Interpreted sequence stratigraphy and gross packaging ofchert and carbonate lithologies in the University WaddellThirtyone chert reservoir.

Thirtyone chert reservoir intervals in University Waddell fieldare developed in the middle and top of the formation.

CHANNELIZED GRAIN-RICH CHERT

Maps of individual grain-rich, cycle-base chert flow units.Note highly variable patterns among units in thickness andφh reflecting the control on reservoir architecture ofdepositional style.

0.5 mm

2.0 cm

Grain-rich spiculitic cherts are channelized grain-rich debris flows that display evidence of rapid, high-energydeposition. Marginal facies are highly burrowed, reflecting slower deposition in low-energy, moderately welloxygenated conditions. Laminated muds document interchannel to distal hemipelagic deposition.

University Waddell field isdeveloped over a simpleanticline that is probablyfault controlled along it’swestern margin.

THIRTYONE STRATIGRAPHY

Pra

gia

nse

qu

ence

2P

rag

ian

seq

uen

ce 1

Architecture of grain-rich, cycle-base chert reservoir units.Note marked differences in thickness and continuity.

2.0 cm

Discovery date: 1949Average depth: 8,600 ft (2,620 m)Area: 8,700 acres (3,510 hectares)Well spacing: 20-40 acres (8-16 hectares)Top seal: Woodford FormationBottom seal: Frame Formation

(Silurian/Devonian Wristen Gp.)Trap: AnticlineHydrocarbon source: Woodford FormationProducing unit: Thirtyone Formation (Lower Devonian)Reservoir lithology: ChertOil-water contact: -6,650 ft (2,030 m) subsea elevationAverage gross pay: 900 ft (27 m)Average net pay: 100 ft (21 m)Average porosity: 9 percentAverage permeability: 1 mdWater saturation: 0.37Residual oil saturation(Sor): 0.23Oil gravity: 44°API @ 60°FOriginal bottom-holepressure: 4,200 psiaTemperature: 140°F (50°C)Formation volumefactor: 1.73 (at original bottom-hole pressure)Oil viscosity: 0.475 centipoise

(at original bottom-hole pressure)Solution gas-oil ratio 1,330 SCF/STB (original)

RESERVOIR CHARACTERISTICSAND VOLUMETRICS

2.0 cm 0.5 mm

DISTAL CHERT RESERVOIRSUniversity Waddell Field

PLATFORM-DERIVEDSKELETAL CARBONATE

Paired core slab and thin-section photographs of thin-beddedto massive skeletal packstone. These rocks contain crinoids,brachiopods, and other minor skeletal debris. Pore space isoccluded by syntaxial cement and carbonate mud.

HEMIPELAGIC LAMINATED CHERT

Paired core slab and thin-section core slabphotographs of laminated chert and limemudstone. These rocks are hemipelagic depositsthat represent the most distal phase of Thirtyonedeposition. They contain no porosity.

Paired core slab and thin-section photographs of thicklylaminated to massive chert. These rocks, which are composedof abundant sponge spicules and carbonate detritus depositedby grain-rich gravity flows, constitute the primary productivefacies in distal chert reservoirs.

Paired core slab and thin-section photographs of burrowedchert. Laminations, graded bedding, and fluid escape structuresare locally common. Although minor spicule-moldic porespace exists, these rocks are mostly nonporous.

DISTAL CHANNEL MARGINBURROWED CHERT

CHANNEL MARGIN DISRUPTEDLAMINATED CHERT

Paired core slab and thin-section photographs of laminated chertwith convolute and disrupted laminations caused by soft sedimentdeformation and burrowing. These rocks, which are nonporous,contain siliceous mud and very fine grained peloid/skeletal debrisdeposited by episodic transport of fine-grained carbonate platformdetritus into a deep-water, outer ramp to slope setting.

CHANNEL MARGIN INTERBEDDEDCARBONATE AND CHERT

Paired core slab and thin-section photographs of nodular chertand limestone formed by burrowing and soft sediment deformationof interbedded and interlaminated chert and carbonate. Minorporosity exists in chert layers; carbonates are nonporous.

0.1 mm

1.0 mm

2.0 cm 0.5 mm

Woodford Fm.

QAc2339(a)c

Up

per

ch

ert

Lowerlst.

Upperlst.

Northwest Southeast

Top Frame Fm.

Top 31 Fm.

A

B

C

Lo

wer

ch

ert

A A'

Channel margin and levee:disrupted laminated chert,nodular chert & limestone,burrowed chert

Channel: thick laminatedto massive chert

Interchannel: laminatedchert & limestone

QAb7651(b)c

Compensation lobes

Bifurcating channelswith attached lobes

Nested channels with levees

Compensation lobes

Bifurcating channelswith attached lobes

Nested channels with levees

Platformmargin

Slope-Basin

Platform

THIRTYONE DEPOSITIONAL MODELDISTAL SETTING

Chert; high-energyturbidite fan deposits

Diagenetically overprintedchert & siliceous limestones

Pelmatozoan packstone& grainstone

Low-energy hemipelagicchert laminates

QAc9145c

STRUCTURAL SETTINGN

QAc2336(b)c

-6250-6300

-6350-6400

-6450-6500-6550

Block B-25

Block 31

161

A

A'

B

B'

