Classification and thermal history of petroleum based on light hydrocarbons

Classifications of oils and kerogens are described. Two indices are employed, termed the Heptane and IsoheptaneValues, based on analyses of gasoline-range hydrocarbons. The indices assess degree of paraffinicity. and allow the definition of four types of oil: normal, mature, supermature, and biodegraded. The values of these indices measured in sediment extracts are a function of maximum attained temperature and of kerogen type. Aliphatic and aromatic kerogens are definable. Only the extracts of sediments bearing aliphatic kerogens having a specific thermal history are identical to the normal oils which form the largest group (41%) in the sample set. This group was evidently generated at subsurface temperatures of the order of 138°–149°C, (280°–300°F) defined under specific conditions of burial history. It is suggested that all other petroleums are transformation products of normal oils.