Abstract
Forty-eight samples of a 370-ft Miocene Monterey Formation core from the Union Leroy 51-18 well, Santa Maria Basin, California, are examined for source rock potential and molecular distributions. Total organic carbon content, Rock-Eval pyrolytic yield, vitrinite reflectance, and distribution of regular steranes, tricyclic and pentacyclic terpanes, and aromatic steroids are measured. The present-day temperature range through the sampled interval is estimated to be less than 5°C; maturity differences among samples are negligible. Source rock potential and molecular distribution differences are considered to arise entirely from the effects of source input, diagenesis (including lithologic effects) and migration. The sampled section is comprised of siliceous and phosphatic/carbonate lithofacies, and covariances between geochemical parameters (source potential and biomarker distribution) and lithofacies are recognized. Several molecular parameters change systematically with depth/lithology, including ββ( αα + ββ) ethylcholestane ratios and C-20 and C-22 sterane and hopane epimer ratios. The siliceous lithofacies possesses higher moretane/hopane, tricyclic/pentacyclic terpane, and sterane/terpane ratios, and lower mono/mono + tri aromatic steroid ratios. Bisnorhopane/hopane ratios are up to 40 times higher in the phosphatic/carbonate lithofacies than in the siliceous lithofacies. Increased relative concentrations of aromatic steroids are attributed to sulfur-sterol reactions during very early diagenesis, whereas increased relative bisnorhopane amounts are considered to result from massive input of specific anaerobic biota. Although kerogen elemental ratios change abruptly at the lithofacies boundary, molecular distributions within the extractable organic matter change gradually, beginning about 30 ft (9m) above this interface. This gradual change is tentatively attributed to upward migration of hydrocarbons originating in the phosphatic/carbonate lithofacies, probably through fractures within the lower part of the siliceous lithofacies interval.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
