Structure architecture interpretation of the middle Frio Formation using 3D seismic attributes and well logs: an example from the Texas Gulf Coast of the United States

8/10/2019 Structure architecture interpretation of the middle Frio Formation using 3D seismic attributes and well logs: an exa

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840 The Leading Edge July 2008

he Oligocene middle Frio Formation is a major

hydrocarbon producer in the U.S. Gulf Coast. Inthe area of the Stratton and Agua Dulce fields, the

middle Frio Formation consists of multiple vertically-stackedreservoir sequences (Figures 1 and 2). Te target for thisstudy was the basal part of the middle Frio Formation (BMF)interval between the G2 (top lower Frio) and the F11 horizons(Figure 3). Te structural setting of the middle Frio in thestudy area is predominantly growth faults and associatedrollover anticlines. Te objective of this work is to show how3D seismic data acquired on the exas Gulf Coast imagessubsurface structure that was impossible to fully interpret on2D seismic and well logs.

Te complex internal architecture of the middle Frio flu-vial reservoirs along with the structural complexity caused bygrowth faults may result in bypassed reservoirs or compart-ments with additional reserves. Previous analysis indicateduntapped and incompletely drained reserves in the middleFrio reservoirs, and inspired using 3D seismic to search forby-passed gas in the BMF interval.

We used two existing (poststack and time-migrated) 3Dseismic surveys to develop a detailed structural interpretationof the middle Frio. Te first is a 90 square-mile data set pro-vided by Anadarko Petroleum Corporation, originally shotin 1993 by Union Pacific Resources (UPR) and processed byWestern Geophysical. Te second is a 7.6 square-mile public

domain 3D seismic survey acquired in 1992 by the exasBureau of Economic Geology (BEG) and processed by Mer-cury International echnology (Figure 1). Te latter survey isreadily accessible through the BEG and has been the subjectof dozens of geophysics and geology papers. It is our hopethat, by presenting this smaller BEG survey in the contextof the larger UPR survey, we might inspire others to furtherrefine the interpretation of the BEG data.

We used the VSP data (Figure 4) acquired by the BEGat the Wardner 175 well in Stratton Field to directly tie theseismic data to the geologic horizons. Tis well is in the heartof the BEG survey and on the southern border of the UPRsurvey. Te two 3D seismic surveys partially overlap in the

northern Stratton field area.

Major and subsidiary faults:Syndepositional or postdepositional?

Te geologic cross-section in Figure 5a illustrates the changein thickness of the interval between the marker for the topE41 horizon (ME41) and the marker for the top lower Friohorizon (MG2) in both the hanging-wall and footwall sidesof the Agua Dulce (AD) fault. Note the change in thicknessof the BMF interval in the hanging-wall side at different lo-cations relative to the AD fault. Te coincident seismic line(Figure 5b) confirms that the AD fault has significant con-

Structural interpretation of the middle Frio Formation

using 3D seismic and well logs: An example from the Texas

Gulf Coast of the United States

INTERPRETERS CORNER Coordinated by MIKECOOPER

HAMEDEL-MOWAFY, Al-Azhar University, Cairo, Egypt

KURTJ. MARFURT, University of Oklahoma, Norman, USA

trol over total thickness variation at different locations withrespect to the fault.

Our BMF isochron maps indicate a thick sediment sec-tion deposited in a time when structurally low areas providedgreater accommodation space. Te thin sediment section wasdeposited in a time when structurally high areas resulted inlittle to no deposition of sediments corresponding to signifi-cantly smaller accommodation space. Te F11-F39 verticalsection (Figure 5) and isochron map (Figure 6) generated us-ing both seismic and well-log data show thickening between

reflectors on the hanging-wall side of the AD fault, suggestingthat the fault was syndepositional.

o test whether the synthetic and antithetic faults associ-ated with the major growth faults were active during depo-sition, we constructed two geologic cross-sections and com-pared them with the coincident seismic lines (Figures 7 and8). Correlating the middle Frio horizons (Figure 7) indicatesthat the gross thickness of the interval between the markertop E41 horizon (ME41) and the marker base F39 horizon(MBF39) is thicker by 126 ft (38 m) in the hanging wall ofthe antithetic fault than in the footwall block. In Figure 8,the gross thickness of the interval between the top F11 and

Figure 1. Index map showing the location of the Stratton and Agua

Dulce fields, the UPR and the BEG 3D seismic surveys, and the well inwhich the VSP survey was run.

