Real-time hydrogen mud logging during the Wenchuan earthquake fault scientific drilling project (WFSD), holes 2 and 3 in SW China

摘要

We present hydrogen gas concentration data for drilling mud extracted during the Wenchuan Earthquake Fault Scientific Drilling (WFSD; holes 2 and 3), measured on-line during drilling in SW China. Hydrogen influx into the well at depth is largely dependent on lithology and fracture density. In hole WFSD-2, the average background concentration of hydrogen is lower in granitic rocks than in sedimentary formations. More than five major hydrogen influxes were detected while drilling through the sedimentary formations, and two hydrogen-rich zones were detected in the granite (1240-1243 m and 1383.5-1405 m depth). In hole WFSD-3, mud extracted from a tectonic breccia contains high hydrogen concentrations at depths of 600-1000 m depth, and relatively low concentrations below 1000 m depth. In both holes, we observe a lack of hydrogen in the center of the fault zone and high concentrations of hydrogen in fractured zones. Hydrogen concentration in both holes displays significant vertical heterogeneity, and is positively correlated with fracture density. Hydrogen was likely sourced from interaction between water and fresh silicate minerals surfaces that were exposed during faulting, from the mantle, and from faulting events. Fracture zones provide the main channels for the migration of hydrogen gas. The variations in background hydrogen concentrations relate to changes in porosity and permeability. The two hydrogen-rich zones (642.36-676.22 m, and 1383.5-1405 m) were likely caused by earthquake activity or far-field triggering during WFSD-2 drilling. The results of this study provide gas data that can be used to model fault activity. It is important to consider the relationship between hydrogen gas and the seismic cycle, and to use such data to identify seismic precursors.