Identification of overpressures resulting from undercompaction and hydrocarbon generation in shale-dominated settings using well-log data

Based on the concepts of organic geochemistry and well-log characteristics, we have determined the differences between undercompaction overpressure (UCOP) and hydrocarbon-generating pressurization (HGP) from well logs in the Chang 7 shale, Ordos Basin. The results revealed that shales have better hydrocarbon generation potential than mudstones. We found that UCOP is characterized by significant changes of the acoustic transit time (AC), compensation neutron (CNL), density (DEN), and resistivity (RT) on the well logs. Although HGP causes obvious changes in AC and RT logs, minor deviations are generally revealed in CNL and DEN logs. Further studies revealed that although UCOP occurs primarily in thick shale layers of the sedimentary center, HGP is predominant in the thin shales of the sedimentary edge. The reason for this difference may be attributed to the ubiquity of the shales associated with HGP, with the variation in well logs caused by the HGP concealed by the abnormal variation of UCOP. These findings thus revealed the distribution and well-log features of abnormal overpressure for the different generating mechanisms, thus providing significant guidance for further exploration and development of the overpressured formations.