Impact of complex tectonics on the development of geo-pressured zones: A case study from petroliferous Sub-Himalayan Basin, Pakistan

In petroliferous basins, integration of regional and local tectonics along with well data is used to understand the generation of geo-pressured variations in different lithological units. In the last 40 years, abnormal geo-pressured zones is the main challenge during hydrocarbons drilling in the tectonically complex Kohat Sub-basin, Sub Himalayas. The trapped fluid in post-Eocene strata generated the over-pressure zones and triggered drilling problems. The present study is an integration of data, by use of regional maps, seismic lines, and formation pressure using Eaton’s equation to understand the possible causes for the development of geo-pressured zones. Where, regional maps and seismic lines were employed for deciphering the regional and local tectonics, respectively. Porosity function and Normal Compaction Trend (NCT) curve were utilized for the identification of abnormal pore pressure zones (over-pressure and under-pressure zones). Three major stress regimes were interpreted. The results show that the mechanism of over-pressure zones developed as a combination of effective horizontal stresses, lithological variations and rapid sedimentation. The Normal Compaction Trend (NCT) line and porosity function indicated that the 1st and 2nd stress regime consists of major abnormal geo-pressured zones. Therefore, it is concluded that these stress regimes experienced more overburden pressure and fracture pressure. Consequently, it is suggested that the results of geo-pressure estimation from this work could be used for the appropriate mud weight during drilling in analogous tectonic settings.