ABSTRACT

Sediments in the south Louisiana portion of the Gulf Coast geosyncline contain at least 90 trillion cubic feet of gas, which is one quarter of the proved ultimate natural gas reserve of the United States. This geosyncline, 750 miles long, began to form in late Triassic time but did not attain maximum development until Cenozoic time. The stratigraphic section in south Louisiana is between 40,000 and 60,000 feet thick, and has a volume of approximately 400,000 cubic miles.

1 This paper was prepared Jointly by the Lafayette Geological Society (L.G.S.) and the New Orleans Geological Society (N.O.G.S.). Each member contributed written material which was organized, rewritten, and edited by A. A. Meyerhoff. For L.G.S.: Chairman: Paine; members: Borland, Kimmey, Meyerhoff, Munchrath, Raspberry, and Roederer; for N.O.G.S., co-chairman: Waters and Spillers; members: Andrews, Baysinger, Cotton, Cristina, Hall, McDougall, Paffett, Rockwood, Stipe, and Woodbury. This paper would not have been possible without the cooperation of scores of the writers' colleagues and of the managements of many oil and gas companies that operate in this area. Special thanks are due Mrs. Kathryn L. Meyerhoff, Mr. Darrel Hebert, Mrs. Carroll Ingram, Mrs. Eva Mae Landry, Miss Betty Martin, Mrs. Flora Brashears, and Miss Helen Dummet for editorial advice and assistance in preparation of the manuscript, tables, and illustrations. Messrs. Gordon I. Atwater, Jules Braunstein, H. N. Hickey, J. K. Larsen, C. M. McLean, L. H. Meltzer, Grover E. Murray, R. D. Ocamb, J. W. Sheller, S. R. Silverman, B. E. Van Arsdale, Jr., and Jay B. Wharton, Jr., kindly reviewed all or parts of the manuscript and offered many valuable suggestions. The writers are especially indebted to Jules Braunstein, H. N. Hickey, and J. W. Low, each of whom spent several long days editing the manuscript prior to submittal. These persons do not necessarily endorse the views expressed herein. The stratigraphic terminology does not reflect the opinions and views of all the reviewers. The aid and cooperation of the Lafayette Geological Society, the New Orleans Geological Society, the Gulf Coast Association of Geological Societies, and the American Association of Petroleum Geologists are acknowledged with gratitude.

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The sedimentation rate within the geosyncline has increased steadily from Triassic time to the present. Since the Jurassic, the depositional axis has shifted progressively gulfward. As a result, the optimum producing trends occur in younger sediments toward the Gulf. Concurrently with gulfward migration of the geosynclinal axis, the depocenter, since Paleocene time, has migrated from south Texas to southeast Louisiana.

The maximum thicknesses of the Cenozoic units, and possibly of some of the Mesozoic ones, are on the downthrown sides of growth faults. Such faults, characteristic of the Gulf Coast geosyncline, permitted great thicknesses of sediments to accumulate in local depocenters.

The concurrence of a large provenance area, a great volume of sediment, abundant organic matter, rapid deposition and burial, basinal subsidence, and movements of an underlying salt layer, which produced syndepositional structural traps in the geosyncline, created optimum conditions for the generation and entrapment of huge hydrocarbon reserves in south Louisiana.

Statistically, more gas than oil occurs below 10,000 feet and in association with sediments deposited in relatively deeper water. Gas is more abundant where shale is a major constituent of the producing section. Gas reserves generally are smaller where the sandstone percentage is greater. With increasing age and depth the atomic hydrogen to carbon ratio of the gas decreases.

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