Depositional Systems and Oil and Gas Plays in the Cretaceous Olmos Formation, South Texas

RI0152. Depositional Systems and Oil and Gas Plays in the Cretaceous Olmos Formation, South Texas, by Noel Tyler and W. A. Ambrose. 42 p., 33 figs., 7 tables, 1 plate, 1986. ISSN: 0082335X: Print.

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ABSTRACT
The Upper Cretaceous Olmos Formation in South Texas continues to be an active exploration target 60 years after oil was first discovered in this clastic assemblage. The shallow, oil-bearing formation was deposited on a broad, wave-influenced shelf. Sand accumulated in two depocenters. Initial deposition took place in a western depocenter during an early phase of wave-dominated deltaic sedimentation. Deposition of the strike-elongate delta preceded two cycles of high-constructive deltaic deposition, during which time sediments prograded seaward over the Lower Cretaceous shelf edge. These three deltaic complexes together compose the Catarina delta system. The focus of sedimentation then shifted eastward to the Big Foot delta system, where again wave-dominated deltaic sedimentation was followed by two episodes of high-constructive but wave-modified delta formation. Sands not retained in the Big Foot delta system migrated alongshore to the west, where they formed a thick retrogradational coastal/interdeltaic complex, named the Rocky Creek barrier/strandplain system. Regional uplift with concomitant erosion removed much of the updip facies tracts of the Olmos. This truncated section was then unconformably covered by Escondido shelf mudstones, thereby creating conditions favorable for stratigraphic entrapment of hydrocarbons migrating updip from the deeper basin through permeable deltaic sandstones.

The resulting stratigraphic trap play is the most prolific of the seven oil and gas plays in the Olmos Formation. Six other plays produce oil and gas from a variety of structural traps, but most of the remaining oil production is from the Charlotte Fault Zone. Youthful shelf-edge gas, condensate, and oil plays are highly productive, and together with wildcat prospects further basinward offer the best potential for continued high-level production from this mature province.

Keywords: Maverick Basin, Olmos Formation, Cretaceous, Gulfian, deltas, barrier/strandplains, oil and gas, Charlotte Fault Zone

CONTENTS

ABSTRACT

INTRODUCTION

Structural framework

Stratigraphy and depositional systems

Objectives and methods

REGIONAL SEDIMENTARY PATTERNS OF THE OLMOS FORMATION

Western depocenter

Eastern depocenter

CATARINA DELTA SYSTEM

Deltaic unit A

Regional extent

Depositional environments

Deltaic unit B

Regional extent

Electric log response

Depositional environments

Deltaic unit C

Regional extent

Lithofacies

Electric log response

Depositional environments

ROCKY CREEK BARRIER/STRANDPLAIN SYSTEM

Shore-zone unit D

Regional extent

Lithofacies

Electric log response

Depositional environments

Shore-zone unit E

Regional extent

Lithofacies

Depositional environments

BIG FOOT DELTA SYSTEM

Deltaic unit F

Regional extent

Depositional environments

Deltaic unit G

Regional extent

Electric log response

Depositional environments

Deltaic unit H

Regional extent

Electric log response

Depositional environments

OLMOS DEPOSITIONAL HISTORY

OLMOS OIL AND GAS PLAYS

Updip stratigraphic traps

Charlotte Fault Zone

Updip structural traps

Volcanic mounds

Deltaic and shorezone sandstone structural traps

Downdip deltaic and shelf tight gas sandstones

Shelf-edge traps

CONCLUSIONS

Future exploration targets

ACKNOWLEDGMENTS

REFERENCES

FIGURES

1. Regional structure map of the Maverick Basin contoured on the' top of the Olmos Formation

2. Structural framework of the Maverick Basin

3. Schematic dip section through the Maverick Basin

4. Well control and location of type logs and stratigraphic dip and strike sections

5. lsopach map of the Olmos Formation

6. Net-sandstone map of the Olmos Formation

7. Type logs of the Olmos Formation from the eastern and western depocenters

8. Simplified stratigraphic strike section A-A'

9. Simplified stratigraphic dip section B-B' from the western depocenter

10. Simplified stratigraphic dip section C-C' from the eastern depocenter

11. Net-sandstone map, unit A

12. Net-sandstone map, unit B

13. Map showing distribution of SP log facies and interpretation of unit B

14. Percent sandstone map, unit C

15. Channel-mouth bar, distributary-channel, and abandoned-channel-fill deposits of unit C, Tesoro No. 3 Gates Ranch core

16. Crevasse splay sandstones interbedded with marsh and shallow-shelf (foundered delta) deposits of unit C, Tesoro No. 3 Gates Ranch core

17. Map showing distribution of SP log facies and interpretation of unit C

18. Net-sandstone map, unit D

19. Barrier and washover sandstones interbedded with shallow lagoon mudstones in unit D, Tesoro No. 2 Gates Ranch core

20. Map showing distribution of SP log facies and interpretation of unit D

21. Percent-sandstone map, unit E

22. Shallow-lagoon sediments truncated by a tidal-channel sandstone in unit E, Tesoro No. 2 Gates Ranch core

23. Net-sandstone map, unit F

24. Net-sandstone map, unit G

25. Net-sandstone map, unit H

26. Early to middle Olmos depositional model

27. Late Olmos depositional model

28. Morphology of sandstone units in the Olmos Formation

29. Discovery history of Olmos oil and gas fields

30. Olmos field size based on cumulative production to 1983

31. Oil and gas fields and major exploration and production plays, Olmos Formation

32. Cumulative oil and gas production from the seven Olmos plays

33. Map showing potential areas of unconformity-related stratigraphic traps

TABLES

1. Olmos plays ranked by cumulative total hydrocarbon production

2. Geologic, engineering, and production characteristics of the stratigraphic trap play

3. Geologic, engineering, and production characteristics of the Charlotte Fault Zone play

4. Geologic, engineering, and production characteristics of the updip structural trap play

5. Geologic, engineering, and production characteristics of a volcanic mound field as exemplified by the Elaine (Olmos) pool

6. Geologic, engineering, and production characteristics of the deltaic and shore-zone sandstone structural trap play

7. Geologic, engineering, and production characteristics of the Olmos downdip deltaic and shelf tight-gas sandstone play

Plate (in pocket of book): Stratigraphic cross sections, Olmos Formation.
A. Updip strike section

B. Western depocenter dip section

C. Eastern depocenter dip section



Citation
Tyler, Noel, and Ambrose, W. A., 1986, Depositional Systems and Oil and Gas Plays in the Cretaceous Olmos Formation, South Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 152, 42 p.