Aromatic biomarkers as indicators of source, depositional environment, maturity and secondary migration in the oils of Cambay Basin, India

Abstract

Aromatic biomarkers have been analysed for source, depositional environment, maturity and secondary migration in crude oils from different formations of Paleocene to Eocene age in the Ahmedabad–Mehsana Block of North Cambay Basin and the Broach Block of South Cambay Basin. North Cambay Basin oils are characterized by a high abundance of 1,2,5- and 1,2,7-trimethylnaphthalenes (TMN) and low to medium ratio of dibenzothiophene/phenanthrene and low ratio of pristane/phytane indicates that these oils have been derived from a source containing predominantly higher plants, and that the organic matter was deposited in restricted marine or lacustrine environments in anoxic to suboxic conditions. The relative abundance of 1,2,7-TMN suggests that it was derived from the structural degradation of oleanane type triterpenoids present in angiosperms. South Cambay Basin oils are less abundant in 1,2,5- and 1,2,7-TMNs and low ratio of dibenzothiophene/phenanthrene and high ratio of pristane/phytane indicating that the oils were derived from higher plant source organic matter deposited under marine or fluvial deltaic environment in oxic conditions. Maturity sensitive ratios based on phenanthrene, naphthalene, dibenzothiophene and their alkyl substituted isomers have been used to calculate the equivalent vitrinite reflectance. These parameters indicate a wide range of thermal maturity of the oils ranging from 0.60% to 0.93% VRc. Methylphenanthrene index and methyldibenzothiophene ratios indicate that the maturity of North and South Cambay Basin oils are different. The North Cambay Basin oils are less mature than South Cambay Basin oils. Pyrrolic nitrogen distribution indicates that the alkylated carbazoles and benzocarbazoles are more abundant in South Cambay Basin oils as compared to North Cambay Basin oils. These data indicate that North Cambay Basin oils are less migrated compared to South Cambay Basin oils. This systematic investigation of the aromatic biomarkers in oils of Cambay Basin has resulted in a new generation, migration, and entrapment model that has led to the development of new exploration leads.