The western Gulf of Mexico province, which lies offshore from the states of Louisiana and Texas, is estimated to contain large resources of natural gas in Miocene, Pliocene, and Pleistocene rocks.

Interpretation of chemical and isotopic analyses of natural gases from 47 fields suggests that the province is important as a gas-producing region for three reasons. (1) Several shallow Pleistocene accumulations are of apparent biogenic origin; this gas is characterized by enrichment of the light isotope C¹² in methane (^dgrC¹³ lighter than -55 parts per thousand) and by large amounts of methane (C₁/C_1-5 >0.99). (2) Many of the Miocene accumulations were generated during the early stages of thermal cracking of liquid hydrocarbons. This type of gas is wetter than biogenic gas (C₁/C_1-5 >0.92) and isotopically heavier (^dgrC¹³ heavier than -43 parts per thousand). (3) Numerous accumulations occur in therma ly immature (with respect to oil generation) rocks in which hydrocarbons, particularly gases, have migrated vertically from deeper, more mature rocks. These gases are relatively dry (C₁/C_1-5 generally >0.90), and have a wide range of carbon isotope values.

The gas occurrences can be related to the sedimentary history and tectonics of the area. The location, areal extent, and thickness of sediments in late Tertiary and Quaternary depocenters controlled the distribution of reservoir and source rocks and the depth of the maturity level for each rock series. Movement of a thick Mesozoic salt section, in conjunction with concurrent subsidence of the Gulf basin and the influx of sediments, resulted in folding and faulting of Cenozoic rocks and the formation of structural traps. Regional growth faults, plus radial faults associated with salt diapirism, provided pathways for the migration of hydrocarbons.

End_of_Article - Last_Page 515------------