INVESTIGATING NATURAL CO2 SOURCES IN SELECTED CALIFORNIA COASTAL BASINS

The U.S. Geological Survey is studying natural carbon dioxide (CO₂) accumulations to predict long-term effects of storing anthropogenic CO₂. This study focuses on characterizing sources of natural CO₂in two petroliferous California coastal basins: Ventura – Santa Barbara and Santa Maria Basins. Ventura – Santa Barbara Basin is structurally complex and filled with primarily marine Cretaceous to Pleistocene sedimentary rocks. Whereas primary hydrocarbon reservoirs in the basin are thick Pliocene and Pleistocene deep-sea sand sequences, the Miocene Monterey Formation contains other important reservoirs. The Monterey Formation is a thick extensive marine deposit consisting predominantly of highly siliceous rocks. Santa Maria Basin is a Mesozoic basement complex composed of metasedimentary and altered mafic volcanic rocks overlain by Miocene to Quaternary marine and non-marine sedimentary rocks. Fractured Miocene Monterey Formation rocks are the primary hydrocarbon reservoirs in this basin.

There are three primary sources of natural CO₂, including thermal destruction of kerogen, thermal destruction of carbonates (decarbonation), and volcanic activity. One source indicator is the carbon isotope ratio δ13C (CO₂). In Ventura – Santa Barbara Basin, sampled CO₂ concentration ranged from 17 to 24 percent and δ13C (CO₂) ranged from -1.0 to 14 permil; and in Santa Maria Basin, CO₂ concentration ranged from 3.5 to 29 percent and δ13C (CO₂) ranged from -5.5 to 14 permil. Ventura – Santa Barbara Basin values match those from a legacy commercial database. Based on these sampled δ13C (CO₂) ranges, results are non-unique, and suggest methanogenesis and petroleum biodegradation or decarbonation as sources of the CO₂. Methanogenesis is a bacterial process synthesizing hydrocarbon gas from CO₂ resulting in isotopically enriched CO₂ and consequently reducing reservoir CO₂ concentration. These gas samples are produced from shallow Monterey Formation reservoirs that contain biodegraded oil, and therefore the CO₂ concentration and isotopic ratio may be due to methanogenesis. Though because of the complex origin and structure of both basins, other sources cannot be ruled out, and additional study is needed.