Real-time drilling mud gas monitoring records seismic damage zone from the 2008 Mw 7.9 Wenchuan earthquake

Abstract

Weinvestigate thedrilling mud gasfrom70m to1200 m borehole depth of theWenchuanearthquake FaultSci- entific Drilling Hole-1 (WFSD-1) in the context of rapid response fault zone drilling to large earthquakes. Com- plete depth profiles are accomplished for CH 4 , He, H 2 , CO 2 , N 2 , Ar, O 2 and N 2 /Ar. 222 Rn is also available for a limited depth. The volcanic hanging wall generates high Rn and He, whereas the CH 4 is low. The sedimentary footwall yields higher CH 4 and CO 2 , with CH 4 peaks up to 2.5 vol.% in the intensely fractured shale and siltstone from 640 m to 715 m. The drilling mud gases distribute asymmetrically across the co-seismic slip plane at 589.2 m, with the bellow 117 m true depth manifests as a prominent gas anomaly zone. Gas concentrations in this zone are much higher and vary more violently than the upper 200 m. The low P-wave velocity and resistivity imply that the strata are highly fractured, and gases intruded the borehole mainly by dissolving in water. We suggest that the gas distri- butionsare partially lithologyaffected,but mostlyfracture constrainedaccording to thelithologyvariations,sub- sidiary fault zone and fracture distribution retrieved from the core. Considering the low deformation rate and seismicity in this region before the earthquake, together with the ductile behavior of the clay-rich fault gouge under slow tectonic loading, and the decreasing gas anomalies away from the slipping plane, we propose that the fractures were mostly generated during the Wenchuan earthquake. It indicates that the Wenchuan earth- quake damagedthefootwall morethanthehangingwall,and the117 m depthmarksthewidthofthefracturing zone. The differential mechanic property between the hanging wall and the footwall, and the asymmetric stress during the rupturing may contribute to the asymmetric fracture distribution.