Seismic restorations: a new technique for sequence boundary identification and correlations

A new technique of using seismic restoration to correlate and identify sequence boundaries is discussed and applied to a post-stack depth-migrated seismic section from offshore of Kushiro, Hokkaido Island, Japan. The seismic section displays shallow marine Cretaceous and younger units. Three major unconformities are recognizable, the two older unconformities are deformed by a reverse fault and folded. Detailed stratigraphic correlation is problematic and sequence boundaries are obscure in the deformed state. Different interpretations and correlations are tested through sequential restorations. One of the viable interpretations is used for tracing all seismic reflections. The interpreted seismic section with traced reflections was restored to different stages in the depositional history by a flexural­ slip algorithm, in which both bed-length and unit area were conserved. Sequence boundaries were picked from the restored-state sections rather than on the faulted and folded seismic section. The depositional history, sequence boundaries, and structural evolution are clearly revealed by this process. Eight depositional sequences and three major unconformities (A, B, and C) have been recognized and correlated in the seismic section from offshore Kushiro, Hokkaido, Japan. There are four sequences between the oldest unconformity (A) and the second unconformity (B). Onlap direction was from northwest to southeast. A reverse fault developed after the formation of the second unconformity. The onlap direction in the two piggyback basins associating the reverse faulting was from southeast to northwest. In the post-faulting three sequences, the onlap direction reversed again, was again from northwest to southeast. The sequence above the youngest unconformity (C) shows parallel horizontal reflections.