Current research suggests that most of the hydrocarbons in petroleum are formed from the organic matrix of sedimentary rock at depths greater than 5,000 ft. The increased generation of light hydrocarbons with depth is offset by decreases in the permeability of source beds and in the volume of migrating fluids. Empirical field studies may outline source-reservoir relations, but the mechanisms by which the hydrocarbons originate, migrate, and accumulate still are poorly defined.

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