Petroleum Potential Offshore Sri Lanka
Even though no oil and gas accumulations have been discovered in the Mannar Basin or in the part of the Cauvery Basin that lies within the jurisdiction of Sri Lanka, discoveries on the Indian side of the Cauvery Basin give clues to the potential for viable petroleum systems offshore Sri Lanka. Exploration efforts in the Cauvery Basin within Indian jurisdiction have resulted in twenty six small and medium sized oil and gas fields with a total resource of approximately 700 million barrels of oil equivalent (Babu and Lakshmi 2004). The closest discovery to Sri Lanka is the PH-9-1 well some 80 km to the north of the island. The well has flowed 1488 barrels per day of 56 API oil from a Cretaceous sandstone on drill stem testing.
The most recent discovery was when in July 2007 Reliance Industries Limited announced the first deep water discovery in the region from an exploration block on the Indian side of the Cauvery Basin, 'Dhirubhai - 35'. The well was drilled in a water depth of 1,185 meters and encountered a clastic reservoir with a gross hydrocarbon column of around 150 m of Cretaceous section. The presence of oil and gas with condensate was confirmed by several tests. During drill stem tests (DST) the well has produced 31 million standard cubic feet of gas with 1,200 barrels of condensate per day from the main zone. Another zone tested below the main zone produced around 550 barrels of oil and 1 million standard cubic feet of gas per day. The well showed that not only the shallow water area but also the deep water area off south India and Sri Lanka could have viable petroleum systems.
Magoon and Dow (1994) defined a petroleum system as the essential elements and processes as well as all genetically related hydrocarbons that occur in petroleum shows, seeps, and accumulations whose provenance is a single pod of active source rock. United States Geological Survey in their 2000 assessment of the world resources introduced the term Total Petroleum System (TPS). The TPS consists of the essential elements (source rock, reservoir rock, seal rock, and overburden rock) and processes (generation-migration-accumulation and trap formation) as well as all genetically related petroleum that occurs in seeps, shows, and accumulations, both discovered and undiscovered, whose provenance is a pod or closely related pods of active source rock (USGS 2000). The TPS is a naturally occurring hydrocarbon-fluid system that can be mapped, and includes the essential elements and processes needed for oil and gas accumulations and presumes the existence of migration pathways, either now or in the past, connecting source rocks with reservoirs and traps
In assessing the total petroleum system in the Cauvery Basin and the Manner Basin one has to determine the possibility of source rocks, reservoir rocks, traps and generation and migration pathways for hydrocarbons to move from source to traps. Source rocks are rocks that contain organic carbon (kerogen) capable of producing hydrocarbons upon undergoing burial in a sedimentary basin. Kerogen is made up from altered remains of marine and lacustrine microorganisms, plants and animals. Upon burial, source rocks get heated up under the geothermal gradient in the region and produces hydrocarbons. The oil and gas production begins at 50° and ends at 150° C with peak oil production around 90° C. Reservoir rocks are rocks with gaps between different grains that make up the rock. These rocks have porosity and permeability so that oil and gas can pass through them or reside in them. When a reservoir rock containing petroleum meets an impermeable rock the oil and gas cannot move any further and get trapped. This way oil and gas can get accumulated in underground pockets where rocks have been faulted or folded creating traps where the reservoir rocks are capped by or juxtaposed against impermeable seal rocks.
Many source rock analyses have been conducted on cuttings from wells drilled in the region which suggest that good quality source rocks occur in the Lower Cretaceous and Jurassic rocks in the Cauvery and the Mannar basins. Especially the Lower Cretaceous rocks in the Pesalai wells seem to have high organic carbon values averaging 3.5 weight percent. However, the samples analyzed are mostly immature as the previous wells have been drilled on structural highs. This may be the reason that the previous wells in the area were dry. Modeling studies (Newsouth Global 2002) have indicated that the source rocks would be in the oil generating window in the deeper parts of the Cauvery and the Mannar basins.
Results from previous drilling indicate potential for good reservoir development in the area. The Pearl-1 well encountered an 850 m thick, good quality Late Cretaceous sandstone reservoir. Seismic data indicate possible development of both shallow marine and deep water submarine channel and submarine fan reservoirs in the area. Late Cretaceous and Tertiary limestone and shale intervals are present to provide adequate seal rocks.
Seismic data reveal a number of large anticlinal closures (traps) that can be interpreted as flower structures associated with northeast trending transfer faults. These faults can be traced to transform faults related to the mid ocean ridge in the Indian Ocean east of Madagascar. Minor faulting and fracturing associated with these large transfer faults would have provided ample pathways for oil and gas produced from the source rocks in the oil window to move into shallower reservoirs and traps. Large oil fields with billions of barrels of oil reserves have been found under similar structural setting offshore Brazil associated with transfer faults
As indicated above the essential elements of a petroleum system include source rock, reservoir rock, seal rock and processes such as hydrocarbon generation, migration-and accumulation and trap formation. Available data and discoveries in the region indicate that the Mannar Basin and the Cauvery Basin offshore Sri Lanka could have viable petroleum systems. This needs to be proved by drilling and until then we only know that there is only potential for oil and gas accumulations in the area. In quantifying this potential, the chance of an adequate source of oil and gas (mature good quality source rocks and migration pathways) could be estimated at 75%, the presence of adequate reservoir rocks in the basin could be estimated at 80% and the possibility of traps could be 100%. Therefore the chance of finding hydrocarbon accumulations may be estimated at (0.75 x O.8 x 1) 60%.