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Petroleum Geology, Stratigrapgy, Sedimentology
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1
Carbonate Reservoirs Carbonate Reservoirs and and
Fractured ReservoirsFractured Reservoirs
Carbonate Sidewall core
1 inch
Carbonate Reservoirs
• Porosity strongly controlled by post-depositional processes
• High initial porosity• Rapid cementation• Dissolution creates secondary porosity
• Usually due to acidic water
Outline• Carbonate Porosity
– Primary vs. secondary porosity
• Reefs• Karst (dissolution)• Dolomitization• Fracture systems• Healed Fractures vs open fractures• Fracture controlled production (example)
Carbonate Porosity TypesIntergranularIntergranular PorosityPorosity
2
VuggyVuggy & & MoldicMoldic PorosityPorosity
Cretaceous Reefs
Water Drive, 33-40 API oil, 200 Million Barrels cum. production since 1915
Golden Lane Facies
Dissolution by Meteoric Water
Allen and Allen, 2002
Karst and Unconformities
3
Persian GulfPetroleum
SystemDolomitization
Ghawar-largest field in the worldPeak production; 5 million barrels per day (1981)
Cumm. Prod. 65 billion blsAverage Porosity: 19%Average perm: 617 mD
Karst (dissolution)
Arab D Reservoir -Ghawar
Dolomite
MoldicPorosity
Mudstone
Dense Dolomite
Partially dolomitizedlimestone
Swart et al. 2005
Daily Oil Production 2008
2.3 MillionNorway92.4 MillionIraq82.6 MillionMexico73.2 MillionCanada63.9 MillionChina54.1 MillionIran48.0 MillionUSA38.1 MillionSaudi Arabia29.9 MillionRussia1
Production (bbl/day)
CountryRank Dolomitization• Dolomite forms during diagenesis or
hydrothermal alteration• CaCO3→ (CaMg)(CO3)2
• 13% volume reduction• New porosity appears• Setting: dry coastal areas
DolomitizationDolomitization
2 mm
Atrim Fm., Michigan
Permian, W. Texas
Calcite (pink)
Dolomite
2 mm
Ordovician Dolomite Cavity,OH
4
Trenton-Black River System-Appalachian Basin
Gloades Corner Reservoir, NY
Fractured Reservoirs
Photo: J. Olson, UT Austin
Evidence for fluid flow along Fractures in Sandstone
Systematic Orthogonal Fractures
Photo: J. Olson, UT Austin
Fracture Spacing/Bedding
Photo: J. Olson, UT Austin
Fractures in Reservoir Simulations
1. Matrix Permeability
2. Fracture Permeability
Distributed Fracture Network
5
Sealed Fractures
Photo: J. Olson, UT Austin (Smosna, 1996) (Gas Atlas)
Example of Fractured Example of Fractured Carbonate ReservoirCarbonate Reservoir
Formation
Gre
enbr
ier
Lim
esto
ne
McCrady Formation
Bluefield
Pickaway
Taggard
Denmar
Hillsdale
Alderson
Union
Southeast West Virginia
Reynolds
Lillydale
MIS
SISS
IPPI
AN
MID
DLE
LOW
ER
AGE
UPP
ER
Maccrady Fm
primary faultplay target
Isopach of Union Oolite, Rhodell
Field Area
(from Gas Atlas)
studyarea
3527’scale in cm
ooids
3527’
intragranular porosity(blue)
1 mm
3527’
oolite rim(8% porosity)
sparry calcite(poor porosity)
6
2000’
2000
’Scale:
Estimated Ultimate Recovery (EUR) Map
C.I. = 200 MMcf
MICAJAH FAULT
PILOT KNOB FAULT
ARISTA FAULT
2000’
2000
’
Scale:
UNION OOLITE
NET PAY ISOPACH
Porosity > 4%
C.I. = 4’
#5834
MICAJAH FAULT
PILOT KNOB FAULT
ARISTA FAULT
#5638
CrossCross--section A section A –– AA’’
A A’
0
-500
-1000
-1500
PILOT KNOB
ARISTA
NW SE
Scale:500’
500’
5645
5644
5639
5646
3035
1271
512
896
1236
652
3444
730
93
UNION
PICKAWAY
DENMAR
LITTLELIME
PRICE
2000’
2000
’
A
A’
PILOT KNOB FAULT
ARISTA FAULT
DEPI #5834 DEPI #5834 (047(047--055055--00238)00238)
UNION OOLITE(Hanging wall)
UNION OOLITE(Foot wall)
PILOT KNOB THRUST FAULT
206’ OFFSET
OPEN FRACTURES30 MMcf natural
50’
DEPI #5834047-055-00238
FMI LOG
openfracture
drillinginduced
fractures
DEPI #5834 DEPI #5834 (047(047--055055--00238)00238)
SIDEWALL SIDEWALL CORESCORES
50’PILOT KNOB THRUST FAULT 206’ OFFSET
7
OPEN FRACTURES – DEPI #5834 (FAULT ZONE)
Highly deformed intervalStrike: NNE – SSWDips: 30-50°
Maximum Horizontal Stress
3446.5’scale in cm
open fracture
300μm micritized ooids
3446.5’
open fracture
3446.5’
open fracture
authigenic, euhedralquartz crystals
3477’scale in cm
calcite filledvugs & fractures
8
(after Nelson, 2001)no scale
UNION OOLITE THRUST MODEL WITH FRACTURE SWARMUNION OOLITE THRUST MODEL WITH FRACTURE SWARM
UNION OOLITE
Avg Mcf/d
050
100150200250300350400450
11/5/01 5/24/02 12/10/02 6/28/03 1/14/04 8/1/04
FAULT WELL PRODUCTIONFAULT WELL PRODUCTION(outside oolite trend)(outside oolite trend)
DEPI #5834 (047DEPI #5834 (047--055055--00238)00238)
Lesser decline(reservoir production)
Steep initial decline(fracture production)
(30 MMcf natural)
Take home ideas
• In carbonate reservoirs secondary porosity is very important
• Dolomitization, dissolution and fracturing create new porosity
• Exposure to meteoric water leads to limestone dissolution
• Natural Fracture networks greatly enhance permeability
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