these calcite cements suggest distinct environments of formation. The microbial rind promotes a rock fabric of well-preserved aragonitic nuclei. Whether aragonitic or calcitic, the permeability of the microbial rinds was sufficiently low at this early stage, to preclude freshwater access to the aragonitic fragments in the nuclei. This exception to a typical diagenetic sequence is well illustrated in thin section and SEM images and has important implications for the dissolution of aragonitic grains during exposure. Furthermore, the recognition of microbial textures, their formation, and distribution within existing facies models may be an exception to the diagenetic paradigm.
REFERENCES CITED IN PRESENTATION
Guyomard, T., Aissaoui, D. M., and D. F. McNeill, 1996, Magnetostratigraphic dating of an uplifted atoll, Maré Island, Loyalty Archipelago, S.W. Pacific: Journal of Geophysical Research, v. 101, p. 601-612.
McNeill, D. F., and A. Pisera, 2010, Neogene lithofacies evolution on a small carbonate platform in the Loyalty Basin, Maré, New Caledonienozoic carbonate systems: SEPM Special Publication, v. 95, p. 243-255.
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Figure 4. (A) Short core drilled June 2012 from the upper unit of the Maré atoll. Depth increases from top left to bottom right. Reefal units comprise the upper and lower portions of the core, with a stratified oncolite unit in the center. (B) Thin section photomicrograph of an oncolite sample from the top of an uplifted isolated reef (faro) near the town of La Roche, Maré, New Caledonia. (C) SEM image of biofilm remnants in the form of calcified cyanobacterial sheaths. (D) SEM image of low-permeability equant calcite crystals in the microbial rind.
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MICROBIAL COMMUNITY CHARACTERIZATION AND FUNCTIONAL GENE DIVERSITY OF OOLITIC GRAINS FROM
GREAT BAHAMA BANK Mara R. Diaz, Alan M. Piggot, Gregor P. Eberli, and James S. Klaus
KEY FINDINGS
Confocal microscopy and measurements of extracellular polymeric substances (EPS) on oolitic grains confirm the ubiquitous presence of attached biofilm communities with variations related to hydrodynamic conditions.
DNA sequencing reveals oolitic sediments harbor diverse bacterial communities that differ between depositional environments (e.g., active ooid shoals, non active ooid shoals, mat-stabilized calcareous sediments).
Differences in microbial functional gene diversity reflect distinct biogeochemical environments associated with active, non-active, and mat-stabilized sediments.
SIGNIFICANCE
Modern marine ooids are found in the Bahamas, the Yucatan, Arabian Gulf, South Pacific, and Shark Bay, Australia. In these regions, ooid shoal complexes can form margin parallel marine sand belts, tidal bar belts or platform interior blankets that stretch up to 100 km. While it is widely recognized that ooid formation and early marine cementation requires water supersaturated with respect to calcium carbonate, and an agitated environment that allows for CO2 degassing, the role of microorganisms has been debated for decades.
The physical presence or metabolic activity of microbes could influence the formation or early cementation of ooids. Microbial metabolism in the form of photosynthesis, anoxygenic photoautotrophy, sulfate reduction, denitrification, and ammonification can promote calcification by creating a more alkaline environment, whereas aerobic respiration, sulfide oxidation and ammonium oxidation can promote dissolution (Dupraz and Visscher, 2005). In addition, production and degradation of EPS by cyanobacteria and heterotrophs can either facilitate or inhibit CaCO3 precipitation. When abundantly present, the EPS acts as a “sponge”, inhibiting carbonate precipitation by trapping free divalent cations (i.e. Ca+2 and Mg+2). However, degradation of EPS by heterotrophs (ie. sulfate reducers, denitrifiers, etc) reduces the cation binding capacity of EPS by releasing free Ca+2, which triggers CaCO3 precipitation. The role of sulfate reducer bacteria (SRB) in EPS production and degradation in the lithifying mats of the stromatolites in the Bahamas has shown the importance of these microbial communities and associated metabolisms, which are considered the environmental engine that drives CaCO3 precipitation in lithifying microbial mats and stromatolites (Reid et al., 2000).
To better understand microbe-carbonate interactions we used a combined approach of clone library sequencing, functional gene-based microarray (GeoChip 4) and confocal laser scanning microscopy (CSLM) to study the microbial structure, functional gene diversity and metabolic potential of microbial communities associated with oolitic grains from active shoals, non-active and mat associated environments.
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RESULTS
EPS determinations
Confocal image analysis of oolitic grains stained with cyanine die-conjugated lectin demonstrated that all three environments harbored EPS-biofilm communities but their densities were different as a clear progression was seen in the amount of EPS coating from active to the mat-stabilized environment. These results are confirmed by phenol-sulfuric acid quantification of ooid EPS levels, which on average showed mat-stabilized EPS content to be ~10- 3.8 fold higher than active and non-active samples, respectively.
Phylogenetic Diversity
Bacterial communities were highly diverse and dominated by Proteobacteria (50-61%). Difference in microbial composition among the three environments were attributed to phylotypes within Proteobacteria and likely related to differences in the hydrodynamic characteristics of each environment, which could affect the biogeochemistry of inorganic nutrients and organic carbon availability. For instance, active ooid shoals are exposed to severe hydrodynamic forces that could hamper deposition of sedimentary organic matter.
Functional Metabolic Genes
To understand the diversity and functional capabilities of oolitic microbial communities, we used a high-throughput functional gene microarray. Geochip 4 gene array contains 83992 distinct probes covering 410 gene families associated with microbial functional processes. This study recovered a total of 12,432 genes, representing ~14.8% of the total number of genes in the array. Phylogenetically, up to 39 different lineages were documented representing 64 species from archaea, 823 from bacteria, 70 from fungi and 10 from others. The total number of genes varied significantly among the environments. Active communities retrieved the least number (8631); followed by mat-stabilized (9455) and non-active communities (10,102).
Carbon fixation. All three environments were characterized by high levels of carbon fixation genes associated with the Calvin–Benson CO2 pathway (RuBisCO). While active environments recovered the lowest signal intensities, non-active and mat environments recovered higher signals (Figure 2). Other autotrophic CO2 fixation processes included carbon monoxide dehydrogenase (CODH), BchY and ATP citrate lyase genes (aclB-ATP). The most significant differences in the level of CODH, was observed between active and non-active communities. In addition, all three environments recovered aclB-ATP and BchY genes, both of which are employed as biomarkers for anoxygenic phototrophs (Figure 2). BchY genes, were found to exhibit significantly higher levels in non-active environments (non-active vs mat: P<0.001; non-active vs active: P<0.001).
Figure 1. CLSM images of ooids and EPS measurements derived from active, non-active and mat-stabilized environments of Joulters Cay, Bahamas.
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Acetogenesis and Methane Metabolism. Methane production (mcrA), methane
oxidation (mmox, pmoA) and acetogenesis (FthFS), do not appear to play a key role in oolitic environments since genes associated with the aforementioned metabolisms were ~5-12 fold lower than other autotrophic CO2 fixation genes e.g. RuBisCO, CODH genes (Figure 2).
Carbon degradation. Carbon degradation appears to be important in oolitic environments as genes involved in carbon degradation were highly enriched. Carbon degradation genes were mostly affiliated with Bacteria, representing 65.3% of the total pool of C degrading genes, whereas fungi (e.g. Ascomycota and Basidiomycota) contributed 33.4%. Other minor contributors included members within the Archaea phyla e.g. Euryarchaeota and Crenarchaeota. The most labile forms of carbon, e.g. starch, hemicellulose and cellulose, recovered a wider array of enzymes (except for van A, acetylglucosaminidase, exochitinase, and mnP) than recalcitrant compounds e.g. aromatics, chitin, lignin and pectin.
Nitrogen Cycling. Five enzymes involved in denitrification including nitrate reductase (narG), nitrite reductase (nirK, nirS), nitric oxide reductase (norB) and nitrous oxide reductase (nosZ) were documented. When pooling all five functional genes, signal intensities were significantly different among sites (P= 0.01 to 0.001), with higher levels documented for non-active and mat-stabilized environments. Besides genes involved in denitrification, Geochip detected two key enzymes involved in ammonification processes e.g. glutamate dehydrogenase (gdh) and urease c (ureC), both of which showed significant differences (P=0.02 to 0.001) among sites.
Sulfur cycling genes. Most functional genes involved in sulfur cycling were detected. Genes related to sulfite reductase (dsrA/B, cysJ/I, SiR) and sulfur oxidation (e.g. soX) were most abundant, whereas genes related to sulfide oxidation (e.g. sqR, fccAB), and dissimilatory adenylsulfate reductase were less abundant (e.g. AprA/B). As previously documented, active environments consistently displayed lower signal intensities as opposed to mat and non-active environments. When combining all sulfite reduction genes, significant differences between pairwise comparisons of active vs mat (P = 0.001) and active vs non-active (P = 0.009) were detected. Sulfur oxidation recovered a total of 65 sox genes, most of which were associated with Alphaproteobacteia and Gammaproteobacteria.
Figure 2. Distribution of key genes involved in carbon cycling processes. The signal intensities were the sum of detected individual gene sequences for each functional gene. Gene classifications are as follows: Rubisco, CODH, aclB and BchY (autrophic Co2 fixation); mmox and pmoA (methane oxidation); mcrA (methane production) and FTHFS (acetogenesis).
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INTERPRETATION AND IMPLICATIONS
Differences in the bacterial communities of active, non-active, and mat-stabilized environments could influence nutrient recycling through variations in the level of both primary production and remineralization processes. Bacterial primary production appears to be driven by a diverse population of autotrophic CO2 fixers among which, Proteobacteria (Alpha-Beta, Gamma), Chlorobi, and Cyanobacteria are the most prevalent. This composite of microbes is not only aerobic phototrophs but includes anaerobic/aerobic anoxygenic phototrophs. All of these can drive the alkalinity towards carbonate precipitation by consuming bicarbonate and increasing local pH.
Carbon degradation appears as a major biological process in oolitic sediments. The widespread array of C substrates suggests these microbial communities have a high plasticity in metabolic pathways that enables them to degrade complex organic matter and exploit limited carbon sources, which are commonly associated with oligotrophic waters. For instance, the potential ability of microbes to use chitin, cellulose, and lignin as major carbon, nitrogen and energy sources has been documented in marine environments (Sieburth, 1976).
The presence of denitrifiers and sulfate reducers in active ooid sediments suggest their metabolic pathways are not constrained to non-active and mat-stabilized environments. Their ubiquitous occurrence is in agreement with earlier studies that documented their prevalence in sediments of the Bahama Bank (McCullum, 1970). In Drew (1911) carbonate precipitation in mud flats (whitings) appears to be mostly driven by the action of denitrifiers on the calcium salts present in seawater.
We also document and measure the abundance levels of EPS on ooid grains. Both, confocal microscopy and phenol-sulfuric acid methods showed increasing amounts of EPS from active to mat stabilized environments. While the synergistic effects of EPS and heterotrophs (i.e. SRB) on micrite laminae and lithification in microbialite systems have provided important insights into the inteplay of microbial communities and chemical processes that regulate CaCO3 precipitation, other studies have focused on the role of EPS and associated photosynthetic communities in the formation of the carbonate cortex in freshwater ooids. The waters of the Bahamas Archipelago are supersaturated with respect to CaCO3 and ideal for precipitation. However, based on pervasive EPS as well as microbes with the potential to influence carbonate precipitation, abiotic and biological factors are probably intertwined in the precipitation processes that form ooids, marine cements, and carbonate mud.
REFERENCES
Dupraz C., and P. T. Visscher, 2005, Microbial lithification in marine stromatolites and hypersaline mats: Trends in Microbiology, v. 13, p. 429-438.
Drew, G. H., 1911, The action of some denitrifying bacteria in tropical and temperate seas and the bacterial precipitation of calcium carbonate in the sea: Journal of the Marine Biological Association, v. 9, p. 142.
McCallum, M. F., 1970, Aerobic bacterial flora of the Bahama Bank: Journal of Applied Bacteriology, v. 33, p. 533-542.
Reid, R. P., Visscher, P. T., Decho, A. W., Stolz, J. F., Bebout, B. M., Dupraz, C., Macintyre, I. G., Paerl, H. W., Pinckney, J. L., Pufert-Bebout, L., Steppe, T. F., and D. J. DesMarais, 2000, The role of microbes in accretion, lamination and early lithification of modern marine stromatolites: Nature, v. 406 p. 989-992.
Sieburth, 1976, Bacterial substrates and productivity in marine ecosystems: Annual Review of Ecology, Evolution, and Systematics: v. 7, p. 259-285.
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PORE STRUCTURE AND PETROPHYSICALCHARACTERIZATION OF MICROBIALITES
Gregor P. Eberli, Ralf J. Weger, Jan Norbisrath, and Giovanna della Porta1
1) University of Milano
KEY FINDINGS
Microbialites such as stromatolites, travertine and microbially cemented hardgrounds have simple pore structures of highly variable size.
The pore structure of microbialites produces a stiff frame that results in:
o high velocity at a given porosity
o good pressure resistance
o maintenance of primary porosity to great burial depth
Resistivity is high compared to other carbonates as a result of scattered and isolated pores.
SIGNIFICANCE
Microbialites are one of the major reservoir facies in the pre-salt offshore Brazil and stromatolites are an important reservoir facies in some Proterozoic carbonate reservoirs. The reason why the microbialites maintain good reservoir quality to a great burial depth is their remarkable amount of preserved primary porosity.
This study investigates the porosity, permeability, sonic velocity and resistivity, in conjunction with the pore structure, to identify the petrophysical characteristics of microbialites. The data set and its interpretation are intended to help explain the petrophysical signature of microbialites in log and seismic data. The incorporation of modern microbialites helps to capture the microbial processes that produce the characteristic petrophysical properties of microbialites.
Figure 1. The data set for this study consists of microbially cemented submarine hardgrounds from the southern end of the Tongue of the Ocean (left), modern stromatolites from the Bahamas (middle), and 70 travertine samples from unknown quarries in Italy (inset = photomicrograph displaying travertine porosity). The travertine samples were collected and classified by Giovanna della Porta and her team.
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INFLUENCE OF MICROBIAL ACTIVITY ON PETROPHYSICAL PROPERTIES
Microbial activity influences petrophysical properties because it 1) promotes mineralization and dissolution, 2) can fuse grains together, 3) can precipitate microbial boundstone, and 4) produces unique pore structures. Most of these processes positively affect the rock stiffness. As a result, microbialites are petrophysically characterized by high velocity, high resistivity and relatively high porosity. The early microbial processes that construct and strengthen the rock are also responsible for their ability to resist compaction. This early stiffening of microbialites is seen in modern hardgrounds and stromatolites where velocity does not, or only slightly increases with pressure (Figure 2). Likewise, travertine samples display a small but consistent increase of velocity with increasing confining pressure.
VELOCITY AND PORE STRUCTURE OF MICROBIALITES
Table 1 provides an overview of the measured petrophysical properties. Porosity is lower in the travertine samples compared to the marine hardgrounds and stromatolites. Microbialites have a high velocity despite their high porosity (Figure 3). In particular, stromatolites and travertine samples display high velocities even at high porosity.
Table 1. Petrophysical Properties
Ø K (mD) Vp (dry) Vs (dry) Vp (wet) Vs (wet) m
Travertine 0.4 – 23.4 0.01 – 1470 4658 – 5976 2611 – 3233 4891 - 6131 2297 - 3183 1.9 – 5.8
Hardground 15 – 41.6 3266 - 4986 1513 - 2662
Stromatolite 12.8 – 30.1 70 - 1694 4407 - 5402 2334 - 2999 2.1 – 2.6
Figure 2.
Velocity evolution of modern hardgrounds (TOTO), stromatolites and travertine samples with increasing pressure.
Most hardgrounds and stromatolites do not display a velocity increase with increasing confining pressure. Travertines have a high acoustic velocity and display a small increase with increasing pressure.
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The high velocities of microbialites are caused by their pore structures. The pore types are mostly interparticle and intraframe (only observed in the travertines). In rock physics models intergranular porosity rocks are usually considered to have compliant pores and low stiffness, and therefore low acoustic velocity. In contrast, rocks with moldic and vuggy porosity are classified as stiff, high aspect ratio rocks, resulting in high bulk modulus and velocity (Lucia and Conti, 1987). Microbialites have a high velocity because microbial processes weld grains together in a form of micritic bridging cement or micritic crusts to form a strong frame of intergranular porosity (Hillgärtner et al., 2001). Despite the microbial bindings the pore structure is relatively simple, resulting in a low Perimeter over Area (PoA) when quantified with digital image analyses. Yet, the pore size varies considerably. Rocks with large simple pores are generally fast (Weger et al., 2009). The measured microbialites follow this trend.
Figure 3.
Velocity -porosity plot of the three data sets and a comparison of the data set of Weger et al. (2009).
In samples with overlapping porosity travertine has the highest acoustic velocity at a given porosity, followed by stromatolites. Marine hardground samples (TOTO) have the highest porosity and high velocity.
Figure 4.
Characterization of the pore system in microbialites with digital image analysis parameters compared to the data set of Weger et al. (2009) and Verwer et al. (2010)
The microbialites have simple (small PoA) pores of variable size (DOMsize).
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RESISTIVITY
Resistivity as expressed by the Formation Factor is high in travertines. The cementation factor “m” in the travertine samples displays a very wide range from 1.9 – 5.8. In comparison, “m” is 1.72 – 4.14 in the data set of Verwer et al. (2011) that included a variety of carbonate textures and pore types.
The high resistivity of the travertines has its roots in the unconnected pore structure and the relatively small number of pores. Macropore structures are simple and variably sized (Figure 4). These pores, like many of the micropores, are very scattered and porous areas are isolated by very dense areas without visible porosity. This, together with the low pore count, results in the high cementation factor and high resistivity.
REFERENCES
Hillgärtner, H., Dupraz, C. and W. Hug, 2001, Microbially induced cementation of carbonate sands: are micritic cements indicators of vadose diagenesis? Sedimentology, v. 48, p. 117-131.
Lucia, F. J. and R. D. Conti, 1987, Rock fabric, permeability, and log relationships in an upward-shoaling, vuggy carbonate sequence: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 87-5, 22 p.
Weger R. J., Eberli, G. P., Baechle, G. T., Massaferro, J. L., and Y. F. Sun, 2009, Quantification of pore structure and its effect on sonic velocity and permeability in carbonates: AAPG Bulletin, v. 93, no. 10, p. 1-21.
Verwer, K., Eberli, G. P., Baechle, G. T., and R. J. Weger, 2010, Effect of carbonate pore structure on dynamic shear moduli: Geophysics, v. 75, p. 1–8.
Verwer, K., Eberli, G. P. and R. J. Weger, 2011, Effect of pore structure on electrical resistivity in carbonates: AAPG Bulletin, v. 95, p. 175-190.
Figure 5.
Formation factor vs. porosity of the travertine compared with stromatolites and the data set of Verwer et al. (2011).
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ROCK FLUID INTERACTION: HOW DISSOLUTION INDUCED CHANGES IN PORE STRUCTURE AFFECT
ACOUSTIC VELOCITYRalf J. Weger, Peter K. Swart, Gregor P. Eberli, and Mark Knackstedt
KEY FINDINGS
Carbonate rocks don’t need millennia to change with respect to acoustic properties.
Observed changes in velocity during dissolution are significantly smaller than expected from porosity increases and predicted by rock physics models.
Dissolution occurs preferentially on sub-micron scales.
Pore structure simplification during dissolution inhibits drastic decreases of acoustic velocity.
SIGNIFICANCE
A large number of publications examine the influence of temporary, reversible effects of pore fluids on the rocks framework and its acoustic properties (e.g. Baechle et. al., 2009). However, assessments of the influence of cementation and dissolution on the rock stiffness, porosity preservation and acoustic properties are rare. This study aims to shed light on how carbonate reservoir rocks behave in contact with formation fluids. Laboratory experiments on precipitation and dissolution of carbonates confirm that the chemical reaction with the pore fluid is a continuous process that changes both fabric and pore structure within days. The changes caused by dissolution and/or precipitation include alteration of the rocks pore structure, porosity, permeability, and its acoustic properties. As a result, the comparison of different vintages of acoustic data either from logs or from time-laps seismic data carries high uncertainties.
Figure 1. High-resolution CT-scan of a rudist grainstone before (left) and after (right) the dissolution experiment. The light color is the solid portion, the gray is the pore space. An increase of the grey color from the left to right documents the increase of pore space by ~10%.
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This study examines and attempts to quantify the changes that occur during precipitation and dissolution in a controlled laboratory setting, specifically those affecting porosity, permeability and sonic velocity in carbonates. We observe both an increase in rock stiffness in the transition from sediment to rock during precipitation, and drastic alterations of the rocks fabric and pore structure during dissolution. CT-scans before and after dissolution are used to correlate the observed changes in physical properties to the changes in the rock fabric and also to determine where exactly the dissolution took place and what portions of the rock were altered (Figure 1).
EXPERIMENTAL SETUP
The experimental setup allows for precipitation or dissolution of calcite cement by filtration of pore fluids and the simultaneous measurement of acoustic velocity. A filtration system provides continuous flow of fluids that are supersaturated (or under-saturated) with respect to CaCO3 through the rock samples and simultaneously monitors the fluid properties. Any changes are immediately compensated for by altering the fluid to maintain the original fluid composition. An Aqua Medic SP 3000 peristaltic pump provides a continuous flow rate of 50 ml/min or less. Temperature and pH of the pore fluid are monitored and logged in 5 sec intervals.
