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Significant amounts of indigenous hydrocarbons have been found in recent sediment cores from the Santa Barbara basin offshore of southern California. The molecular composition of these hydrocarbons is like that of crude oil and appears to be uniform across the basin and not altered by depth of burial. The molecular composition of this incipient crude oil differs from local crude oils and oil seeps and from hydrocarbons contained in underlying, geologically older rocks. Phytoplankton, which are so abundant in the nutrient-rich waters of the Santa Barbara Channel, appear to be the major source of the hydrocarbons. Study of these cores seems to be leading to the general conclusion that the process of oil generation starts very early in the depositional cycle.
One piston core from the basin contained a 2-m thick sand and shell layer buried 2 m below the mud line. In contrast to the surrounding clay muds, a large fraction of the extractable organic matter from the sand-shell layer was hydrocarbons, and the hydrocarbons exhibited a more crude oil-like composition than the hydrocarbons in the clay. Thus, it appears that hydrocarbons have migrated from organic-rich, clay muds into a reservoir bed within a few thousand years.
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