The Origin of Overpressure in the Cooper Basin, Australia

The most commonly cited mechanisms of overpressure generation are burial-driven disequilibrium compaction and hydrocarbon generation. However, it is unlikely that these mechanisms generated the overpressure in the Cooper Basin. The last significant burial event in the Cooper Basin was the Cretaceous deposition of the Winton Formation (-90 Ma). Maximum temperature was attained in the Cretaceous, with cooling beginning prior to 75 Ma. Hence, overpressure related to rapid burial or paleo-maximum temperatures (e.g. hydrocarbon generation) must be at least 75 million years old. To hold overpressure in sedimentary basins over this time scale requires average pressure seal permeabilities at or below the lowest published shale permeability measurements. Moreover, the Cooper succession has cooled since the Late Cretaceous, and cooling is an underpressure generating mechanism. Therefore, it is unlikely that burial or temperature driven processes generated the overpressure witnessed in the Cooper Basin. The Cooper Basin has been subjected to an increase in horizontal compressive stress during the Tertiary, and thus an increase in mean stress. The presence of polygonal faulting the Late Cretaceous Eromanga sequence enabled the paleo-stress to be estimated. Layer bound polygonal faulting occurs in a normal fault regime with low differential stress (σv ˃ σH = σh). The contemporary stress regime in the Cooper Basin, measured using the density log, well tests and wellbore deformation modeling, is on the boundary between reverse and strike-slip (σH ˃ σh = σv). The largest measured overpressure in the Cooper Basin is approximately 14.5 MPa in excess of hydrostatic pressure at 3780 m in the Kirby 1 borehole. Mean stress has increased by 46 MPa at 3780 m in the Cooper Basin since the Late Cretaceous. The maximum increase in pressure that can be generated via disequilibrium compaction is equal to the increase in applied stress. Therefore, the increase in mean stress in the Cooper Basin is sufficient to explain the magnitude of the observed overpressure. Additionally, velocity / effective stress analyses on 9 wells indicate that the overpressure was generated by disequilibrium compaction related to increases in mean stress.