Downloaded 25 Oct 2009 to 157.182.210.138. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/


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the base F39 in well Gruene 9 in the hanging-wall block ofsynthetic fault B is 44 ft thicker than the same interval in wellGruene 21 in the footwall block. Tese two examples indicatethat the synthetic and antithetic faults associated with themajor AD growth fault are not just accommodation faults butare syndepositional faults that cut through the BMF interval(the interval between G2 and F11). Further investigation isneeded to determine if these secondary faults are conduitsor seals to the migrating hydrocarbons in Stratton and AguaDulce fields.

Te middle Frio Formation is composed of fluvial de-posits in the form of point bar and crevasse splay sandstonesisolated within floodplain mudstones and siltstones. Tis

heterogeneity makes it possible that even small vertical faultoffsets will juxtapose low-permeability floodplain depositsagainst permeable sandstone reservoir rocks. Terefore, smallvertical offsets along these faults may form lateral flow bar-riers. Another way in which these synthetic and antitheticfaults may compartmentalize the middle Frio reservoirs is byproviding seals which follow the fault as gouge or cementedbreccias which formed along fault planes due to mechani-cal breakdown of rock during fault movement. Terefore thesecondary faults interpreted from the 3D seismic data sets(Figures 5-8) may impact the lateral continuity and result inmore compartmentalization of the middle Frio, meandering,fluvial sandstone reservoirs.

Structural framework

Although the previously constructed structure mapsbasedon well data onlyclearly showed the AD fault, the moresubtle synthetic and antithetic faults were not recognized.Figure 9 is a contour map showing Yangs (1998) structureconfiguration of the F11 horizon in Stratton Field. Otherthan the major AD fault, only one minor fault (in the south-east corner) is mapped. Figure 10, a two-way time structuremap for the same F11 horizon constructed from the BEGand the UPR 3D surveys, shows two major faults boundingthe western side of the image area and several subtle faults,

not shown before, at the F11 stratigraphic level and scatteredat different parts of the rollover anticlinal structure in the

hanging-wall side of the AD fault. Figure 11 is a 3D view ofthe F11 horizon mapped from the BEG 3D survey. Note thatthe surface irregularities in the rollover anticline reflect thelocations of subtle faults in the hanging wall of the AD fault.At the deeper horizons of the middle Frio Formation (suchas the F39 horizon in Figures 7b and 8b), the hanging-walldeformation is more pronounced than at shal lower horizons.Based on seismic data only, the structure of the F39 horizonexhibits rollover amplitudes (structural closures) that rangefrom 150 to 700 ft. At the shallower F11 stratigraphic level,the rollover amplitudes decrease and range from 50 to 500ft.

Figure 2. Interpreted 2D seismic line through Stratton Field showingrotation of fault blocks in the lower Frio and Vicksburg formations andthe lack of faulting in the middle Frio Formation (from Levey et al.,1993).

Figure 3. Logs from well Wardner 177 showing reservoir subdivisionsand markers of the basal middle Frio interval (BMF) used in this study. = top, B= base, and m = marker.

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Nature of the growth faults

Te nature of the subsidiary synthetic and antithetic faultswas estimated by looking at several vertical seismic linesfrom both 3D surveys. Te crossline in Figure 12a illustratesthat the BMF interval is deformed by the subsidiary faults onthe hanging-wall side of the AD fault and is accommodatedmainly by synthetic faulting. Te difference between thelevel of detail in this 3D seismic crossline and that presentedby Levey et al. (Figure 2) is dramatic. Te deformation ismore pronounced closer to the AD growth fault. Approxi-mately 1.5 miles from AD fault in the down-basin direction,we note an average of five secondary faults that affect thestratigraphic units of the BMF interval (F39-F11), with the

deeper horizons (e.g., F39) being more deformed than theshallower horizons (e.g., F11).

Figure 12b illustrates the deformation of the deeper mid-dle Frio in the footwall side of the AD fault. It shows thatantithetic faulting accommodates the deformation in the ADfault footwall, the opposite case from the hanging-wall defor-mation, and is more intense in the northern part of the UPR3D volume (southern Agua Dulce Field) than in the south-ern part (northern Stratton Field). Several synthetic and anti-thetic faults affect the deeper middle Frio interval (G2-E41)and younger horizons. Te coherence slices in Figure 13 in-dicate that the BMF interval is clearly affected at different

stratigraphic levels by several growth faults. Figure 14 showsmaximum positive and maximum negative curvature attri-butes that also indicate that the BMF interval is deformedby many faults. Tese faults are assumed to result in morecompartmentalization of the fluvial channels embedded inthe BMF interval.