Although precipitation of calcite cement is relatively fast, it takes several days-weeks to produce a rock from loose sediment grains. Initially the sediment is compacted to 5 MPa and subsequently saturated. A stepwise initial pressurization to 40 MPa ensures full compaction. Throughout the experiment the confining pressure is held at 10 MPa and the pore pressure is given by the flow rate. Acoustic velocity is measured in one-hour intervals. After the experiment samples are analyzed under SEM to determine where and how many crystals have formed during the experiment. Several precipitation experiments were performed during which the CaCO3 concentration was kept close to but above saturation for ~90 hrs.
Dissolution occurs much faster than precipitation; initial experiments were performed for 5 days, and a subsequent dissolution experiment was performed for 3 days while maintaining the pore fluid at a pH no greater than six. Both SEM images and CT –scans where acquired before and after the experiment to visualize and analyze the changes (Figure 1). Particular attention was paid to determining where the dissolution of calcium carbonate had occured. CT-scan data was calibrated and segmented into pore space, rock space, and intermediate (microporous) space (Figure 2).
PRECIPITATION EXPERIMENTS
All experiments produced permanent alteration of the rocks within only hours or days. The precipitation experiments show small but measurable increases in acoustic velocity and decrease of porosity. SEM analysis revealed variable amounts of small crystals precipitated on the ooids. Two distinct types of crystallization are identified: 1) crystals formed directly at the grain to grain contact, fusing the ooids together into a more coherent unit, causing an increase in acoustic velocity of ~50 m/s to ~300 m/s; 2) precipitation occurred in the form of needle-like crystals.
In samples where precipitation creates a stiffer framework by fusing grains together the observed increase in velocity is comparable with model prediction (Extended Biot Theory model) and can be attributed to the fusing of grain-grain contacts. In other samples, velocity increased throughout the experiment from ~2200 m/s to ~2500 m/s (3400 m/s to 3700 m/s), but the change is disproportionally small with respect to the
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porosity decrease from over 36% (35%) to ~25% (28%) that occurred during the experiment. This small velocity increase is attributed to the needle-like crystals that formed in these samples, which do not increase the stiffness of the rock but produce small particles in the inter-grain space and create a more complex pore-system.
DISSOLUTION EXPERIMENTS
Several dissolution experiments were performed during which the CaCO3 concentration was kept close to but below saturation for 3-5 days. Pleistocene ooid grainstone and Cretaceous rudist grainstone samples were used, and both SEM images and CT–scans were acquired to analyze the changes. During all experiments porosity increased drastically by 10-15%. The dissolution experiments show small decreases in acoustic velocities from ~3200 m/s to ~3000 m/s.
In the first two samples, channel-like dissolution is observed and appears to leave the stiff part of the rock frame intact while substantially increasing the rocks porosity. Small fragile particles and small grains appear to be dissolved first and the proportion of microporosity decreases. CT-scan analysis shows that the proportion of microporosity drastically decreases during the experiment (Figure 3).
Measuring the Perimeter over Area (PoA) and Dominant (DOM) size using digital image analysis indicates that the process of dissolution simplified the overall pore geometry (Figure 4). CT-scan derived parameters show clear increases in DOM size and decreases in PoA.
Figure 2. Comparison of high-resolution CT-scan slices before and after the experiment. On these slices pore space is black. The difference in pore space before and after the experiment is given in grey tones in the picture to the right. This provide the visualization of the porosity enhancment and illustrates that alterations are expanding upon existing porespaces and pathways, and decrease the amount of microporosity.
Figure 3. Comparison of macro and micro porosity derived from individual slices of CT-scans before (blue) and after (red) dissolution experiment. After dissolution experiment, nearly all micro porosity has disappeared.
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The large increase in porosity combined with the simplification of the pore geometry due to selective dissolution make the frame flexibility substantially lower after dissolution than it was before. The lower frame flexibility and the less complex pore geometry result in only moderate velocity decrease during dissolution.
Figure 4. (Bottom) Velocity-porosity cross plots of a diverse set of samples (Weger et al., AAPG 2009) with theoretical values for frame flexibility in the background. Color represents geometrical parameters Perimeters over Area (PoA) and Dominant Size (DomSize). (Top left) Thin section images corresponding to the nine enlarged dots on the cross-plots. (top right) Digital image parameters PoA and DOMsize obtained from CT-scans of dissolution sample before and after dissolution. (Inside red eclipse) Dissolution sample before and after dissolution with DIA parameters in color.
REFERENCES
Baechle, G. T., Eberli, G. P., Weger R. J., and J. L. Massaferro, 2009, Changes in dynamic shear moduli of carbonate rocks with fluid substitution: Geophysics, v. 74, no. 3, p. 135-147
Weger R. J., Eberli, G. P., Baechle, G. T., Massaferro, J. L., and Y. F. Sun, 2009, Quantification of pore structure and its effect on sonic velocity and permeability in carbonates: AAPG Bulletin, v. 93, no. 10, p. 1-21.
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NEW INSIGHTS INTO SLOPE PROCESSES FROM THE BAHAMAS AND WEST FLORIDA
Gregor P. Eberli, Donald F. McNeill, Thierry Mulder1, Emanuelle Ducassou1, Dierk Hebblen2, Claudia Wienberg2, and
Paul Wintersteller2 1) University of Bordeaux)
2) University of Bremen
KEY FINDINGS
New data from the north slope of Little Bahama (LBB), the western Great Bahama Bank (GBB) and the west Florida shelf refine existing models of processes, morphology and facies distribution along carbonate slopes.
The steep platform margin is less prone to failure than previously thought. In contrast, large-scale slope failures occur on the low-angle mid–lower slopes that produce extensive carbonate mass transport complexes (MTC) on the toe-of-slope.
The slope is dissected by a variety of incisions that include regularly spaced gullies (20 – 30 m deep), a series of channels (up to 60 m deep), and spectacular slope canyons (up to 200 m). Channelized and canyon-dissected areas abruptly end laterally.
Creeping of the slope is common on many places along strike.
The size and abundance of the debris of the MTC is highly variable but where present the debris serves as a foundation for an extensive deep-water ecosystem.
SIGNIFICANCE
Modern carbonate platforms occur in a variety of settings as attached or isolated platforms and may be surrounded by steep slopes that can exceed 45º. Early work in the Bahamas contributed to the understanding of the slope anatomy and facies distribution, in particular carbonate turbidites (Mullins et al., 1984). One of the objectives of the first leg of the Ocean Drilling Program (ODP) was to explain the slope variability of accretionary, bypass and erosional slopes (Austin et al., 1988). The evolution of the prograding platform and slope within a sequence stratigraphic context was documented by seismic and coring during ODP Leg 166 (Eberli, 2000). Subsequent submersible expeditions added information about the role of early diagenesis on slope stability (Grammer et al., 1993) and carbonate redeposition in respect to sea level and ocean currents (Anselmetti et al., 2000). Several of these studies have made it into textbooks as modern analogs for carbonate slopes (Playton et al., 2012).
Yet, what was still missing was a comprehensive overview of the seafloor morphology that would delineate the dimension and relationships of the different facies elements of the margin and slope. New multibeam and seismic data, combined with piston coring and visual observation with a Remote Underwater Vehicle, that were collected in two recent cruises in the Bahamas and Florida provide for the first time high-resolution digital elevation maps of the slope on LBB and GBB and the edge of the West Florida shelf.
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Taken together these new data refine existing models of processes, morphology and facies distribution along carbonate slopes. In particular, they indicate the importance of slope instability on the lower slope, the lateral variability of slope canyons and the extent of debris fields on the toe-of slope that serves as the habitat of a diverse deep-water ecosystem.
DATA SETS
Figure 1. The data sets for this study are of two generations. In the eighties submersible dives at Chub Cay and the Tongue of the Ocean investigated the platform margin and uppermost slope. Multichannel seismic data along the western margin of GBB were collected in the nineties. The new generation of data are collected in the last 10 years from a multitude of platforms: ship based multibeam data for the slope of LBB in 2002; AUV- based high-resolution multibeam; side scan sonar and sub-bottom profiles at five sites for cold-water coral environments in 2005; multibeam, single channel and multichannel seismic data and piston cores were collected onboard R/V Le Suroit in 2010 north of LBB and west of GBB during the Carambar cruise, chief scientists on board were Thierry Mulder and Emanuelle Ducassou of the University of Bordeaux; multibeam acoustic and single channel seismic data, grab samples and piston cores, as well as visual observations with an ROV revisited two sites at the western side of GBB and also on the western edge of the West Florida shelf (not shown on map) in the spring of 2012 on board R/V Maria Merian during cruise MSM 20-4 with Dierk Heblen as “Fahrtleiter”. The Carambar cruise was financed by the French National Science Foundation and cruise MSM 20-4 was paid for by the German Funding agency.
THE PLATFORM MARGIN AND THE ONLAPPING WEDGE
On most modern carbonate platforms the platform edge is between 100 m to a few kilometers seaward in water depths of around 20 – 30 m. Seaward of the platform edge, a near vertical cemented slope of approximately 100 m height, called either the wall or
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escarpment, develops on all Bahamian carbonate platforms on both the windward and the leeward side (Grammer et al., 1993). The steep slope (45 - 70 ) beneath the escarpment is also cemented with a thin veneer of sediment and occasionally large boulders and talus debris. For example along the southern side of LBB, only 2 - 28 boulders per kilometer are observed on multibeam data. No large-scale margin collapse is observed on either LBB or on the northernmost 180 km along western GBB.
The steep uppermost slope beneath the escarpment is onlapped by soft sediment that can reach up to 125 m in thickness in the Holocene. The wedge often displays a moat at the point of contact indicating strong currents along the upper slope. The wedge thins downslope and interfingers with coarse-grained sediment and debris from mass wasting.
THE SLOPE
The slope of western GBB where declivity is less than 4 contains five major morphological elements (Figure 2). They are 1) regularly-spaced gullies in the soft sediment of the onlapping wedge, 2) numerous small and a few large-scale slope failures in the slope of less than 2 that produce scars up to 100 m in height and several kilometers in length, 3) kilometer long scars that dissect the soft sediment, indicating the continuous movement on the low angle slope. This scar is associated with features that are interpreted as creeping of the slope. Single-channel seismic data and multibeam data provide evidence that creeping is major slope process that is occurring along the entire slope (Figures 3 and 4). Creeping seems to be restricted to the soft sediment portion of the slope because it decreases as the sediment thickness of the mud wedge decreases. At the toe-of-slope, the mud wedge thins and as a result older boulders and blocks from earlier mass transport events litter the sea floor (Figure 4).
Figure 2. Multibeam bathymetry of approximately 50 km of the slope of western GBB, displaying four major slope features. (1) Soft sediment wedge that is dissected by regularly spaced gullies, (2) large scars and mass transport complexes, (3) long scars that displace the soft sediment wedge, and (4) partly buried debris field on the toe-of-slope that documents earlier slope failure and mass transport debris.
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Figure 3. (Left) Multibeam bathymetry of western GBB with location of close-up. (Right) Close-up of slope above the scar of the MTC. The muddy slope shows an incipient slope failure with buckled sediment below that are interpreted as the result of slope creeping.
Figure 4. (Left) Single-channel seismic profile taken during the MSM 20-4 cruise across a slope scar and the modern sediment. The scar is approximately 50 m high. The most recent sediment is imaged as a thinly layered succession. Down slope of the scar, these sediments form a wrinkled sea-bottom morphology due to sliding along a surface (red in Figure). (Right) Multibeam bathymetry of the slope beneath the scar. Divergent ridges that are dotted with boulders dominate the seascape; they are the debris field of an earlier partly buried MTC .
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Figure 5. Detail of slope canyons on the north slope of LBB (above) and a multichannel seismic section across these canyons. The 3-D view of three canyons with coalescing arcuate scars (As) forming the amphitheatre envelope, retrogressive erosion (Re) and terraces (T), talweg incision (Th) and channel (Ch). Canyon numbering in red. Bottom) High-resolution multichannel seismic profile showing talweg incision through morphologic terraces (from Mulder et al., 2012b)
Slope failures in the form of mass transport complexes (MTC) occur at several locations on the lower slope The largest of these mass transport events consists of three scarps of over 9 km length (Figure 2). The scar height ranges from 80 to 110 m (Figure 3). The entire MTC covers an area of about 300 km2 (Mulder et al., 2012a). The slope angle where this MTC is located is less than 2 . At the west Florida shelf the entire shelf breaks off at this low angle, indicating that major mass wasting events in carbonates do not require a steep slope.
ROV observations, grab sampling and coring in both the channelized slope and the MTC of GBB and West Florida document that the mass wasting debris is the foundation for cold-water corals mounds. This colonization of antecedent topography explains why neither mound shape nor orientation of the coral mounds along western GBB correlate to the local current pattern (Correa et al., 2011). ROV images show boulders either partly or completely covered with a diverse cold-water coral fauna (Figure 6).
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Figure 6. (Left) Boulder partly colonized by deep-water fauna with surrounding coarse sediment. Water depth 720 m; location west of Bimini (above). (Right) Block covered by cold-water corals and associated fauna. Water depth 650 m, western margin of GBB. Photos taken with ROV during cruise MSM 20-4, 2012.
REFERENCES
Anselmetti, F. S. Eberli, G. P., and Z.-D. Ding, 2000, From the Great Bahama Bank into the Straits of Florida: A margin architecture controlled by sea level fluctuations and ocean currents: Geological Society of America Bulletin, v. 112, p. 829-846.
Austin, J. A., Jr., Schlager, W., et al., 1988, Leg 101—an overview: Proceedings ODP, Scientific Results, v. 101, p. 455-472.
Correa, T. B. S., Grasmueck, M., Eberli, G. P., Reed, J., Verwer, K., and S. Purkis, 2012,Variability of cold-water coral mounds in a high sediment input and tidal current regime, Straits of Florida. Sedimentology, v. 59, p. 1278-1304, doi: 10.1111/j.1365-3091.2011.01306.x
Eberli, G. P., 2000, The record of Neogene sea-level changes in the prograding carbonates along the Bahamas Transect—Leg 166 synthesis: Proc. ODP, Sci. Results, 166: College Station, TX (Ocean Drilling Program), v. 166, p. 167–177.
Grammer, G. M., Ginsburg, R. N., and P. M. Harris, 1993, Timing of deposition, diagenesis, and failure of steep carbonate slopes in response to a high-amplitude/high-frequency fluctuation in sea level, Tongue of the Ocean, Bahamas: AAPG Memoir 57, p. 107-131.
Mulder, T., Ducassou, E., Eberli, G. P., Hanquiez, V., Gonthier, E., Kindler, P., Fournier, F., Leonifde, P., Billeaud, I., Marsset, B., E., Reijmer, J. J. G., Bondu, C., Joussiaume, R., Pakiades, M., 2012a, Morphology and sedimentary processes along a carbonate slope. Example of the Great Bahama Bank: Geology, v. 40, p. 603-606, doi:10.1130/G32972.1
Mulder, T., Ducassou, E., Gillet, H., Hanquiez, V., Tournadour, E., Combes, J., Eberli, G. P., Kindler, P., Gonthier, E., Conesa, G., Robin, C., Sianipar, R., Reijmer, J. J. G., and A. François, 2012b, Canyon morphology on a modern carbonate slope of the Bahamas: evidence of a regional tectonic tilting: Geology, v. 40, p. 771-774.
Mullins, H. T., Heath, K. C., Van Buren, H. M., and C. R. Newton, 1984, Anatomy of a modern open-ocean carbonate slope: northern Little Bahama Bank: Sedimentology, v. 31, p. 141-168.
Playton, T. E., Janson, X., and C. Kerans, 2012, Carbonate slopes. In: Walker, R. G., and N. P. James (Eds.), 2012, Facies model, response to sea level change: Geological Association of Canada Press, p. 447-474.
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VARIABILITY OF SLOPE AND BASIN FLOOR MORPHOLOGYALONG SOUTHWESTERN GREAT BAHAMA BANK
Andrew Jo, Gregor P. Eberli, and Mark Grasmueck
KEY FINDINGS
Newly acquired multibeam bathymetry, backscatter, and sub-bottom profile data provide magnificent visualization of sea bottom morphology that is highly variable along strike.
The steep margin is onlapped by an up to 125 m thick mud wedge that forms a continuous moat of up to 30 m depth along the margin.
A multiphase mass transport complex with an 11 km long slump scar in the upper slope sheds large blocks (30 m high, 500 m length) 13 km into the basin.
Backscatter and sub-bottom profile data reveal an unexpected but consistent sediment distribution along the slope. The mud wedge forms the muddy ~ 6- 7 km wide upper slope, the middle to lower slope is ~ 20 km wide with coarse-grained deposits supporting channels and boulders that finally transition into the fine-grained basin floor.
SIGNIFICANCE
Carbonate slope and basin floor reservoirs are considered underdeveloped in hydrocarbon exploration. Sedimentary processes on carbonate slopes vary greatly depending on various external and internal controls and hence such deposits are very heterogeneous in composition, architecture, and lateral continuity (Playton et al., 2010). This data set allows the heterogeneity of slope and basin floor morphology to be documented along dip and strike. The over 100 km long data set is unique in assessing the distribution, dimensions and variability of the slope facies that, together with ground truthed data, will improve our understanding of slope to basin floor depositional processes.
DATA SET
The data for this study was acquired by the Bahamas Petroleum Company. The study area is located at the confluence of the Old Bahama and the Santaren Channel and covers the western slope of Great Bahama Bank (GBB), the basin floor and northeast dipping seafloor covering the SE-NW trending Cuban fold and thrust belt. The data include high resolution multibeam bathymetry, backscatter, and single channel seismic survey in an area of 6,512 km2, 742 lines totaling 5,342 km. The data provide unprecedented visualization of slope and basin floor morphology along southwestern GBB (Figure 1).
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THE DEPOSITIONAL ENVIRONMENTS OF SOUTHWESTERN GBB
Three main depositional areas are recognized in the study area (Figure 1). 1) The GBB margin and slope has slope angles that vary from 4 - 76 and is
approximately 25 km wide. The main morphologic elements of the slope are: a steep margin with an onlapping mud wedge, channelized slope with blocks and lobes of redeposited carbonates, and mass transport complexes.
2) The basin floor is about 15 km wide and has homogeneous sediment cover. In some places boulders and distal portions of the lobe reach the basin floor.
3) A northeast sloping seafloor with a slope angle of 0.08 in the southwestern portion of the study area is characterized by pockmarks in soft sediment.
Figure 1. Location of study area in the southwestern Great Bahama Bank and three different depositional environments.
Upper Slope Morphology: margin and onlapping mud wedge
The morphology of the margin and upper slope varies from the northern to southern portion of the study area. In the northern and mid portion, the margin has a declivity of up to 30° (Figure 2.a), while in the southernmost portion of the study site, it is up to 76°. A nearly transparent mud wedge onlaps the margin and thins basinward. At its maximum extent it reaches 125 m in thickness and extends 4.2 km from the margin (Figure 2.b). Outside of the study area where the wedge was cored it consists of 90% fine-grained aragonite mud and is dated as less than 11 kyrs in age (Wilber et al., 1990; Malone et al., 2001). This mud wedge is mostly off-bank transported lime mud. In the study area the wedge is not just onlapping but it forms a moat that is 20- 30 m deep and runs for tens of kilometers (Figure 2.b).
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Figure 2. (a) Multibeam bathymetry from -50 to -350 m along Great Bahama Bank, displaying the steep margin and the onlapping mud wedge with the characteristic moat. (b) Sub-bottom profile across the thick mud wedge onlapping the margin. The wedge thins basinward across buried blocks.
Middle and Lower Slope Morphology
Gullies perpendicular to the margin run from the mud wedge into the middle slope that contains abundant features of redeposition. Several large-scale channel systems with fan-shaped terminations occur along the slope. They are of variable lateral extent but similar in length, typically 15 – 18 km. One of these terminal fans is over 3.3 m wide and 35 m thick (Figure 3). In the north, large blocks and mounds are scattered as far as 20 km into the basin. The mounds are slightly elongated and surrounded by a moat; they are likely cold-water coral mounds sitting on boulders and large blocks.
Further to the south, the sediment apron with occasional lobes covers 49 km of the middle-lower slope. The lobes extend up to 18 km into the basin. Regularly spaced gullies incise the middle slope and act as sediment transport pathway to the basin floor.
b
a
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Figure 3. Multibeam bathymetry of two channelized slope areas with fan-like terminations and an area with scattered mounds. Inset = Single channel seismic profile across fan deposit at the mouth of a single channel.
The southernmost area exhibits intensive slumping and mass transport deposition. A
slump scar up to 40 m in height runs for 11 km high up on the margin, indicating a major margin collapse (Figure 4). The debris traveled as far as 13 km basinward. Blocks are up to 30 m high and 500 m in diameter. A second mass wasting event indicates repeated slope failure in this part of GBB.
Figure 4. (a) Steepness generated from bathymetric map showing high slope angle (~ 76° = yellow line) in the southernmost area. Slump scars up to 40 m in height are also observed in the lower slope. Debris from this process are dispersed as far as 13 km into the basin. (b) Three dimensional view of the margin with onlapping wedge, lower slope scars and debris field.
b a
Slope AngleValue
High : 76
Low : 0
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SEDIMENT DISTRIBUTION
Backscatter and sub-bottom profile calibrated with ground-truthed data from northern part of Great Bahama Bank gives insight into sediment characteristics and their consistent distribution. The margin and upper slope portion are covered by muddy sediments of 6-7 km wide. It changes abruptly in the ~20 km wide middle to lower slope, where it is covered by large blocks of redeposited carbonate, channels, and sediment apron. The sediment finally transitions into fine-grained basin floor.
