New understanding of overpressure responses and pore pressure prediction: Insights from the effect of clay mineral transformations on mudstone compaction

Chemical compaction dominated by clay mineral transformations, commonly develops in deeply buried mudstones, and prominently affects the petrophysical properties of mudstones and fluid flow. This paper takes the Neogene mudstones in the Ledong Slope of the Yinggehai Basin as research object, analyzes the relationship between chemical compaction and overpressure, establishes a new multisegment normal compaction trends (NCTs) and reevaluates the overpressure mechanisms. The results show obvious clay mineral transformation with increasing depth, and the top of the clay mineral transformations is approximately equal to the depth of the overpressure. The logging responses of overpressured mudstones are affected by clay mineral diagenesis, showing a 45% increase in the sonic transit time at 3500-5000 m and a 10% decrease in the density at the same interval, and the density response exhibits a relative lag. A method comprising an inversion algorithm calibrated by measured pressure data is proposed to establish multisegment NCTs. Furthermore, new loading-unloading curves comprising a mechanical loading curve and chemical loading curve are established to reevaluate the overpressure mechanisms. The loading-unloading curves show that the pressure gradient generated by mudstone compaction maybe less than 1.8. While the pressure data with pressure gradient larger than 1.8 fall on the unloading curve, indicating elevated sandstone pressures rather than in situ mudstone pressures. The multisegment NCTs were proven to provide a satisfactory overpressure estimate in the Ledong Slope as well as the relative contribution of different overpressure mechanisms. This paper provides new insight into mudstone compaction and overpressure responses, as well as a meaningful reference for the identification of multiple-overpressure generation mechanisms and the prediction of pore pressure in deeply buried formations.