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
