Lower Miocene (Fleming) Depositional Episode of the Texas Coastal Plain and Continental Shelf: Structural Framework

RI0150. Lower Miocene (Fleming) Depositional Episode of the Texas Coastal Plain and Continental Shelf: Structural Framework, Facies, and Hydrocarbon Resources, by W. E. Galloway, L. A. Jirik, R. A. Morton, and J. R. DuBar. 50 p., 20 figs., 4 tables, 1 appendix, 7 plates, 1986. ISSN: 0082235X: Print.



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ABSTRACT
The Fleming Group and its basinward equivalents constitute the stratigraphic record of one of the major Cenozoic depositional episodes of the northern Gulf Coast Basin. The depositional sequence representing the episode is bounded above by the Amphistegina B shale and below by the Anahuac shale. Initially, lower Miocene (Oakville) progradation advanced across the broad submerged shelf platform constructed during earlier Frio deposition. When outbuilding reached the Frio paleocontinental margin, the rate slowed as large-scale growth faulting created a narrow lower Miocene expansion zone. The later portion of the lower Miocene episode, generally equivalent to the Lagarto Formation, was characterized by long-term shoreline stability and retreat punctuated by local, temporary progradation.

In South Texas, the lower Miocene depositional framework includes the Santa Cruz fluvial system and the North Padre delta system. The bed-load fluvial complex fed a wave-dominated delta, constructing a broadly convex deltaic headland across the foundered Frio Norias delta system. Extensive wave reworking and longshore transport of sand and mud nourished a broad barrier island/ lagoon and strandplain complex that extended along the central and much of the northeastern Texas coast. This well-known barrier/strandplain system was bounded updip by a coastal plain traversed by numerous, small, intrabasinal streams. Near the present Sabine River, westernmost deposits of a continental-scale mixed-load fluvial and equivalent delta system extend beneath the Texas Coastal Plain and shelf from the Miocene depocenter in Louisiana. Here, the early phase of lower Miocene progradation was also complicated by the incision and filling of numerous submarine gorges.

Lower Miocene reservoirs have produced nearly 4 billion barrels of oil equivalent of petroleum from nine identified plays in the Texas Coastal Plain and shelf. The most prolific play, the Houston Embayment salt domes, accounts for nearly all the oil and more than two-thirds of the total production from deposits of the episode. Four offshore plays offer the greatest area for discovery of substantial new reserves, primarily of gas. To date, however, the yield per volume of reservoir sandstone for Miocene plays remains low relative to more prolific units, such as the Frio Formation.

Keywords: Fleming, Oakville, lower Miocene, petroleum (oil and gas), depositional system

Contents

Abstract

Introduction

Stratigraphic nomenclature and zonation

Depositional setting

Episode boundaries, correlation, and operational map units

Objectives

Structural Framework

Reactivated structures

Contemporaneous structures

Lower Miocene Depositional Framework

Sources of data

Lower Miocene depositional systems

Santa Cruz fluvial' system

North Padre delta system

Moulton/Point Blank streamplain

Matagorda barrier/strandplain system

Newton fluvial system

Calcasieu delta system

Depositional history and paleogeography

Origin and Distribution of Hydrocarbons

Indigenous source rocks and maturation history

Migration and entrapment

Occurrence of oil and gas

General features of lower Miocene plays

Exploration potential

Onshore and State submerged lands

Outer Continental Shelf

Conclusions

Acknowledgments

References

Appendix: Tabulation of fields by play

Figures

1. Gulf Coast Cenozoic depositional episodes

2. Lithostratigraphic and biostratigraphic subdivision of the lower Miocene section, Northwest Shelf of the Gulf of Mexico

3. Index map and depositional framework of the lower Miocene (Fleming) depositional episode

4. Schematic reconstruction of the stratigraphic architecture of a Gulf Coast Cenozoic depositional sequence

5. Electric log response of typical facies sequences within the depositional complex of a Gulf Coast Cenozoic episode

6. Schematic cross section of the lower Miocene depositional sequence, Texas Coastal Plain and shelf, showing the lithostratigraphic and biostratigraphic features used for correlation

7. Structural framework of the lower Miocene sequence

8. Net-sandstone isopach map of the Oakville (Anahuac shale to Marginulina a.) operational unit

9. Percent-sandstone map of the Oakville (Anahuac shale to Marginulina a.) operational unit

10. Net-sandstone isopach map of the Lagarto (Marginulina a.to Amphistegina B shale) operational unit

11. Percent-sandstone map of the Lagarto (Marginulina a. to Amphistegina B shale) operational unit

12. Generalized paleobathymetry of Oakville and Lagarto map interval mudstones

13. Lower Miocene depositional systems and their component elements

14. Paleogeographic reconstruction during later Oakville progradation and middle Lagarto coastal plain aggradation

15. Interpretive morphologic profiles across the lower Miocene coastal plain, shoreline, shelf, and slope

16. Depositional architecture of the lower Miocene coastal plain and shoreline

17. Distribution of lower Miocene oil and gas fields that have produced more than 1 million boe of hydrocarbons

18. Cumulative oil and gas production for each of the nine lower Miocene plays

19. Total hydrocarbon yield factors calculated on the basis of estimated total rock volume and total sandstone volume contained within each of the nine Miocene plays

20. Distribution of total producible reserves in 32 Miocene fields in Federal OCS areas

Tables

1. Total organic carbon content of lower Miocene mudstones

2. Summary of geologic characteristics and remaining potential of lower Miocene oil and gas plays

3. Cumulative production and sedimentary volumes of lower Miocene plays

4. Projected volumes of undiscovered recoverable hydrocarbons

Plates (in pocket of book)

I. Structural cross section 1-1'

II. Structural cross section 12-12'

III. Structural cross section 19-19'

IV. Net-sandstone isopach map, Oakville operational unit

V. Percent-sandstone map, Oakville operational unit

VI. Net-sandstone isopach map, Lagarto operational unit

VII. Percent-sandstone map, Lagarto operational unit



Citation
Galloway, W. E., Jirik, L. A., Morton, R. A., and DuBar, J. R., 1986, Lower Miocene (Fleming) Depositional Episode of the Texas Coastal Plain and Continental Shelf: Structural Framework, Facies, and Hydrocarbon Resources: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 150, 50 p.