The Rifted Basins of Eastern India

Abstract The rifted basins of eastern India contain three distinct tectono-stratigraphic units. Tectonostratigraphic Unit 1 (the oldest) occurs in grabens and half grabens that extend far into the Indian craton. The Upper Carboniferous to Lower Cretaceous strata of Unit 1 are primarily coarse- to fine-grained clastics and coals of fluvial and lacustrine origin. The Permian to Early Cretaceous age structures include major basement-involved normal faults that bound the grabens and minor normal faults and folds within the grabens. Unit 1 can be found in the Satpura, Pranhita-Godavari, Son-Mahanadi, Damodar, and West Bengal Basins. Tectonostratigraphic Unit 2 occurs in broad basins along the eastern Indian continental margin. The Upper Jurassic to Eocene strata of this unit are nonmarine to marine in origin and are predominantly clastics. The Late Jurassic to Eocene age structures include basement-involved normal faults and gentle folds. Most major faults strike NE-SW parallel to the continental margin. Unit 2 can be found in the Cauvery, Palar, Godavari-Krishna, Mahanadi-Brahmani, and West Bengal Basins. Tectonostratigraphic Unit 3 occurs in the same basins as Unit 2. The Eocene to Recene strata of Unit 3 are nonmarine to marine in origin and are both clastics and carbonates. The Eocene to Recent age structures include a) shale diapirs; b) listric, basement-detached normal faults and c) minor normal faults and gentle folds produced by reactivation of the basement-involved normal faults of Unit 2. The strata and structures of Unit 1 reflect the continental rifting that preceded the breakup of Gondwanaland. The strata and structures of Unit 2 reflect the breakup and early spreading of India from Australia and Antarctica. The intense Late Jurassic/Early Cretaceous deformation is related to breakup, whereas the Late Cretaceous deformation is related to changes in spreading direction and location in the eastern Indian Ocean. The strata and structures of Unit 3 reflect the maturity and stability of the eastern Indian continental margin and the adjacent eastern Indian Ocean by Eocene time. Each tectonostratigraphic unit has a unique hydrocarbon potential. Unit 1 has possible hydrocarbon traps produced by faulting and folding as well as nonmarine seals, sources and reservoirs. In particular, the abundant coals in Unit 1 may be gas sources. Both Units 2 and 3 have possible hydrocarbon traps produced by faulting and folding as well as nonmarine to marine seals, sources and reservoirs. The source rock possibilities for Unit 2 are particularly good for two reasons. First, Unit 2 deposition occurred synchronously with Cretaceous oceanic anoxic events marked by organic carbon-rich marine sedimentation. Second, Unit 2 deposition began during or immediately after the breakup of India from Antarctica and Australia. High geothermal gradients associated with breakup may have enhanced hydrocarbon maturation. Combinations of tectonostratigraphic units have better hydrocarbon potentials. A Unit 2/Unit 3 combination provides two levels of traps and ensures deep burial of the Unit 2 source rocks. This combination exists in the Cauvery Basin and probably in the Palar Basin. A Unit 1/Unit 2/Unit 3 combination has allof the advantages of the Unit 2/Unit 3 combination. In addition, structural and combined structural/stratigraphic traps may exist beneath the Unit 2 Cretaceous unconformities in the Unit 1 strata. This combination may exist in the Godavari-Krishna, Mahanadi-Brahmani, and West Bengal Basins.