Analysis of overpressure and its generating mechanisms in the northern Carnarvon Basin from drilling data

Abstract

The Northern Carnarvon Basin is one of Australia’s most prolific hydrocarbon basins. Overpressure has been encountered in numerous wells drilled in the Northern Carnarvon Basin. Knowledge of overpressure distribution is important for drilling and exploration strategies, and understanding the origin of overpressure is essential for applying reliable pore pressure prediction techniques. Unconventional pore pressure indicators—primarily drilling kicks and the presence of connection gas—were used to improve an updated distribution of overpressure and to investigate the origin of overpressure in the Northern Carnarvon Basin. This unconventional dataset was compiled from 45 wells. Overpressures are observed in 40 wells and tend to occur near, or on, the Rankin Platform, Alpha Arch, and Barrow Trend. The presence of overpressure in this area coincides with the region of maximum Cenozoic deposition. Overpressured strata in the Northern Carnarvon Basin occurs through a wide stratigraphic range, from Late Triassic to Paleocene sequences. Generally, post Paleocene sequences in the Northern Carnarvon Basin are considered to be normally pressured. Porosity-vertical effective stress analysis in shale lithologies was used to investigate the origin of overpressure in the Northern Carnarvon Basin. Porosity-vertical effective stress plots from 28 wells in the Northern Carnarvon Basin identified 20 wells where the overpressure appears to be generated by disequilibrium compaction, and eight wells where the overpressure appears to be generated by a component of fluid expansion. Disequilibrium compaction mechanisms were the predominant cause of overpressure in wells around the Rankin Platform and areas located further away from the coast. Conversely, fluid expansion mechanisms were the predominant cause of overpressure in wells around the Alpha Arch and Bambra Trend, and an area located closer to the coast. These results broadly confirm those obtained from earlier studies and highlight the usefulness of kick and connection gas data in overpressure analysis.