Organo-Clay Mineral
Interactions: From Preservation to Oil Generation
Bristow, Thomas F.1, Martin J.
Kennedy1, Arkadiusz Derkowski1
(1)
In many source rocks the main phase of
hydrocarbon generation coincides with illitization of
smectitic clays, supporting the hypothesis that
expulsion of organic compounds from interlayer sites of 2:1 clay minerals plays
a role in hydrocarbon generation and cracking rather than just reactions with
pore waters. In marine sediments ~90% of organic material sediments cannot be
separated from the mineral phase and intercalation of organics within clay
mineral interlayers leads to enhanced preservation.
Sorption is reversed during diagenetic alteration of
clays as expandable layers collapse (illitization)
and mineral surface area is reduced, leading to expulsion of organic matter,
making it more susceptible to hydrous cracking reactions. This is demonstrated
by progressive steepening of TOC vs
total mineral surface area trends of three source rocks with smectitic, mixed layer and illitic
mineralogies. Our model reconciles organic-mineral
interaction involvement in preservation and in protokerogen
formation with observations of discrete kerogen
particles that do not show intimate relationships with clays in rocks that have
passed through the oil window. Processes leading to organic expulsion include
physical collapse of interlayer sites and increased layer charge repelling
non-polar sections of hydrocarbon molecules away from mineral surfaces.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California