POCKMARKS
The northeast dipping slope is dotted by 22 pockmarks with differing morphologies. They range in diameter from 100 to 2200 m, and depth from 10 to 130 m. There is no correlation between pockmark diameter and depth due to sediment infill through time (Figure 5).
Figure 5. (Top) Map of the pock marks on the slightly dipping seafloor. (Below) Plot of pockmark depth versus diameter. The diameter of pock marks has no relationship with the depth. Units in meters.
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INTERPRETATION AND IMPLICATIONS
The margin parallel distribution of the mud wedge, the apron of coarse to very coarse redeposited sediment and the muddy basin floor is consistent for over 100 km along the slope of southwestern GBB. Yet the slope and basin floor morphology varies along strike. Margin declivity is approximately 30 but changes abruptly to approximately 76 in the southernmost portion of the study area. This sudden increase might be caused by large scale margin failure.
The moat between the onlapping mud wedge and the margin might be generated by currents sweeping down and along the margin. Similar moats have been observed in submersible dives in the southern Exumas.
The sharp transition from mud to a slope apron made up of redeposited sediment has not been reported from any other locality. The variety of channelized systems, fan shaped lobes, and debris fields give evidence of the multitude of sedimentary processes that feed the apron.
The pockmarks in the southwest corner of the study area are located at the outermost boundary of the Cuban fold and thrust belt. It is likely that migrating fluids and/or hydrocarbons along the basal detachment are contributing to the high amount of pock marks that are the surface expression of fluid escape (Sun et al., 2011).
REFERENCES
Malone, M. J., Slowey, N. C., and G. M. Henderson, 2001, Early diagenesis of shallow-water periplatform carbonate sediments, leeward margin, Great Bahama Bank (Ocean Drilling Program Leg 166): Geological Society of America Bulletin, v. 113, p. 881-894.
Playton, T. E., Janson, X. and C. Kerans, 2010, Carbonate slopes, in James, N. P., and R. W. Dalrymple, eds., Facies Models 4: St. Johns, Newfoundland, Canada, Geological Association of Canada, p. 449-476.
Sun, Q. L., Wu, S. G., et al., 2011, The morphologies and genesis of mega-pockmarks near the Xisha Uplift, South China Sea: Marine and Petroleum Geology, v. 28, no. 6, p. 1146-1156.
Wilber, R. J., Milliman, J. D. and R. B. Halley, 1990, Accumulation of bank-top sediment on the western slope of Great Bahama Bank: rapid progradation of a carbonate megabank: Geology, v. 18, p. 970-974.
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COMPOSITION OF COLD-WATER CORAL MOUND“MATTERHORN” AND ITS SURROUNDING SEDIMENTS IN
THE STRAITS OF FLORIDARani Sianipar, Gregor P. Eberli, and Emmanuelle Ducasou1
1) University of Bordeaux
KEY FINDINGS
A 7.03 m core into the 110 m high “Matterhorn” mound retrieved an unlithified succession of coral floatstone with variable matrix composition.
The percentage of corals within the coral floatstone peaks in the fine-grained matrix unit with a 49.1% coral content.
Grain size alternations in the matrix of the floatstone are interpreted to be related to glacial and interglacial deposition. Corals grow during both times but are more abundant in the interglacial periods.
The mineralogy displays a downcore trend of decreasing aragonite and increasing low magnesium calcite that is related to early diagenetic changes.
Cross-bedded pteropod-foraminifera grainstone sediments surround the “Matterhorn” giving the sedimentary record of the bi-directional tidal current regime in the Straits of Florida.
SIGNIFICANCE
The Straits of Florida has received increasing attention over the last decades in term of cold-water coral (CWC) mound ecosystems (Neumann and Ball, 1970; Neumann et al., 1977; Paull et al., 2000; Grasmueck et al., 2006; Reed et al., 2006; Roberts et al., 2006; Correa et al., 2012). New seafloor mapping technologies and considerable research efforts in the Straits of Florida have documented that a variety of factors influence the initiation, growth and distribution of CWC mounds. Research using an autonomous underwater vehicle (AUV) and submersible observations at 3 sites on Great Bahama Bank (GBB) slope (Figure 1) documented that variability in sedimentation rates, current regime, and underlying
Figure 1. Overview of the study area in the Straits of Florida with white boxes showing location of AUV survey (Modified from Grasmueck et al., 2007).
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topography control the distribution, development, and morphology of CWC mounds (Correa et al., 2012). Simultaneously, Rosenberg (2011) measured temperature ranging from ~4 to 100C, salinity from ~33.6 to 35.3, and seawater density from ~ 27.35 to 27.8 kg/m3 around the CWC in this area.
The sediments within and adjacent to the mounds had only been sampled on the surface preventing estimates of sediment accumulation rates and recognition of sedimentary structures. This study fills these gaps by analyzing two cores retrieved during the CARAMBAR cruise in November 2010 on the slopes of GBB. Both cores were taken in the area where Correa et al. (2012) identified the largest mound in the AUV data set, specifically in GBB site 3 west of Bimini (Figure 2).
Figure 2. (B) High-resolution bathymetry map of GBB site 3 western Bimini on the slope of GBB with the red box outlining the Matterhorn (C) Location of piston cores (CARKS 15 and CARKS 16).
MATTERHON MOUND SITE
The largest mound at GBB site 3, hereafter called the “Matterhorn”, reaches 110 m in height (Correa et al., 2012). A 7.03 m gravity core (CARKS 15) from the Matterhorn allows the top portion of the mound to be investigated with regards to internal structure, composition and growth rate of the mound. A second, 3.26 m long core (CARKS 16) was taken in the sediments adjacent to the mound. Current data revealed a bidirectional current regime produced by an internal tide that switches direction from N to S every 6 hours (Grasmueck et al., 2006). The sediment core is expected to record this current regime in its sedimentary structures.
SEDIMENT CHARACTERISTIC OF THE “MATTERHORN”
Core CARKS 15 from the flank of the “Matterhorn” is basically a coral floatstone with variable amounts of corals in a matrix of changing composition. The succession can be divided into five sedimentary units based on the lithology and the different geophysical and geochemical properties (Figure 3). (A) Coral floatstone in coarse grained matrix, (B)
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Coral floatstone with wackestone intercalation, (C) Coral floatstone interbedded with grainstone, (D) Coral floatstone in fine grained matrix, (E) Lithic rudstone. The matrix of the coral floatstone is mainly composed of pteropods, planktonic and benthonic foraminifera, with admixed coral fragments, sponge spiculae, mollusks, and echinoids. The lithic rudstone unit at the lower portion of the core is mainly composed of micrite and a few echinoids, planktonic and benthonic foraminifera.
Figure 3. Core photograph, textural description, fossil content and facies and unit boundaries of core CARKS 15 of the “Matterhorn” on the slope of GBB. Variable amounts of corals are present throughout the core but the matrix changes in each facies unit.
Table 1. Result of coral percentage measurement at certain depths of core CARKS 15 showing a maximum, minimum and average value of coral content in each facies.
Unit Depth (cmbsf) Facies Maximum (%) Minimum (%) Average (%) A 135 – 150 Coral floatstone 8.9 2 5.1 B ? Coral floatstone 14.6 1.7 6.5 C 342 – 354.6 Coral floatstone 28.9 3.9 18.6 C 354.6 – 357 Skeletal grainstone 10 6 7 C 433 – 448 Coral floatstone 28.9 4 16.7 D 507 – 519.9 Coral floatstone 17.9 1.8 10.6 D 519.9 – 522 Skeletal packstone 6.3 4.6 4.7 D 570 – 585 Coral floatstone 49.1 8.8 29.4 D 627 - 642 Coral floatstone 22.8 5.4 17.9
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Coral Quantification
The distribution of the corals in the top meters of the “Matterhorn” is assessed with tomographic (CT) scan imagery over several intervals of core CARKS 15. Coral density varies between 1.7% to 49.1%. The highest amount of coral, 49.1%, is in the interval 570 - 585 cmbsf (cm below seafloor) where corals float within a fine-grained matrix (Table 1).
Geophysical Logging
The Matterhorn core was analyzed with a GEOTEK Multi Sensor Core Logger at a resolution of 1 cm, measuring natural gamma radiation (cps) and resistivity (mV). Gamma density and resistivity reflect changes in the lithology and porosity. The Gamma raw bulk densities measured on core CARKS 15 display relatively parallel trends. The densities vary between 4,107 to 6,875 cps. The resistivity has values between -1 to 125 mV with an average of 81 mV and displays an inverse correlation with the density measurement.
Geochemistry
The mineralogy consists mostly of aragonite and low magnesium calcite (LMC), but X-Ray Diffraction (XRD) documents a decreasing trend of aragonite and increasing trend of low magnesium calcite (LMC) from top to bottom of the core. In unit A through unit B, aragonite content varies between 70% to 85%, while LMC varies between 15% to 30% for sediments of all size fractions. The high percentage of aragonite is in the smallest size fraction, <63 μm, confirming that the fine mud matrix is likely to be sourced from the platform top. At a depth of 360 cmbsf in unit C, a significant increase in LMC content is identified. LMC reach 75% in coral floatstone facies within the size fraction >150 μm, while the mud matrix still is 84% aragonite. At a depth of 580 cmbsf, however, the fine-grained matrix has a relatively low 54-65% aragonite content. In unit D where grains larger than 150 μm are lithoclasts, aragonite reaches 49% and LMC 41% while the mud fraction has 32% aragonite and 68% LMC. The decrease of aragonite and the coeval increase of lithification and LMC document early diagenetic alterations within the mound.
Down core element intensities of Fe, K and Si, determined with an X-Ray Fluorescence (XRF) core scanner, are relatively constant downcore except at a depth of 613 to 643 cmbsf, where Fe, K, and Si are enriched, which indicates the presence of clay minerals or iron-minerals. Visually, this interval is characterized by darker streaks due to presence of pyrite. Sr intensities are strongly inversely correlated with Fe, K and Si. For example, in unit D (500 to 600 cmbsf) Sr content increases, while Fe, K, and Si intensities decrease. The opposite trend can be seen over the lower part of the core at depths of 600 to 660 cmbsf.
SEDIMENT CHARACTERISTICS SURROUNDING “MATTERHORN”
Core CARKS 16 from the side of the “Matterhorn” consists of a coarse-grained skeletal grainstone to rudstone. The skeletal components are predominantly pteropods, benthonic and planktonic foraminifera. Portions of the core display large-scale cross-bedding. The core can be divided into three lithologic units based on the composition and depositional texture (Figure 4 & 5).
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Unit A: Pteropod grain-rudstone (0-113 cmbsf); the light yellowish brown pteropod grain-rudstone is poorly sorted and displays a coarsening upward trend. The grains consist of abundant pteropods, planktonic and benthonic foraminifera, and sponge spiculae. Unit B: Foraminifera grainstone (100-293 cmbsf); the pale brown unit is a well-
Figure 4. Photograph and lithology of off-mound core CARKS 16, displaying the brownish coarse-grained grainstone-rudstone and the lighter colored packstone to grainstone. The top portion is massive, while the lower portion of the core displays large scale cross-bedding.
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sorted silt to very coarse, sand-sized foraminiferal grainstone. The unit is slightly burrowed but still shows inclined bedding and bi-directional cross-bedding. Unit C: Foraminifera packstone (294-326 cmbsf); the very pale-brown foraminifera packstone is moderately sorted, clay to medium sand-sized and composed of mostly planktonic foraminifera and micrite.
Figure 5. Composition of core CARKS 16. (A) & (B) Pteropod grain-rudstone showing coarse grained pteropods with finer-grained planktonic and benthonic foraminifera. (C) Foraminifera grainstone showing abundant planktonic and benthonic foraminifera and some pteropods and echinoids. (D) Foraminifera packstone with planktonic and benthonic foraminifera in muddy matrix. Pf-planktonic foraminifera, Bf-benthonic foraminifera, P-pteropods, and E-echinoids.
INTERPRETATION AND IMPLICATIONS
In the Matterhorn core, coral fragments up to 30 mm in length are irregularly distributed throughout the mound core. The matrix of the coral floatstone, however, alternates between coarse-grained and fine-grained. The coarse-grained matrix is likely to reflect times of increased bottom currents such as those associated with lowered sea level. The fine-grained matrix is mostly composed of aragonite mud that is likely sourced from the platform top during high sea-level. Both of these matrixes contain abundant coral fragments but the highest amount is present in an interval characterised by a muddy matrix. This indicates that cold water corals are growing during both glacial and
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interglacial times in the Straits of Florida, but living conditions might be slightly better during interglacials.
The downcore decrease of aragonite content and concomitant increase of LMC is mostly explained by early diagenesis. The high value of aragonite in the top portion of core CARKS 15 is from corals and pteropods, while LMC mainly comes from foraminifera and, to a lesser extent, from mollusks and echinoids. Downcore LMC increases in the muddy sediment supporting transformation of the metastable aragonite to the more stable LMC during early diagenesis.
Off-mound core shows layers and lenses of coarse and fine-grained sediments, while the core from the mound shows alternations of coral-rich and coral-poor layers. The off mound sediments are mainly composed of coarse pteropod grainstone showing inclined bedding and bi-directional cross bedding documenting internal tidal current environment.
REFERENCES
Correa, T. B. S., Grasmueck, M., Eberli, G. P., Reed, J., Verwer, K., and S. Purkis, 2012, Variability of cold-water coral mounds in high sediment input and tidal current regime, Straits of Florida: Sedimentology, v. 59, p. 1278-1304.
Foubert, A. and J. Henriet, 2009, Nature and significance of the recent carbonate mound record: the mound challenger code: Springer-Verlag Berlin Heidelberg.
Grasmueck, M., Eberli, G. P., Viggiano, D. A., Correa, T., Rathwell, G., and J. Luo, 2006, Autonomous underwater vehicle (AUV) mapping reveals coral mound distribution, morphology, and oceanography in deep water of the Straits of Florida: Geophysical Research Letters, v. 33, L23616.
Grasmueck, M., Eberli, G. P. Correa, T. B. S., Viggiano, D. A. , Luo, J., Reed, J. K., Wright, A. E. and P. A. Pomponi, 2007, AUV-based environmental characterization of deep-water coral mounds in the Straits of Florida: OTC 18510, Houston, p. 1-11.
Neumann, A. C. and M. M. Ball, 1970, Submersible observations in Straits of Florida - Geology and Bottom Currents: Geological Society of America Bulletin, v. 81, p. 2861-2873.
Neumann, A. C., Kofoed, J. W., and G. H. Keller, 1977, Lithoherms in Straits of Florida: Geology, v. 5, p. 4-10.
Rosenberg, A., 2011, Insight from the depth of the Straits of Florida: assessing the utility of Atlantic deep-water coral geochemical proxy techniques: University of Miami M.S. Thesis, Open Access Theses, Paper 244, http://scholarlyrepository.miami.edu/oa_theses/244.
Paull, C. K., Neumann, A. C., Ende, B. A. A., Ussler, W., and N. M. Rodriguez, 2000, Lithoherms on the Florida-Hatteras slope: Marine Geology, v. 166, p. 83-101
Reed, J. K., Weaver, D., and S. A. Pomponi, 2006, Habitat and fauna of deep-water Lophelia pertusa coral reefs off the Southeastern USA: Blake Plateau, Straits of Florida, and Gulf of Mexico: Bulletin of Marine Science, v. 78, p. 343-375.
Roberts, J. M., Wheeler, A. J., and A. Freiwald, 2006, Reefs of the deep: The biology and geology of cold-water coral ecosystems: Science, v. 312, p. 543-547.
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PETROPHYSICAL PERSPECTIVE OF CRETACEOUS-TERTIARY RE-DEPOSITED CARBONATES FROM THE
APENNINES AND THE ADRIATIC SEA, ITALYIrena A. Maura, Gregor P. Eberli, and Daniel Bernoulli1
1) Geological Institute, University Basel, Switzerland
KEY FINDINGS
Re-deposited carbonates have a good reservoir potential with the following petrophysical charateristics:
At the same porosity, re-deposited carbonates have higher permeability and are faster than the background sediment.
In terms of age, Cretaceous re-deposited carbonates are more porous and permeable than Tertiary ones. Yet, Cretaceous sections have faster velocity compared to the Tertiary counterparts.
The pore structure is dominated by large simple pores producing fast high permeability rocks. In contrast the pelagic background sediments are dominated by compliant micropores and are, thus, slower at any given porosity.
CONTROLS ON RE-DEPOSITED CARBONATE RESERVOIR QUALITY
Re-deposited carbonate reservoirs are a challenging prospect for hydrocarbon exploration. The reservoir quality and potential of these deposits is still questionable. However, reservoirs in carbonate mass gravity flow deposits exist, indicating that these deposits can be a good reservoir.
Re-deposited carbonates are like their shallow-water counterparts susceptible to diagenetic processes that alter their original mineralogy and pore structure (Eberli et al., 2003). Diagenetic alterations create a complex pore structure that will contribute to variations in the petrophysical properties such as porosity, permeability and sonic velocity (Weger et al., 2009). Likewise, the amount of microporosity also plays an important role in the various petrophysical properties observed (Baechle et al., 2008).
Figure 1. (Left) Study sites; the yellow dot indicates location that was visited in 2006, the blue dot indicates the location of Well 1, and the red dot indicates locations that were visited in 2012. (Upper Right) Coarse breccia bed (above back pack) in a series of thin calcareous turbidites in the Valle di Pennapiedimonte (Maiella). (Lower Right) Monte Conero section.
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In this study the different petrophysical properties of Cretaceous - Tertiary re-deposited carbonates and their background sediment will be assessed in outcrops and the subsurface. The rock samples were collected from several sites in the Abruzzi and Apennines in Italy (Figure 1).
DATASETS AND METHODS
Over 210 plugs from the outcrop and 85 core plugs are analyzed. The samples were collected from three different areas (1) Maiella platform margin, an isolated Mesozoic to Mid-Tertiary carbonate platform margin located in the southern part of Italy (Figure 1); (2) Monte Conero, a section located 150 km to the north of the Maiella mountain which contains mostly turbidites and few breccia beds (Figure 2); and (3) Well 1, an offshore well in the Adriatic Sea, that penetrated the basinal portions of a buried carbonate platform.
Each plug was measured for porosity and permeability. In addition, 87 plugs from the outcrop and 40 core plugs were measured for sonic velocity. Petrographic description and Digital Image Analysis (DIA) using thin sections were used to examine composition and pore structure. In addition, Scanning Electron Microscope (SEM) analyses from some core samples were used to study the composition and micro-pore structure.
Figure 2. (Upper left) Sassi Neri outcrop location (Monte Conero area). (Lower left) Outcrop photo of a exceptionally thick Cretaceous turbidite bed with flute cast. (Right) Stratigraphic columns of 4 Sassi Neri outcrop.
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FACIES OF RE-DEPOSITED CARBONATES
The studied calcareous mass gravity flow deposits, hereafter called “re-deposited carbonates” are comprised of breccias, turbidites, slumps and calcisiltites. The perennial background sediment is either periplatform ooze, or, in the distal positions, fine pelagic sediment. In Well 1 the dominant facies in the Tertiary is megabreccia (28.11 %), followed by turbidite (12.17 %), whereas in the Cretaceous the facies is dominated by turbidite (36.59 %) and megabreccia (8.82 %), followed by calcisiltite (7.72 %) (Figure. 3). The carbonate turbidites in Well 1 core are dominated by fine-grained distal turbidites. A similar distal turbiditic facies is found in outcrops along the Marchean anticlinorium I and is the closest outcrop to Well 1 (Montanari et al., 1989). The re-deposited carbonates in the Maiella outcrop tend to be coarser because they are in a very proximal position to the platform.
POROSITY, PERMEABILITY AND SONIC VELOCITY
I. Porosity and Permeability of Maiella outcrop versus Well 1 core
Maiella outcrop samples of re-deposited carbonates and the background sediment have a larger range and higher porosity and permeability compared to Well 1 core data (Figure 4). The higher porosity and permeability of the Maiella samples compared to those from Well 1 is related to the coarse composition and the proximal location to the source. In contrast,the Monte Conero samples have a similiar range in porosity (2– 15 %) to Well 1 samples. The re-deposited carbonates tend to be more permeable than the background sediments (Table 1, Figure 4).
Table 1. Porosity – permeability value of re-deposited carbonates versus background sediments from the Maiella outcrop and Well 1 core samples.
Re-deposited carbonates Background sediments Dataset Maiella
Outcrop Well 1 Core
Monte Conero
Maiella Outcrop
Well 1 Core
Monte Conero
(%) 0.40 - 32 0.4561 - 22 2 – 15 1.96 - 27 1.19 - 11 2 – 15 (md) 0 - 522 0 - 23 - 0 - 72 0 - 1.5 -
Figure 3. Facies distribution histogram of Cretaceous – Tertiary re-deposited carbonate facies from Well 1 (modified from Maura et al., 2011).
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Figure 4. Re-deposited carbonates (left) and background sediments (right) porosity – permeability plot from Maiella (red dots) and Well 1 (blue dots) samples.
II. Porosity and permeability of Re-deposited Carbonate versus Background Sediment
At a given porosity, re-deposited carbonates tend to be more permeable than the background sediments. Porosity can reach 32 % with permeability up to 522 md. While porosity of the background sediment can reach 27 % and permeability 72 md (Table 2).
