CARBON-ISOTOPE GEOCHEMISTRY OF PETROLEUM-ASSOCIATED GASES IN KRISHNA-GODAVARI BASIN, INDIA

This paper presents the results of a comprehensive study of the compos itional and stable carbon isotopic characteristics of 23 natural gases from 13 structural areas from different tectonic blocks of the Krishn a-Godavari Basin. The deltaC-13 methane, deltaC-13 ethane and deltaC-1 3 propane values of reservoired associated gases have been used in fur thering an understanding of origin and occurrence of these hydrocarbon s. Three aspects have been discussed: (1) genetic correlation of gases ; (2) evaluation of maturity level of the source of these gases by Jam es' method; and (3) delineation of major hydrocarbon generation centre s for reservoired petroleum. The isotopic data reveal a multiplicity o f hydrocarbon generation centres, in terms of inter-basinal and inter- structural (within a basin) variations in maturation and organic facie s. The study illustrates the use of gas isotopic data for predicting t he source of petroleum, and for developing a model of source-accumulat ion relationships across the basin. In the West Godavari Basin, oil ha s been generated at a maturity of 1.0% VR(o) which corresponds to sour ces in the vicinity of these accumulations. In the East Godavari Basin , hydrocarbons have been generated at maturities of 0.55-1.4% VR(o) wh ich correspond to sources adjacent to reservoirs. In the Godavari Offs hore Basin, oil has been generated at 0.78% VR(o) which corresponds to a deeper source and medium distance upward migration. The validity of the methodology of isotopic gas to gas, and gas to source, correlatio n for modelling the origin and migration of petroleum is confirmed by the hydrocarbon occurrence pattern in the Krishna-Godavari Basin.