Plio-Pleistocene Depositional Sequences of the Southeastern Texas Continental Shelf and Slope:

RI0200. Plio-Pleistocene Depositional Sequences of the Southeastern Texas Continental Shelf and Slope: Geologic Framework, Sedimentary Facies, and Hydrocarbon Distribution, by R. A. Morton, R. H. Sams, and L. A. Jirik. 80 p., 40 figs., 1 table, 1 appendix, 3 pls., 1991. Print.

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ABSTRACT
Plio-Pleistocene strata of offshore Texas having hydrocarbon exploration potential are restricted to the southeastern continental shelf and upper continental slope. This thick wedge of nearshore and deep-marine sediments firms the west margin of a late Neogene and Quaternary depocenter. The wedge is divided into eight genetic sequences on the basis of well logs, paleontological reports, and seismic profiles. These same data are also used to establish the structural framework, determine the timing of deformation, and map the principal depositional systems of the western Gulf Coast Basin during the past 3 m.y. Contemporaneous sea-level fluctuations, salt migration, and shifting sites of sedimentation caused substantial lateral variations in sedimentary facies as well as structural styles. The resulting complex geologic history of this part of the basin directly controlled the migration and entrapment of hydrocarbons.

Early Pliocene inundation of the continental platform and attendant deposition of marine mudstones was interrupted by a fall in sea level and development of a broad entrenched system landward of the shelf margin. Repeated excavation of the entrenched system during the middle and late Pliocene promoted the deposition of successive sand-rich submarine channels and fans. These lowstand deposits extended more than 100 mi (160 km) basinward of the paleomargin. Overlying Pleistocene sediments were deposited mainly by mud-rich fluvial-deltaic systems of moderate size. These rivers and shelf-edge deltas constructed a broad continental platform that buried the submarine fans and prograded the shelf margin tens of miles basinward. Sand-rich delta-front deposits were faulted and slumped along unstable shelf margins and were resedimented on the continental slope. As a result, the Pleistocene sandstones were trapped by growth faults and in withdrawal synclines and therefore are not present very far downslope of the extant shelf margin.

Plio-Pleistocene reservoirs contain an estimated 1.5 billion barrels of oil equivalent in at least 75 different fields. The fields are located near diapirs or faults associated with late salt movement. Structural style, reservoir facies, and hydrocarbon composition are used to group the fields into six exploration plays. The most prolific play encompasses stacked sandstone reservoirs and broad rollover anticlines located between the Trimosina regional and counterregional fault systems. Each play produces gas and some oil; however, most of the oil comes from the oldest lowstand submarine fans (Globoquadrina altispira and Lenticulina 1). All six plays offer some potential for either new field discoveries or reserve growth.

Keywords: depositional system, genetic sequence, Gulf Coast, hydrocarbons, Plio-Pleistocene, Texas

CONTENTS

Abstract

Introduction

Exploration history and regional setting

Sources of data

Electric logs Paleontological reports

Seismic profiles

Climate and eustatic fluctuations

Pliocene-Pleistocene boundary

Structural framework

Principal structural features

Extensional faults

Shale ridges and salt diapirs

Unconformities

Formation temperatures

Structural styles

Timing of structural movement

Depositional system s, systems tracts, an d genetic sequences

Depositional systems

Shelf-edge delta systems

Slope systems

Depositional systems tracts

Sequence boundaries and offshore sequences

Onshore formations

Plio-Pleistocene depositional framework

Mapping depositional systems and shelf margins

Characteristics of Plio-Pleistocene genetic sequences

Pre-Buliminella 1 (not mapped)

Buliminella 1 to Globoquadrina altispira (unit 1)

Globoquadrina altispira to Lenticulina. 1 (unit 2)

Lenticulina 1 to Angulogerina B (unit 3)

Angulogerina B to Hyalinea balthica (unit 4)

Hyalinea balthica to Trimosina A (unit 5)

Trimosina A to Globorotalia flexuosa (unit 6)

Post-Globorotalia flexuosa (unit 7

Summary of depositional and structural history

Late Miocene and early Pliocene

Middle Pliocene

Late Pliocene

Early Pleistocene

Middle Pleistocene

Late Pleistocene

Hydrocarbon generation and distribution

Source rock potential, thermal maturation, and petroleum migration

Spatial distribution of hydrocarbons

Delineation of hydrocarbon plays

Play characteristics and exploration potential

Play 1

Play 2

Play 3

Play 4

Summary and conclusions

Acknowledgments

References

Appendix: Cumulative production and recoverable reserves in Plio-Pleistocene fields, by play

