Downslope variability in the seal capacity of fine-grained sediments in the Kazusa forearc basin, Japan: Insights into the seal risk assessment for geological carbon storage

• High-spatial-resolution analysis of downslope variability in seal capacity. • Lowest and slight basinward increase in seal capacity in the outer shelf deposits. • Significant increase from the shelf margin to the base of the slope deposits. • Highest and nearly constant seal capacity in the basin floor deposits. • The findings can be a useful geological analog to aid seal risk assessment. The large uncertainty in seal risk assessment owing to limited subsurface datasets is a major concern for geological carbon storage. An understanding of the spatial variations in caprock properties helps seal risk assessment and supports the design of an adequate appraisal program. The objective of this study is to quantify the spatial variability in the seal capacity of fine-grained sediments from the Kazusa forearc basin, Japan, to reduce uncertainty in seal risk assessment for geological carbon storage. We examined the mudstone-dominated intervals in the Otadai, Kokumoto, and Kakinokidai formations of the group. Samples were collected from closely spaced outcrops arranged in a northeastward downslope direction and the sampling horizons were correlated with regional tephra key beds. This enabled a high-spatial-resolution analysis of the downslope variability in the seal capacity of the three stratigraphic horizons. Mercury injection capillary pressure analysis revealed a systematic downslope variation in the seal capacity across the basin. Seal capacity is the lowest and increases slightly basinward in the outer shelf samples over at least 10 km. Significant increases in seal capacity are observed from the shelf margin to the base of the slope samples within relatively short distances (i.e., 10–19 km). The seal capacity is the highest and nearly constant in the basin floor samples over a distance of at least 16 km. The findings of the present study provide insights into seal risk assessment for regional screening and site characterization in the basin, as well as in other sedimentary basins with similar geological settings.