Abstract

The Barmer Basin of northwest India is a failed intracontinental rift that has become an established prolific hydrocarbon province in the last decade. Primary source rocks in the basin are the diatomites and interbedded lacustrine shales of the Paleocene Eocene Barmer Hill and Dharvi Dungar formations, although subordinate lacustrine shales in Lower Cretaceous sediments beneath the main rift basin are also high-quality source rocks.

Synrift deposition commenced in the Paleocene and peaked in the Eocene, with Barmer Hill Formation shales maturing as early as 55 Ma. Generation continued across the basin through to the early Miocene. Kinetic variations in the Barmer Hill Formation source rock enabled the northern prolific type I lacustrine facies algal kerogen to mature at lower temperatures and generate more oil than the leaner but more deeply buried, deep water southern facies type III land plant kerogen equivalent.

Migration modeling indicates that cross-fault juxtaposition of Paleocene and Lower Cretaceous reservoirs against downthrown mature source rocks is sufficient to charge all the known giant oilfields in the northern part of the basin. Accumulations in the shallower Dharvi Dungar and Thumbli formations in the central and southern parts of the basin require longer distance vertical and lateral migration from the mature Barmer Hill Formation through thick shale sequences, via fault linkages, fill and spill migration, and top-seal leakage.

Post-Miocene, Himalayan-related collision inverted and tilted the northern part of the Barmer Basin, terminating generation in this area, while the southern basin kitchens continued subsiding and expelling hydrocarbons. Extensive residual oil shows attest to widespread, large-scale re-migration of reservoired hydrocarbons in the uplifted northern basins as tilted structures spilled up-dip or were breached during inversion erosion. As a result, many of the present-day accumulations are significantly smaller than peak burial accumulations.

Simple mass-balance calculations indicate that more oil was generated in the basin than has so far been discovered. As much as 90% of oil generated in the basin was lost from the basin during uplift and tilting as breached structures were successively exposed. Such extensive loss of accumulated hydrocarbons is likely to be typical of inverted rift basins.