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Abstract: Gas Generation - A Major Cause of Deep Gulf Coast Overpressures and Hydrocarbon Expulsion

John M. Hunt, Jean K. Whelan, Lorraine B. Eglinton, Lawrence W. Cathles III

Direct measurements of porosities of Tertiary and Cretaceous shales in the Texas-Louisiana Gulf Coast show that in many areas shale porosity is either constant or increasing at the depths where overpressures occur and where hydrocarbons are being generated. In the absence of a decrease in porosity with sediment load (depth) gas generation becomes the principle cause of over-pressuring and hydrocarbon migration. The porosities at which shales reach a relatively constant value depends on the internal surface areas of the shales. Shales containing minerals with small, internal surface areas such as fine-grained quartz and carbonates stop compacting at porosities around 3% whereas shales containing minerals with large surface areas such as illite stop compacting around 10%. This minimum compaction usually is reached at depths around 9,000 to 12,000 ft. prior to the development of high overpressures and the generation of hydrocarbons. Model studies indicate that gas generation is the dominant force causing these deep overpressures.

Model studies also suggest that secondary shale porosity (decompaction) may develop in some high pressure compartments under topographic highs. However, many deep overpressured shales onshore in the Gulf Coast show no evidence of undercompaction.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France