Plio-Pleistocene Genetic Sequences of the Southwestern Louisiana Continental Shelf and Slope: Geologic Framework, Sedimentary Facies, and Hydrocarbon Distribution

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.
Authors
Robert A. Morton
Walter B. Ayers, Jr.
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.

Code
RI210
ISSN
2475-367X
Number
210
Number of figures
35
Number of pages
77
Publisher
The University of Texas at Austin, Bureau of Economic Geology
Series
Report of Investigation
Year
1992