Abstract:

Development and continuity of shear clay smear is the main cause of lateral and vertical seal of faults. And SSF is referred  as the ratio of throw of fault to thickness of mudstone. Physical modeling and outcrop observation confirm the clay smear is not continuous combined with faults vertically leakage when SSF is greater than a constant value. The critical SSF value of the vertical fault seal is 5.0 of faults-related trap in Tanan depression，Hailar-Tamuchage basin. Most of faults are sealed vertically for oil and gas. However, formation period and post-action of faults must be taken into consideration. Shear clay smear couldn’t develop during basin inversion and uplift, and fault inversion causes failure of the clay smear and hydrocarbon leakage. The gouge in the fault zone comes not only from clay smear, but also phyllosilicate from sandstones, therefore SSF is not fully representative of clay content in fault zone, and SGR is the most effective parameter of the clay content in fault zone, especially for sand-sand juxtaposition seal. Considering the minimum SGR of fault plane as the critical value of faults lateral seal in fault-controlling oil reservoir, we regard the throw corresponding to the critical SGR as risk throw, then, faults with throw smaller than risk throw are at high risk of laterally sealing. Using these two methods, we can rapidly evaluate the risk of fault-traps during early exploration, and then reduce the risk of drilling.

Key words: fault seal, shear smear, continuity, risk throw, quick evaluation, hydrocarbon