III. Porosity and permeability of Cretaceous versus Tertiary Re-deposited Carbonate
Both Cretaceous and Tertiary re-deposited carbonates have a good reservoir potential. In addition, the Cretaceous has a better reservoir quality compared to the Tertiary. The porosity in Cretaceous re-deposited beds can reach 32 % with 522 md, while in the Tertiary the highest porosity is 23 % and 17 md permeability (Figure 5). The difference in reservoir quality is related to the original mineralogy and the resulting diagenetic potential. During the Cretaceous, calcite was the predominant carbonate mineral, while in the Tertiary most carbonate production was aragonite. Metastable aragonite is subject to dissolution and re-precipitation. This process produced more cement and tighter rocks in the Tertiary.
IV. Sonic velocity
At the same porosity the re-deposited carbonates are faster than the background sediment but they are more permeable than the background sediment. This discrepancy is related to the differences in pore structure in the mostly skeletal grainstone to rudstone in the redeposited beds compared to the dominance of microporosity in the background sediments.
Table 2. The range of porosity – permeability value of Cretaceous versus Tertiary re-deposited carbonates and background sediments.
Re-deposited carbonates Background sediments Dataset Cretaceous Tertiary Cretaceous Tertiary
(%) 0.40 - 32 1.61 – 23.89 5.19 – 26.95 1.96 – 24.42 (md) 0 - 522 0 – 17.381 0.045 -36.35 0 - 72
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Figure 5. (Left) Porosity – permeability plot of Cretaceous and Tertiary re-deposited carbonates and background sediments. (Right) Thin section photos of A and B in the plot.
Figure 6. Plot of porosity and velocity. Colorbar represents: (A) Perimeter Over Area (POA), (B) Dominant pore size (DOMsize). We overlay our data with Weger’s (2009) data to compare the trends and the distributions of the two datasets. Higher DOMsize indicates larger pores, and higher PoA indicates higher complexity in the pore structure. Large and simple pores are the dominant pore structure of the Maiella samples.
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PORE STRUCTURE OF RE-DEPOSITED CARBONATES
Pore structure is one of the important factors that can affect the velocity in carbonates. Digital Image Analysis (DIA) shows that the majority of pores in re-deposited carbonates are large pores dominated by simple pores (Figure 6A and B). Such pore structures produce a fast velocity at a given porosity (Weger et al., 2009)
Another significant factor that affects the velocity behavior in carbonates is the percentage of microporosity in the rock. Figure 7 shows that at the same porosity, samples with higher amounts of microporosity are slower. The reason is that the small pores are composed of soft compliant pores (Baechle, 2008).
Figure 7. Plot of porosity and velocity. Colorbar represents the percent of microporosity. This plot shows that the samples with fewer micropores and lower porosity will have a faster velocity.
REFERENCES
Baechle, T. G., Colpaert, A., Eberli, G. P., and R. J. Weger, 2008, Effect of microporosity on sonic velocity in carbonate rocks: The Leading Edge, p. 1012-1016.
Eberli, G. P., Baechle G. T., Anselmetti, F. S., and M. L. Incze, 2003, Factor controlling elastic properties in carbonate sediments and rocks: The Leading Edge, v.22, p. 654-660.
Maura, I. A., Eberli, G. P., and D. Bernoulli, 2011, Comparison of Cretaceous-Paleocene carbonate turbidite successions from core and outcrop adjacent to the Maiella platform margin, Italy: CSL Annual Meeting, p. 107-112.
Montanari, A., Chan, L. S., and W. Alvarez, 1989, Synsedimentary tectonics in the Late Cretaceous-Early Tertiary pelagic basin of the Northern Apennines, Italy: controls on carbonate platform and basin development: SEPM Special Publication 44, p. 379-399.
Weger, R. J., Eberli, G. P., Baechle, G. T., Massafero, J. L., and Y. F. Sun, 2009, Quantification of pore structure and its effect on sonic velocity and permeability in carbonates: AAPG Bulletin, v. 93, p. 1297-1317.
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SEDIMENTOLOGY, GEOMETRIES AND LINK TO THE SUBSURFACE FROM A FIELD-SCALE ANALOG:
THE SIERRA DE LA VACA MUERTAMichael Zeller, Samuel B. Reid, David L. Giunta1, Ralf J. Weger,
Gregor P. Eberli, and Jose Luis Massaferro1
1) YPF, Buenos Aires, Argentina
KEY FINDINGS
The Quintuco-Vaca Muerta System displays a strong cyclicity that is linked to eustatic sea-level fluctuations despite some tectonic overprint.
Small-scale heterogeneities follow sequence stratigraphic units and boundaries and can therefore be identified through seismic stratigraphy studies.
Synthetic seismic models of both distal and proximal outcrop areas display very similar geometries to real subsurface seismic data.
Local tectonic pulses produce differential developments from the Valanginian onwards in the outcrop and subsurface areas.
SIGNIFICANCE
The Quintuco - Vaca Muerta System in the Neuquén Basin in Argentina, is considered to be one of the most promising unconventional plays outside the US. The Vaca Muerta shale represents the source rock for most petroleum systems in the basin and its thickness (up to > 1km) and high TOC values make it a prime target for unconventional hydrocarbon exploration.
In order to identify best exploration and production techniques, the stratigraphic architecture as well as the bed-scale heterogeneities have to be understood. They are best studied in field-scale outcrop analogs. The Sierra de la Vaca Muerta (SdlVM) represents a key study area with excellent exposures displaying the architecture of the mixed carbonate siliciclastic system, while being easy to access and tectonically relatively undisturbed. Using the detailed facies descriptions from the outcrop and a geometrical framework derived from high-resolution satellite imagery, a new large-scale correlation provides insights into the sequence architecture and places the observed small-scale heterogeneities within this framework.
This information is most useful, if directly linked to the subsurface exploration fields. Synthetic seismic modeling in combination with 2D lines acquired across the study area provide the connection to subsurface studies and can help to understand the subsurface architectural elements and stratigraphic composition, which play an important role in unconventional evaluation of, for example, carbonate content and brittleness.
METHODS
This study is an integrated approach to document the outcrop facies distribution and geometries, illustrate their seismic expressions, and correlate between the outcrop area and the subsurface producing fields. Therefore the workflow is subdivided into 3 main
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portions: 1) Stratigraphic Architecture, 2) Synthetic Seismic Model, and 3) Seismic Outcrop – Subsurface Correlation.
Stratigraphic Architecture
Figure 1 illustrates the applied workflow, which was used to develop the field-scale facies correlation. During fieldwork, detailed sedimentological descriptions are collected in both vertical and lateral sections in order to document spatial and temporal variations of the depositional system. All points of observation are recorded with GPS coordinates and facies types are defined and grouped into facies associations.
In a second step, newly acquired high-resolution satellite imagery (Worldview II, 0.5 m resolution) is draped on top of digital elevation model (DEM, ASTER, 15m resolution) in order to build a photorealistic digital outcrop model (DOM). The high quality images together with the correct elevations allow tracking of 49 beds within a 1km thick stratigraphic section. This information is used to construct a geometry model and calculate true stratigraphic thickness based on dip and distance measurements.
Finally, observed color transitions on the satellite imagery together with information of the topographic dip angles from the DEM, in combination with the logged beds and sections, facilitate interpretation of facies and stratigraphy in inaccessible areas.
Petrophysics and Synthetic Seismic Modeling
The synthetic seismic modeling follows the same workflow that was successfully applied at the Picún Leufú Anticline (Zeller et al., 2011a). In the first step, the facies model (Figure 2) is used as the geometrical input and is placed at 2km depth. 532 facies bodies are defined and 333 traces along the 10km section ensure sufficient lateral resolution.
Petrophysical properties of 70 samples, covering all facies types, are measured. They include dry vp, dry vs (both from 5 – 60 MPa), bulk and grain densities, porosity and carbonate content. The velocity values (for 50MPa ~ 2km burial depth) are corrected from dry to realistic wet state using the Gassmann fluid substitution equations. The average values determined for each facies (Table 1) are then distributed according to the facies model. The resulting acoustic impedance model (based on the velocity and density input) is then convolved with a 5-10-50-60 Butterworth wavelet and models for 0,10, 20 and 30 degree incident angles are stacked to create a synthetic seismic secti0n that can be compared with real subsurface datasets.
Table 1. Average petrophysical properties for different facies associations after Gassmann Fluid Substitution. These values are used for property distribution within the geometry model and basis for the synthetic seismic modeling. vp and vs values for 50 MPa effective pressure.
facies shale calc. shale calc.silt calc.sand pure sandvp (m/s) 3547 4370 4676 4849 4215vs (m/s) 2109 2518 2687 2830 2623(g/cm3) 2.25 2.45 2.53 2.51 2.34(fract) 0.07 0.04 0.07 0.07 0.11
Carb (fract) 0.20 0.51 0.60 0.36 0.02facies WS PS oyster FS GS deltaic sandvp (m/s) 5652 5365 5650 5375 4174vs (m/s) 3090 2972 3077 2934 2466(g/cm3) 2.63 2.56 2.58 2.56 2.38(fract) 0.02 0.04 0.05 0.06 0.10
Carb (fract) 0.72 0.66 0.74 0.81 0.17
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Seismic Outcrop Subsurface Correlation
During exploration a fairly dense grid of 2D seismic lines was shot also throughout the western portions of the Neuquén Basin, covering large parts of the study area. 2D lines from the western area are tied to the 3D volumes in the eastern portion of the basin and allow direct connection between outcrop and subsurface areas. A key reflection within these regional lines reaches the surface in the study area, which allows correlation of this reflector to the existing digital outcrop model.
Figure 1. Outcrop correlation workflow. Geometry and facies distribution are based on the combination of field observations and digital outcrop model interpretations.
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RESULTS AND INTERPRETATIONS
Large-Scale Architecture
Lithologically the studied interval represents a truly mixed succession. Both pure carbonate and pure siliciclastic portions are very scarce. Overall the package follows a prograding/shallowing upward trend with lower basinal deposits overlain by shelfal sediments that finally shifts rapidly into deltaic deposits (Figure 2). This long-term trend is repeatedly interrupted by marine transgressions on the shelf. The succession is subdivided into 8 depositional sequences (Figure 2, I-VIII). The sequences are commonly asymmetric with thick regressive portions on the shelf, while this pattern is slightly alleviated in the basinal portions with the thickening of the transgressive portions.
Depositional geometries are mostly gentle with dip angles around 0.5-1°, with the
exception of sequence V, where a pure carbonate system produces a strong break along the shelf and a depositional dip of around 3° along its lower portion.
The largest shift of the depositional environment occurs within sequence VIII with the onset of an unidirectional prograding deltaic facies (Figure 3B) on top of the otherwise tidally dominated shelfal succession. This strong change could have been associated with a change in subsidence rate and tectonic movement in the hinterland area, increasing the siliciclastic input to the study area.
Small-Scale Heterogeneities
Within this large-scale facies architecture the outcrops at the SdlVM also allow more detailed insights into the smaller scale heterogeneities of the mixed system. These include cyclic variations, lateral facies transitions and event beds.
The Los Catutos Member (Figure 3G) is a meter scale alternation of skeletal wackestones with marls (Figure 3E) . Shale successions follow the same pattern and display on a meter scale variations of more and less carbonate content. These cycles are laterally associated with more carbonate-rich systems on the shelf. Similar limestone – marl successions in the northern part of the Neuquén Basin have been associated with orbital cyclicity and interpreted as regressive carbonates covered by transgressive marl layers (Kietzmann et al., 2011).
Lateral facies transitions occur in both clastic and carbonate dominated environments. As illustrated in Figure 3A (see also Zeller et al., 2011b), the high energy carbonate system, consisting of oolitic grainstone and skeletal packstone shifts laterally into a mixed shelfal system and finally into slope facies (similar to Los Catutos) and basinal calcareous shale. Likewise, siliciciclastic sandstones develop downdip to sand-siltstone mixtures into pure shale deposits along several kilometers.
Oyster float- and framestone are the most common carbonate dominated facies in the system and can occur as extensive dm-m beds (Figure 3C) or as up to 4m buildups. One particular buildup is mapped out and shows rapid thickening from 0.1 to more than 3m (Figure 3F).
In addition to the cyclic and lateral variations, some sedimentary structures provide evidence for sedimentation events during the time of deposition. Soft sediment deformation is a common feature within the regressive portions of siliciclastic-dominated sequences (Sequences VI-VIII) and can be explained by the rapid sedimentation of silt and sand on top of the shale. In contrast mixed carbonate clastic turbidites (Figure 3D) can be found just above carbonate dominated sequence tops
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(sequences I and IV) and might have been triggered by the steeper depositional profile built by the precursor carbonate systems.
Figure 4. Petrophysics and synthetic seismic model. A) Vp vs. porosity, color-coded with carbonate content, B) vp/vs vs. porosity, color-coded with carbonate content. Both plots illustrate that porosity combined with the carbonate content exert the main control on the sonic velocities. C) Synthetic seismic model based on the acoustic impedance model from facies distribution, the petrophysical measurements at 50 MPa.
Petrophysics and Seismic Expressions
Porosity and carbonate content have the strongest impact on sonic velocity in the mixed system of the Vaca Muerta (Figure 4A). This finding corroborates the results from the more proximal succession at the Picún Leufú Anticline (PLA) (Zeller et al., 2011a), Overall carbonate-rich facies show higher velocities and higher vp/vs ratios and are commonly less porous than clean siliciclastics due to cementation (Figure 4A-B). Carbonate content has a particularly strong effect on shale properties with calcareous
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shale having considerably higher sonic velocities than their pure shale equivalents (Table 1).
In the resulting synthetic seismic model the general character of the prograding – aggrading succession is well preserved (Figure 4C). In the lower portion the beds downlap on the Los Catutos Member. The strongest geometrical anomaly is represented by the carbonate succession of sequence IV. The following beds onlap onto this key reflection and are overlain by a more aggradational succession, which exposes quite commonly lateral variations in amplitudes and are very well-defined as downlapping beds in the lower portion.
Identification of depositional sequences is not as straight forward as from the facies correlation. Some of the thin sequences would most likely be missed in seismic interpretation of real subsurface seismic data with similar resolution.
IMPLICATIONS
Sequence Stratigraphy
In outcrop the succession can be subdivided into 8 depositional sequences. Based on biostratigraphic studies (Leanza et al., 2011) the studied interval extends from the Early Tithonian to the Middle – Late Berriasian, a time span of approximately 10 Myr. In subsurface seismic data 7 sequences are identified that correspond to the 7 deepening and shallowing cycles represented in the eustatic sea-level curve proposed by Mitchum and Uliana (1985). The similar number of sequences indicates a similar stratigraphic framework for both outcrop and subsurface, which are controlled by eustatic sea-level changes. This is important, since the value of the outcrop as an analog is greatly enhanced if both areas were controlled by similar depositional processes.
Heterogeneities
Small-scale variations of depositional facies follow lateral and sequence stratigraphic trends. Even event beds like soft sediment deformation structures and turbidites occur in predictable intervals within the sequence stratigraphic framework. This could have a considerable impact on identification of potential sweet spots for unconventional exploration. Intervals with higher carbonate content represent best spots for hydraulic fracking, while intervals with high turbidite frequency would offer relatively elevated porosity and permeability streaks within an otherwise tight surrounding.
Regional Seismic Architecture (Outcrop – Subsurface)
With now two synthetic seismic models in place, both in proximal (PLA) and distal (SdlVM) positions, comparisons of the regional architecture of outcrop and subsurface hold very interesting insights.
Figure 5 is a combination of a regional subsurface line from the producing fields towards the NW and the two synthetic seismic datasets from this study. Placed with the same scales, they show very similar characteristics. Proximal Portions (PLA and E subsurface) are relatively thin, have reflectors with gentle dips up to 1° and no breaks along their profile. In contrast distal portions (SdlVM and W subsurface) are very thick, both show steeper dip angles in the lower portion and contain reflections with clear breaks.
Again, these similar developments enhance the value of the outcrops as analogs and point to similar depositional processes and environments in the different areas over most of the studied interval.
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Figure 5. Comparison subsurface with synthetic seismic data from outcrop areas. Top: Regional seismic line (modified from Leanza et al., 2011); Base: synthetic seismic sections from this study, in same scale as the subsurface dataset.
Differential Development Outcrop Subsurface (Quintuco vs Quintuco sensu strictu)
Based on the direct outcrop subsurface correlation, marginal marine (Picún Leufú Anticline) and deltaic deposits (SdlVM) are time equivalent to widespread mixed carbonate clastic cycles in the subsurface area (Figure 6). These observations suggest that the western (outcrop) portions were subject to a major siliciclastic input due to a tectonic uplift in the south, while the eastern areas remained tectonically rather calm in their shallow marine setting. This finding could help to explain the major discrepancies between the (outcrop) Quintuco s.s. and the subsurface time-equivalent Quintuco Formation, which was for a long time a major topic for discussions on the applicability of the outcrops as analogs for the subsurface.
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.
Figure 6. Comparison of outcrop and subsurface lithologies. A) Regional outcrop correlation (modified from Leanza et al., 2011), B) Regional subsurface model based on subsurface data (Mitchum and Uliana, 1985). Dotted red line marks the time equivalent surface in outcrop (A) and subsurface (B).
REFERENCES
Kietzmann, D. A., Martin-Chivelet, J., Palma, R. M., Lopez-Gomez, J., Lescano, M., and A. Concheyro, 2011, Evidence of precessional and eccentricity orbital cycles in a Tithonian source rock, The mid-outer carbonate ramp of the Vaca Muerta, northern Neuquen Basin, Argentina: American Association of Petroleum Geology Bulletin, v. 95, no. 9, p. 1459-1474.
Leanza, H., Sattler, F., Martinez, R. S., and O. Carbone, 2011, La formacion Vaca Muerta y equivalentes (Jurassico Tardio – Cretacico Temprano) en la Cuenca Neuquina. In: Leanza, H., Arregui, C., Carbone, O., Danieli, J. C., and Valles, J. M., Geologia y recursos naturales de la provincia del neuquen: Relatorio del XVII Congreso Geologico Argentino, p. 113-130.
Mitchum, R. M., and M. A. Uliana, 1985, Seismic stratigraphy of carbonate depositional sequences, Upper Jurassic-Lower Cretaceous, Neuquen Basin, Argentina. In: Bero, B. R., and D. G. Wooverton, 1985, Seismic stratigraphy: an integrated approach to hydrocarbon exploration: American Association of Petroleum Geology Memoir 39, p. 255-274.
Zeller, M., Weger, R. J., Eberli, G. P., Giunta, D. L., and J. L. Massaferro, 2011a, Seismic expressions of a field-scale Quintuco Vaca Muerta outcrop analog – implications for seismic interpretation of mixed carbonate-siliciclastic systems: CSL Abstracts 2011, p. 61-68.
Zeller, M., Reid, S. B., and Eberli, G. P., 2011b, The distal mix – shelf to basin transitions of the mixed Quintuco-Vaca Muerta System: CSL Abstracts 2011, p. 69-73.
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DECOUPLED INORGANIC AND ORGANIC CARBON ISOTOPE RECORDS: A GLOBAL SIGNAL UNRELATED TO GLOBAL
CARBON CYCLING?Amanda M. Oehlert and Peter K. Swart
KEY FINDINGS
The carbon isotope composition ( 13C) of pelagic carbonates deposited on the Walvis Ridge in the south Atlantic do not show a positive covariation through time, contrary to theoretical expectations.
The 13C of the organic and inorganic fractions from carbonate slope sediments show different relationships depending on platform margin architecture.
- Inorganic and organic 13C records in a transect of Ocean Drilling Project (ODP) cores off the margin of the Great Barrier Reef shows no significant relationship through time.
- ODP cores from the Great Australian Bight show a variable relationship between inorganic and organic 13C records through time.
- Compared to published results from the Great Bahama Bank, each of these three slopes exhibits different relationships, suggesting that local depositional processes and platform architecture significantly influence the 13C records.
SIGNIFICANCE
Synchronous excursions in inorganic and/or organic 13C records from globally distributed basins have been used to create chemostratigraphic correlations in a variety of carbonate deposits. Carbon isotope chemostratigraphy of either inorganic or organic
13C records is especially useful in deposits where other age dating techniques lack resolution. Of major concern, however, is the impact of open-system diagenesis, where the isotopic composition of the bulk inorganic carbonate is subjected to significant isotopic alterations. One method used to prove the primary nature of the inorganic 13C record is to conduct a paired carbon isotope analysis. A positive covariation between inorganic and organic 13C records has been used as a tool to prove that the bulk inorganic 13C isotopic record represents changes in the global carbon cycle. This analysis has been applied to both shallow and deep marine settings in studies of global carbon cycling in time periods spanning the Proterozoic to the present; however, the ability of the shallow marine and deep marine settings to record the same trends has not been evaluated. An evaluation of this assumption needs to be conducted so that interpretations and stratigraphic correlations of 13C excursions accurately reflect the processes that generate the bulk 13C signal.
In order to test this assumption, this study has quantified the relationship between
inorganic and organic 13C records in both a pelagic setting, and in two platform-to-basin transects of ODP cores. The pelagic relationship, analyzed in pelagic carbonates from
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Walvis Ridge, was then compared to relationships produced from new records at the Great Barrier Reef and the Great Australian Bight, and published records from the Great Bahama Bank. In each of these cases, a variable relationship between inorganic and organic 13C values was observed, suggesting that local controls significantly influence the bulk isotopic composition of inorganic and organic carbon in these environments. As a result, a knowlege of the processes that contribute to the bulk isotopic composition of organic and inorganic 13C records will improve interpretations of the significance of excursions in the 13C values of marine carbonates and sedimentary organic matter. A better understanding of the significance of the excursions may aid in chemostratigraphic correlations.
