Burial and thermal history of the Cauvery Basin, Sri Lanka A Basin Modeling Approach

Abstract

Cauvery Basin is located between the southeastern onshore areas of India and the northwestern onshore areas of Sri Lanka. The basin has evolved due to crustal extension between India and Sri Lanka as a result of the breakup of East Gondwana. The rifting began in the Hauterivian and continued until the Cenomanian. The Indian side of the basin accommodates commercially viable oil and natural gas deposits. Hydrocarbon exploration in the Sri Lankan sector of the basin commenced as early as 1957. Six exploration wells were drilled in Sri Lanka during 1972 to 1981. None of these wells encountered any economically viable hydrocarbon deposits. Little further exploration took place in the Cauvery Basin in Sri Lanka to this date. However, the Sri Lankan government has indicated its plan to resume hydrocarbon exploration in the Cauvery Basin after the discovery of natural gas deposits in the Mannar Basin in 2011. The Mannar Basin is considered to be the southern extension of the Cauvery Basin. Only a handful of studies is available on the Sri Lanka sector of the basin in the public domain, though plenty is available on the Indian counterpart. The objective of this study is to understand the burial and thermal history of the Sri Lankan part of the Cauvery Basin based on basin modeling techniques. Petromod 1D software (v. 12.2) was used to simulate the burial and thermal history of the Pesalai-1, Palk Bay-1, Delft1 and Pedro-1 wells using data from unpublished well reports held at Petroleum Resources Development Secretariat in Sri Lanka. In addition, data available in the public domain were also used to derive some input data and boundary conditions. Results of the study show that southern part of the Cauvery Basin in Sri Lanka has experienced higher heat flows. The present-day geothermal gradient in Pesalai-1 and Palk Bay-1, Delft-1 and Pedro-1 wells are 25.5o C/km, 25.0o C/km, 21.3o C/km and 18.5o C/km, respectively. The present-day geothermal gradient of the basin increases from the north to the south. The highest bottom hole temperature (~90o C), was recorded in the Pesalai-1 well. The Pesalai area has experienced higher heat flows than the Palk Bay area. Current heat flows in Pesalai and Palk Bay areas are around 52 mW/m2 and 48 mW/m2 , respectively. The pre-rift heat flow in the Sri Lankan sector of the Cauvery Basin varied between 46 -40 mW/m2 . Area where the Delft-1 and Pedro-1 well are located have undergone upliftment during the Albian to the end of the Eocene (around 100-34 Ma). The Sri Lankan Cauvery Basin has had a relatively a higher sediment influx after the Eocene. The sediment thickness decreases towards the north of the basin. Sediments penetrated by the Palk Bay-1, Delft-1 and Pedro-1 wells are thermally immature. However, Albian sediment penetrated by the Pesalai-1 well is marginally matured and has just entered the oil window