Formation and evolution of clay smear and hydrocarbon migration and sealing

Abstract：Clay smear is one of the most important mechanism for fault sealing. In order to establish a reasonable method for fault sealing assessment, we systematically studied the pattern of clay smear, formation and evolution rule. The results indicate that there are mainly three types of shale smear： shear, abrasion and injection. Mudstone, shale, coal-bed, gypsum bed, argillaceous halite, carbonate rock and siltstone can be applied to the original rock. Extensional relay, which is determined by the different strengths of the mudstones and the surrounding rocks, is the key geological factor controlling the formation of clay smear. The continuity of clay smear is controlled by overlapping zone geometry, the ratio between fault throw and thick- ness of mudstone and the effective normal stress. For a sub-seismic fault （a fault throw less than 15 m）, the maximum critical value of shale smear factor （SSF） can be up to 50, whereas for a large scale fault （a fault throw more than 15 m）, the critical value of SSF is usually 5-8. The larger the effective normal stress, the larger the critical values of SSF. A continuous shale smear results into a vertical sealing of the fault in the caprock. Fault lateral sealing depends on fault gouge content applied for several times. We could establish the relationship among the shale gouge ratio （SGR）, SSF, clay smear potential （CSP） and the pressure difference acrossthe fault plane, and consequently calculate the maximum gas column height that the fault can seal to achieve quantitative evaluation of fault sealing.