RESULTS
Pelagic Setting: Walvis Ridge in the South Atlantic, DSDP Site 525
DSDP Site 525 is located on the Walvis Ridge in the South Atlantic (Figure 1a) and has been described as a sequence of well-preserved pelagic carbonates. These sediments were studied in 1984 by Shackleton and Hall who related a trend in the carbon isotope record to a progressive increase in the transfer of organic carbon to the inorganic carbon reservoir in what has become a seminal paper in global carbon cycling research. The data produced in the Shackleton and Hall (1984) paper is the foundation for quantitative carbon cycling models through time (Kump and Arthur, 1999; Shackleton, 1985), which suggests that the relationship between inorganic and organic 13C values at the DSDP Site should exhibit a positive covariation through time as predicted by theoretical fractionation calculations. However, the results of the paired isotope analysis conducted
Figure 1.
(A) Location of DSDP Site 525 (From Bergren et al., 2003).
(B) Relationship between inorganic and organic carbon isotope records from Site 525.
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during this study at Site 525 revealed no significant correlation between inorganic and organic 13C records (Figure 1b).
Periplatform Setting: Great Australian Bight, ODP Leg 182
The Great Australian Bight is a sub-tropical, cool water carbonate factory located on the southern coast of Australia. ODP Sites 1128, 1134, and 1132 were selected for paired carbon isotope analysis (Figure 2). These sites are aligned in a roughly proximal to distal transect, spanning the platform margin to basin transition south of the Outer Eucla Shelf. Of the three sites selected for analysis, ODP Site 1132 is the most proximal drill site, while ODP Site 1128 is the most distal. The results of the paired inorganic and organic 13C record analysis showed that the relationship between inorganic and organic
13C values is variable in this transect.
Periplatform Setting: Great Barrier Reef, ODP Leg 133
ODP Leg 133 was drilled off the eastern coast of Australia, near Cairns (Figure 3a). The Great Barrier Reef margin is a mixed siliciclastic-carbonate, reef-rimmed margin. Three sites (820, 823 and 811) located in a basin-to-platform transect were selected for this analysis. No significant relationship between inorganic and organic 13C records was observed at any location (Figure 3b).
INTERPRETATION AND IMPLICATIONS
The results of this study highlight the importance of local depositional processes and global sea-level variability. While some of these variables, like eustatic sea-level changes, exert a global influence on carbonate slopes around the world, processes like depositional patterns and margin architecture seem to play a large role in determining the isotopic signature in the bulk inorganic and organic 13C records. However, many
Figure 2. Location of cores analyzed from ODP Leg 182. From Feary et al., 2003.
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Figure 3. Relationship between inorganic and organic carbon isotope records at the Great Barrier Reef transect.
published studies use a positive covariation between inorganic and organic 13C records to prove the global nature of the inorganic record (Swanson-Hysell et al., 2010; Johnston et al., 2012; Fischer et al., 2009; Grotzinger et al., 2011). Furthermore, these studies have also used a positive correlation to reconstruct ancient atmospheric concentrations of carbon dioxide (Hayes et al., 1999; Rothman, 2002; Jasper and Hayes, 1994; Fike et al., 2006). In many of these cases, the local processes that seem to play an important role in determining the composition of the inorganic and organic 13C are not considered.
Theoretical calculations of the fractionation of dissolved inorganic carbon by photosynthesis suggest that the inorganic and organic carbon fractions should be offset by a consistent and predictable fractionation factor. As a result, excursions in the isotopic composition of organic and inorganic carbon should be simultaneous. This would result in a positive covariation between inorganic and organic 13C records through time. However, the results of this study suggest that inorganic and organic 13C records derived from pelagic carbonates may not always co-vary through time as expected. The inorganic and organic carbon isotope records produced from pelagic carbonates at Site 525 show no relationship (r2=0.08). Interestingly, the range in organic 13C values (-28 to -21‰) at Site 525 may provide some evidence of terrestrial organic matter contributions to the deposit. Pelagic organic material typically falls around -21‰ (Laws et al., 1995), while some types of terrestrial organic material are more isotopically depleted. The depleted organic 13C values measured in the sediments from Site 525 may imply that even pelagic sequences may be considered a mixed system, depending upon the regional oceanographic circulation.
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Figure 4. Relationship between inorganic and organic carbon isotope records along the basin-to-profile transect of the leeward margin of Great Bahama Bank (From Oehlert et al., 2012).
Shallow marine and periplatform carbonates have been substituted for pelagic 13C
records in geologic strata older than 200 Ma. In the majority of cases, pelagic carbonates older than 200 Ma have been subducted, making it necessary to find another source of inorganic and organic carbon to analyze in order to reconstruct the ancient global carbon cycle. This substitution has not been evaluated, and the ability of periplatform carbonates to record the same type of information about global carbon cycling has not yet been tested. Recently, it has been shown that the relationship between inorganic and organic 13C records can be more complicated than previously thought. Within a 30 km basin-to-platform transect, the relationship between inorganic and organic 13C varied between a strong positive correlation (r2=0.64) in the basin to no relationship at the top of the slope (Oehlert et al., 2012, Figure 4). Therefore, along with the analysis of a pelagic carbonate setting at DSDP Site 525, the analyses of the relationship between inorganic and organic 13C records in slope carbonates conducted during this study provide a quantitative appraisal of whether or not the slope carbonates 13C values record the same information as the pelagic 13C values.
The results of the analyses on the slope carbonates from the Great Australian Bight and that Great Barrier Reef agree with the findings of Oehlert et al. (2012) in that the relationship between inorganic and organic 13C records at a cool-water carbonate factory and a mixed clastic-carbonate margin were observed to be variable. These results suggest that local factors override the signature of the global carbon cycle in carbonate slope environments. Local factors such as depositional processes, margin architecture, mineralogy of the inorganic carbon, and the variability in the organisms that are contributing organic carbon to the slopes are important factors to consider when interpreting the significance of excursions in inorganic and organic 13C records sourced from shallow marine carbonates.
Finally, the inconsistency in the relationship between inorganic and organic 13C records in both environments suggests that using paired isotope analyses to prove the
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original nature of a bulk inorganic 13C record should be reconsidered. From the results of these analyses, sedimentological and sequence stratigraphic characteristics of the deposit are key variables that should be incorporated into any interpretation of the significance of excursions in 13C records.
REFERENCES
Fike, D. A., Grotzinger, J. P., Pratt, L. M., and R. E. Summons, 2006, Oxidation of the Ediacarn ocean: Nature, v. 444, p. 744-747.
Fischer, W. W., Schroeder, S., Lacassie, J. P., Beukes, N. J., Goldberg, T., Strauss, H., Horstmann, U. E., Schrag, D. P., and A. H. Knoll, 2009, Isotopic constraints on the Late Archean carbon cycle from the Transvaal Supergroup along the western margin of the Kaapvaal Craton, South Africa: Precambrian Research, v. 169, n. 1-4, p. 15-27.
Grotzinger, J. P., Fike, D. A., Fischer, W. W., 2011, Enigmatic origin of the largest-known carbon isotope excursion in Earth’s history: Nature Geoscience, v. 4, p. 285-292.
Hayes, J. M., Strauss, H., Kaufman, A. J., 1999, The abundance of 13C in marine organic matter and isotopic fractionation in the global biogeochemical cycle of carbon during the past 800 Ma: Chemical Geology, v. 161, p. 103-125.
Jasper, J. P., Hayes, J. M., Mix, A. C., and F. G. Prahl, 1994, Photosynthetic fractionation of 13C and concentrations of dissolved CO2 in the central equatorial Pacific during the last 255,000 years, Paleoceanography, v. 9, n. 6, p.781-798.
Johnston, D. T., Macdonald, F. A., Gill, B. C., Hoffman, P. F., and D. P., Schrag, 2012, Uncovering the Neoproterozoic carbon cycle: Letters to Nature, v. 483, p. 320-323.
Kump, L. R. and and M. A. Arthur, 1999, Interpreting carbon-isotope excursions: carbonates and organic matter: Chemical Geology, v. 161, p. 181-198.
Laws, E. A., Popp, B. N., Bidigare, R. R., Kennicutt, M. C., and S. A. Macko, 1995, Dependence of phytoplankton carbon isotopic compositions on growth-rate and [CO2] (Aq)-theoretical considerations and experimental results: Geochimica, Cosmochimica Acta, v. 59, p. 1131-1138.
Oehlert, A. M., Lamb-Wozniak, K. A., Devlin, Q. B., Mackenzie, G. J., Reijmer, J. J. G., and P. K. Swart, 2012, The stable carbon isotopic composition of organic material in platform derived sediments: implications for reconstructing the global carbon cycle: Sedimentology, v. 59, p. 319-355.
Rothman, D. H., 2002, Atmospheric carbon dioxide levels for the last 500 million years: PNAS, v. 99, n. 7, p. 4167-4171.
Shackleton, N. J., 1985, Atmospheric carbon dioxide, orbital forcing, and climate. In: The Carbon Cycle and atmospheric Co2: natural variations Archaen to Present: Geophysical Monograph (Eds. E. T. Sundquist and W. S. Broecker), v. 32, p. 412-417.
Shackleton, N. J. and M. Hall, 1984, Carbon Isotope Data from Leg 74, In: Initial Reports Deep Sea Drilling Project (Eds. J.T. Moore and P. Rabinowitz), v. 74, p. 613-619.
Swanson-Hysell, N. L., Rose, C. V., Calmet, C. C., Halverson, G. P., Hurtgen, M. and A. C. Maloof, 2010, Cryogenian glaciation and the onset of carbon-isotope decoupling: Science, v. 328, p. 608-611.
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APPLICATION OF CAVITY RINGDOWN SPECTROSCOPY TO STABLE ISOTOPIC MONITORING OF CO2 SEQUESTRATION
DURING ENHANCED OIL RECOVERYBenjamin T. Galfond, Daniel D. Riemer, and Peter K. Swart
KEY FINDINGS
Routine monitoring of the concentration and 13C of gases can be accomplished under field conditions using a Cavity Ring Down Spectrometer (CRDS).
The CRDS is superior to traditional Isotope Ratio Mass Spectrometry in that there is rapid access to data enabling real time decisions to be made based on the changes in concentration and 13C values.
Even subtle variations in the concentration and composition of CO2 due to biogenic activity can be reliably measured.
BACKGROUND
Atmospheric concentrations of carbon dioxide (CO2) have rapidly increased over the past two centuries as a result of the burning of fossil fuels. One approach to deal with the increase is to store or sequester the CO2 underground. With viable injection sites all over the globe, millions of tons of gas may be stored for millennia in underground reservoirs, and when coupled with practices such as enhanced oil recovery this can even be an economical process.
One question which must be addressed is whether the CO2 remains within the storage site. Geochemical observations of CO2 concentrations provide an insight into possible leaks, although with a large number of natural sources, flux measurements may not always represent reservoir leakage. By examining the stable isotopic signature of the emitted CO2, we can better attribute increased soil gas emissions to their sources, be they natural or a result of the sequestration process. When coupled with the other geological data, we can better attribute the CO2 signature we see near the surface with subterranean seismic activity and gas migration.
Traditional stable isotope ratio mass spectrometry (IRMS) is poorly suited to high temporal resolution in-situ studies, because of size, power, and cryogen requirements. In contrast, a new optical technique, Cavity Ringdown Spectroscopy (CRDS), has the potential to play an important role in such measurements (Figure 1) because of the instrument’s small size and low power requirements.
INSTRUMENT DEPLOYMENT
A site was chosen in Texas, where CO2 derived from a natural source (Jackson Dome) was being injected to support enhanced recovery. In order to monitor potential leakage a range of analytical methods were deployed at the site including synthetic aperture radar (SAR), Global Positioning System (GPS), and Seismic analysis. Deployment of the CRDS to the site occurred in February 2012. The instrumentation setup consists of a
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centrally located shed that houses the bulk of the equipment and 13 sampling positions spread radially approximately 150 m from the center covering an approximate 70,000 m2 footprint, allowing for topographical constraints (Figures 2 and 3). A PVC pipe is sunk two feet into the soil (Figure 4) and terminates above the surface in a polyurethane tube that runs back to a vacuum manifold located inside the shed. All tube lengths are kept identical in order to ensure an equal distribution of vacuum between the lines.
Figure 1. The CRDS in the lab at RSMAS prior to deployment at the Texas site.
Figure 2. Sampling manifold which allows the sampling of 13 locations plus three standards.
Figure 3. Sampling locations. Analyzer is located near position 1.
Figure 4. Sampling point
Constant suction by a KNF rotary pump ensures that the sample taken from each line is
a current representation of soil gas conditions rather than an amalgam of stagnated gasses remaining since the last time the line was sampled. Prior to entering the vacuum manifold, each line is tapped into by a VICI Valco 16 position valve (Figure 2). The electrically actuated valve allows for the automated selection of any sampling position. The valve feeds sample gas directly to the Picarro CRDS on site and is programmed and actuated by the system as well. This system is housed inside a shed on site. A steel space frame covered in pressure treated plywood provides a stable, dry foundation for the structure. Power is wired directly into the shed from nearby overhead lines, with one circuit devoted to a power conditioning battery backup unit and a second circuit for cooling purposes.
As a result of the remote nature of the selected site, anthropogenic contributions from industry and transportation should be at a minimum. The predominant signal aside from any potential leakage would thus be from the local plant and soil microbial community. In order to better assess such contributions, we have begun a
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comprehensive assessment of the area soils and vegetation for carbon and nitrogen content and isotopic composition. By combining the range of varied data being collected at the site with geochemical modeling we are able to constrain the measurements obtained by the CRDS instrument.
Figure 5. The inversely correlated diurnal patterns of CO2 concentration and isotopic composition over the course of one week are easily identified.
INTERPRETATION AND IMPLICATIONS
With the high temporal resolution provided by the CRDS instrument, we could observe diurnal trends in CO2 uptake and emission by plant and microbial sources, as well as seasonal shifts in biogenic activity (Figure 5). When local weather data is cross-referenced with CO2 values, the influence of precipitation can be observed as well. The isotopic values of the CO2 contributed by these sources was monitored and recorded over time, helping to develop a baseline of expected emissions against which an emission from Enhanced Oil Recovery (EOR) related activities would be more easily identified.
With ambient CO2 13C value at ~ -8‰ and the biological and anthropogenic contributions far more depleted (-20 to -30‰), the 13C signature of the injectant at about -3.6‰ to -2.6‰ (Zhou et al., 2003) is easily identifiable. An above average methane concentration can also be found in the injectant and can be measured by our field instrument. An injectant emission event would thus be characterized by an increase in CO2 and CH4 concentration, enrichment of the CO2 isotopic composition, and a Keeling plot of delta value vs. 1/CO2 concentration yielding an intercept consistent with the 13C of the injectant. An expected emission event during a routine site maintenance activity provided an excellent opportunity to verify the integrity of our detection system. Event detection and analysis was conducted prior to being informed of the gas release. As expected, we saw an increase in concentration of the species of interest and a positively trending isotopic composition. When further analyzed, we found the contributing gas to have an isotopic composition of -3.9‰. As a mix of both the
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injectant gas and the more negative biogenic sources that characterize a normal background emission, this is an excellent match to the injectant being released (Figure 6).
Figure 6. Increased CO2 concentration mirrors an isotopic enrichment. This excess gas matches the isotopic fingerprint of the injectant at -3.9‰
CONCLUSIONS
CRDS is an emerging technology that is already in a position to supplant traditional IRMS for high resolution field monitoring analysis. Though there are many factors to consider when deploying instrumentation remotely, this system can produce reliable continuous measurements with minimal required on-site interaction. The data from such a system is able to observe and easily identify the release of injectant CO2, as well as the more subtle diurnal and seasonal variations in ambient CO2. As a key component of a multifaceted geophysical and geochemical monitoring regimen, CRDS is a proven asset in the monitoring of carbon sequestration operations.
REFERENCES
Zhou, Z., Ballentine, C. J., Schoell, M., and S. H. Stevens, 2003, Noble gas tracing of subsurface CO2 origin and the role of groundwater as a CO2 sink: American Geophysical Union, Fall Meeting, v. V51H-0379.
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SUB-MICRON DIGITAL IMAGE ANALYSIS (BIBSEM-DIA),PORE GEOMETRIES AND ELECTRICAL RESISTIVITY IN
CARBONATE ROCKSJan H. Norbisrath, Gregor P. Eberli, Ralf J. Weger, Klaas Verwer,
Janos Urai, Guillaume Desbois, and Ben Laurich
KEY FINDINGS
DIA from large-scale BIBSEM mosaics and from Optical Light Microscopy (OLM) images can be combined for multi-scale analysis, ranging from 20 nm to cm-scale.
Because the Pore-size Density Distribution (PsDD) follows a power law, the amount of micropores can be extrapolated from OLM imagery.
The Total Pore Density (TPD) has a dominant control on electrical resistivity but the rock type has to be taken into account because pore network architecture controls the connectivity.
Pore network connectivity can be predicted from a combination of pore shape and spatial analysis parameters (NNCF - Nearest Neighbor Connectivity Factor).
SIGNIFICANCE
Verwer et al. (2011) postulate that electrical resistivity and Archie’s cementation factor m are directly related to the number of pores and pore throats. This hypothesis is based on Digital Image Analysis (DIA) of thin sections with Optical Light Microscopy (OLM) that has a limited resolution of 6-7 μm/pixel. Subsequent tests by analyzing high-resolution Micro-CT scans and relating pore throat distributions from MICP measurements to electrical resistivity corroborated Verwer’s findings (Norbisrath, 2011). However, finite element resistivity modeling performed on the μ-CT tomograms indicates the importance of the micropores on electrical properties, which lie below the resolution of the Micro-CT method.
Figure 1. Zoom into Broad-Ion-Beam (BIB) cross-sectioned and investigated area (red outline) of sample 22. Cut-out on right shows virtually a two-dimensional surface with sub-micron-scale pores ready for subsequent segmentation and quantification.
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To assess the influence of the micropores in a quantitative manner, we use a new method that combines Broad-Ion-Beam milling (Desbois, 2011) and subsequent SEM image mosaic acquisition (BIBSEM). With this method the sub-micron architecture of the rock becomes quantifiable and the DIA can be extended from millimeter down to nanometer scale (Figure 1). This multi-scale quantification is particularly necessary in carbonate rocks, which are heterogeneous across several length scales.
METHOD AND DATASET
The cutting-edge new BIBSEM technique was performed at the RWTH Aachen in a joint collaboration. The method utilizes a JEOL SM-09010 cross section polisher to produce nanometer-precision flat surfaces by milling the rock down with an argon ion beam. The BIB-milling step is necessary because quantification of nanometer-scale pores requires large nanometer-precision flat surfaces. BIB milling produces surfaces of up to 2 mm2 total area, in contrast to Focused Ion Beam milling (FIB), which often introduces surface damage and where investigated areas are 100 times smaller. The large BIB surfaces are investigated at 5000x and 15000x magnification (resolution: 58.6 nm/pixel and 18.5 nm/pixel, respectively). The acquired BIBSEM mosaics are composed of around 500 images each, covering up to 1 mm2. Segmentation into pore and solid phase then allows for Digital Image Analysis. Results from BIBSEM-DIA are combined with DIA from OLM for multi-scale analysis of pore geometry, and detected pore sizes span from tens of nanometers to tens of millimeters.
Four samples were chosen from different depositional and diagenetic environments in order to compare their distinct microstructures. Two of the samples have previously been imaged in 3D with Micro-CT scanning. All of the samples have been measured for their electrical resistivity with a NER AutoLab 1000 system, analyzed with Mercury Injection Capillary Pressure (MICP) methods, and investigated on their macropore structure with DIA from OLM on epoxy impregnated thin sections. The four rock samples are: 1)Oolitic Grainstone, 2) Wackestone, 3) Travertine and 4) Dolomite. All the samples have a similar porosity of around Ø = 16% to minimize the effect of differing porosity but to allow the assessment of controls of the pore geometry on electrical resistivity, i.e., the cementation factor m.
Pore Size Density Distribution (PsDD)
PsDDs are analyzed by distributing the pores into logarithmically spaced bins according to their areas. Rather than their size equivalent diameter the total cross-sectional areas are used because electrical current uses the entire area of the opening to flow, unlike fluids which are more dependent on the shape of the opening due to capillary forces. The PsDDs are then normalized by total area and bin width and the resulting normalized pore densities are plotted on log scale (Figure 4).
Pore Network Connectivity (NNCF)
The Nearest Neighbor Connectivity Factor (NNCF) gives an indication of the connectivity of the pore network. The hypothesis is that the closer the next pore, the more likely a connection exists. The NNCF is calculated by relating the perimeter of the pore to the distance to the nearest neighboring pore (NND). This indirect approach has to be taken as 3D pore network connectivity is very hard to assess from 2-Dimensional imagery. Finally the values for all the pores in each sample are averaged (Table 1; Figure 5, left).
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Figure 2. Overviews of the BIB-cross-sectioned areas of the 4 samples with accompanying high-resolution images at 15000x magnification.
RESULTS
The large BIBSEM image mosaics reveal the diverse microarchitectures of the different rock types. Around 80-90% of the detected pores were invisible with previous imaging techniques. Binarized into solid phase and pore phase the mosaics illustrate the micropore structure of the rock (Figure 3).
Table 1. Petrophysical parameters for the 4 analyzed samples, including the cementation factor m, Helium Porosity, the Nearest Neighbor Connectivity Factor (NNCF), the Total Pore Density (TPD) for both OLM+5kx and OLM+15kx magnification imagery, and the slope of the regression line of the Pore Size Density Distributions (PsDD) from Figure 4.
