Flow rates in extremely low permeability shale gas reservoirs depend on the total area of permeable fractures that are hydraulically connected to the well and the matrix permeability of the shale formation. We study how co-seismic slip induced on pre-existing natural fractures and faults during hydraulic stimulation evolves the cumulative area of fractures contributing to flow. The study area is the Barnett gas field. We base our modeling on field data obtained during multistage hydraulic fracturing. We assume that only the propped hydraulic fractures and pre-existing fractures and faults that are stimulated in shear, are hydraulically conductive and contribute to flow. Thus, stimulation not only creates hydraulic fractures but also induces slip on a network of pre-existing fractures and faults, leading to a significant increase in the hydraulically active percolation zone responsible for production. We model slip on these pre-existing faults using two-dimensional plane-strain dynamic rupture models featuring a strongly rate-weakening friction and off-fault Drucker-Prager plasticity. Modeling predicts the formation of strain localization features (indicative of new fractures) at the tips of poorly oriented faults. The new fractures increase the percolation zone (and hence flow rate) by providing a larger cumulative area of fractures hydraulically connected to the well (larger area for gas to diffuse out of the formation) and by increasing the connectivity of the pre-existing fracture network (more reservoir penetration). This study, therefore, suggests three primary contributors to the increase in the percolation zone - the hydraulic fractures themselves, the pre-existing percolating fractures stimulated in shear, and the newly-formed and newly-connected percolating fractures. As the contribution from the hydraulic fractures is relatively small in the very low permeability Barnett (~100 nd) the magnitude of increase in the percolation zone and hence the amount of stimulation is directly correlated to the initial fracture population and is extremely important in controlling flow.