The role of faulting in Northern Carnarvon’s sub-basins in transferring very high overpressure

Abstract

Australia’s Northern Carnarvon Basin has experienced a varied geological history with periods of extension and thermal subsidence giving rise to separate sub-basins. Between the sub-basins the sequences of strata, their age, depth and rate of burial, and rock type vary greatly. Contrasts in rock type and burial rates are key variables that control generation of overpressure and, as such, create a wide range of pore pressure conditions from hydrostatic pressure to overpressure levels approaching values of vertical stress. Often these contrasting pressures can be situated relatively proximal to one another in both a horizontal and lateral sense. To improve pre-drill prediction capabilities for well planning in Northern Carnarvon, a study was completed in 2013 that included over 600 exploration wells and included reporting on areas at increased risk of high overpressure. One of the study findings was that reservoirs at sub-basin boundaries were often where the pressure was most variable and difficult to predict. The reason pressure was difficult to predict in these areas was due to high overpressure being transferred via faults at the margin of early Jurassic to early Cretaceous depocentres. In 2015, Hoskin et al. published the paper ‘Influence of faulting on reservoir overpressure distribution in the Northern Carnarvon Basin’ and this extended abstract looks at the Winchester-1 well, drilled after data were compiled for the original study, that validates the model presented in 2015, and also poses additional questions about the transfer of pressure.