Introduction

The U.S. Geological Survey (USGS) assessed the potential for undiscovered, technically recoverable conventional oil and gas resources in 10 basins of India and Sri Lanka as part of the ongoing assessment of conventional resources of the world (fig. 1; table 1). The potential for undiscovered conventional resources was assessed in the Upper Assam, Mahanadi, Krishna-Godavari, Cauvery, Mannar, Kerala-Konkan, Mumbai Offshore, Cambay-Barmer, Kutch, and Laxmi Basins. The formation of these basins and the composite total petroleum system (TPS) defined for this assessment are related to the tectonic evolution of India (Rao, 2001; Bastia and others, 2010; Desa and others, 2013; Kalra and others, 2014; Nair and Pandey, 2018; Pandey and others, 2018; Srivastava and others, 2023; Unnikrishnan and others, 2023). Gondwana began to fragment along Neoproterozoic lines of crustal weakness in the Permian to Triassic, forming the Karoo rifts, which contain coal beds and lacustrine mudstones as potential petroleum source rocks (Raza Khan and others, 2000). Rifting continued in the Late Triassic and Early Jurassic due to uplift related to the thermal effect of the Reunion mantle plume. In the Late Jurassic, Greater India (India, Madagascar, Seychelles, and Sri Lanka) rifted from northeast Africa, forming—or further rifting—the Kerala-Konkan, Mumbai Offshore, Cambay-Barmer, and Kutch Basins along the western passive margin. Madagascar rifted from India in the Late Cretaceous, and at the end of the Cretaceous, the Seychelles block rifted and separated from western India, representing the final phase of rifting and forming the Laxmi Basin. Each rift phase resulted in extended continental crust, in which grabens may contain synrift source rocks overlain by Cretaceous postrift marine source rocks. In the Early Cretaceous, Antarctica rifted and separated from eastern India, forming the Mahanadi, Krishna-Godavari, Cauvery, and Mannar Basins along the eastern passive margin (fig. 1). Around the time of the Cretaceous–Paleocene boundary, the Reunion plume caused an outpouring of volcanic rocks as much as 2,000 meters thick, forming the Deccan traps, which covered a large area of southwestern India, including the Kerala-Konkan and Mumbai Offshore Basins (Srivastava and others, 2023). Subvolcanic source and reservoir rocks may occur in these basins. Uplift related to the plume occurred along the western cratonic margin, causing India to tilt eastward, which reoriented the major fluvial drainages from west to east flowing (Sahu and others, 2013). Reorientation of the fluvial systems led to the progradation of major Paleogene clastic sequences along the eastern margin of India. Minor clastic progradation occurred at this time along the western passive margin due to small sediment supply. Carbonate deposition dominated the Mumbai-Saurashtra Basin during this time. In the late Eocene, India began to collide with the southern margin of Eurasia, and contractional deformation peaked in the Oligocene, resulting in the deposition of as much as 20 kilometers of sediment in the large south-directed deltaic systems of the Indus River and Ganges River drainages (Desa and others, 2013). Collision of the West Burma terrane with Eurasia from the Oligocene to Pliocene created the Naga Thrust Belt and foreland of the Upper Assam Basin (Kent and Dasgupta, 2004).

Total Petroleum System and Assessment Units

The Mesozoic–Cenozoic Composite TPS was defined for this assessment to encompass oil and gas generated from source rocks ranging in age from Permian to Miocene. Geochemical data are meager or not available for most of the postulated source-rock intervals within this composite TPS (Qin and others, 2017; Kumar, 2018). Limited offshore drilling has focused source-rock sampling to shallower proximal sediments, which may be organically lean compared to deeper marine mudstones. Deep synrift mudstones and distal offshore marine mudstones as young as Miocene are greatly underrepresented in the sampling, which is significant because organic matter in condensed sections of distal marine oil-prone mudstones may be major petroleum source rocks. Sampling for source rocks in the shallow proximal offshore has been mainly from large deltaic systems dominated by gas-prone rather than oil-prone organic matter (Kumar, 2018; Mishra and others, 2023).

Postulated or known source rocks in the India and Sri Lanka basins, in order of importance based on meager data, are (1) Paleogene marine mudstones, (2) Lower Cretaceous synrift and postrift mudstones, (3) Upper Cretaceous marine mudstones, (4) Permian coals, and (5) Upper Jurassic–Lower Cretaceous synrift lacustrine mudstones (Rao, 2001; Pahari and others, 2008; Qin and others, 2017; Rammurthy and others, 2017; Kumar, 2018; Kularathna and others, 2020; Mishra and others, 2023). Additional geochemical data would greatly assist in defining viable source rocks in all the basins of India. Paleogene oil-prone mudstones are generally considered the main source rocks, along with possible gas-prone terrestrial organic matter in most basins. Lower and Upper Cretaceous marine source rocks have been suggested for several basins, but the presence of viable Cretaceous source rocks is based on meager data. Upper Jurassic mudstones are possible source rocks in the Kutch Basin and Cambay-Barmer Basin (Rammurthy and others, 2017). Permian coal beds have been interpreted as one source of gas in the Krishna-Godavari Basin (Raza Khan and others, 2000; Rao, 2001). Gas hydrates with mixed thermogenic and biogenic gases in the Krishna-Godavari Basin and Mahanadi Basin suggest that, in addition to thermal gas from deeper sources, there is a Miocene–Pliocene source rock for biogenic gas (Datta and others, 2012).

Ten assessment units (AUs) were defined within the composite TPS: the Upper Assam Basin Reservoirs AU, Mahanadi Basin Reservoirs AU, Krishna-Godavari Basin Reservoirs AU, Cauvery Basin Reservoirs AU, Mannar Basin Reservoirs AU, Kerala-Konkan Basin Reservoirs AU, Mumbai Offshore Basin Reservoirs AU, Cambay-Barmer Basin Reservoirs AU, Kutch Basin Reservoirs AU, and Laxmi Basin Reservoirs AU. The assessment input data for the 10 conventional AUs are summarized in table 1 and in Schenk (2025).

Undiscovered Resources Summary

The USGS quantitatively assessed undiscovered conventional oil and gas resources in 10 AUs in India and Sri Lanka (table 2). The estimated mean resources are 1,010 million barrels of oil (MMBO), or 1.0 billion barrels of oil, with an F95–F5 range from 280 to 2,507 MMBO; 53,402 billion cubic feet of gas (BCFG), or 53.4 trillion cubic feet of gas, with an F95–F5 range from 19,122 to 107,365 BCFG; and 2,135 million barrels of natural gas liquids (MMBNGL), or 2.1 billion barrels, with an F95–F5 range from 760 to 4,304 MMBNGL.

For More Information

Assessment results are also available at the USGS Energy Resources Program website, https://www.usgs.gov/programs/energy-resources-program.

India and Sri Lanka Assessment Team

Christopher J. Schenk, Tracey J. Mercier, Phuong A. Le, Andrea D. Cicero, Ronald M. Drake II, Sarah E. Gelman, Jane S. Hearon, Benjamin G. Johnson, Jenny H. Lagesse, Heidi M. Leathers-Miller, and Kira K. Timm