Analyses of petroleum and sedimentary hydrocarbons have provided a number of clues to the origin and primary migration of petroleum from source sediments and can reasonably be expected to continue to be a fruitful

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field of investigation. A review of recent contributions in identification of hydrocarbon structures in petroleum supports and reaffirms earlier evidence that some of the hydrocarbons in petroleum have been derived from the residues of biological materials.

The hydrocarbons distributed in sediments are observed to be more petroleum-like after burial and compaction. There is no systematic change with depth or age. Large variations may occur between different formations and different facies of the same formation. Mild metamorphism can change the kind, and reduce the amount, of liquid and solid hydrocarbons in rock.

A number of mechanisms have been suggested for the primary migration and collection of the finely disseminated oil from the presumed source rocks. These modes of migration, however, should explain observed differences between reservoir and sediment hydrocarbons, including the larger percentages of alkanes in the heavy saturated hydrocarbons of crude oils.

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