Sample m Porosity [%]
NNCF TPD 5kx [pores/mm²]
TPD 15kx [pores/mm²]
Slope
22 2.61 16.2 2.09 82786 222450 -1.66 49b 1.72 15.9 1.85 134042 414225 -1.80
ST36A 3.36 14.0 1.06 25800 222671 -1.69 WR56.15 2.20 16.8 1.40 14351 44228 -1.53
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Total Pore Density (TPD)
The amount of pores per square millimeter varies drastically depending on imaging resolution. Pore count per square millimeter is 3 to 8 times higher when analyzed with 15000x magnification instead of 5ooox magnification (Table 1). At all resolutions, Total Pore Density (TPD) is highest in the Wackestone (22) and lowest in the Dolomite (WR56.15). This is also directly evident from the binarized mosaics (Figure 3). Not directly visible, however, is that TPD at high magnifications is equally high (around 222000 pores/mm²) in the Travertine (ST36A) as in the Wackestone (49b).
(Micro-) Pore Structures of Different Carbonate Rock Types
The Ooid Grainstone (sample 22) has a well-connected pore network between isopachous, bladed cements, covering the partially microporous ooid grains. This pore network is considered to allow high electrical conductivity, but pore count is low and tortuosity is increased because the interior of the large ooid grains is not very well connected. As a result the ooid grainstone has a high cementation factor (m = 2.61).
The Wackestone (sample 49b) is made up almost entirely of a matrix of homogeneous microspar (microspar crystals: ~1 μm diameter) with very little shell fragments or larger pores. These small crystals form a very well-connected pore network with low tortuosity and a very high pore density, which results in the low measured cementation factor (m = 1.72).
Figure 3. Binarized microstructures of the 4 different rock samples at 5000x magnification. Cementation factor m, Total Pore Density (TPD), and Nearest Neighbor Connectivity Factor (NNCF) are shown for each sample. Scale bar is 100 μm.
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The Travertine (sample ST36A) has a very scattered and diverse porosity network. Porous areas are isolated by very dense areas without visible porosity. This, together with the low pore count, results in the highest observed cementation factor (m =3.4).
The Crystalline Dolomite (sample WR56.15) is characterized by a wide and angular intercrystalline pore network, which is similar in structure to the Wackestone’s pore network, just at a larger scale (dolomite rhombs: ~100 μm diameter). The decreased pore density, however, makes conditions less favorable for the conduction of electrical charge, which results in a higher cementation factor (m = 2.2).
Figure 4. Pore-size Density Distributions (PsDD). Log of pore densities in exponentially growing pore size bins, colors represent different imaging techniques and resolutions. Data exhibits a linear behavior, i.e. predictability at each resolution with high R² values. The regression line is steeper for the more microporous samples. Pore areas are expressed in size equivalent diameters for better comprehensibility.
Pore Size Density Distribution (PSDD)
The most arresting finding from this multi-scale pore-structural investigation is that the PSDDs show a linear behavior at all scales and resolutions (Figure 4) when plotted on log-log paper. The slope of the regression line through the combined PsDDs is steepest for the sample with the highest TPD (49b – Wackestone) and lowest for the one with the lowest TPD (WR56.15 – Dolomite).
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INTERPRETATION AND IMPLICATIONS
(Micro-) Pore Structures and Pore Network Connectivity (NNCF)
Visual analysis of micro-architectures from BIBSEM mosaics of different rock types can qualitatively describe their pore network connectivity (Figure 3). It is evident from the mosaics that the Ooid Grainstone (sample 22) has the best connected pore network and that the Travertine (sample ST36A) has the least inter-connected pore structure. Visual comparison between the two crystalline pore structures of the Wackestone (sample 49b) and the Dolomite (sample WR56.15) is more difficult from the binarized overview images (Figure 3), because the excellent connectivity of the Wackestone only becomes evident at higher resolutions (Figure 2; top right).
By using spatial analysis software (GIS), this visual comparison can be enhanced and the connectivity can be quantified. The NNCF can separate rocks with well-connected pore networks (e.g. sample 22 - Ooid Grainstone – high NNCF = 2.09) from those which are less well connected (e.g. sample ST36A – Travertine – low NNCF = 1.06), where TPD are the same.
A combination of TPDs with NNCFs can give a good estimate of the electrical flow properties of a pore network (Figure 5).
Figure 5. Cementation factor plotted against NNCF (left) and TPD (right). A correlation is visible, but it is not yet statistically relevant due to the so far limited dataset.
Electrical transport properties, however, are not directly linked to fluid transport properties, because electrical current (electrons) can still flow where surface tensional effects hinders large water molecules to flow (diameter of water molecule is about 0.3 nm). For example, the Wackestone sample shows a very low cementation factor, but the gas permeability is negligible. This low permeability is caused by the very narrow and intricate pore network of the sample.
Pore Size Density Distributions (PSDD)
The linear behavior of the PSDDs is the most interesting finding from the new multi-scale analysis. This implies that TPD can be estimated from analysis of the pore network at a limited range of resolution, i.e. a single imaging technique (OLM or BIBSEM). In combination with pore throat size distributions from MICP measurements, one could calculate the pore cavity to pore throat size ratio, which is a very important factor for electrical transport properties, as big parts of larger pores are "dead volume" behind narrow pore throats. However, when reaching the limit of the resolution at around 20 nm (at 15000x magnification) and 60 nm (at 5000x magnification), the pore size information becomes less reliable. This is evident from a kink at the upper end of the Pore-size Density Distributions (Figure 4).
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Total Pore Density (TPD)
Pore count statistics from OLM and BIBSEM still show a general trend of increased electrical conductivity (lower m) with increased pore density (Table 1; Figure 1). However, the effect of pore network architecture on the different rock types has to be considered. The dolomite sample WR56.15 shows comparably less pores, but the excellent connectivity (higher NNCF) and low tortuosity of the crystalline microstructure results in a moderate cementation factor (Table 1 or Figure 5).The increased TPD at higher resolution illustrates how visible porosity and pore detection are resolution-dependent. The higher the resolution, the more pores you resolve. The question remains, up to which resolution will you find more pores (i.e. what is the physical lower limit of pore sizes)? It is also necesary to determine an endpoint when extrapolating amounts of (nano-)pores from the linearly distributed PsDDs.
Implications
To understand and predict the effect of the pore structure on the electrical behavior in carbonate rocks, it is paramount to analyze both the macro- and microstructure of their pore system (Figure 6). This is a consequence of their multi-scale heterogeneity. Nevertheless, this study indicates that pore size distributions can be predicted from DIA from a single image source (i.e. at a single imaging resolution), as they follow a power law. The amount of pores (pore density) at macro- and microscales has proven to be an essential factor when predicting electrical resistivity; hence this finding could become useful for reservoir characterization. However, it has to be tested on a larger dataset, one that consists of a broader range of rock types and also different amounts of porosity.
REFERENCES
Desbois, G., Urai, J. L., et al., 2011, High-resolution 3D fabric and porosity model in a tight gas sandstone reservoir: A new approach to investigate microstructures from mm- to nm-scale combining argon beam cross-sectioning and SEM imaging: Journal of Petroleum Science and Engineering, v. 78, p. 243-257.
Norbisrath, J. H., Eberli, G. P., Weger, R. J., Knackstedt, M., and K. Verwer, 2011, Modeling electrical resistivity in carbonates using micro-CT scans and assessing the influence of microporosity using MICP: CSL Annual Review Meeting.
Verwer, K., Eberli, G. P., and R. J. Weger, 2011, Effect of pore structure on electrical resistivity in carbonates: AAPG Bulletin, v. 95, p. 175-190.
Weger, R. J., Eberli, G. P., Baechle, G. T., Massaferro, J. L., and Y.-F. Sun, 2009, Quantification of pore structure and its effect on sonic velocity and permeability in carbonates: AAPG Bulletin, v. 93, no. 10, p. 1297-1317.
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USING CLUMPED ISOTOPES TO UNDERSTANDEARLY DIAGENESIS
Peter K. Swart, Monica M. Arienzo, Sean T. Murray, Yula Hernawati, James S. Klaus, and Donald F. McNeill
KEY FINDINGS
Using the clumped isotope thermometer in combination with conventional stable oxygen isotopic analysis we have determined the 18O of fluid involved in the formation of corals, bird eggs, and early diagenetic carbonates.
In every instance in which the 18O of the depositional fluid was calculated, the values reflected the composition of the fluids which were actually present.
The use of clumped isotopes will revolutionize the interpretation of diagenetic process in carbonate rocks.
SIGNIFICANCE
Several seminal papers have established patterns in the behavior of geochemical tracers during early diagenesis (Allan and Matthews, 1982; Lohmann, 1987). These include the identification of enrichments in the 18O at sub-aerial exposure surfaces, the inverted-J isotopic signal, the relatively constant 18O values within vadose and freshwater phreatic zones, a gradual enrichment within the mixing zone, and heavy 18O values within the marine phreatic zone. All of these signatures are predicated on the basis of a constant temperature with the 18O reflecting a change in the source or evaporation history of the water. The use of the clumped isotopic measurements allows an assessment of the temperature and therefore using the conventional 18O measurement the true 18O of the water can be measured. This knowledge can lead to substantially improved interpretation of the paragenetic sequence.
STUDY AREAS
In order to assess the potential for clumped isotopes to reveal information on the 18O of the fluids involved in precipitation of carbonates we have chosen materials from modern, Holocene, and Pleistocene carbonates. In each of these cases we have measured the 47 to determine the temperature and then used this temperature in conjunction with known temperature- 18O calibrations to calculate the 18O of the precipitating fluid. The samples include the following.
Modern Corals: Modern corals have been chosen from areas such as Tobago which are known to be influenced by riverine sources of water (Orinoco) and also corals which have not experienced any freshwater influence (coastal reefs from Florida). These different areas should have different 18O of the waters. Bird Eggs: The 18O of bird eggs have been suggested to be related to the 18O of the water in which the birds forage. We have compared the clumped isotope signature of
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eggs from the same species (Great Egrets) living in hydrologically distinct regimes as well as eggs from Black Skimmers and Cormorants living in northern latitudes which have isotopically more depleted 18O values. Clumped Isotopic Signature in Characteristic Diagenetic Zones: The inverted J signal is a commonly seen signal in early diagenetic carbonates. It is believed to result from the alteration of the marine sediments by a large pool of meteoric water which imparts its
18O upon the carbonate leading to the formation of a meteoric carbonate line (MCL). The materials used for determining these trends are taken from cores drilled during the Bahamas Drilling Project and the Dominican Republic Drilling Project.
RESULTS
Modern Corals: The 18O of the water calculated using the Ghosh et al. equation (Ghosh et al., 2006) equation and the Leder et al. equation (Leder et al., 1996) yield values close to modern seawater for corals collected from Florida. Corals which have been collected from regions clearly influenced by freshwater have much more depleted 18O values than than corals living under normal marine conditions. Bird Eggs: The calculated temperatures of the shells from the Everglades are within the expected error (40-44oC), while the birds from NY and CT have lower temperatures (Figure 1). Specimens collected from the Everglades had calculated 18O values of the water which were significantly more positive than samples from Florida Bay. In contrast samples of Black Skimmers and Cormorants, from New York and Connecticut respectively, had more depleted calculated 18O water values. The calculated water compositions are in agreement with values measured in the environment from which they were collected.
Figure 1. Upper panel shows the 13C and 18O of eggshells from three species of birds, 2 specimens of Great Egrets from South Florida (Everglades and Florida Bay), a Black Skimmer (NY) and a Cormorant (CT). The calculated temperatures of the shells from the Everglades are within the expected error (40-44oC), while the birds from NY and CT have lower temperatures. The calculated water compositions are in agreement with values measured in the environment from which they were collected.
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Clumped Isotopic signatures associated with the Inverted J: The inverted J pattern has been described as the pathway by which carbonate samples are altered when exposed to a large volume of meteoric fluid with a constant 18O value (Lohmann, 1987). On a plot describing 13C and 18O, the pathway of evolution describes an inverted J pattern forming a meteoric calcite line (MCL) which is representative of calcite in equilibrium with the meteoric fluid at the temperature of diagenesis (Figure 2). Within a singal geological deposit multiple MCLs can be recognized representing alteration by a variety of fluids at different times. Analyses of such samples from a meteorically altered deposit in the Dominican Republic not only confirms the notion that the different MCLs represent alteration in different fluids, but also that they have been altered in different temperatures (Figure 3). It is suggested that this alteration occurred at progressively lower stands of sea level, with the heaviest and coldest MCL having taken place at the lowest sea-level stand.
INTERPRETATION AND IMPACT
The data presented in this abstract clearly show the potential of using clumped isotopes in conjunction with conventional stable isotope analyses. In all instances in which modern carbonates were formed in environments with elevated 18O values, the calculated 18O of the fluids were also elevated. In instances where the 18O were supposedly depleted, the calculated 18O values were also depleted.
Figure 2. 13C and 18O values from Pleistocene cores from the Dominican drilling project. Trends describe three different MCL lines. The green box represents approximate composition of the original sediment.
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REFERENCES
Allan, J. R., and R. K. Matthews, 1982, Isotope signatures associated with early meteoric diagenesis: Sedimentology, v. 29, p. 797-817.
Ghosh, P., Adkins, J., Affek, H., Balta, B., Guo, W. F., Schauble, E. A., Schrag, D., and J. M. Eller, 2006, C-13-O-18 bonds in carbonate minerals: a new kind of paleothermometer: Geochimica et Cosmochimica Acta, v. 70, p. 1439-1456.
Leder, J. J., Swart, P. K., Szmant, A. M., and R. E. Dodge, 1996, The origin of variations in the isotopic record of scleractinian corals: I. Oxygen: Geochimica et Cosmochimica Acta, v. 60, p. 2857-2870.
Lohmann, K. C., 1987, Geochemical patterns of meteoric diagenetic systems and their application to the study of paleokarst, in James, N. P., and P. Choquette, Eds., Paleokarst: Berlin, Springer -Verlag, p. 58-80.
Figure 3. Temperature and water 18O calculated using clumped isotopes for samples shown in Figure 2. Each MCL defines a separate trend with the most depleted 18O values representing alteration at progressively lower temperatures and probably lower sea levels. The most positive values represent the original 18O of the seawater in which the carbonates were formed.
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NEW INSIGHTS INTO DOLOMITIZATION USINGCLUMPED ISOTOPES
Sean T. Murray, Monica M. Arienzo, and Peter K. Swart
KEY FINDINGS
Through the measurement of the 47 of dolomites, it has become possible to constrain the temperature and oxygen isotopic value of the fluid involved in the formation of dolomite. The oxygen isotopic composition in turn relates to the salinity of the fluid.
The analysis of dolomites from locations in which the temperature and environment of dolomitization is reasonably well constrained allows us to determine the most suitable equation with which to back calculate the 18O of the fluid involved in dolomitization.
SIGNIFICANCE
The ever present “Dolomite Problem” has made the understanding of dolomite systems difficult in the past (Land, 1980). With the advent of clumped isotopes (Ghosh et al., 2006), it has become possible to approach questions on the formation of dolomites from a new angle that is not confounded by unknowns such the 18O of the fluid. Despite the promise of an independent thermometer provided by clumped isotopes, old problems regarding which of the five equations link temperature to the 18O of the fluid are still present. However, by utilizing young dolomites that formed in an environment that is reasonably well constrained, this study is able to make a best guess at which of these equation is most suitable.
STUDY AREAS
In this study we utilize samples from two areas, The Bahamas, a locality with a reasonably well constrained diagenetic history and an older dolomite of Carboniferous age with a more complicated history. The younger rocks are derived from a 168m deep-core drilled on the island of San Salvador in the Bahamas and extensively studied (Dawans and Swart 1988; Supko 1977; Swart et al., 1987). This core displays an extended dolomite section replacing middle Miocene to late Pliocene carbonates. These dolomites are texturally mature but formed as recently as 150,000 yr BP. Through extensive isotopic and petrographic studies, it has been determined that these dolomites were formed in two phases by seawater with near normal composition.
The older dolomites are derived from the Mississippian aged Madison Formation at Sheep Mountain in Wyoming. Dolomites from this locality are fine-crystalline and have been interpreted as forming during transgressive sea-level cycle changes associated with the reflux of hypersaline brines from evaporitic lagoons (Smith et al., 2004). This interpretation was supported by the existence of solution collapse breccias which are associated with the existence of evaporitic lagoons (Sonnenfeld, 1996). The dolomites are then believed to have been altered by meteoric waters which reset some of the dolomites isotopically (Moore, 2001).
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The data from these two localities are then compared with the Latemar, in the Italian Alps formed during the middle Triassic. The dolomites at this location show varying degrees of textural maturity with the most mature in the center of the buildup, but there is a sharp transitional boundary with the out-lying calcites. The Latemar dolomites form a mushroom shaped cap approximately 2.5 km across and 400 m high which cross cuts multiple formations and depositional features. Their formation has been suggested to be either unmodified, hydrothermal sea water driven by plutonic activity (Wilson et al., 1990) or modern diffuse flow fluids derived from the mid-ocean ridges (Carmichael and Ferry, 2008; Carmichael et al., 2008). These dolomites were the subject of a recent paper utilizing clumped isotopes by Ferry et al. (2011).
RESULTS
Measurements using the clumped isotope method were made on a section of the San Salvador core spanning 54m to 67m depth and on the Sheep Mountain samples from 299m to 317m. The temperatures were calculated using the equations of Ghosh et al. (2006) and corrected using the methods of Dennis et al. (2011). The samples from San Salvador have a calculated temperature of formation of 31 to 39°C with an average standard error of ±1.7°C. The temperatures show a slightly increasing arc down core.
Samples from Sheep Mountain display a temperature range of 40 to 62°C with an average standard error of ±1.5°C. The temperatures trend towards higher temperatures with increased amounts of dolomite present in the sample.
Although the clumped
isotope method provides temperature, the calculation of the fluid isotopic composition necessitates the use of one of the five equations (Fritz and Smith, 1970; Northrop and Clayton,
Figure 1. (A) The lines represent the interpretation using the dolomite-temperature line from Vasconcelos (2005). The San Salvador data plots as forming from a fluid with a 18O between +2‰ to +4‰. This is too high and inconsistent with previous interpretations.
Figure 1. (B) These are the same data, but adjusted using the constraints of Shepard and Schwarz (1970) which puts the San Salvador data in range consistent with formation from normal marine waters.
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1966; O'Neil and Epstein, 1966 ; Sheppard and Schwarcz, 1970; Vasconcelos et al., 2005) which link the temperature of formation and the 18Owater with temperature. This problem has been recognized as a major stumbling block in dolomite interpretation over the past 30 years (Land, 1980). In order to determine which is the most appropriate equation we used the dolomites from San Salvador which are known to have formed at near surface temperatures and from fluids which were near seawater or maybe slightly elevated in their 18Owater values. The most recent equation (Vasconcelos et al. 2005) of the five provides fluid compositions greater than+4‰. We consider these values too positive to be realistic. The other extreme, the equation of Northrop and Clayton (1966), gives water values which are -4‰. The equation of Sheppard and Schwarz (1970) provides fluid values which are in closest agreement with the previous interpretations (Figure 1).
INTERPRETATION AND IMPACT
San Salvador: The dolomitized interval from San Salvador represents an alteration from a hard crystalline dolomite showing mimetic replacement of the precursor to a
Figure 2. The alternation (LH) between the different types of dolomite described in the text used to select the samples for temperature measurements (figure from Dawans and Swart, 1988). Temperatures (RH) increase into the middle of the alternation and then decrease towards the base. The high temperature at the base represents the start of the next alternation. See Figure 3 for 18Owater values.
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microsucrosic dolomite. These alterations occurred repeatedly throughout the core and each one was associated with a sub-aerial exposure surface and a change in the stoichiometry, Sr content, and 18O (Figure 2).
Within this one alternation of
dolomite types, the measured temperature ranged from 31 to 29oC with higher values being prevalent in the central portion of the alternation (Figure 2). The change in temperature is positively correlated with the 18O of the fluids so that the warmer fluids represent fluids elevated in 18O (Figure 3). The range of temperatures measured in the San Salvador dolomites seem too high by 10-15oC, perhaps a result of the fact that the Ghosh et al., (2006) equation may not be directly applicable to dolomites. It should
be noted that in the previous study on dolomites (Ferry et al., 2011) a completely theoretical approach (Guo et al., 2009) was used which tends to yield slightly lower temperatures than those presented here. Lower temperature would in turn lead to lower calculated fluid values.
Sheep Mountain: Temperatures calculated for the Sheep Mountain dolomites show higher values (40 -62oC) than those from San Salvador. These higher values could reflect partial resetting of originally lower formation temperatures during burial. As these rocks are not 100% dolomites (in contrast to San Salvador), we have estimated the
18O of the fluid using a combination of the calcite-water (Kim and O'Neil, 1997) and dolomite-water equations (Sheppard and Schwarcz, 1970). These rocks show a range of values from ~0‰ to +4‰ (excluding 2 points which have significantly lower
18O values) with a tendency for values to be the most positive immediately below the breccia (Figure 4). This would tend to support the hypothesis that the dolomitization is associated with the collapsed breccia as previously proposed.
Figure 3. Correlation between temperature and fluid 18Owater from San Salvador.
Figure 4. The 18O of the fluids from Sheep Mountain. The collapsed breccia is situated around 430 m and so values become more enriched towards the feature.
