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Petroleum Systems in the World's Richest Petroliferous Basin, Los Angeles, California


Based on a training set of 112 nonbiodegraded or mildly biodegraded crude oil samples, a chemometric decision tree was constructed which identifies five tribes and 12 genetic oil families in the Los Angeles basin. The decision tree classified more than 50 additional samples that were affected by severe thermal maturation or biodegradation. The families occur in different parts of the basin and exhibit different bulk properties, such as API gravity, sulfur and metal content. Unlike previous publications, which propose that these differences result from maturation or biodegradation of similar oil, this work shows that these properties are largely controlled by different source rock organofacies. The character of the source rock was inferred from the geochemistry of each family and their distinct stratigraphic occurrence. Carbon isotope ratios for saturate and aromatic fractions indicate marine Neogene source rocks. Tribes 1–2 consist of five oil families found mainly east of the Newport-Inglewood Fault Zone (NIFZ). These oils show evidence of proximal, clay-rich source rock deposited under suboxic conditions with more higher-plant and especially angiosperm input than Tribes 3–6 (e.g. elevated C19 tricyclic terpane, oleanane/hopane, C31R/hopane, and C24/C23 tricyclic terpane ratios). Tribes 1–2 originated from Upper Miocene (Delmontian) shale in the Central Trough. Tribes 3–6 originated from distal, clay-poor source rock deposited under anoxic conditions with less angiosperm input. Tribes 3–6 also show elevated bisnorhopane/hopane, supporting anoxic conditions. Tribe 3 consists of three families generated from Upper Miocene (Middle-Upper Puente) marl source rock west of the NIFZ. These oils occur from Torrance in the north to the giant Wilmington and Huntington Beach fields in the south. Tribe 4 consists of two families west of the NIFZ that likely originated from Middle-Upper Miocene (Mohnian) marl source rock. The oils range from Playa del Rey in the north to Wilmington field in the south. Tribe 5 occurs southwest of the NIFZ mainly in the Wilmington and Huntington Beach fields and originated from Middle-Upper Miocene (Lower Puente) nodular shale with significant higher-plant plant input. Tribe 6 occurs northwest of the NIFZ from Beverly Hills in the north to the El Segundo field in the south. These oils were generated from Middle-Upper Miocene (Lower Modelo) nodular shale equivalent with significant higher-plant plant input.