ORGANIC GEOCHEMISTRY AND COMPARATIVE DIAGENESIS: MIOCENE MONTEREY FORMATION, LOST HILLS OIL FIELD AND VICINITY, WEST SAN JOAQUIN BASIN, CALIFORNIA (BIOMARKERS, GAS CHROMATOGRAPHY/MASS SPECTROMETRY)

Much of the Miocene Monterey Formation of California is rich in biogenic sediment, especially organic matter and silica. Because of the geologic structure, the Monterey in the subsurface near Lost Hills in the San Joaquin Basin forms a natural laboratory for the study of the diagenetic responses of these materials. Rocks of similar age and lithology are buried to depths ranging between 500 and 3500 m, and are thus exposed to a temperature range of 45-130(DEGREES)C.; Silica phases exhibit a clear diagenetic progression with depth from the opal-A of the diatom frustules to opal-CT and ultimately to microquartz. The d(,(101)) spacing of opal-CT decreases from 4.088 to 4.044 (ANGSTROM) with depth. However, diagenetic indicators need to be established below the depth of complete conversion of opal-CT to quartz, which is very shallow (1500 m or less at Lost Hills). Conventional maturity indicators are problematic.; Biomarker geochemistry provides effective alternative maturity indicators. Of particular interest are the stereochemical variations observed in the assemblages of steranes and triterpanes extracted from oil well core samples. For example, the 20S/20R ratio of 5(alpha),14(alpha),17(alpha)(H) 24-ethylcholestane increases from 0.09 to 0.55 over the depth range cited.; Samples from petroleum production zones are distinguished by anomalously mature biomarker assemblages considering their depth of burial. For instance, the ethylcholestane 20R/20S ratios are between 0.45 and 0.64. It appears that any immature indigenous bitumen in production zone samples is overwhelmed by a mature component, which presumably migrated updip from deeper, warmer strata.; Since much oil is produced from shallow, organic-rich fractured Monterey shales, early in situ generation has previously been hypothesized. However, while both source and reservoir rock are lithologically similar and lie within the same formation, biomarker geochemistry indicates that substantial generation occurs only in deeply buried Monterey shales.; The predominance of C(,27) and C(,28) (alpha)(alpha)(alpha) 20R steranes over C(,29) and the low pristane/phytane ratios are indicative of aquatic organic sedimentation. While otherwise similar, indigenous bitumens are distinguished by varying amounts of 28-norhopanes, major occurrences of which are coincident with quasi-stromatolitic structures suggestive of fossil bacterial mats. (Abstract shortened with permission of author.)...