Figures

1. Chronostratigraphic and biostratigraphic subdivision of Plio-Pleistocene strata in the western Gulf Coast Basin

2. Schematic cross section of Plio-Pleistocene strata in the western Gulf Coast Basin showing the lithologic and biostratigraphic criteria used to correlate genetic sequences

3. Regional depositional framework of Plio-Pleistocene strata

4. Structural features of the southeastern Texas continental shelf and slope, Plio-Pleistocene productive trend

5. Subsea depths to the 150°F isotherm

6. Subsea depths to the 200°F isotherm

7. Structural cross section derived from seismic profile illustrating lithofacies, stratigraphic subdivisions, and structural style in the northern sector of the study area

8. Structural cross section derived from seismic profile illustrating Iithofacies, stratigraphic subdivisions, and structural style near the east margin of the study area

9. Structural cross section derived from seismic profile illustrating Iithofacies, stratigraphic subdivisions, and structural style near the center of the study area

10. Structural cross section derived from seismic profile illustrating lithofacies, stratigraphic subdivisions, and structural style near the west margin of the study area

11. Structural cross section derived from seismic profile illustrating lithofacies, stratigraphic subdivisions, and structural style of the upper continental slope

12. Areas affected by the periods of principal structural deformation in the northern East Breaks and western Garden Banks Areas

13. Typical electric log responses

14. Net-sandstone map of unit 1(Buliminella1 to Globoquadrina altispira)

15. Percent-sandstone map of unit 1 (Buliminella 1 to Globoquadrina altispira)

16. Seismic facies map of unit 1 (Buliminella 1 to Globoquadrina altispira)

17. Depositional systems map of unit 1 (Buliminella 1 to Globoquadrina altispira)

18. Net-sandstone map of unit 2 (Globoquadrina altispira to Lenticulina 1)

19. Percent-sandstone map of unit 2 (Globoquadrina altispira to Lenticulina 1)

20. Seismic facies map of unit 2 (Globoquadrina altispira to Lenticulina 1)

21. Depositional systems map of unit 2 (Globoquadrina altispira to Lenticulina 1)

22. Net-sandstone map of unit 3 (Lenticulina 1 to Angulogerina B)

23. Percent-sandstone map of unit 3 (Lenticulina 1 to Angulogerina B)

24. Seismic facies map of unit 3 (Lenticulina 1 to Angulogerina B)

25. Depositional systems map of unit 3 (Lenticulina 1 to Angulogerina B)

26. Net-sandstone map of unit 4 (Angulogerina B to Hyalinea balthica)

27. Percent-sandstone map of unit 4 (Angulogerina B to Hyalinea balthica)

28. Seismic facies map of unit 4 (Angulogerina B to Hyalinea balthica)

29. Depositional systems map of unit 4 (Angulogerina B to Hyalinea balthica)

30. Net-sandstone map of unit 5 (Hyalinea balthica to Trimosina A)

31. Percent-sandstone map of unit 5 (Hyalinea balthica to Trimosina A)

32. Seismic facies map of unit 5 (Hyalinea balthica to Trimosina A)

33. Depositional systems map of unit 5 (Hyalinea balthica to Trimosina A)

34. Net-sandstone map of unit 6 (Trimosina A to Globorotalia flexuosa

35. Percent-sandstone map of unit 6 (Trimosina A to Globorotalia flexuosa)

36. Seismic facies map of unit 6 (Trimosina A to Globorotalia flexuosa)

37. Depositional systems map of unit 6 (Trimosina A to Globorotalia flexuosa)

38. Distribution of Texas Plio-Pleistocene oil and gas fields and their subdivision into principal exploration plays

39. Sizes of known Texas Plio-Pleistocene oil and gas fields determined on the basis of recoverable reserves

40. Aggregated volume of cumulative production (1985) and recoverable reserves for each Plio-Pleistocene play

Table

Table 1. Summary of geologic characteristics and remaining exploration potential of Plio-Pleistocene hydrocarbon plays, offshore Texas

Plate

1. Structural cross section 3'-3"

2. Structural cross section 5'-5"

3. Structural cross section 8'-8"


Citation
Morton, R. A., Sams, R. H., and Jirik, L. A., Plio-Pleistocene Depositional Sequences of the Southeastern Texas Continental Shelf and Slope: Geologic Framework, Sedimentary Facies, and Hydrocarbon Distribution: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 200, 80 p.