ABSTRACT: Critical moments for the petroleum systems of the Cauvery basin

Thomas, N. J. and U. Samanta , Oil & Natural Gas Corporation Ltd, Dehradun, India

Hydrocarbon discoveries have been made in different sub-basins of the Cauvery basin of India in reservoirs ranging in age from the Precambrian to the Oligocene. Source rock evaluation of the drilled sequences indicated only poor to fair sequences in fine-grained Albian and younger sequences, which cannot account for the accumulations in the basin. The shales in the Andimadam Formation, which have been penetrated in the sub-basin flanks, have better source potential which is expected to improve towards basinal areas. This is substantiated by the source rock log of a well drilled near the depocentre of the Tranquebar sub-basin. Several hypothetical petroleum systems are proposed in the sub-basins assuming Andimadam source rocks and different primary reservoirs. Critical moments, the time of filling of the primary trap, have been computed for each of these petroleum systems. The time at which the source attained a maturity equivalent to 0.70 Vro has been defined as the critical time in the Cauvery basin based on MPI values of the accumulated oils. For the petroleum system in the northern Ariyalur-Pondicherry sub-basin, the critical moment is 50 MYBP. The critical moment becomes younger from north to south. It is 25 MYBP in Tranquebar, 15 MYBP in Nagapattinam and 3 MYBP in Ramnad sub-basin. Source rock in the western Tanjavur sub-basin has not attained the required maturity. The computed critical moments coupled with regional geology explain the hydrocarbon occurrence pattern in the basin.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia