Mark A. Beeunas¹, Martin Schoell², John Zumberge³

(1) Chevron USA Production Company, New Orleans, LA
(2) Chevron Research and Technology Co, Richmond, CA
(3) GeoMark Research Inc, Houston, TX

ABSTRACT: Habitat of Natural Gas in the Gulf of Mexico Shelf

Natural gas in Tertiary reservoirs of the Gulf of Mexico shelf (GOMS) are related to two primary petroleum systems. a) an eastern and basinward, oil rich, Middle Cretaceous sourced system and b) a western and landward, gas/condensate rich, Tertiary sourced system. A number of fields across the Central GOMS contain hydrocarbons that are mixtures of these two sourced systems. In some fields these source systems also contribute to a later stage thermally mature charge of gas and condensate.

Production from the GOMS is predominantly around salt domes and growth fault systems associated with the formation of mini basins where late stage evacuation of salt provides accommodation space for a thick, rapidly deposited section. The gases from 14 fields across the eastern GOMS have the character of pervasively mixed gases of thermogenic and bacterial origin. Most importantly, many of the gases associated with oils have bacterial gas signatures. We can demonstrate within single field complexes that,

a) the mixing occurs such that individual reservoirs within a field have different proportions of bacterial and thermogenic gases and

b) thermogenic gases have different maturity signatures, suggesting multiple charges of thermogenic gases.

Pervasive mixing of bacterial and thermogenic gases can be explained with a 2 step formation model that is related to the evolution of the mini-basins in the GOMS: 1) the mini-basins are prolific kitchens of bacterial gases that migrate towards growth faults and salt ridges; 2) maturation of deeper source rocks generates thermogenic gases, which migrate along deep-rooted faults into the accumulation areas of bacterial gas.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado