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