Te vertical displacements of the major and subsidiarygrowth faults were measured from both vertical seismic sec-tions (inlines and crosslines) and well-log cross-sections. Basedon the two 3D seismic volumes, it was found that the amountof throw of major and minor faults varies from line to line inthe seismic surveys and increases in the northern part of theUPR 3D survey in the area of Agua Dulce Field. Te amount

of vertical separation of the AD growth fault increases withdepth and ranges from 850 to 1600 ft. For the subsidiarysynthetic and antithetic faults, vertical displacements rangefrom 15 to 145 ft. Tese subtle small-throw faults can be sig-nificant in the creation of multiple reservoir compartmentswithin the middle Frio Formation.

Structural model for the middle Frio Formation

One objective of this study was to test whether synthetic andantithetic faults cut through the middle Frio Formation inthe area of the Stratton and Agua Dulce fields. Investigatingthe BMF interval (G2-F11) from the 3D seismic volumes and

Figure 4.VSP data from well Wardner 175 (from Hardage et al., 1996). Horizons of interest include F11 and F39 at ~1.58 s and ~1.65 s,respectively. Te peaks (black-filled to the right) are the tops of reservoir units.

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well-log data (Figures 5-14) shows that the middle Frio hori-

zons (e.g., F11 and F39) become segmented or compartmen-talized by the effect of the subsidiary synthetic and antitheticfaults associated with the major growth faults.

Based on these findings, we developed a new structuralmodel (Figure 15) for the BMF interval. In this model, theinterval between F11 and G2 has been broken into severalcompartments by major and secondary growth faults. Atabout 2.75 miles from the trace of the AD growth fault, theBMF horizons split into several reservoir compartments dueto, on average, seven synthetic and antithetic faults cuttingthrough this interval. Movements along these individualfaults may result in several compartmentalized traps ratherthan one undeformed larger rollover anticlinal trap. In the

footwall block of the AD fault, antithetic faults are commonand displace the BMF and younger horizons.

In the middle Frio Formation at Stratton Field, Kerr andJirik (1990) and Yang (1998) recognized two architecturalend members: the concentrated and the dispersed sandstonebodies. Tese architectural styles were noted on the hanging-wall side of the AD growth fault. Figure 16 is a robust rolloveranticline from Stratton Field that shows good examples ofthe concentrated and the dispersed facies architectural styles.In this interval, well data and 2D seismic data suggest thestructure attitude is mostly affected by the master growthfaults while all the antithetic and synthetic faults are dimin-

ished. Tis facies architectural model proposed by Kerr andJirik (1990) and Yang (1998) did not account for the effectof the secondary faults on the architecture of the middle Friostrata.

Figure 17 shows how this model can be modified to in-clude the effects of the major and subsidiary faults togetheron the middle Frio interval as presented by this study. In ad-dition, syndepositional movements of these growth faultsmay result in the creation of more accommodation space andin the avulsion of the BMF fluvial channel belts.

Figure 6.Isochron map between F11 and F39 reflectors extracted fromthe UPR 3D volume. Tick isochrons (dark colors) indicate the growthof the faults.

Figure 7. (a) Well log cross-section showing that the minor antitheticfaults associated with the AD fault are also syndepositional (growth)faults. Te growth of the fault as measured between ME41 andMBF39 is approximately 126 ft in the hanging-wall side of the fault.Tis growth is evident in the thickening of the individual layers suchas F11 and F39. Location of the cross-section is indicated in Figure9. (b) Coincident seismic line (from the BEG 3D survey) showing theantithetic fault (AF) in Figure 7a.

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Conclusion

Information extracted from two 3D seismic data volumesresulted in more understanding of the structural styles andarchitecture and in accurate delineation of reservoir blocks ina study area from the exas Gulf Coast. Te results suggest

more development opportunities in the Stratton-Agua Dulcearea in the FR-4 (Frio gas play). Te proposed structural andfacies models show that the Vicksburg and AD faults and nu-merous subsidiary synthetic and antithetic faults are synde-positional and cut through the BMF and younger horizons.Tese faults affect the architecture of the sandstone bodiesstacked in these intervals.

