Geochemical characterization of oils and their source rocks in the Barmer Basin, Rajasthan, India

The Barmer Basin is a failed continental rift of late Cretaceous–Eocene age in Rajasthan, NW India, containing prolific hydrocarbon resources, with 33 discoveries having been made in the last decade. The basin is predominantly oil-prone, although gas discoveries have been made in the deeper parts of the basin. Oils in the Barmer Basin are highly waxy, a result of the lacustrine nature of the source rocks that dominate the sedimentary fill of the basin. Detailed interpretation of the molecular composition of the oils defines three main oil groups that can be related to differing sources. The oils are all distinctively lacustrine in origin, although differing in specific source-facies characteristics. All of the oils are isotopically light, mostly in the −29‰ to −33‰ range.

Most oils in the northern Barmer Basin (groups 1A and 1B) are interpreted to have been generated from the Late Paleocene Barmer Hill Formation, an excellent oil-prone source rock with predominantly Type I lacustrine algal and bacterial kerogen. Group 2 oils are subordinate in abundance, occurring only in the southern part of the basin, and are interpreted to be at least partly sourced from the overlying Early Eocene Dharvi Dungar Formation, which is characterized by mixed Type I and Type III kerogen, and attains oil maturity only in the southern basin. Group 3 oils are less common, and are of higher maturity than the Group 1 oils, but also appear to have been generated from the Barmer Hill Formation where it was buried more deeply in the central and southern parts of the basin. However, recognition of probable Mesozoic sediments in sub-basins beneath the Tertiary Barmer Basin introduces a further source-rock candidate for the Group 3 oils. A high maturity hydrocarbon charge that is recognized in the gasoline-range hydrocarbons in the Group 2 oils of the southern Barmer Basin may also be from a Mesozoic source rock, or from the Barmer Hill Formation that is much more deeply buried in this part of the basin than in the north, and represents a more mixed oil- and gas-prone source.