Modelling of Hydrocarbon generation, migration and accumulation in the northern Mannar Basin offshore Sri Lanka

Abstract

The Gulf of Mannar, a Mesozoic rift basin evolved during Gondwana beak up, extends in 10-3000 m water depths between India and Sri Lanka. It has evolved due to crustal extension between the Indo-Lanka landmasses. The rifting period might have lasted for about 55 Ma from the latter part of the Aptian to the end of the Cretaceous. The Sri Lankan sector of the Gulf of Mannar is commonly referred to as the Mannar Basin. The northern part of the basin has present to probably Late Jurassic or older sediments. The stratigraphic thickness ranges from 4 km in shallow water depths to more than 6 km in deep water depths towards the south. The occurrence of an active petroleum system in the basin was confirmed for the first time by the discovery of natural gas from two exploration wells in 2011. However, the petroleum system of the Mannar Basin is poorly understood. In this study, an integrated two dimensional modelling of hydrocarbon generation, expulsion, migration and accumulation was performed along a stratigraphic section in the northern part of the Mannar Basin by using the SIGMA-2D basin modelling software. The exploration wells drilled in the basin to date have not penetrated the Early Cretaceous and older sediments, which could be the potential source rocks in the northern part of the Mannar Basin. As a result, the source character of the potential source rocks is uncertain. Therefore, scenario modelling was performed on the source character of the Early Cretaceous and older potential source rocks. The results of the basin and petroleum system modelling show if the Santonian to Cenomanian stratigraphic interval has a good hydrocarbon source potential they could have generated and expelled a smaller quantity of oil and natural gas. The productivity of this stratigraphic interval as a hydrocarbon source may increase towards the southern tip of the Mannar Basin. If the Early Cretaceous and older claystones have a good hydrocarbon source potential they would have expelled oil and natural gas mainly during the late Cretaceous. A couple of minor expulsion phases could be seen in the same stratigraphic sections in the Palaeogene and Neogene. Even if the Early Cretaceous and older potential source rocks have a relatively poor source potential (TOC ~0.5% and 100% Type III kerogen), they could still give rise to economically feasible natural gas deposits. Thermal cracking of oil generated in the Early Cretaceous and older source rocks increases towards the southern tip of the basin giving rise to a higher gas potential in the southern part of the Mannar Basin. The time lag between the oil generation and expulsion is around 5 Ma. Oil cracking starts at 120-140 oC. The potential source rocks in the Mannar Basin could be expected to have an average TOC content of at least 1% and 10-15% contribution from Type II kerogen. In this case, economically feasible oil and gas deposits could occur mainly in Cretaceous sandstone in the Mannar Basin