Re-evaluation of the genetic mechanism of overpressure: Western slope zone of the Xihu Depression, East China Sea Shelf Basin

Abstract

The accurate prediction of pore pressure relies on a comprehensive understanding of the genetic mechanism of overpressure and its impact on rock's physical properties. The western slope zone of the Xihu Depression in the East China Sea Shelf Basin serves as a valuable natural experimental site for re-evaluating the mechanism of overpressure formation due to significant differences in scholars' understanding of this phenomenon. This study uses a combination of multilogging analysis, the Bowers method, and density-velocity crossplotting to demonstrate that overpressure in the area is not attributed to disequilibrium compaction, combined with the geological background of overpressure generation. By considering the types of clay minerals, their diagenetic transformation, temperature, and current overpressure distribution, the main genetic mechanisms of overpressure are determined to be chemical compaction and fluid expansion. Fluid expansion is primarily associated with hydrocarbon generation, as evidenced by the correspondence between the overpressure top interface and chloroform asphalt “A,” hydrocarbon generation potential and TOC (total organic carbon) distribution. Furthermore, the initial scale generation of hydrocarbons is consistent with the current overpressure top interface. A mathematical model is developed to quantify the proportion of overpressure by considering the identified genetic mechanism and the vertical variation characteristics of mudrock porosity resulting from original components, mechanical compaction, and chemical compaction. The proposed “one point-two lines” model characterizes the contributions of fluid expansion and chemical compaction to overpressure formation during the intensive chemical compaction stage. The overpressure generated by hydrocarbon generation through fluid expansion likely induces formation fractures, leading to the episodic expulsion of oil and gas into reservoirs. This charging pattern of overpressure exhibits fast speed and low migration loss, which holds significant implications for forming large and medium-sized oil and gas fields with high enrichment.