Light hydrocarbon compositions of petroleums are genetically informative. They reflect nature and maturity of original materials under the influence of secondary processes such as migration and transformation within reservoirs.
The three indices based on light hydrocarbon compositions, proposed by Hiratsuka (1976) are very useful for classification of oils and condensates in the oil-field region of northeast Honshu, whereas the criteria by Thompson (1981) is not applicable to it.
The hydrocorrosion index is the ratio of 2, 3 dimethylpentane to n-heptane and shows a degree of hydrocorrosion including water washing and biodegradation. Hydrocorrosion appears to have no effect on two other indices in the oils and condensates when a hydrocorrosion index is less than 0.25.
Two other indices include the ratio of n-heptane to 1 trans 3 dimethylcyclopentane and the ratio of toluene to methylcyclohexane, termed as evolution index and environment index, respectively. From a crossplot of both parameters, the oils and condensates can be valuably classified and two types of condensate, which are due to phase redistribution and maturation, are differentiated. Comparision of evolution index with carbon isotopic composition and biomarker composition of the same oil or condensate suggests that evolution index is a reliable maturity index.