Abstract

Crude oils from Miocene and Pliocene reservoirs from the Elk Hills field in California's San Joaquin basin were analyzed for stable carbon isotopes and biomarkers. Cluster analysis of geochemical variables defines five principal oil families, all derived from different organic-rich facies of the Miocene Monterey Formation. Carbon isotope analysis indicates no contribution from the basin's other major source rock, the Eocene Kreyenhagen Formation. Oil families show a strong correspondence to stratigraphic intervals. Oils from pre-Monterey reservoirs were probably generated from the lowermost organic-rich facies of the Monterey and are the most thermally mature. Upper Miocene Stevens zone turbidite reservoirs contain oils of various thermal-maturity stages, but mature light ends are abundant and are likely generated from the floors of the adjacent subbasins located north and south of Elk Hills. The relatively minor presence of low-thermal-maturity biomarkers that are typically characteristic of Monterey oils may indicate that Stevens traps did not form until after the source intervals were at a higher level of thermal maturity. All oils in Stevens porcelanite reservoirs contain a higher concentration of low-maturity biomarkers, which may indicate derivation from more localized areas on the flanks of the Elk Hills anticlines. The shallow Pliocene oils have suffered biodegradation to different degrees, and the lowest API gravities occur on the flanks of the anticline. The carbon isotopic composition of these oils suggests yet another Monterey source facies that charged the Pliocene reservoirs and is not simply the result of vertical leakage from the older Miocene reservoirs.

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