Plio-Pleistocene Genetic Sequences of the Southwestern Louisiana Continental Shelf and Slope:

RI0210. Plio-Pleistocene Genetic Sequences of the Southwestern Louisiana Continental Shelf and Slope: Geologic Framework, Sedimentary Facies, and Hydrocarbon Distribution, by R. A. Morton and W. B. Ayers, Jr. 77 p., 1 table, 35 figs., 1 appendix, 1992. Print.


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ABSTRACT
The Plio-Pleistocene hydrocarbon fairway of offshore western Louisiana is restricted to the outer continental shelf and upper continental slope where a thick wedge of nearshore and deep-marine sediments was deposited. Electric logs, paleontological reports, and seismic profiles form the basis for (1) dividing the wedge of Plio-Pleistocene strata into eight genetic sequences, (2) establishing the structural framework, (3) determining the timing of deformation, and (4)mapping the principal depositional systems of the West Cameron and western Garden Banks Areas during the past 5 m.y. Sedimentary facies and structural styles in this part of the Gulf Coast Basin are highly variable owing to contemporaneous sea-level fluctuations, salt migration, and shifting sites of deltaic, shelf, and slope sedimentation. The complex geologic history of this part of the basin controlled the generation, migration, and entrapment of hydrocarbons.

During the early Pliocene the continental platform became inundated, and a thick succession of marine mudstones was deposited. About 3 mya this accumulation of deep-water mudstone was interrupted by deposition of sand-rich submarine channels and fans associated with a lowering sea level. These lowstand deposits extended at least 55 mi (90 km) basinward of the paleoshelf margin. Overlying Pleistocene sediments were deposited mainly by prograding mud-rich fluvial-deltaic systems of moderate size. The rivers and shelf-edge deltas constructed a broad continental platform that buried the submarine fans and prograded the shelf margin approximately 70 mi (110 km) basinward. During this rapid outbuilding, slumping and other gravity-driven mass transport processes removed sand-rich delta-front sediments from unstable shelf margins and redeposited them on the continental slope.

Plio-Pleistocene reservoirs in the West Cameron and western Garden Banks Areas contain more than 1.1 billion barrels of oil equivalent distributed among at least 100 different fields or local accumulations. Most of the fields are located near faults associated with relatively recent salt movement, although a few fields are located near shallow salt diapirs. The fields have been grouped into six exploration plays on the basis of structural style, reservoir facies, and hydrocarbon composition. The most prolific play has stacked delta-front reservoirs, which produce from broad rollover anticlines that formed in a graben complex between the Trimosina regional and counterregional fault systems. Each play primarily produces gas, although some oil is produced from fields beneath the continental slope. New field discoveries and reserve growth are likely in all six plays because of deeper drilling or off-structure exploration in former intraslope subbasins.

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

CONTENTS
Abstract
Introduction
   Regional Setting
   Sources of Data               
      Electric Logs
      Paleontological Reports
      Seismic Profiles
   Climatic and Eustatic Fluctuations

   Pliocene-Pleistocene Boundary

Structural Framework

   Principal Structural Features

      Extensional Faults

      Shale and Salt Diapirs
      Unconformities

   Structural Styles

   Timing of Structural Movement

Depositional Systems, Systems Tracts, and Genetic Stratigraphic Sequences

   Depositional Systems

      Shelf-Edge Delta Systems

      Slope Systems

Depositional Systems Tracts

Sequence Boundaries and Offshore Sequences

Onshore Formations

Plio-Pleistocene Stratigraphic Framework

Mapping Depositional Systems and Shelf Margins

Characteristics of Plio-Pleistocene Genetic Stratigraphic Sequences

Pre-Robulus E (not mapped) Robulus E to Buliminella 1 (Sequence 1)

Buliminella 1 to Globoquadrina altispira (Sequence 2)

Globoquadrina altispira to Lenticulina 1 (Sequence 3)

Lenticulina 1 to Angulogerina B (Sequence 4)

Angulogerina l3 to Hyalinea balthica (Sequence 5)

Hyalinea balthiea to Trimosina A (Sequence 6)

Trimosina A to Globorotalia flexuosa (Sequence 7)

Post-Globorotalia flexuosa (Sequence 8)

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             
         Play 1

         Play 2

         Play 3

         Play 4

         Play 5

         Play 6

Summary and Conclusions

Acknowledgments

References

Appendix: Plio-Pleistocene recoverable reserves and cumulative field production 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 West Cameron and western Garden Banks Areas showing the lithologic and biostratigraphic criteria used to correlate genetic sequences

3. Regional depositional framework of Plio-Pleistocene strata

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

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

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

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

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

9. Structural cross section derived from seismic profile illustrating lithofacies, stratigraphic subdivisions, and structural style of the upper continental slope
10. Typical electric log responses

11. Net-sandstone map of sequence 1 (Robulus E to Buliminella 1)

12. Percent-sandstone map of sequence 1 (Robulus E to Buliminella 1)

13. Depositional systems map of sequence 1 (Robulus E to Buliminella

14. Net-sandstone map of sequence 2 (Buliminella 1 to Globoquadrina altispira)

15. Percent-sandstone map of sequence 2 (Buliminella 1 to Globoquadrina altispira).

16. Depositional systems map of sequence 2 (Buliminella 1 to Globoquadrina altispira)

17. Net-sandstone map of sequence 3 (Globoquadrina altispira to Lenticulina 1)

18. Percent-sandstone map of sequence 3 (Globoquadrina altispira to Lenticulina 1)

19. Depositional systems map of sequence 3 (Globoquadrina altispira to Lenticulina 1).

20. Net-sandstone map of sequence 4 (Lenticulina 1 to Angulogerina B 21. Percent-sandstone map of sequence 4 (Lenticulina 1 to Angulogerina B)

22. Depositional systems map of sequence 4 (Lenticulina 1 to Angulogerina B)
23. Composite net-sandstone map for pre-Hyalinea balthica sand-rich submarine fans penetrated in the northern Garden Banks Area
24. Net-sandstone map of sequence 5 (Angulogerina B to Hyalinea balthica)

25. Percent-sandstone map of sequence 5 (Angulogerina B to Hyalinea balthica)

26. Depositional systems map of sequence 5 (Angulogerina B to Hyalinea balthica)

27. Net-sandstone map of sequence 6 (Hyalinea balthica to Trimosina A)

28. Percent-sandstone map of sequence 6 (Hyalinea balthica to Trimosina A)

29. Depositional systems map of sequence 6 (Hyalinea balthica to Trimosina A)

30. Net-sandstone map of sequence 7 (Trimosina A to Globorotalia flexuosa)

31. Percent-sandstone map of sequence 7 (Trimosina A to Globorotalia flexuosa)

32. Depositional systems map of sequence 7 (Trimosina A to Globorotalia flexuosa)

33. Distribution of Plio-Pleistocene oil and gas fields, West Cameron and western Garden Banks Areas and their subdivision into principal exploration plays

34. Sizes of known Plio-Pleistocene oil and gas fields, West Cameron and western Garden Banks Areas, determined on the basis of recoverable reserves

35. Cumulative production and recoverable reserves for each Plio-Pleistocene play, West Cameron and western Garden Banks Areas

Table

1. Summary of geologic characteristics and exploration potential of Plio-Pleistocene hydrocarbon plays, West Cameron and western Garden Banks Areas

Plates

1. Structural cross section 1-1'

2. Structural cross section 2-2'

3. Structural cross section 3-3'


Citation
Morton, R. A., and Ayers, W. B., Jr., 1992, Plio-Pleistocene Genetic Sequences of the Southwestern Louisiana 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. 210, 77 p.