Abstract: Nature of Shales and Dynamics of Hydrocarbon Expulsion in Gulf Coast Tertiary Section (AAPG Short Course: Physical and Chemical Constraints on Petroleum Migration)

Henry H. Hinch

For years shales have been recognized as the most probable source beds of hydrocarbons. It has been natural to attribute to expulsion migration of those hydrocarbons into adjacent carrier and reservoir beds together with shale pore water as the shales compact. Few geologists recognized that to explain expulsion of hydrocarbons from shales in this manner might be difficult. However, in the last 10 years, many reasonable doubts have arisen as to the mechanism of hydrocarbon expulsion. By the time hydrocarbons are generated in significant amounts, most of the shale pore water has already been expelled and it is highly questionable, considering the low solubilities of most hydrocarbons, whether the amounts of pore water remaining are sufficient to flush the hydrocarbons from the shale source beds.

Our more recent studies of Gulf Coast Tertiary shales suggest that hydrocarbon expulsion from Gulf Coast Tertiary shales is due to preferential diffusion of hydrocarbon molecules through the shale pore system rather than flushing of the hydrocarbons by water expelled during compaction. This diffusion process is the result of mechanisms related to the physical properties of the shales and their pore fluids and the molecular interaction between rock grains and pore fluids.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma