Abstract: Integration of Natural Gases in Hydrocarbon Systems of the Northern Gulf of Mexico Basin

Alan T. James, Kenneth C. Hood, Lloyd M. Wenger, Stanley C. Harrison, Oliver P. Gross

Associated and non-associated natural gases of the northern Gulf of Mexico Basin have been characterized as part of Exxon's multidisciplinary study of hydrocarbon source, maturation and migration. Analyses of over 600 natural gases were integrated within the hydrocarbon (HC) system context provided by evaluation of more than 2000 reservoired oils, 1200 HC-bearing sea bottom drop cores, and extensive source rock data, all synthesized within a geologic framework developed from 2-D and 3-D seismic.

The molecular and isotopic compositions of gases can vary due to type of organic matter and level of maturity of the source, mixing with biogenic methane, and biodegradation. Organic matter type is a primary control on the compositions of natural gases, as documented by the close correspondence of gas compositions to HC systems defined from oil and source rock characterization. For example, the predominance of relatively dry gas on the Texas shelf primarily reflects generation from a terrestrial, gas-prone source. The recognition of significant source controls on gas composition has important implications to evaluation of gas maturity and inferred timing of generation. Complications from widespread occurrence of biogenic gas are incorporated in gas interpretations.

Integration of these gas and oil data strongly supports the concept that most associated thermogenic gases have been cogenerated with oils within the normal oil window. On the slope, large seeps contain early mature oils and abundant wet thermogenic gases, documenting comigration and suggesting that the two have a common origin. In contrast, high maturity gases are distinctly absent.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela