ABSTRACT: Stratigraphic Variations in the Biomarker Distribution of the Moreno Formation: Their Correlation with San Joaquin Basin Oils

Margaret G. Bac, Benjamin J. Schulein

Variability in the biomarker compositions of petroleums is typically employed in the recognition and distinction of contributions from different source rocks. We demonstrate that the fluctuations in the biomarker distributions of different intervals within a single source rock sequence appear to account for specific compositional differences in a suite of oils from the San Joaquin basin, California.

Rock-Eval pyrolysis studies of a 100-m-thick immature, laminated, marine shale sequence within the Upper Cretaceous to lower Paleocene portion of the Moreno Formation reveals TOC (total organic carbon) contents consistently around 2% and moderate hydrogen indices (i.e., 175-300 mg HC/g org. C) characteristics suggestive of a uniform depositional sequence with source rock potential. Analyses of the extractable aliphatic hydrocarbons of cored samples taken at approximately 10-m intervals from the sequence reveal significant variability in biomarker distributions. Such differences are exemplified by the triterpenoids (as seen in m/z 191 chromatograms from GC-MS and GC-MS/MS analyses) where the dominant component fluctuates from a 17^agr(h),21ß(H)-30-norhopane to 28,30-18^agr(H)-bis orhopane to 20S and 20R dammar-13(17)-enes. Some components are dominant in one interval, but are not detected in others, suggesting discrete stratigraphic variations in the biomarker characteristics of the Moreno.

Similar discrepancies in biomarker distributions are evident in the aliphatic hydrocarbons of the suite of oils. The three petroleums reservoired in the San Carlos sandstone member of the Lodo Formation, which directly overlies the Moreno, reflect biomarker contributions from a Moreno source, including compound distributions (e.g., prominance of C₂₈ steranes relative to C₂₇ and C₂₉ homologs), and the occurrence of both alkanes (e.g., 28,30-bisnorhopane) and alkenes (e.g., dammarenes and diasterenes). However, the distribution of the aforementioned components in the oils reservoired in the San Carlos is sufficiently distinctive to argue for their derivation from different source rocks, whereas it probably reflects generation from discrete intervals of t e Moreno sequence.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990