Steranes and triterpanes generated from kerogen pyrolysis in the absence and presence of minerals.
Steranes and triterpanes generated from pyrolysis of immature Monterey Formation kerogen in the presence and absence of calcite, illite and montmorillonite reveal results that are both consistent and divergent with published data that reflect the use of these biological markers as maturation indicators.  The extent  of isomerization of biomarkers generated from pyrolysis of kerogen at 300 degrees C for 2 hours, at C-20 in 14 alpha(H), 17 alpha(H)-steranes, at C-22 in 17 alpha(H),21 beta(H)-hopanes and of 17 beta(H),21 beta(H)-hopanes correspond to early diagenetic stages in rock extracts from sedimentary basins.  Isomerization increases with heating time and, after 1000 hours, attains values which correspond to the catagenetic stage in sedimentary basins, or equivalent to that of mature oil.  Stepwise pyrolysis of the kerogen indicates faster isomerization rates for steranes and triterpanes in the bitumen than those retained in the kerogen structure, confirming earlier studies.  Presence of a mineral matrix can influence the isomerization of steranes and triterpanes considerably.  Comparisons with results from kerogen heated alone, for a given maturation stage, show that calcite inhibits, illite catalyzes slightly and montmorillonite has a pronounced catalytic effect on these reactions.  This effect results in early isomerization of steranes and hopanes corresponding to the catagenetic stage in the presence of montmorillonite, while kerogen or kerogen with calcite held at the same temperature (300 degrees C) and time (10 hours) only yield isomerized products which correspond to a diagenetic stage.  Further, illite and montmorillonite affect various isomerization reactions differently.  The fastest reaction is the isomerization at C-20 in 14 alpha(H),17 alpha (H)-steranes followed by that at C-22 in 17 alpha(H),21 beta(H)-hopanes and the slowest is the formation of 14 beta(H),17 beta(H) steranes.  These results show that maturation measurements of rock or oil samples from basins which use biological markers have to take into account the mineral matrix effects, which have been largely ignored until present.
Tannenbaum
E
E
Institute of Geophysics and Planetary Physics, University of California, Los Angeles 90024, USA.
Ruth
E
E
Kaplan
I R
IR
D001546:Bentonite / Q000737:chemistry; D015415:Biomarkers / Q000032:analysis* / Q000737:chemistry; D002119:Calcium Carbonate / Q000737:chemistry; D002140:California; D019015:Geologic Sediments / Q000737:chemistry; D006358:Hot Temperature; D006838:Hydrocarbons / Q000032:analysis / Q000737:chemistry*; D007536:Isomerism; D008903:Minerals / Q000737:chemistry; D011108:Polymers / Q000032:analysis / Q000737:chemistry*
D015415:Biomarkers; D006838:Hydrocarbons; D008903:Minerals; D011108:Polymers; C099089:illite; D001546:Bentonite; D002119:Calcium Carbonate