PETROPHYSICAL RELATIONSHIPS

QAa1497(a)c

2520151050.01

.1

1

10

100

WHOLE CORE

Translucent chertChert in limestoneLimestone

Porcelaneous porous chert

Porosity (%)

Perm

eabi

lity

(md)

Porcelaneous porous chert

CORE PLUG

24

18

12

6

00

Cor

e po

rosi

ty (p

erce

nt)

r = 0.894

6 12 18 24Neutron porosity (percent)

QAa9856(a)c

Although there is a good relationship between true matrix porosity andwireline log values, neutron logs typically underestimate porosity.

COMPARISON OF LOG ANDCORE POROSITY

BURROWED POROUS SPICULITIC CHERT

Core slab of typical productive chertreservoir facies at Three Bar field.Small f ractures are commonthroughout; larger fractures are moreabundant near apparent fault zones.

Photomicrograph of Thirtyonereservoir chert showing abundanceof monaxon sponge spicules.

SEM photomicrographshowing spicule-moldicand microcrystallinepores in Thirtyone chertat Three Bar field.

SEM photomicrographshowing microporositydeveloped betweenaggregates of 1-µmellipsoids.

2.0 cm

0.1 mm

0.05 mm

.01 mm

0.5 mm

FAULT-INDUCED COMPARTMENTALIZATION

ft m30

150 m0

100

0

500 ft

QAa7702(a)c

80

00

80

00

80

00

TBU TBU TBUGR N GR Son GR N

NorthSouth

Fault

Thirtyone FormationSkeletalpackstone

Porous chert

Nonporous chert/limestone

Permian

Frame Fm

Unconformity

-1500

-5000

-1400

-4800

-4600ft m

Depth belowmean sea level

80008000

GR

GR

Son

GR

GR Son

GR

SP Res

GR Den

Cored interval

QAa7052c

Silurian-DevonianFrameFormation

PermianWolfcampian

EastWest

8000 8000

N

Son

8000

N

Porous spiculitic chert

Skeletal packstone andnonporous chert

Argillaceous limemudstone

“Permian detrital”

Perforations

0

0 600 m

2000 ft

Unconformity

8000 8000

DevonianThirtyoneFormation

A A'STRUCTURAL SETTING

UDA A'

Three Bar field is developed along the western margin of a breachedPennsylvanian anticline. The reservoir is truncated to the east by erosion.Top seal is formed by Lower Permian shales.

9 10

34

16 15

22

18

2019

Uni

vers

ityB

lock

11

UD

UD

UDD

U

UD

UD

DU D

U

UD

UD

DU

DU

UD

UD

DU

UD

DU

UD

DU

DU

UD

7

2119

13 2

PSLBlockA-54

-470

0-480

0

-490

0

-500

0

-510

0

-490

0

-4800

-4700

-4600

-500

0

-490

0-4

800

-470

0

-4600

-4700

OWC

Three BarDevonian

Unit

ParkerUnit

U

6

QAa7060c

N

0 4000

0 1200Contour interval 50 ft

Datum sea level

OWC Oil-water contactUD Inferred fault

Well

Truncated chertreservoir interval

Thirtyone Fm absent

Cored well

UD

THIRTYONE FORMATION STRUCTURE

0.01 mm

PERMIANUn

itDepth(ft)

Th

irty

on

e F

m.

8200

54

321

8400

Fra

me

Fm

.

PR

AG

IAN

LOCHKOVIAN

Density(g/cm3)

2.0 3.0

GR (API)

1000

6

1

Chert–pebbleconglomerate

Translucent, spiculiticnonporous chert

Perforations

Argillaceous limemudstone

Burrowed spiculiticporous chert

Unconformity

Skeletal packstone/grainstone

Reservoir unit

RESERVOIR FACIES

QA18039c

SUMMARY OF HETEROGENEITY IN PROXIMAL THIRTYONE RESERVOIRS

The Thirtyone chert section in proximal reservoirs is remarkably continuous, being traceablethroughout more than 250 mi2 display. Despite this continuity, there are significant causes ofinternal heterogeneity that affect fluid flow and recovery in these reservoirs. Primary causes ofheterogeneity and incomplete drainage and sweep of remaining mobile oil at Three Bar are (1)faulting and fracturing, (2) carbonate dissolution, and (3) small-scale facies architecture. Faultsand fractures appear to variably facilitate or inhibit fluid movement. In some parts of the field,zones of abundant faults and fractures are associated with areas of high productivity, whereasin other areas faults separate distinct reservoir compartments.

