Chemometric recognition of genetically distinct oil families in the Los Angeles basin, California

The prolific Los Angeles basin in California may be the most petroliferous province on Earth per volume of sedimentary fill. However, because most exploration in the basin occurred prior to the advent of modern geochemical methods, genetic relationships among the various petroleum accumulations and their source rocks have remained speculative. A training set of 24 source-related biomarker and stable carbon isotope ratios for 111 non- or mildly biodegraded oil samples from the basin was used to construct a chemometric (multivariate statistics) decision tree. The decision tree allows genetic classification of additional oil or source-rock extract samples that might be collected. The decision tree identifies 6 tribes and a total of 12 genetically distinct oil families. The families have different bulk properties, such as API gravity and sulfur content, which were previously explained as resulting from secondary processes, including thermal maturity or biodegradation. However, the chemometric assignments are based on genetic properties that reflect distinct organofacies. The oil families occur in different locations and reservoir intervals in the basin, consistent with their origins from different organofacies of active source rock. The source-rock depositional environment for each oil family can be inferred using biomarker and isotope ratios. The samples show stable carbon isotope ratios for saturate and aromatic hydrocarbons that indicate different organofacies of Miocene marine source rocks. Tribes 1 and 2 straddle the central trough, mainly occur east of the Newport-Inglewood fault zone (NIFZ), and show evidence of proximal, clay-rich source rock deposited under suboxic conditions with elevated angiosperm input. Tribes 3-6 occur west of the NIFZ and show evidence of more distal, clay-poor source rock deposited under anoxic conditions. Geochemistry and stratigraphy of the oil tribes (1-6 below) suggest the following source-rock organofacies: 1. Suboxic upper Miocene (Delmontian) proximal clay-rich shale generated low-sulfur tribe 1 and 2 oil types (similar to 0.55-1.06 wt %) east of the NIFZ that show higher thermal maturity than tribes 3-6. The source rock for tribe 1 was slightly more reducing (lower redox potential or Eh) than that for tribe 2. 2. Low-sulfur tribe 2 oil (similar to 0.20-0.23 wt. %) is significantly more mature, and the source rock is more clay rich and received more angiosperm input than tribe 1. 3. Anoxic upper Miocene (middle upper Puente) distal shale source rock generated sulfur-rich tribe 3 oil (similar to 1.42-1.58 wt. %) west of the NIFZ. The source rock received less higher-plant input than the source rocks for tribes 1 and 2. 4. Suboxic anoxic Mohnian(?) clay-poor shale or marl source rock generated tribe 4 oil west of the NIFZ. One oil family in tribe 4 has low sulfur, whereas the other has high sulfur content. Higher-plant input is comparable to tribe 3. 5. Anoxic middle upper Miocene (Lower Puente nodular shale) distal shale generated sulfur-rich (similar to 1.24 wt. %) tribe 5 oil to the southwest of the NIFZ. 6. Anoxic middle upper Miocene (Lower Modelo nodular shale equivalent) distal shale or marl generated high-sulfur tribe 6 oil (similar to 2.42 wt. %) to the northwest of the NIFZ at lower levels of thermal maturity than the other tribes.