Sand Smear Along Normal and Reverse Faults in Mud-Dominated Marine Sequences: A Case Study from Boso Peninsula, Japan, and Implications for Fault Sealing Assessment in Deepwater Basins

Sorkhabi, Rasoul¹ (1) Energy and Geosciences Institute, Salt Lake City, UT

“Shale smear” in normal faults has been recognized as an effective fault sealing process and algorithms using this phenomenon are widely and sometimes successfully used for fault prospect analyses in sand-mud sequences. Several approaches have been suggested to explain the shale smear. One approach suggests that undeformed clastic sediments with a clay content of over 40 percent essentially deform by smear on fault planes. A second approach classifies shale smears into shale on sand abrasion smears, brittle shear smears, and gravitational injection smears. Fault-related shale injection is thought to be due to a mechanical contrast between ductile mudstone and relatively hard sandstone layers. This study shows that (1) rock smear is a more general phenomenon in sedimentary basins and is necessarily not restricted to shale, and thus, given an appropriate rheological setting, sand can also create fault smear; and that (2) rock smear is not limited to normal faults but may also develop in reverse fault zones. Evidence for these observations comes from the late Miocene Nishizaki Formation outcropped along the coast on Boso Peninsula, Japan, where both reverse and normal faults have deformed a mud-dominated sequence interbedded with sand-sized volcaniclastic sediments. In this sequence of marine slope sediments, the thick mudstone has predominantly deformed in a brittle manner while the thin, high fluid-pressure volcaniclastic layers have smeared along fault planes. This process of fault-related sand injection in reverse faults has critical relevance for fault sealing reassessment in mud-dominated deepwater basins.