Suggested reading.Te impact of 3D seismic data on ex-ploration, field development, and production by Nestvold(in Applications of 3D Seismic Data to Exploration and Pro-duction, SEG, 1996); Structural Geologyby wiss and Moore(Freeman, 1992). Interpretation of Tree-Dimensional SeismicData by Brown (AAPG Memoir 42, Fifth Edition, 1999);Fluvial architecture and reservoir compartmentalization ofthe Oligocene Middle Frio Formation, South exas by Kerrand Jirik (Gulf Coast Association of Geological Societiesransactions, 1990).3D Fluvial Facies Architecture Simulationof Middle Frio Formation, Stratton Field, South exas by Yang

Figure 8. (a) Well log cross-section showing that the subsidiarysynthetic faults associated with the AD fault are syndepositional (growth)

faults. Te interval between F11 and F39 is thicker in the hanging wall(by 44 ft) than in the footwall side of the synthetic fault. Location of thecross-section is indicated in Figure 9. (b) Coincident seismic line fromthe UPR 3D survey showing the synthetic fault (SF) in (a).

Figure 9.Contour map constructed on top of the F11 horizon in Strat-ton Field based on well data, showing only the AD fault and one sub-sidiary fault (from Yang, 1998).

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Figure 10.ime structure map constructed on top of the F11 horizonfrom the BEG and the UPR 3D surveys. Fault polygons indicate thelocations of the major faults and several secondary faults. Tesesecondary faults were not shown before (compare with Figures 2 and 9)at the F11 stratigraphic level.

Figure 11. 3D view of the F11 structure from the BEG 3D surveyclearly shows surface irregularities in the hanging-wall rollover anticlinethat indicate the locations of the secondary faults associated with themajor AD fault.

Figure 12. (a) Seismic crossline from the UPR 3D survey showing thedeformation on the hanging-wall side of the AD fault. All subsidiary

faults are affecting the deeper middle Frio (G2-F11). Note thedifference in structural details in this seismic line and that shown inFigure 2. (b) Another seismic crossline from the same survey showingdeformation in the footwall block of the AD fault.

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Figure 13.Tree coherence slices nearthe top (a), near the middle (b), andnear the bottom (c) of the BMF intervalcorresponding to the UPR survey, show-ing that the whole BMF stratigraphicinterval is clearly affected by many largeand subtle faults.

Figure 14.Maximum positive (a), (c) andmaximum negative (b), (d) curvature 3Dattributes extracted near the top (a), (b) andnear the bottom (c), (d) of the BMF interval.Tese attribute maps clearly show the loca-tions of the faults that affect the BMF inter-val.

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Figure 16. Regional well-log structural cross-section showing good example of a rollover anticline developed on the hanging wall of the major AguaDulce fault in the Stratton Field area. Te stacking patterns of the BMF fluvial sandstone reservoirs range from dispersed architecture(multiple vertically stacked sand bodies that have limited lateral continuity and separated vertically by thick layers of floodplain mudstones) close tothe major growth fault to concentrated architecture (multiple laterally stacked sand bodies that have more lateral continuity and separated verticallyby thin layers of floodplain mudstones) along the crest of the rollover anticline. Location of the cross-section is indicated in Figure 9.

Figure 15.Structural model based on seismic and geologic observations showing the effects of the major and subsidiary growth faults on the deepermiddle Frio succession in the Stratton and Agua Dulce fields. Te faults displace the horizons into several compartments. Deformation is more

pronounced in the hanging-wall sides of the growth faults. Deformation is common in the AD fault footwall in northern Stratton Field andsouthern Agua Dulce Field.

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Figure 17. Schematic 2D facies architectural model of the BMF stratigraphic interval. Deformation caused by growth fault (major andsubsidiary) activity is one possible factor controlling the BMF fluvial architectural styles in the study area. Tickness and number of sandstone bodiesincrease on the hanging wall-sides of the growth faults relative to the footwalls (modified from Kerr and Jirik, 1990; and Yang, 1998).

(masters thesis, Te University of ulsa, 1998). 3D seismicimaging and interpretation of fluvially deposited thin-bed res-

ervoirs by Hardage et al. (inApplications of 3D Seismic Datato Exploration and Production, SEG, 1996). Secondary NaturalGas Recovery: argeted echnology Applications for Infield Re-serve Growth to Fluvial Reservoirs. Statton Field, South exasby Levey et al. (Report for Gas Research Institute, 1993).

Acknowledgements: Te first author is grateful to the Geosciences

INTERPRETERS CORNER

Department of the University of ulsa, especially Dennis Kerrand Christopher Liner (now at the University of Houston) for

their guidance and help. Special thanks to Anadarko PetroleumCorporation for providing the UPR 3D seismic survey and to theBureau of Economic Geology for providing seismic data licenses tothe University of ulsa for use in education and research.

Corresponding author: [email protected]

Documents

Structure architecture interpretation of the middle Frio Formation using 3D seismic attributes and well logs: an example from the Texas Gulf Coast of the United States