Application of Re-Os Geochronolgy to Offshore California Basins



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The rhenium-osmium (Re-Os) isotope system has historically been used with geochronological applications in the dating of igneous and metamorphic rocks, and in constraining the Earth’s geochemical evolution. However, various sources of Os contribute to the ocean and ultimately become sequestered in organic rich sediments, these sediments will reflect the 187Os/188Os signature of seawater at the time of their deposition. The Re-Os isotope system is therefore a reliable tracer and dating tool in organic rich sediments that act as excellent source rocks for petroleum. Oil generation and migration is thought to reset the initial 187Os/188Os of oil, reflecting the 187Os/188Os of the source rock at the time of generation. This resetting of the Re-Os geochronometer implies that a Re-Os age from a suite of oils should reflect the timing of generation and migration unless post-emplacement alteration has taken place. This study set out to determine the Re-Os isotopic compositions of 16 oil samples that originate from the offshore Santa Maria basin (Field 395 oils) and the northwestern Santa Barbara Channel (Electra and Santa Maria field oils). The Re-Os age determined for the Electra and Santa Maria field oils was 30 + 3.1 Ma and determined that these oils are more than likely sourced from the Monterey Formation, but have undergone alteration by a mantle-derived fluid that has overprinted the original 187Os/188Os isotopic signature. The age of 30 + 3.1 Ma aligns perfectly with the shift from subduction tectonics to transform and oblique subduction between the Farallon-Pacific plate and the California margin that may have initiated hydrocarbon generation in Eocene source rocks, provided these oils are not sourced from the Monterey Formation. There is significant scatter among the data in Field 395 oils interpreted as an overprint caused by mantle-derived fluid interactions facilitated by nearby faults or alteration by thermochemical or bacterial sulfate reduction that is taking place to this day. This study and Barnette (2015), the only Re-Os studies published in the California area, show the viability of acquiring essential generative information of an oil field using only Re-Os geochemical techniques in the absence of additional geochemical data, and confirm the results of previous Re-Os studies regarding oil-fluid interactions.



Geology, Geochemistry, Geochronology, Rhenium, Osmium, Hydrocarbons