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. 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 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 (version. 12.2) was used to simulate the burial and thermal history of the Pesalai-1, Palk Bay-1, Delft-1 and Pedro-1 wells using data from unpublished well reports held at Petroleum Resources Development Secretariat in Sri Lanka.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 25.5 o C/km, 25.0 o C/km, 21.3 o C/km and 18.5 o C/km, respectively. The present-day geothermal gradient of the basin increases from north to south. The highest bottom hole temperature (~90 o C), was recorded in 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/m 2 and 48 mW/m 2 , respectively. The pre-rift heat flow in the Sri Lankan sector of the Cauvery Basin varied between 46 -40 mW/m 2 .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 Pesalai-1 well is marginally matured and has just entered the oil window.

Modeling of the Petroleum System in the Cauvery Basin, Sri Lanka

“…A basement depth isopach map of the Sri Lankan sector of the Cauvery Basin, generated using seismic data, shows that the stratigraphic thickness at some places in the Pesalai-Palk-Bay depression reaches 3000 m (Cantwell et al, 1978). In line with this observation, the Pesalai-3 well researched a total depth of 2850 m. Kerogen maturation Profiles available in Premarathne (2020) show that the thermal maturity of sediments currently buried at 2800 m depth has reached the vitrinite reflectance value of around 0.7%Ro. Therefore, it is possible that Cretaceous potential source rocks at that depths had generated and expelled a significant quantity of hydrocarbons.…”

“…Erosion is in Ma. The unit of HI is mgHC/g TOC (data in the first 10 columns are from Premarathne, 2020).…”

“…Stratigraphy, tectonic setting, and hydrocarbon prospectivity of the Indian side of the Cauvery Basin have been addressed in detail by Sastri et al (1973;1981); Chandra et al (1991); Ramana et al (1995); Murthy et al (2008); Rana et al (2008); Lal et al (2009); Rao, et al (2010); Chaudhuri et al (2010); Chakraborty et al (2018); Bastia and Radhakrishna, (2012), etc. Those facts on the Sri Lankan counterpart have been addressed by Cantwell et al, (1978); Premarathne, (2008Premarathne, ( , 2015Premarathne, ( , 2017Premarathne, ( , 2020; Bandara et al, 2020). Most of this information comes from gravity, magnetic and seismic data, and well reports gathered as a result of hydrocarbon exploration in the basin.…”

“…Chandra et al, 1991;Chari et al, 1995;Baillie et al, 2003;Premarathne, 2008;Rana et al, 2008;Rao et al, 2010) consider the Mannar basin to be the southern extension of the Cauvery Basin. The petroleum geology of the Sri Lankan sector of the basin has been addressed by Cantwell et al (1978) and Premarathne (2015Premarathne ( , 2020. However, as to whether the Sri Lankan sector of the Cauvery Basin has an active petroleum system of its own or whether the petroleum system in the Indian side extends towards the Sri Lankan side still remains unknown due to the lack of well penetration and scarcity of data.…”

“…Model validation was carried out using the kerogen maturation profiles presented in Cantwell et al (1978) for the four wells. The method used to validate the model is described in detail in Premarathne (2020).…”