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Abstract

The distribution pattern and occurrence of aromatic hydrocarbons as influenced by thermal maturity and the nature of Aptian to Campanian source rocks from the Orange Basin were investigated to gain more understanding of the petroleum generation and accumulation potential of the basin. Gas chromatography–mass spectrometry was used to determine the aromatic hydrocarbon content of rock extracts. The aromatic hydrocarbon composition showed abundant alkylated aromatic hydrocarbons. The most abundant of the naphthalene series is trimethylnaphthalene, and the most abundant of the phenanthrenes is methylphenanthrene. The distribution of the aromatic hydrocarbons across the seven wells investigated in this basin revealed that the naphthalenes are relatively more abundant than the phenanthrenes. Both the naphthalene and the phenanthrene distribution patterns are strongly controlled by thermal maturity of the organic matter. The type of organic matter and age of the rock also have a significant impact on the distribution and abundance of the aromatic hydrocarbons. The distribution of both naphthalenes and phenanthrenes indicates strong input of marine organic matter. The rock samples have enhanced concentrations of 1,2,5-trimethylnaphthalene compared with 1,2,7-trimethylnaphthlene, which is a typical characteristic of mature rocks. The values of methylnaphthalene ratio and dimethylnaphthalene ratio of the rock samples from the seven wells investigated revealed that Coniacian, Albian, and Aptian source rocks are more thermally mature than other source rocks in this basin that are not of these ages. Phenanthrene thermal maturity indices indicate that Turonian source rocks are the least thermally mature, whereas Santonian, Coniacian, Albian, and Aptian source rocks have a relatively higher thermal maturity. The effect of burial depth on thermal maturity of the source rocks appears to be more significant than the age of the rocks.