Deep basin gas: A new exploration paradigm in the Nappamerri Trough, Cooper Basin, South Australia

Abstract

Deep basin hydrocarbon accumulations have been widely recognised in North America and include the giant fields of Elmworth and Hoadley in the Western Canadian Basin. Deep basin accumulations are unconventional, being located downdip of water–saturated rocks, with no obvious impermeable barrier separating them. Gas accumulations in the Nappamerri Trough, Cooper Basin, exhibit several characteristics consistent with North American deep basin accumulations. Log evaluation suggests thick gas columns and tests have recovered only gas and no water. The resistivity of the entire rock section exceeds 20 Ωm over large intervals, and, as in known deep basin accumulations, the entire rock section may contain gas. Gas in the Nappamerri Trough is located within overpressured compartments which witness the hydraulic isolation necessary for gas saturation outside conventional closure. Furthermore, the Nappamerri Trough, like known deep basin accumulations, has extensive, coal–rich source rocks capable of generating enormous hydrocarbon volumes.The above evidence for a deep basin–type gas accumulation in the Nappamerri Trough is necessarily circumstantial, and the existence of a dee gas accumulation can only be proven unequivocally by drilling wells outside conventional closure. Exploration for deep basin–type accumulations should focus on depositional–structural–diagenetic sweet spots (DSDS), irrespective of conventional closure. This is of particular significance for a potential Nappamerri Trough deep basin accumulation because depositional models suggest that the best net/gross may be in structural lows, inherited from syndepositional lows, that host stacked channel sands within channel belt systems. Limiting exploration to conventionally–trapped gas may preclude intersection with such sweet spots.