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Latemar: There has been only one previously published article on the application of
clumped isotopes to the study of dolomitization (Ferry et al., 2011). These workers measured a temperature range of between 50 to 70 °C with the 18Owater averaging -0.3 ± 1.7 ‰. This is in stark contrast to a previous study which measured fluid inclusion temperatures above 150oC (Wilson et al., 1990). The later study concluded that the fluid inclusions had been reset by a later event. It was suggested that the temperature range of 50-70oC reflected a partial resetting of the temperature during later burial. It is also important to note that this study used the Vasconcelos et al. (2005) equation to calculate the 18Owater values. Based on our work and considering the nature of the samples used to construct that relationship we feel the Sheppard and Schwarcz (1970) equation may be more appropriate. Changing the equation would produce diagenetic fluids with lower
18Owater values than originally suggested.
CONCLUSIONS
Although the application of clumped isotopes has the potential to revolutionize the study of dolomitization, there are still some uncertainties. These include:
1) Which is the correct temperature-water relationship to use when calculating the 18O
of the dolomitizing fluid?
2) Is the Ghosh et al. (2006) equation applicable to dolomite?
3) To what degree does the original temperature signature imparted during the formation of the dolomite, become altered during burial?
REFERENCES
Carmichael, S. K., and J. M. Ferry, 2008, Formation of replacement dolomite in the Latemar carbonate buildup, Dolomite, northern Italy: Part 2. Origin of the dolomitizing fluid and the amount and duration of fluid flow: American Journal of Science, v. 208, p. 885-904.
Carmichael, S. K., Ferry, J. M., and W. F. McDonough, 2008, Formation of replacement dolomite in the Latemar carbonate buildup, Dolomites, northern Italy Part 1. Field relations, mineralogy, and geochemistry: American Journal of Science, v. 308, p. 851-884.
Dawans, J., and P. K. Swart, 1988, Textural and geochemical alternations in late Cenozoic Bahamian dolomites: Sedimentology, v. 35, p. 385-403.
Dennis, K. J., Affek, H. P., Passey, B. H., Schrag, D. P., and J. M. Eiler, 2011, Defining an absolute reference frame for 'clumped' isotope studies of CO2: Geochimica et Cosmochimica Acta, v. 75, p. 7117-7131.
Ferry, J. M., Passey, B. H., Vasconcelos, C., and J. M. Eiler, 2011, Formation of dolomite at 40-80 degrees C in the Latemar carbonate buildup, Dolomites, Italy, from clumped isotope thermometry: Geology, v. 39, p. 571-574.
Fritz, P., and D. G. W. Smith, 1970, The isotopic composition of secondary dolomites: GCA, v. 34, p. 1161-1173.
Ghosh, P., Adkins, J., Affek, H., Balta, B., Guo, W. F., Schauble, E. A., Schrag, D., and J. M. Eller, 2006, C-13-O-18 bonds in carbonate minerals: A new kind of paleothermometer: Geochimica et Cosmochimica Acta, v. 70, p. 1439-1456.
Guo, W., Mosenfelder, J. L., Goddard, W. A., III, and J. M. Eiler, 2009, Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: insights from the first
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principles theoretical modeling and clumped isotope measurements: Geochimica et Cosmochimica Acta, v. 73, p. 7203-7225.
Kim, S. T., and J. R. O'Neil, 1997, Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates: Geochimica et Cosmochimica Acta, v. 61, p. 3461-3475.
Land, L. S., 1980, The isotopic and trace element geochemistry of dolomite: the state of the art: SEPM Special publication 28, p. 87-110.
Moore, C., 2001, Carbonate reservoirs: Porosity evolution and diagenesis in a sequence stratigraphic framework: Developments in Sedimentology, v. 55: Amstterdam, Elsevier, 444 p.
Northrop, D. A., and R. N. Clayton, 1966, Oxygen isotope fractionation in systems containing dolomite: J. Geol., v. 74, p. 174.
O'Neil, J. R., and S. Epstein, 1966, Oxygen isotope fractionation in the system dolomite-calcite carbon dioxide: Science, v. 152, p. 198-201.
Sheppard, S. M. F., and H. P. Schwarcz, 1970, Fractionation of carbon and oxygen isotopes and magnesium between coexisting calcite and dolomite: Contrib. Mineral Petrol., v. 26, p. 161.
Smith, L. B., Eberli, G. P., and M. Sonnenfeld, 2004, Sequence-stratigraphic and paleogeographic distribution of reservoir-quality dolomite, Madison Formation, Wyoming and Montana, in Grammer, G. M., Eberli, G. P., and P. M. Harris, Eds., Intergration of outcrop and modern analogues in reservoir modeling: American Association of Petroleum Geologists Memoir, p. 94-118.
Sonnenfeld, M. D., 1996, Sequence evolution and hierarchy within the lower Mississippian Madison Limestone of Wyoming, in Longman, M. W., and M. D. Sonnenfeld, Eds., Paleozoic Systems of the Rocky Mountain region, Society for Economic Paleontologists and Mineralogists (Society for Sedimentary Geology) Rocky Mountain Section, p. 165-192.
Supko, P. R., 1977, Subsurface dolomites, San Salvador, Bahamas: Journal of Sedimentary Petrology, v. 47, p. 1063-1077.
Swart, P. K., Ruiz, J., and C. W. Holmes, 1987, Use of strontium isotopes to constrain the timing and mode of dolomitization of Upper Cenozoic sediments in a core from San Salvador, Bahamas: Geology, v. 15, p. 262-265.
Vasconcelos, C., McKenzie, J. A., Warthmann, R., and S. M. Bernasconi, 2005, Calibration of the 18O paleothermometer for dolomite precipitated in microbial cultures and natural
environments: Geology, v. 33, p. 317-320. Wilson, E. N., Hardie, L. A., and O. M. Phillips, 1990, Dolomitization front geometry, fluid-flow
patterns, and the origin of massive dolomite - the Triassic Latemar buildup, Northern Italy: American Journal Of Science, v. 290, p. 741-796.
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SPELEOTHEMS: A MODEL SYSTEM FOR THE STUDY OF FLUID INCLUSIONS AND CLUMPED ISOTOPES
Monica M. Arienzo, Sean T. Murray, Hubert B. Vonhof1, and Peter K. Swart
1) Vrije Universiteit Amsterdam, Amsterdam, The Netherlands
KEY FINDINGS
Oxygen isotopic analysis of fluid inclusions, combined with clumped isotopes, and the in situ monitoring of calcite precipitation in caves allows for the determination of the competing influences of temperature and water isotopic composition in controlling the oxygen isotopic composition of carbonates.
The example presented here, a speleothem from the Bahamas, demonstrates that by applying these various methods we can develop a clear understanding of the fluids which form these carbonates.
The lessons from these examples can be applied to a wide range of diagenetic carbonates.
SIGNIFICANCE & BACKGROUND
Traditionally carbon and oxygen isotope analyses have been used to unravel the depositional and diagenetic history of carbonates. However, the 18O of a carbonate is dependent both upon the variations in temperature as well as the 18O of the water. In order to solve for the second unknown, an additional proxy is needed which can provide information on one of the two unknowns. Such proxies might include the ratio of certain trace elements relative to calcium, fluid inclusions, and/or clumped isotopes.
The motivation for this study is to utilize a speleothem from the Bahamas as a case study for the application of clumped isotopes and stable isotopic analysis of fluid inclusions. By applying these various methodologies we hope to gain a better understanding of the factors which control the oxygen isotopes of the speleothem.
METHODS
This study will focus on a speleothem, sample DC-09, collected from Dan’s Cave on Abaco Island, Bahamas. Sampling of the speleothem for stable C and O isotopes was conducted using a computerized micromill and analysis was performed on the Delta Plus mass spectrometer. Fluid inclusion analysis is the analysis of microscopic, water filled cavities within the stalagmite. These cavities preserve drip water at the time of formation and allow for the direct measurement of the 18O composition of the formation water. Fluid inclusion analyses were conducted at a resolution of about one sample every 1.5 cm. Fluid inclusion isotopes were analyzed at Vrije Universiteit Amsterdam. Analysis was conducted utilizing the “Amsterdam Device”, an instrument built specifically for the extraction of water from fluid inclusions (Vonhof et al., 2006). The extracted water was then measured for oxygen and hydrogen isotopes.
Carbonate clumped isotope analysis is based on the measured abundance of the rare isotopes in the carbonate, such as the 13C-18O bonds (Ghosh et al., 2006) . The 13C-18O
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‘clump’ is of interest to geochemists because as temperatures decrease, clumping increases, independent of the isotopic composition of the formation water . This method therefore allows temperature to be determined without prior knowledge of the d18O of the fluid involved in precipitation. The same samples utilised for fluid inclusion analysis were also measured for clumped isotopes allowing for a direct comparison between the two proxies. Clumped isotope analyses were conducted at the University of Miami and have been standardized using the method from Dennis et al. (2011). Carbonate stalagmite samples have been run in triplicate with an average 47 standard error of 0.007 which equates to a 1.5 °C range in temperature. The clumped isotope analyses have been conducted using the Thermo MAT-253 in the Stable Isotope Laboratory.
RESULTS
Plotting the 18Oc of the calcite with respect to age demonstrates that there are significant variations in the oxygen isotopic value of the calcite (Figure 1). There is an observed increase in the 18Oc followed by a very rapid decrease in the 18Oc value. As discussed above, determining the drivers of these changes is problematic in that they could either be temperature related or reflect variations in the 18Ow of the fluid.
The fluid inclusion and clumped isotope records support a change in the 18Oc of the fluid precipitating the stalagmite. The fluid inclusion 18Ow (water) increases in concert with increases in the 18Oc (Figure 1). This is then followed by a decrease in the 18Ow as the carbonate oxygen isotopes decrease. Utilizing the 18Oc and 18Ow results, temperature can be calculated. For stalagmites, this relationship was determined from cave monitoring experiments by Tremaine et al., 2011. Applying this equation supports additional temperature variation associated with the changes in the 18Oc and 18Ow
(Figure 2).
Figure 1. Blue line represents the 18Oc record from the calcite. Blue squares are the 18Ow derived from the fluid inclusions and orange circles are 47 measurements, note the axis is reversed.
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The clumped isotope record supports the fluid inclusion data (Figure 1). Temperature from Figure 2 is calculated using the Ghosh et al. (2006) equation. The 18Ow is calculated from temperature using the equation from Tremaine et al., (2011). There is an offset between the 47 and fluid inclusion 18Ow records and also an offset between the two temperature records (Figure 2). The calculated 18Ow from the clumped isotope record is more positive than the fluid inclusion data, with an average offset between the records of about 0.6 ‰. The temperature offset between the two records is about 9 °C (Figure 2). This offset is not unique to this speleothem sample. The observed offset is consistent with other speleothem studies and is thought to be driven by the way the calcite precipitates in speleothems (Affek et al., 2008; Kluge and Affek, 2012). However, overall the clumped isotope record supports similar trends to the fluid inclusion results, with an increase in 18Ow associated with the 18Oc increase and associated temperature variation.
Although there are still uncertainties regarding the interpretation of the clumped isotope signal in stalagmites, the 18Ow record from the fluid inclusions is supported by the calculated oxygen isotope ratio of the water based on the clumped isotopes.
FLUID INCLUSION ANALYSIS AT THE UNIVERSITY OF MIAMI
Recent work has been conducted on developing a fluid inclusion extraction device at the University of Miami. The “Miami Device” is unique because the setup utilizes cavity-ring down spectroscopy (CRDS) for 18O and 2H analysis of fluid inclusions. The extraction line at the University of Miami is an in-line system directly interfaced with the Picarro CRDS isotopic water analyzer and the design of the line is similar to the
Figure 2. (Left) Blue line with triangles represents 18Ow derived from the fluid inclusions and orange line with triangles is 18Ow derived from 47 measurements. (Right) Blue line is the water from fluid inclusions, orange line is the water calculated from 47.
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Amsterdam Device (Vonhof et al., 2006). The extraction line consists of a crusher which is a modified valve unit, a septum port for the direct injection of water and a water trap (Figure 3a). The CRDS technique is based on using a near infrared laser to scan over the H2O spectral range and by measuring the absorption spectra using a ring-down method, to determine isotopic abundances (Figure 3b). Preliminary results demonstrates a 0.3 L water injection provides ample signal for isotopic analyses with an average standard deviation of 0.29 ‰ for 18O and 2.2 ‰ for 2H, which is comparable to other fluid inclusion extraction devices (Figure 3b). This setup will now enable in house isotopic measurements of fluids from carbonates.
Figure 3. (Top) “Miami Device” on the right with the Picarro CRDS on the left. (Bottom) Screen shot of the Picarro analyzing 0.3 L of water injected into the Miami Device. Top view shows the ppm of H2O, middle view of the 18O, and bottom view is of the 2H.
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BAHAMAS CAVE MONITORING
In the summer of 2012, cave monitoring began in Eleuthera, Bahamas. Here, the temperature of the cave and relative humidity are measured every 2 hours. Additionally, we are conducting in situ monitoring of calcite precipitation (Figure 4). Calcite is “farmed” by placing glass slides on top of actively forming stalagmites within the cave with the slides being collected every 2-3 months (Figure 4). This allows for direct comparison between the cave environment and the chemistry of the calcite. The carbonate is then removed from the glass slide and analyzed for stable C and O isotopes and clumped isotopes. In Figure 4, preliminary results from clumped isotopes are shown. The offset between the clumped isotope and actual temperature is 3.4°C , with the clumped isotopes giving a warmer temperature. As demonstrated, there are still uncertainties regarding the interpretation of the clumped isotope signal in stalagmites and by continually monitoring a Bahamas cave and collecting calcite as it forms, a better understanding of clumped isotopes in speleothems can be developed. Through continual monitoring, this work may potentially aid in the development of a calibration equation of
47 to temperature for speleothems.
Figure 4. (Top left) A student descends into the monitoring cave in the Bahamas. Top right: Calcite farming set up within the cave. (Bottom left) Microscope slide after 2 months in the cave, calcite was removed from the slide and analyzed for clumped isotopes. (Bottom right) Actual measured temperature compared with preliminary clumped isotope derived temperature from the calcite farming.
Slide 6 - HBC
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INTERPRETATION AND IMPLICATIONS
This case study demonstrates how the application of the clumped isotope and fluid inclusion methods can aid in understanding the fluid from which carbonate precipitated. This study demonstrates that changes in the calcite isotopes are a result of changes in the calcite precipitating fluids, as well as temperature. Combining these two methods may aid our understanding of diagenesis and of the fluids precipitating carbonates. Furthermore, future cave calcite farming may provide valuable insight regarding the processes driving geochemical records.
REFERENCES
Affek, H. P., Bar-Matthews, M., Ayalon, A., Matthews, A., and J. M. Eiler, 2008, Glacial/interglacial temperature variations in Soreq cave speleothems as recorded by 'clumped isotope' thermometry: Geochimica et Cosmochimica Acta, v. 72, p. 5351-5360.
Dennis, K. J., Affek, H. P., Passey, B. H., Schrag, D. P., and J. M. Eiler, 2011, Defining an absolute reference frame for 'clumped' isotope studies of CO2: Geochimica et Cosmochimica Acta, v. 75, p. 7117-7131.
Ghosh, P., Adkins, J., Affek, H., Balta, B., Guo, W. F., Schauble, E. A., Schrag, D., and J. M. Eller, 2006, C-13-O-18 bonds in carbonate minerals: A new kind of paleothermometer: Geochimica et Cosmochimica Acta, v. 70, p. 1439-1456.
Kluge, T., and H. P. Affek, 2012, Quantifying kinetic fractionation in Bunker Cave speleothems using 47.: Quaternary Science Reivews, v. 49, p. 82-94.
Tremaine, D. M., Froelich, P. N., and Y. Wang, 2011, Speleothem calcite farmed in situ: Modern calibration of 18O and 13C paleoclimate proxies in a continuously-monitored natural cave system: Geochimica et Cosmochimica Acta, v. 75, p. 4929-4950.
Vonhof, H. B., van Breukelen, M. R., Postma, O., Rowe, P. J., Atkinson, T. C., and D. Kroon, 2006, A continuous-flow crushing device for on-line 2H analysis of fluid inclusion water in speleothems: Rapid Communications in Mass Spectrometry, v. 20, p. 2553-2558.
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SEISMIC AND GPR IMAGING OF FRACTURES IN CARBONATE RESERVOIRS USING 3D DIFFRACTION RESPONSES CAUSED BY FRACTURE INTERSECTIONS
Mark Grasmueck, Tijmen Jan Moser1, and Michael A. Pelissier2 1) Moser Geophysical Services, Den Haag NL
2) Marathon Oil Company, Houston USA
KEY FINDINGS
Due to kinematic similarity of GPR and seismic wave propagation, GPR data can be used as a scaled proxy for seismic data to study the use of diffractions for fracture imaging below the resolution limit of the reflection seismic method.
Intersections of perpendicular fracture sets create dihedral and trihedral scatterers causing recordable diffractions from fracture systems with little or no vertical displacement and 1/500th of a wavelength fracture width.
Fracture intersections and hence the resulting diffractions are direct indicators of fracture connectivity.
Seismic diffraction signal levels are lower than their GPR counterparts. In order to use diffractions at the reservoir level, special care to preserve and enhance diffractions must be taken during acquisition and processing.
INTRODUCTION AND SIGNIFICANCE
Typically sub-vertical fracture mapping based on seismic data relies on displacements of otherwise continuous reflections. Semblance or coherency attributes help make small displacements visible. In theory, vertical displacements as small as one quarter wavelength of the highest frequency are resolvable. For a typical carbonate reservoir, a velocity of 5000 m/s and seismic signal frequency of 50 Hz the smallest visible fracture displacement of a stratigraphic reflection is 25 m. Such a fracture is a rather large fault. Fractures with small or zero vertical displacement are thus beyond the resolution of classical reflection seismic fracture mapping. Borehole imaging and structural modeling may help estimate the distribution of smaller fractures but are limited by spatial uncertainty. Besides carbonate reservoirs, tight shale reservoir production and stimulation would benefit from laterally extensive information about connected fracture networks with below quarter wavelength displacements.
Diffractions are a promising source for sub-wavelength 3D fracture information. Diffractions originate from small-scale discontinuities in the subsurface and are normally treated as noise in conventional seismic processing as they interfere with continuous reflections. Diffractions already have been successfully used to define oil bearing karst caverns which previously had not been resolved (Yang et al., 2011). Similarly, Pomar (2010) showed how diffractions recorded in dense 3D Ground Penetrating Radar (GPR) data can be used to image complex karst and fracture networks. Li et al. (2012) propose a geologically plausible model for the origin of karst diffractions with sub-wavelength spherical or random bodies of slow material (2500–3200 m/s) embedded in fast carbonate hostrock (6000 m/s). The objective of this paper
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is to find a geological model for the origin of fracture related diffractions where no karst voids are present.
HYPOTHESIS AND APPROACH
Our Hypothesis is that seismically recordable diffractions are caused by intersections of thin fractures with no displacement.
We use high-resolution 3D GPR data as a bridge between synthetic modeling and seismic reservoir imaging. Natural fracture networks of outcropping reservoir analogs can be efficiently imaged with 3D GPR and interpreted with the help of the nearby outcrop (Grasmueck et al., 2012). Due to the kinematic similarity of GPR and seismic wave propagation, the GPR data can used as a proxy for seismic data to help develop new diffraction based fracture imaging workflows. Through scaling relationships the GPR findings are applied to the seismic method and provide guidance for the use of diffractions for determining previously seismically unresolvable fracture systems at reservoir depth.
RESULTS
3D GPR Response of a vertical X subhorizontal Fracture Intersection
Figure 1 is a small subvolume of the larger Cassis 3D GPR survey acquired with 5 x 10 cm trace spacing and 200 MHz antennae (Grasmueck et al., 2012). The low amplitude 11º dipping subhorizontal reflections are caused by around 1 mm open joints following stratigraphic boundaries. When following the reflection bands of these subhorizontal fractures in timeslices they are lined by small bright spots. The center of the timeslice in Figure 1 is located on such a bright spot. On unmigrated data the bright spot corresponds to the apex of a diffraction cone. For a slightly deeper timeslice the spot has the shape of a circle (Figure 1b). In the 3D migrated data in Figure 1c), d) the diffraction is focused in a small and elongate high amplitude anomaly. The long axis of the migrated anomaly is aligned with the intersection line between the two joints (Figure 1 e). Within the entire 3D GPR survey volume hundreds of such small bright spots can be observed. Laterally extensive fractures are defined by multiple such bright spots aligned in the same fracture plane.
In the nearby outcrop wall the subhorizontal fractures are continuous over tens of
meters and relatively smooth (Figure 2a). In contrast, the vertical fractures consist of multiple segments belonging to the same fracture trend. Fracture opening is less than 1 mm, similar to the horizontal fractures. The size of continuous fracture segments is typically less than 0.5 m, thus smaller than the wavelength of the GPR signal. At the intersection of vertical and horizontal fractures sharp corner geometries are formed leading to a blocky appearance of the quarry wall. The dimensions of these blocks are between 0.1 and 0.5 m (Figure 2b). Within the 3D rock volume the corners form dihedrals for 2 intersecting fractures or trihedrals (also known as cateye or corner reflector) when 3 perpendicular fractures intersect. Dihedrals and Trihedrals are known to be efficient wavefield scatterers. The same geometrical configuration is used in retro reflectors for the safety of vehicles and back scattering targets in satellite remote sensing applications. In the case of fractured media the intersection of near perpendicular fractures creates natural scatterers causing diffractions in GPR and seismic data.
