Biomarker geochemistry of the Miocene Monterey Formation, West San Joaquin Basin, California: Implications for petroleum generation

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°C. The diagenetic progression of silica from opal-A to opal-CT to microquartz is well-documented in Monterey burial history studies. However, diagenetic indicators need to be established below the depth of complete conversion of opal-CT to quartz, which may be very shallow (1500 m or less at Lost Hills). Conventional maturity indicators are problematic. The scarcity of vitrinite in many Monterey samples hampers reflectance measurements. In addition, values that are measured may be anomalously low. Maximum pyrolysis temperatures are depressed, due in part to high heavy bitumen content. 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 5α(H), 14α(H), 17α(H)-24-ethylcholestane increases from 0.09 to 0.56 over the depth range cited. Samples from petroleum accumulation zones are distinguished by anomalously mature biomarker assemblages considering their depth of burial. This indicates that these bitumens are as mature as the deepest non-production zone sample (3430 m), even though they are from much shallower depths (580-1572 m). Other maturity (and/or migration) indicators confirm this, such as the relative increases in amounts of 14β(H), 17β(H)-steranes, rearranged steranes and tricyclic terpanes. Thus, 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.