Petroleum Geochemistry of the Cauvery Basin, India

Singh, Harvir¹, Basant Giri Goswami¹, S. Pahari¹, I.V.S.V. Prasad¹, Ram Raj Singh¹ (1) KDM Institute of Petroleum Exploration, Oil & Natural Gas Corporation Ltd, Dehradun, India

Cauvery Basin, a pericratonic rift basin covering an area of 27850 km2 on land and about 35000 km2 offshore is located on the eastern continental margin of India comprising mainly of six sub-basins; Ariyalur-Pondicherry, Tranquebar-Tanjore, Nagapattinam, Palk-Bay, Ramnad and Gulf of Mannar. Hydrocarbons have been discovered in reservoirs ranging in age from Precambrian to Oligocene. A detailed work on source rock evaluation, thermal maturity modeling, stable carbon isotopic composition and biomarker studies on about 150 well's sediments and 100 Oils/ bitumens have been carried out in order to identify the active petroleum systems in the basin.

The post rift Cretaceous shales in Portonovo, Kudavasal, Sattapadi and Synrift Andimadam formations are the potential source rocks in different sub-basins. The Andimadam and Sattapadi shales contain almost similar type of mixed organic matter (Type III + II kerogen) with significant contribution from Type II kerogen (30-40%), deposited in marginal to shallow marine environment. Andimadam Formation containing mainly amorphous organic matter has better source rock characteristics than Sattapadi Formation in wells closer to depocentres. Both the source rocks have attained the peak oil generation stage in all the sub-basins. These source rocks have experience higher maturity in the Ramnad and Tanjore sub-basins as compared to Nagapattinam and Tranquebar sub-basins.

The study also suggests that oils and the source rock extracts from the Andimadam and the Sattapadi formations are genetically correlatable. The presence of 24-n-propyl cholestane and 4-methyl steranes in these oils and rock extracts indicate the contribution from marine organic matter. Each sub-basin has its own independent petroleum system. Major petroleum systems in the basin are: Andimadam-Andimadam, Andimadam/Sattapadi-Bhuvanagiri, Andimadam/Sattapadi-Nannilam, Andimadam/Sattapadi-Kamalapuram and Andimadam/Sattapadi-Niravi.