Insights on fault-valve behavior gained from the Ms5.1 isolated shallow earthquake in Moxi gas reservoir, Sichuan Basin, China

Abstract:

We investigated in detail the 2010 Ms5.1 (Mw4) Suining earthquake (a temporally and spatially isolated event) centered in Moxi gas field which is located in the tectonically stable central region of Sichuan Basin, China. The focal depth is estimated to be approximately 2.4 km and thus the earthquake is nucleated in Triassic marine sediment, coincident with the depths of the top boundary of Jialingjiang limestone formation (T₁) which is an over-pressured (R~2.2) gas reservoir. The strike/dip/rake of source fault is 209/33/102, showing a rupture of a blind reverse fault in agree with the geological structures and regional stress regime of this area. Through an integrated analysis of several issues, including the focal parameters, the local petroleum geology and radon anomaly, and numerical stress analysis for gas production and injection, we propose that the associated fault slip of this earthquake was most probably initiated and driven by gas injection from the underlying over-pressured reservoir into the shallower gas reservoir of normal pressure. With consideration of the fact that the injection-induced seismicity in nearby gas fields shows very low levels of aftershock productivity, no aftershocks following the Suining earthquake indicates that the gas injection from the deeper reservoir is somewhat episodic and a fault-valve behavior.