Pore pressure and gas saturation distribution in the forearc basin of the Nankai subduction zone inferred from high-resolution Vp and Vs

Pore pressure and gas saturation distribution in sedimentary basins are difficult to predict and quantify because these two factors produce the same impact on reducing P-wave velocity (Vp). To tackle this issue, the modified Eaton's equation for Vp and S-wave velocity (Vs) ratio derived from automatic velocity analysis and a pre-stack inversion was applied to estimate pore pressure distribution, and Rock Physics Templates (RPTs) was performed to estimate hydrate and gas attributes (i.e., saturation). We applied this methodology in the Kumano forearc basin, southwest Japan. As a result, pore pressure distribution can be quantified from gas saturation effects. The pore pressure distribution in this basin is nearly hydrostatic. According to RPTs, hydrates and gas saturation are up to 50% and 20%, respectively. Besides, the clustering zones of overpressure, gas, hydrate, and brine can be differentiated from each other using estimated Vp and Vp/Vs. Based on these results, it can be concluded that the dynamic deformation associated with the Nankai plate subduction process mainly controls pore fluid connected pathways (i.e., dipping strata, fractures, and faults) in the shallow forearc basin sequence. This shallow sequence therefore cannot trap the excess pore pressure.

Automated summary

Predicting and quantifying pore pressure and gas saturation distribution in sedimentary basins is challenging due to their similar impact on reducing P-wave velocity. Through a study in the Kumano forearc basin, it was found that pore pressure distribution can be quantified from gas saturation effects, with hydrate and gas saturation levels reaching up to 50% and 20% respectively. The dynamic deformation associated with the Nankai plate subduction process plays a significant role in controlling pore fluid pathways in the shallow forearc basin sequence.