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A diverse suite of 76 oils was analyzed for light C4 to C7 hydrocarbons (LHC). Indices of paraffinicity, termed the "heptane value" (HV) and "isoheptane value" (IV) were examined. These paraffin-to-naphthene concentration ratios had provided reliable measures of the catagenetic grade of sedimentary rocks, also a means of determining paleotemperatures, employing autochthonous LHC. The goal of the present study was an assessment of the conditions of generation of oil.
Heptane values in the sampled oils range from 0.5 to 60.9, but possess a near-normal distribution. The principal group (31 samples, 41%) is within the range 18.0 to 22.0. The modal class (HV 19.0 to 20.0) comprises 14%. The isoheptane value is similarly distributed about a modal class (18%) of 0.90 to 1.0. It is concluded that most oils retain evidence of generation in an extremely limited range of subsurface temperatures. The temperatures are of the order of 280 to 300°F (138 to 149°C), assessed from curves relating HV and IV to maximum-attained subsurface temperature for sediments bearing aliphatic, petroleum-source kerogens.
The oils of the principal group (HV 18.0 to 22.0) are termed "normal, paraffinic." Twenty percent of the analyzed oils are naphthenic (HV 0.0 to 18.0). Their compositions differ from those of low-temperature sediment extracts: they are identified as biodegraded, not immature, oils. Forty percent of the oils have an HV exceeding 22.0 (mature oils); some exceed 30.0 (supermature oils). Both classes have undergone protracted heating. The analytic methods and parameters provide a scheme of oil classification containing substantial geologic information. In addition, the plot of HV versus IV for sediment extracts provides clear distinction between aromatic, coaly kerogens and aliphatic, sapropelic kerogens.
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