Distribution of Overpressure and its Generating Mechanisms in the Northern Carnarvon Basin
A J Iko Sagala
Master of Science 2010
University of Adelaide
The Northern Carnarvon Basin is one of the Australia’s most prolific hydrocarbon basins. Overpressure has been encountered in numerous wells drilled in the Northern Carnarvon Basin and knowledge of overpressure distribution is important for drilling and exploration strategies. Moreover, understanding the origin of overpressure is essential for applying reliable pore pressure prediction techniques.
Unconventional pore pressure indicators, kicks and connection gases, were used to improve an updated distribution of overpressure and to investigate the origin of overpressure throughout the Northern Carnarvon Basin. This unconventional dataset was compiled from 45 wells throughout the Northern Carnarvon Basin. Overpressures are observed in 40 wells and tend to occur near, or on, the Rankin Platform, Alpha Arch, and Barrow Trend. The occurrence of overpressure in this area coincides with regions of maximum deposition of Tertiary. Overpressured strata in the Northern Carnarvon Basin occur over a wide stratigraphic range, from Late Triassic to Paleocene sequences, while post Paleocene sequences are normally pressured.
Porosity-vertical effective stress analysis in shale lithologies was used in this study to investigate the origin of overpressure in the Northern Carnarvon Basin. Porosity-vertical effective stress plots from 28 wells in the Northern Carnarvon Basin have identified 20 wells where the overpressure appears to be generated by disequilibrium compaction and 8 wells where the overpressure appears to be generated by a component of fluid expansion. Disequilibrium compaction mechanisms were predominantly identified in wells around the Rankin Platform and located further from the coast (outboard). Conversely, fluid expansion mechanisms were predominantly identified in wells around Alpha Arch and Bambra Trend and located closer to the coast (inboard).