The Mannar Basin is a Late Jurassic – Neogene rift basin located in the Gulf of Mannar between India and Sri Lanka which developed during the break‐up of Gondwana. Water depths in the Gulf of Mannar are up to about 3000 m. The stratigraphy is about 4 km thick in the north of the Mannar Basin and more than 6 km thick in the south. The occurrence of an active petroleum system in the basin was confirmed in 2011 by two natural gas discoveries following the drilling of the Dorado and Barracuda wells, located in the Sri Lankan part of the Gulf. However potential hydrocarbon source rocks have not been recorded by any of the wells so far drilled, and the petroleum system is poorly known. In this study, basin modelling techniques and measured vitrinite reflectance data were used to reconstruct the thermal and burial history of the northern part of the Mannar Basin along a 2D profile. Bottom‐hole temperature measurements indicate that the present‐day geothermal gradient in the northern Mannar Basin is around 24.4 oC/km. Optimised present‐day heat flows in the northern part of the Mannar Basin are 30–40 mW/m2. The heat flow histories at the Pearl‐1 and Dorado‐North well locations were modelled using SIGMA‐2D software, assuming similar patterns of heat flow history. Maximum heat flows at the end of rifting (Maastrichtian) were estimated to be about 68–71 mW/m2. Maturity modelling places the Jurassic and/or Lower Cretaceous interval in the oil and gas generation windows, and source rocks of this age therefore probably generated the thermogenic gas found at the Dorado and Barracuda wells. If the source rocks are organic‐rich and oil‐ and gas‐prone, they may have generated economic volumes of hydrocarbons.
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.
The Gulf of Mannar and adjoining Cauvery basin to the north between India and Sri Lanka are associated with a failed rift, which initiated during the late Jurassic to early Cretaceous as a precursor to the breakup of East Gondwana. Despite the occurrence of igneous rocks that can be noted in seismic profiles, offshore, and deep seated occurrence of those have lead only to the limited understanding of igneous activity in the Mannar basin. Rock cuttings recovered in the Barracuda exploratory well in the Mannar basin shows approximately 700 m thick basalt rock sequence interlayered with sediments at a depth of 3500-4200 m below mean sea level. Here, we analyzed samples recovered from the Barracuda well for major and trace element composition.Major and trace element data suggest that the basalts were crystallized from two different degrees of partial melts from a similar source. Chondrite normalized rare earth element (REE) patterns indicate that the basalts are similar to continental flood basalt, though they show a distinct Ba positive anomaly. Importantly, supported with previously available K-Ar data, we decipher that these basalts are contemporaneous with the Deccan traps. Rifting between Seychelles and India which had occurred at 62 Ma approximately 3.5 Ma after the main Deccan eruption is synchronous with the Barracuda volcanism suggesting coeval rifting between Seychelles-India and India-Sri Lanka. Thus, our data suggest simultaneous rifting between Seychelles-India and India-Sri Lanka. Large plate reorganizations that took place during this time period in the Indian Ocean have likely caused consequent passive rifting in the Mannar basin.
A part of the southeastern onshore area of India and the northwestern onshore area of Sri Lanka, and the Palk Strait come under the Cauvery Basin. The Indian side of the Cauvery Basin has proven hydrocarbon reserves, while the Sri Lankan sector remains relatively under-explored. Only six exploration wells drilled in the Sri Lankan sector from 1972 to 1981 was dry. Since little further exploration has taken place from 1981 to this date, the existence of an active petroleum system in the Sri Lanka sector of the basin is not known. The objective of this study was to simulate the petroleum system in the Sri Lankan sector of the Cauvery Basin on Petromod 1D software (v.12.2) using data from unpublished reports and data in the public domain. The model was validated using the measured vitrinite reflectance data. Results of the study showed that Albian and Late Cretaceous shale in the Pesalai area have relatively higher hydrocarbon source potential and that hydrocarbon exploration in the Sri Lankan sector of the Cauvery Basin should mainly target Cretaceous and Eocene clastic and carbonate reservoirs associated with faults and anticlinal traps. Hydrocarbon potential in the Pesalai area is relatively higher than that in the Palk-Bay area.
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