Comparison of Porphyrin and Hydrocarbon Biomarker Data with Other Indicators of Maturity and Oil Generation

L. D. Raedeke, P. Sundararaman, J. M. Moldowan

A single well in the San Joaquin Valley penetrates six Miocene shale formations through a depth interval of more than 10,000 ft. Eighty-three samples through the interval were analyzed by Rock-Eval to determine zones of maturity and oil generation. The maturity of select samples was also determined using porphyrins, steranes, and aromatic steroids.

Rock-Eval T_max increases smoothly with depth through the interval, reaching 435°C (the shallow limit of the oil window) at about 9,500 ft. Increase in production index [S1/(S1 + S2)] and decrease in hydrogen index (S2/TOC × 100) indicate beginning of oil generation at about 10,000 ft and increasing generation with depth. The vanadyl porphyrin maturity parameter, C₂₈ETIO/(C₂₈ETIO + C₃₂ DPEP) obtained by HPLC, shows a gradual increase with depth to about 10,000 ft and then a rapid increase to a maximum of 0.85 at 12,000 ft; the zone of rapid increase corresponds to the zones of maturity and of oil generation indicated by Rock-Eval parameters. The biomarker maturity parameters based on C²⁹ sterane isomerization and C₂₉ monoaromatic steroid aromatization showed a systematic increase through the interval. Isomerization and aromatization levels observed in these bitumens were comparable to local oils, thus confirming the top of the oil generation window.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.