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Environmental factors in a depositional basin may control the quantity and nature of hydrocarbons initially deposited with the sediments. The hydrocarbon mixtures in sediments increase in quantity and become more like petroleum after long periods of burial and compaction, providing evidence that petroleum hydrocarbons are generated in non-reservoir rock.
Ratios of odd to even carbon numbered heavy n-paraffins in shale may in some instances be regarded as indicators of conversion of organic material to hydrocarbons. This parameter, supplemented by infrared measurements, enables the detection of petroleum-like mixtures of hydrocarbons in possible "source beds." An estimated 30 per cent of the shales in petroleum provinces contain petroliferous mixtures of hydrocarbons. There is a parallelism between amount of organic carbon, quantity of hydrocarbons, frequency of occurrence of dispersed oil in non-reservoir rock, and the occurrence of petroleum accumulations. Although the statistical nature of this parallelism provides opportunity for exceptions, it appears that both kind and quantity of hydrocarbons are important parameters for recogni ion of source rock.
In a number of instances similarities as well as systematic differences have been observed between hydrocarbons in oil and the corresponding fractions in the presumed source rock.
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