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Ray-Born Synthetic Modeling of Fracture Intersections
As shown in Figure 2c) a simple cross represents the basic geometrical element of a fracture intersection. Ray-Born synthetic modeling (Moser, 2012) of crosses with different sizes (Figure 3a) show that for dimensions of less than one quarter wavelength point diffraction responses are generated. The larger the cross the higher the amplitude. The point diffraction does not resolve the individual arms of the cross (Figure 3b). When exceeding a quarter wavelength, also known as the Rayleigh resolution limit, the diffraction tails split up as the arms of the cross generate individual diffractions interfering with each other. As a result the amplitude is concentrated in the apex and weaker on the split diffraction tails (Figure 3c) also leading to multiple interfering circles on timeslices as seen for example in Figure 1b.
Figure 1. Subvolume of larger 3D GPR survey acquired in Cassis Quarry (France). The bright spot in the center of the top face is caused by the intersection of a vertical with a sub-horizontal fracture. a) and b) are unmigrated data. In b) the top face is 5 cm deeper than in a) showing circular diffraction pattern. c) and d) are the corresponding 3D migrated cubes where the diffraction hyperboloid has been focused into a small high amplitude anomaly. e) Fracture Interpretation. Vertical exaggeration of the cube display is 3.3x.
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UPSCALING OF DIFFRACTION RESPONSE FROM OUTCROP GPR TO RESERVOIR DEPTH SEISMIC DATA
While many clear diffraction signatures are observed in dense 3D GPR data, seismic diffraction examples from reservoir depth are still rare. Table 1 compares the GPR and seismic parameters relevant for the detection of diffractions in the near surface and at a typical carbonate reservoir depth of 4 to 5 km. Central frequency of the seismic data is assumed to be 50 Hz.
The absolute seismic scattering strength for oil- or water-filled fractures is about half of the air-filled limestone fractures of the Cassis outcrop. The 1 mm fracture opening observed in outcrop translates into 20 cm fracture width at reservoir level. Dihedrals and trihedrals at the intersections of fracture segments with extents of less than 25 m cause point diffractions responses as shown in the modeling results of Figure 3. For proper sampling of the full diffraction signals seismic surveys need to be acquired with a single sensor trace spacing of less than 12.5 m to also include the higher than 50 Hz frequency content of the wavelet. Many recently acquired seismic surveys satisfy this spatial sampling requirement.
The maximum reflection depth recorded in the Cassis 3D GPR survey is 10 m corresponding to twenty wavelengths. Using the number of wavelengths for the seismic cases translates into 1600 m which is too shallow for most reservoirs. The GPR data also show that clear diffractions are only visible to half or even just a quarter of the maximum reflection depth. Two factors are responsible for this strong reduction of depth for the observation of diffractions: 1) Diffraction signals experience spherical spreading amplitude decay for the down-going and the up-going wavefield. For reflections spherical spreading affects only the down-going part as the up-going wave can be approximated by a plane wave. 2) Diffractions depend on the recording of wide apertures. At a radiation angle of 60º measured from vertical, only half the vertical depth can be reached with the same signal amplitude. With these amplitude reductions on the order of one magnitude, seismic diffractions can only be recorded to less than 1000 m depth. In fact, most published seismic diffraction examples are from seismic data sets (Berkovitch et al., 2009). In order to overcome this depth limitation and see clear diffractions at the reservoir level three measures must be taken:
1) Acquire very dense, vertically stacked single sensor data with a sufficiently
high SN ratio. While GPR equipment only has 16 bit dynamic range, the standard 24 bits of seismic AD converters are sufficient to also record weak diffraction signals originating at reservoir depth.
2) Preserve all diffracted Energy during processing. 3) Separate reflection and diffraction parts of the seismic signal and perform
diffraction analysis on the diffraction only part (Moser and Howard, 2008).
IMPLICATIONS AND CONCLUSIONS
Diffractions are not just caused by karstic voids and caverns. Our hypothesis that diffractions originate at the intersection of thin fractures with no displacement is supported by high-resolution 3D GPR data compared to the nearby outcrop and confirmed by synthetic modeling. Quarter wavelength or smaller dihedrals, trihedrals created at the intersections of perpendicular fracture sets are efficient point scatterers with omnidirectional radiation patterns. The non-random distribution of these sub-Rayleigh size discontinuities is caused by fracture trends and patterns providing information about fracture spacing and fracture continuity of fractured domains. By
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their nature fracture intersections and hence the resulting diffractions are direct indicators of fracture connectivity. Seismic diffraction signals from 4-5 km deep carbonate reservoirs are by an order of magnitude weaker than reflections from the same depth. To fully harness diffractions and their information about reservoir fracture systems seismic data need to be acquired densely with high signal-to-noise ratio coupled with processing optimized for diffraction signal preservation and separation.
Figure 2. Quarry wall below the site where the 3D GPR of Figure 1 were acquired. Subhorizontal fractures are intersected by vertical fracture segments and form dihedral scatterers.
Figure 3. Ray-Born synthetic data of line crosses with different sizes. Until quarter wavelength cross diameter point diffraction responses with increasing amplitudes are generated.
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Table 1. GPR vs. Seismic Scale Comparison. To reach sufficient seismic diffraction strength at reservoir level an order of magnitude signal dynamic range gain is needed.
REFERENCES
Berkovitch, A., Belfer, I., Hassin Y., and E. Landa, 2009, Diffraction imaging by multifocusing: Geophysics, v. 74, WCA75–WCA81. doi:10.1190/1.3198210.
Grasmueck, M., Coll, M., Eberli, G. P., and K. Pomar, 2012, Diffraction imaging of sub-vertical fractures and karst with full-resolution 3D GPR: Accepted for publication in Geophysical Prospecting.
Li, F., Di, B., Wei, J., and X. Li, 2012, Volume estimation of the carbonate fracture-cavern reservoir - A physical model study: 74th EAGE Conference & Exhibition.
Moser, T. J., and C. B. Howard, 2008, Diffraction imaging in depth: Geophysical Prospecting, v. 56, p. 627–641. doi: 10.1111/j.1365-2478.2007.00718.x.
Moser, T. J., 2012, Review of ray-Born modeling for migration and diffraction analysis: Studia Geophysica et Geodætica, In Press.
Pomar, K., 2010, Visualization and quantification of fractures and karst in Cretaceous carbonates, Cassis, France: University of Miami M.S. Thesis, Open Access Theses, Paper 76. http://scholarlyrepository.miami.edu/oa_theses/76
Yang, P., Liu, Y. L., Li, H. Y., Dan, G. J., An, H. T, and Y. M. Shao, 2011, Fractured-vuggy reservoir characterization of carbonate, Tarim Basin, Northwest China: 73rd EAGE Conference & Exhibition.
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4D GPR FOR CHARACTERIZATION OF FLUID FLOW IN CARBONATES: INSIGHTS FROM STRUCTURAL- VS.
STRATIGRAPHIC-CONTROLLED DOMAINS AND COMPARISON WITH ECLIPSE DYNAMIC MODELING
Pierpaolo Marchesini, Mark Grasmueck, Gregor P. Eberli, and Ralf J. Weger
KEY FINDINGS
4D GPR monitoring of gravity flow in reservoir analogues allows the characterization of fluid dynamics at the 1-10 m scale.
Small-scale stratigraphic boundaries control fluid flow in high-porosity, non-fractured carbonates.
Fluid migration in fractured domains is governed by structural discontinuities such as faults and deformation bands.
Deformation bands are responsible for fluid compartmentalization in highly-saturated conditions.
Standard approach in dynamic modeling fails to capture the influence of small-scale heterogeneities on fluid dynamics.
INTRODUCTION AND SIGNIFICANCE
Lateral variability makes the characterization of fluid dynamics in carbonate reservoirs an open challenge. Even a precise reconstruction of stratigraphic boundaries and fracture networks does not guarantee a comprehensive knowledge of preferential flow paths. Current characterization of hydraulic parameters largely relies on 0.01-0.1 m scale laboratory experiments, sample plug measurements, and modeling. However, a large degree of upscaling prevents these methods from fully reproducing realistic flow conditions. In this study we used time-lapse 3D GPR (4D GPR) to non-invasively track and quantify the evolution of fluid flow over time (2-15 hours interval) and space (1-10 m scale). The goal is to compare results from two field-scale experiments. The first experiment was in the fracture-controlled Madonna della Mazza quarry. The second was conducted in the undisturbed oolitic limestone in Ingraham Park. The purpose is to assess the role of stratigraphic versus structural heterogeneities on fluid flow in high-porosity carbonates. Furthermore, comparison between 4D GPR results and Eclipse dynamic simulation offers insights to optimize workflows for more detailed flow models and residual fluid recovery.
FIELD SITES DESCRIPTION
Madonna della Mazza quarry
The Madonna della Mazza quarry (MdM) is cut into the Orfento Formation situated on the inner part of the Majella anticline (Southern Italy). The Upper Cretaceous formation is composed of eroded subangular rudist fragments ranging in size from silt to rudite.
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Porosity values range from 25% to 35% and permeability from 150 mD to 630 mD. The stratigraphy of the quarry is characterized by prograding beds of grainstones interbedded with thinner, fine-grained carbonate layers slightly dipping to the NE. Previously conducted structural assessments of the entire quarry revealed the presence of two main types of fractures: faults and deformation bands (Tondi et al., 2006). Deformation bands are thin sheets of reduced porosity in the fault zone. These cataclastic features form preferentially in the quarry uppermost, high-porosity, massive grainstones due to micro-mechanical grinding of grains and do not show discontinuity surfaces. Deformation bands generally have sealing properties for cross-fault fluid flow and present porosities from 25% to 29%, lower than the grainstones with 32-35%. This reduction of porosity results in a decrease of permeability up to half of the values measured in the intact grainstones.
Ingraham Park
The field site of Ingraham Park is located southwest of Miami, between Coral Gables and Coconut Grove. The park lies on a barrier oolitic bar which is part of a complex and heterogeneous shoal system deposited during an interglacial Pleistocene sea-level high-stand. Heterogeneities in the structurally undisturbed oolitic system are influenced by grain size distribution, geometry of depositional bodies, and stratigraphy. The highly variable rock matrix results in a wide variety of porosities ranging from 40% to 60% and permeabilities from 600 mD to 1500 mD. Diagenesis and large-scale karstic dissolution increase the degree of heterogeneity. Based on 3D GPR surveys covering the entire park area, the 4D GPR site was selected to avoid the 1-4 m diameter filled-in dissolution holes reported by Truss et al. (2007). Three main GPR facies, presenting high horizontal and vertical variability, are observed in the 3D datasets: 1) bioturbated facies characterized by worm burrows (lowermost); 2) perpendicular geometries interpreted as migrating oolitic bars (middle); 3) cross-bedded continuous layers representing the prograding shoal complex (uppermost). The majority of the connected porosity within the uppermost reservoir unit is related to thin, coarse-grained shell hash beds typically forming at the base of channels (Neal et al., 2008).
THE 4D GPR METHOD AND DATASETS
4D GPR is the acquisition of repeated 3D GPR surveys with identical geometries. The purpose is to compare pairs of surveys and extract physical changes related to the fluid migration while the surrounding matrix, not affected by water content changes, remains unaltered.
In MdM 2952 liters of water were infiltrated from the surface into the host matrix over a period of 30 hours. In Ingraham Park 3200 liters were infiltrated in 5 hours. Both infiltrations were performed using 4 m diameter, temporary polyethylene pond walls (circular for MdM, squared for Ingraham). The time intervals, measured after the end of the infiltration, of repeated 3D GPR surveys used to compute snapshots of local water content changes were: 1) 2-4 hours, 7-9 hours, 12-15 hours for MdM, and 2) 2-5 hours, 5-8 hours, 8-11 hours for Ingraham Park. Table 1 shows acquisition parameters and survey information for the two field sites.
Table 1. Survey information for the MdM quarry and Ingraham Park field sites.
Field Site GPR system, Frequency Inline Spacing
Crossline Spacing
Survey Volume (X, Y, max. depth)
MdM Dual-channel, 200 MHz 5 cm 2.5 cm 20 x 20 x ~12 m Ingraham Park Single-channel, 250 MHz 10 cm 5 cm 18 x 20 x ~8 m
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Water decreases the speed of electromagnetic waves and, as a consequence, increases the traveltime of subsurface reflections. The application of a 3D crosscorrelation algorithm (also known as WARP) allows automatic extraction of local timeshifts between pairs of time-lapse surveys. Using the Topp petrophysical transfer function (Topp et al., 1980) local water content changes can be calculated and displayed as a semi-transparent attribute overlaying standard 3D GPR data (Fig. 1). Pairs of repeated surveys are used to create snapshots of different stages during the waterbulb infiltration experiment. In each snapshot the boundaries of the waterbulb are the draining zone (top) and the wetting zone (bottom), the portions of matrix with negative and positive water content changes.
COMPARISON OF 4D GPR RESULTS FROM MDM AND INGRAHAM PARK
In MdM quarry structural features, such as deformation bands, play a key role in influencing the fluid migration. Timeslices in Figure 1 (top part) show a pronounced asymmetry of both draining (D) and wetting (W) zones, experiencing higher water content changes over 2 hours in the undisturbed matrix (D: -3.5/-3%; W: +3.5/+4%) than in the deformation bands area (D: -1.5/-1%; W: +1.5/+2%). Moreover, the wetting zone shows a noticeable lateral distribution up-dip along the fault plane, indicating its role as a preferential flow path (Marchesini et al., 2010).
Figure 1. Timeslices showing the evolution of draining (D) and wetting (W) zones for MdM (top part, 2-4 hours snapshot) and Ingraham Park (lower part, 2-5 hours snapshot). Draining zone is the upper boundary of the waterbulb and wetting zone the lower. Depths of timeslices are marked in Figure 2. Black circles (top part, for MdM) and black squares (lower part, for Ingraham Park) mark the boundaries of the temporary, 4 m diameter polyethylene pond walls.
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At Ingraham Park (Fig. 2, lower part) water content changes over 3 hours are larger across the pond infiltration area in both draining and wetting zones (D: -5.5/-4.5%; W: +6.5/+9%) suggesting a faster fluid migration compared to MdM. The upper waterbulb boundary is within the pond perimeter while the lower boundary is shifted down-dip: fluid migration in the wetting zone follows stratigraphy.
These observations are confirmed when comparing results of 4D GPR analysis in inline direction near the center of the pond in Figure 2. Deformation bands in MdM (Fig. 2, top part) compartmentalize the fluid while higher water content changes are experienced in the undisturbed matrix. Stratigraphy has a lesser effect on fluid flow since both draining (D) and wetting zone (W) do not dramatically exceed the vertical projection of the pond into the subsurface.
On the contrary, water content changes in Ingraham Park are more laterally developed and the shape of the water bulb follows stratigraphic boundaries (Fig. 2, lower part). As a general trend also observed in timeslices, magnitudes of water content changes are higher suggesting that higher porosity and permeability values of the host rock can be related to more rapid fluid migration after the infiltration. The most lateral elongated bodies of high water content changes follow shell hash beds and thin cemented calcite layers (Grasmueck et al., 2007). The isolated water content change peak in the left portion of the wetting zone is interpreted to be the result of out of plane fluid flow.
Figure 2. Inline sections corresponding to timeslices in Figure 1. Water bulb geometry in MdM (top part) is affected by deformation bands while in Ingraham Park (lower part) draining and wetting zones mainly follow stratigraphic layers.
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HYDRAULIC HEADS AND WATER CONTENT CHANGES
Measurements of hydraulic heads over time, and in different locations within the pond infiltration area, allow the fluid migration in the two different domains to be compared and understood (Fig. 3). The hydraulic head of a moving water mass is a measure of the gravitational force that causes groundwater to flow. A practical way to estimate this quantity is to measure the height of the saturated water column or, in other words, the local difference in depth between the bottom of the draining zone and the top of the wetting zone. Hydraulic heads are measured at several XY locations within the pond infiltration areas as the water content changes volumes. The results are plotted in Figure 3 against correspondent maximum volumetric water content changes measured in the wetting zone of each hydraulic head evaluation point. Data show that in the case of MdM there is a sharp difference in magnitude of water content changes between undisturbed matrix and deformation bands, showing their role as fluid barriers for cross-fault fluid migration, especially in fully-saturated conditions (i.e. the early stages of infiltration). The influence of the deformation bands on fluid flow diminishes over time and water content changes converge to similar magnitudes for lower values of hydraulic heads.
On the contrary, in Ingraham Park there are no separate trends of 4D GPR derived
water content changes and hydraulic head measurements, indicating homogeneous flow across the entire pond area. Moreover, higher magnitudes of water content changes are experienced when comparing all the three temporal snapshots: faster fluid migration is achieved despite lower values of measured hydraulic heads. Less gravitational force is needed to produce large water content changes suggesting that higher values of porosity and permeability are responsible for faster fluid migration. This corroborates the observations made for timeslices and inlines in Figure 1 and Figure 2. As a final remark, hydraulic heads and correspondent water content changes plot on straight lines indicating linear relationship between fluid velocity and pressure gradient, which is a key condition to describe the infiltration mechanism as Darcy flow.
Figure 3. Measurements of hydraulic heads plotted against computed water content changes for MdM (red to yellow) and Ingraham Park (shades of blue).
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4D GPR RESULTS COMPARED WITH DYNAMIC FLUID FLOW MODELING
A static reservoir model of the surveyed portion of the MdM quarry was constructed from detailed geological interpretation of the 3D GPR volumes and integration with petrophysical parameters derived from sample plugs. This static model was the input for a dynamic fluid flow simulation generated with Eclipse (Schlumberger Dynamic Simulation Software) using the same infiltration settings as in the real MdM controlled experiment presented in this study (2952 liters of water, 5 days infiltration time span). As a preparation for the dynamic simulation, the original 3D stratigraphic and structural interpretation had to be adapted in terms of: 1) precision: while strike orientation and main geometries remained the same, small-scale heterogeneities (<30 cm) were lost. Zig-zag shapes, adopted for both faults and deformation bands, did not preserve original crosscutting relationships; 2) resolution: minimum cell size was set to 30 cm (total of ~150.000 cells) due to computational constrains while the original resolution of GPR volumes is 5 cm (correspondent to ~4.000.000 cells). In addition, faults and deformation bands have been set as zero-transmissibility surfaces in the orthogonal direction. The simulation produced snapshots of absolute water saturation covering a time span of 5 days with data reported every 12 hours (first snapshot: 2 hours after infiltration).
Figure 4. Visualization of absolute values of water saturation from Eclipse dynamic fluid flow modeling after 38 hours from the beginning of infiltration: inline section (top part) and timeslices (lower part) at correspondent depths of draining and wetting zones as for the 4D GPR experiment.
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There are differences when comparing results from 4D GPR method with dynamic fluid modeling. The data show that in inline direction (Fig. 4, top part) the effect of deformation bands on the fluid migration is completely lost: in the deformation bands area higher values of water saturation are experienced while 4D GPR showed the opposite behavior. The dynamic model fails to capture and visualize the role of structural heterogeneities on the fluid behavior also in timeslice (Fig. 4, lower part) showing a homogeneous distribution of water saturation across the whole pond area in both sections corresponding to draining and wetting zones as described in Figure 1.
CONCLUSIONS AND IMPLICATIONS FOR RESERVOIR CHARACTERIZATION
The 4D GPR method was successfully applied to two reservoir analogues in gravity flow experiments to visualize and quantify the effect of heterogeneities on fluid migration at the 1-10 m scale and time intervals of 2-15 hours. The presented study shows different fluid flow behaviors occurring in stratigraphic versus structural-controlled carbonate domains offering insights to: 1) conduct more efficient reservoir characterization and 2) to reduce uncertainties when upscaling from plug to field scale. Stratigraphic boundaries have a relevant control on fluid migration in high-porosity, non-fractured domains. In addition, structural heterogeneities (such as deformation bands) should be taken into consideration when building static reservoir models for dynamic simulations. Realistic flow models should always include both structural and stratigraphic heterogeneities in order to improve reservoir kinematic studies and residual fluid recovery.
REFERENCES
Grasmueck, M., and D. Viggiano, 2007, Flowzone detection with time-lapse GPR water content change measurements: CSL Annual Review Meeting.
Marchesini, P., Grasmueck, M., Eberli, G. P., and R. Van Dam, 2010, Tracking and quantifying fluid flow in fractured Cretaceous carbonates with 4D Ground Penetrating Radar (GPR): Madonna della Mazza Quarry, Italy: CSL Annual Review Meeting.
Neal, A., Grasmueck, M., McNeill, D. F., Viggiano, D. A., and G. P. Eberli, 2008, Full-resolution 3D radar stratigraphy of complex oolitic sedimentary architecture: Miami Limestone, Florida, USA: Journal of Sedimentary Research, v. 78, p. 638-653.
Tondi, E., Antonellini, M., Aydin, A., Marchegiani, L., G. Cello, 2006, The role of deformation bands, stylolites and sheared stylolites in fault development in carbonate grainstones of Majella Mountain, Italy: Journal of Structural Geology, v. 28, p. 376-391.
Topp, G. C., Davis, J. L., and A. P. Annan, 1980. Electromagnetic determination of soil water content: Measurements in coaxial transmission lines: Water Resource Research, v. 16, no. 3, p. 574-582.
Truss., S., Grasmueck, M., Vega, S., and D. A. Viggiano, 2007, Imaging rainfall drainage within the Miami oolitic limestone using high-resolution time-lapse ground-penetrating radar: Water Resource Research, v. 43, W0345, doi: 10.1029/2005WR004395.
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