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Organic matter, including parent materials of hydrocarbons and porphyrins, is produced by phytoplankton living in oceanic surface waters in quantities far greater than required for petroleum. Mostly, however, these materials are regenerated in the water and at the bottom, and thus are lost to petroleum. An understanding of petroleum genesis includes knowledge of how they can be preserved.
Radiocarbon age determinations at several depths in cores from basins off southern California show that the rate of deposition of total sediment varies from 8 to 125 mg/cm2/year. Highest rates occur in basins close to shore where rates are similar to those for the now-filled Los Angeles and Ventura basins during the Pliocene epoch; the grain size, content of calcium carbonate, and interruption by turbidity currents also are similar. Most of the sediment is detrital silt and
clay from the mainland. Organic matter is diluted by this detrital sediment so that it forms a lower percentage of total sediment in nearshore than in offshore basins. Even though hydrocarbons and porphyrins are also diluted by detrital sediments in nearshore basins, both are much more abundant (constituting a higher percentage of total sediment) in nearshore than in offshore basins. Thus, it is evident that both hydrocarbons and porphyrins are more easily oxidized than is total organic matter and that their preservation is greatly enhanced by rapid burial which removes them from contact with the oxidizing overlying water. Comparison with estimated ultimate petroleum recovery from Los Angeles basin shows that far more organic matter, hydrocarbons, and porphyrins were produced and depo ited in the basin sediments than were required to form the petroleum. Nevertheless, present production of petroleum from Los Angeles basin is at a rate that appears to be about 150,000 times greater than its rate of formation.
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