Trace elements and organic geochemical fingerprinting of natural crude oils from the Monterey Formation, offshore Santa Maria Basin, California

Based on our recent analytical method development using microwave digestion and triple-quad ICP-MS, we have analyzed up to 57 elements for 20 natural crude oil samples from the offshore Santa Maria Basin of California, a prolific producing basin thought to be sourced dominantly from the Monterey Formation. Our goal is mainly to see if there were oil-oil correlations between wells (fractured reservoir mixing) or no correlation (locally sourced from an unconventional restricted depth reservoir or compartment), and which part (upper and/or lower) of the Monterey Formation acted as the primary source of oil. The results show distinct fingerprints based on V/Mo and Ni/Mo ratios derived from each of the wells where the oils were sampled during stem tests, indicating that they might be partially correlated with restricted intervals of the stratigraphy (or lithofacies). The V/Mo and Ni/Mo ratios of the oils also show that the lower Phosphatic-carbonate lithofacies of Monterey Formation may be a primary source of the oils. Vanadium and Ni contents, V/(Ni + V) ratios, and sulfur content of the oils indicate that the Monterey Formation was deposited under reducing marine conditions. The linear correlation between Mg and Ca contents and non-linear correlation between P and Ca contents in the oil samples may indicate that dolomite-rich rather than apatite-rich portions of Monterey Formation may be a critical source rock component. The similarity between REY distribution patterns of the oils and authigenic carbonate implies that carbonates might have been correlated with formation of the oils. Positive Eu anomalies in most oils reveal possible hydrothermal influences associated with magmatic and slab window effects as a consequence of the Farallon-Pacific Ridge subduction, higher heat flow, and later transtensional and transpressional deformation during deposition of Monterey Formation. The comparable REY distribution patterns for the oils and outcrop shales of Lions Head reveal potential oil-source correlations.