India’s state-run ONGC Ltd. is preparing to launch exploratory drilling in waters of Palk Bay Shallow (PBS) concession (PBS-1-1 Extension) located in Park Strait between India and Sri Lanka.

The state-run upstream company has firmed up a plan to drill six exploratory wells in the shallow-water PBS-1-1 extension block located offshore in the southern part of the Cauvery Basin’s Ramnad sub-basin.

“Based on detailed G&G [geological and geophysical] studies, about six exploratory locations have been identified in PBS-1-1 extension area to test the prospectivity of the Nannilam, Bhuvanagiri and Andimadam formations. Play maps prepared for different formations have clearly brought out the leads,” ONGC said in a report.

The proposed six exploration wells will be drilled, with a target depth of 3,000 to 4,000 m (9,843 to 13,123 ft), at the identified locations in the water depth of up to 7 m (23 ft) in the shallow-water block with an estimated investment of about $38 million.

An ultra-shallow jackup rig is proposed to be deployed to drill the wells in PBS-1-1 Ext, which is spread across an area of about 86 sq km (33 sq miles). The wells will be drilled up to 3.45 km (2.1 miles) from the coast.

The exploration targets are the Upper and Lower Cretaceous formations.

“The hydrocarbon potential of Upper Cretaceous reservoirs ranging in age from Turonian to Campanian (Bhuvanagiri and Nannilam formations) is well established,” according to the report. “[With] the Lower Cretaceous being the source rocks, there is a fair chance of hydrocarbon accumulation in these sequences at favorable places.”

ONGC claims that drilling the proposed “six locations are expected to accrete 15 MMt (million tons) in this part of the basin. The provisional initial in-place reserves in the block are estimated to be about 30 MMt of oil and oil equivalent gas.”

The operator sees prospects in this area considering the adjacent PBS Field, with the similar geological and geophysical structures, has hydrocarbon reserves of commercial quantity. Gas is being produced commercially from Upper Cretaceous reservoirs in the PBS Field, which is estimated to have about 1.5 MMt of initial in-place reserves of oil and oil equivalent gas.

The operator found gas in the Nannilam, Bhuvanagiri and Lower Kamalapuram formations in 29 of 55 exploratory/development wells drilled in the PBS and four adjacent fields: Pariyapattinam, Perungulam, Ramanavalasai and Kanjirangudi. The Nannilam Formation is main hydrocarbon producer.

The PBS-1-1 extension concession and a large part of Ramnad sub-basin are considered prospective areas for hydrocarbons. The sub-basin—flanked by the Pattukottai-Manargudi ridge to the west and northwest, Mandapam delft ridge to the east and southeast and the Gulf of Mannar to the south—holds sediments that are more than 6,000 m (19,685 ft) thick. The synrift sedimentary column comprises mainly shale and sandstone in the Andimadam Formation. The sedimentation represented in the upper Cretaceous Bhuvanagiri, Kudavasal Shale, Nannilam and Portonovo shale is predominantly sand shale alternations with minor limestone development.

Two regional fault trends—the older NNE-SSW curvilinear fault set intersected by a younger EW fault set—played a vital role in the formation of the structures, subsequent charging and entrapment in Ramnad sub-basin. The established pools at the upper Cretaceous reservoir levels are all located along or at the intersection of these two trends, according to the report.

—Ravi Prasad