-6200

H.H. No.14

1 mi0

0 1 kmC.I. 50 ft

Coredwell

Well Well withdigital logs

THIRTYONE RESERVOIR ARCHITECTUREDISTAL SETTINGS

500 ft

100 m00

30mft

100

Porosityzones

8800

9000

9200

8800

9000

9200

Lowerporosityinterval

Middleporosityinterval

Upperporosityinterval

QAc1286(b)c

GR NPHIGR PHI GR PHI

GR PHIGR PHI

GRSoutheastNorthwest

B'B

Depth(ft)

Depth(ft)

PHIDepth

(ft)Depth

(ft)Depth(ft)

Depth(ft)

8800

9000

9200

8800

9000

9200

8800

9000

9000

9200

C

Oil completions Oil recompletionopportunities

Injection recompletionopportunities

Depthft m

0

100 30

0

3000 ft

900 m

0.25 mm1.0 cm

SUMMARY OF HETEROGENEITY IN DISTALTHIRTYONE RESERVOIRS

In contrast to proximal Thirtyone chert reservoirswhere there is a single, continuous porous chertreservoir, distal chert reservoirs contain numerousseparate and discontinuous stacked porous chertunits. In distal reservoirs, lack of continuity is theprimary contributing factor to heterogeneity and lowrecovery efficiency. The distribution of porous chertin distal settings is a function of sediment geometriesassociated with submarine fan and debris-flowdeposition. Episodic downslope transport ofsiliceous spiculitic sediment along the margins ofthe carbonate platform has resulted in verticalsegregated and laterally discontinuous chertreservoir intervals. These deposits are interbeddedwith and grade laterally into lower energy mud-richsediments that typically have low porosity andpermeability. Although these muddy rocks are notflow barriers, they do act as baffles to flow andimpact recovery efficiency. Detailed correlation andmapping of individual porous chert layers is criticalfor establishing a reservoir framework that can serveas a basis for defining recompletion and infill drillingtargets. Such an approach has led to theidentification of several drilling and recompletionprospects in Waddell field.

CONCLUSIONS

Devonian chert reservoirs in WestTexas contain a large remaining oilresource that is a target for more efficientexploitation techniques based on a betterunderstanding of the geological controlson heterogeneity. Because these controlsdiffer systematically between chertreservoirs developed in updip, proximalsettings and downdip, distal settings, itis crucial that both regional and localgeologic controls be examined andintegrated into modern reservoircharacterization and exploitation efforts.The Three Bar and University Waddellfields provide fundamental models of thestyles of heterogeneity expected inproximal and distal settings, respectively.

FLOW UNIT CONTINUITY

QAc6231(a)c

B'

B

CI 2 ft

1 mi0

0 1 km

Well locationsWell with digital logsPetrophysical analysis QAc3512c

P.S.L.

Block B-25

University

Lands

Block 31

THICKNESS(ft)

6–84–6

>108–10

2–4 10

11

3

2

1

QAc2347c

P.S.L.

Block B-25

B'

B

QAc1284c

B'

BP.S.L.

Block B-25

CI 5 ft

Well locationsWell with digital logsPetrophysical analysis

THICKNESS(ft)

15–2010–15

>20

5–10

THICKNESS(ft)

8–124–8

>1612–16

N

10

11

3

2

1

10

11

3

2

1

CI 0.5 phi•h

1 mi0

0 1 km

Well locationsWell with digital logsPetrophysical analysis

QAc6083c

P.S.L.

Block B-25

University

Lands

Block 31B'

B

1.0

2.0

0.9

0

10

11

3

2

1

P.S.L.

Block B-25

University

Lands

Block 31 1.5

1.0

0.5

0.51.0

B'

B

QAc1246c

P.S.L.

Block B-25

University

Lands

Block 31

2.03.0

1.0

1.0

1.0

2.0

B'

B

QAc3960c

CI 0.5 ft phi•h

1 mi0

0 1 km

Well locationsWell with digital logsPetrophysical analysis

CI 0.5 ft phi•h

1 mi0

0 1 km

Well locationsWell with digital logsPetrophysical analysis

N

10

11

3

2

1

10

11

3

2

1

1 mi0

0 1 km

NN

CI 4 ft

1 mi0

0 1 km

Well locationsWell with digital logsPetrophysical analysis

University

Lands

Block 31

University

Lands

Block 311.0

N N

Pra

gia

n s

equ

ence

2P

rag

ian

seq

uen

ce 1

30φ

0N

C

B

A

GRWoodfordFormation

Frame Fm.

Th

irty

on

e F

orm

atio

n

8500

8700

8600

9200

9400

9300

Thick laminated tomassive chert

Burrowed chert

Disrupted laminated chert

Nodular chert/limestone

Laminated chert/limestone

Skeletal packstone

Completion

RESERVOIR FACIES

Typical vertical section in distal Thirtyonereservoirs. Highest porosity is associatedwith high-energy deposits of spiculiticchert that are interbedded with lowerenergy, mud-rich facies.

QAc4149(b)c

University Waddell 161

THIRTYONE TYPE LOGCHEVRON U.S.A.

Texas “H.H.” No. 14

QAb9005(b)c

Majorreservoirinterval

0 100

100 200

GR0.3 0

DPHI

0.30 0NPHI

Dep

th (

ft)

9000

8900

8800

8700

8600

8500

FrameFormation

9500

9400

9300

9200

9100

WoodfordFormation

ThirtyoneFormation

C

A

B

WinkFormation

2.0 cm