Leaching and dissolution of carbonate was caused by fluids that entered the top of the Thirtyonesection and along the truncated and exposed updip margin of the field during the Pennsylvanian.Evidence of carbonate dissolution is apparent especially in areas with greater fault densities,suggesting that faults have acted as flow pathways for diagenetic fluids. In updip parts of thefield, vertical communication has been enhanced and productivity increased by this diagenesis.

Complex chert/carbonate interbedding in chert section has created poor lateral and verticalcommunication between high-matrix-porosity chert beds within the reservoir section. These faciesvariations, which are the result of combined original depositional facies patterns and subsequentdiagenesis, may contribute to significant mesoscale reservoir compartmentalization.

QAc9144c

Well

Cored well

UD Inferred

fault

Oil-watercontact

OWC

PRIMARY RECOVERY

N

QAa7072c

8 9 1034

18 17 16 15

222119

Univ.Block

11

UD

UD

UD

DU

UD

UDD

UDU

UD

UD

DU

UD

DU

UD

UD

DUU

D

DU

UD

DU

200

400

200

400

600

400

200

400

600

200

400

200

200

400

OWC

Three BarDevonian

Unit7

0-400

>400

0 2000 ft

0 600 mC.I. 100 MSTB

Oil-water contact

Three Bar Devonianunit

Thirtyone Fm absent

Inferred fault

Cored wellWell

UD

7 8 934

18 17 16 15

22212019

4

OW

C

Univ.Block

11

Three BarDevonian

Unit

N

U

D

U

D

UD

DU

UD

DU U

DUD

D

UD

U

U

DUD

UD

U UD

UD

UDU

D

DU

Mapped phi*h values conform to net chert thickness maps. Primaryproduction patterns closely match phi*h trends documenting the dominanceof matrix permeability on recovery.

RESERVOIR PHI*H

8-16

< 8

16-24

>24

QAa7071(a)c

C.I. 4 porosity-ft

0 2000 ft

0 600 m

Discovery date: March 6, 1945Average depth: 8,100 ft (2,470 m)Area: 3,640 acres (1,470 hectares)Well spacing: 40 acres (16 hectares)Top seal: Pennsylvanian/Permian “detrital”Bottom seal: Frame Formation (Silurian Wristen Group)Trap: Updip pinchoutHydrocarbon source: Woodford FormationProducing unit: Thirtyone Formation (Lower Devonian)Lithology: ChertOil-water contact: -5,050 ft (1,540 m) subsea elevationAverage gross pay: 90 ft (27 m)Average net pay: 70 ft (21 m)Average porosity: 15%Average permeability: 5 mdWater saturation: 0.37Residual oilsaturation (Sor): 0.14 (measured from mercury injection)Oil gravity: 41.5°API @ 60°FOriginal bottom-hole pressure: 3,200 psiaTemperature: 121°F (50°C)Formation volumefactor: 1.595 (at original bottom-hole pressure)Oil viscosity: 2.38 centipoise (at original bottom-hole pressure)Water salinity: 92,548 ppm NaClWater saturation (Sw): 0.37 (measured from mercury injection)Original oil in place: 131.1 MbblCumulative production: 36.1 Mbbl (1989)Recovery efficiency: 27%

RESERVOIR CHARACTERISTICSAND VOLUMETRICS

Photomicrographof nonporous chertshowing spiculesreplaced bychalcedony ormicroquartz orfilled with quartz.

SEM Photomicrographof nonporous chertshowing absence ofmicrocrystalline porespace.

Core slab of nonporouschert facies at Three Barfield. Fractures are moreabundant in nonporouscherts and indicative ofmore brittle deformation.

Proximal Thirtyone reservoirs are characterized by a basal chert reservoir sectionand an overlying carbonate section that is commonly much less productive.

Core analysis permeability values commonly reflect the presence offractures in the Thirtyone. Selected fracture-free samples (circles) definethe matrix porosity/permeability transform.

Apparent faults defined from structural mapping suggest the possibilityof partial reservoir compartmentalization due to flow unit offset.

PROXIMAL CHERT RESERVOIRSTHREE BAR F IELD

2.0 cm

BureauofEconomic

Geology