Analysis of overpressure and its generating mechanisms in the northern Carnarvon Basin from drilling data
Iko Sagala A and Mark Tingay BA The University of Adelaide
B Australian School of Petroleum, The University of Adelaide
The APPEA Journal 52(1) 375-390 https://doi.org/10.1071/AJ11030
Published: 2012
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.
Iko Sagala is a geologist in the exploration department at Santos Indonesia. Iko completed a Bachelor of Engineering from the Institut Teknologi Bandung in Indonesia and, in 2010, completed a MSc in Petroleum Geoscience at the Australian School of Petroleum. His thesis was about overpressures in the Northern Carnarvon Basin. Iko has worked as a mine geologist with Freeport Indonesia, and as an associate log analyst with Halliburton Indonesia. iko.sagala@santos.com |
Mark Tingay is a senior lecturer at the Australian School of Petroleum, where he examines the petroleum geomechanics, overpressures and the tectonic evolution of sedimentary basins in Australia and southeast Asia. In particular, he specialises in studying the stresses in sedimentary basins and the mechanics of rock failure and fluid mobilisation in zones of very high pore pressure, including oil field blowouts and natural features, such as mud volcanoes. Mark graduated with a PhD in geophysics from the Australian School of Petroleum in 2003, worked at the World Stress Map Project in Germany from 2003–06, and has been a lecturer in petroleum geology and drilling engineering at the University of Adelaide and Curtin University since 2006. He has published more than 45 peer-reviewed papers, taught industry short courses on petroleum geomechanics to companies and for the American Association of Petroleum Geologists and Society of Petroleum Engineers, and routinely provides media interviews on geoscience and petroleum industry issues. mark.tingay@adelaide.edu.au |
