Sealing faults and fluid-induced seismicity

Sealing faults are nearly impermeable barriers that can form boundaries between subsurface pore-pressure domains. In hydrocarbon systems, sealing faults commonly form part of a structural trap; they are thus important elements for future storage of CO2 and other gases in depleted reservoirs. The Triassic Montney Formation in western Canada hosts low-permeability gas reservoirs containing sealing faults that have previously been assumed to compartmentalize pressure domains. In this study, we show that the distribution of induced seismicity associated with hydraulic fracturing (HF) exhibits a statistically significant spatial correlation with zones of high lateral gradient in pore pressure. These high-gradient zones are interpreted as sealing fault systems. The largest induced seismicity sequence, including a 4.5 ML mainshock on 30 November 2018, occurred during HF treatments in two horizontal wells, between which there is an exceptionally large contrast (~10 MPa) in measured pore pressure. Numerical simulation of a simplified model of a hydraulic fracture intersecting a nearby vertical fault, followed by fault rupture using rate-and-state friction rheology, generates results that are in good agreement with observed strike-slip faulting near one of the HF wells. Our study demonstrates that sealing faults exhibit previously unrecognized behaviour that may be important for understanding induced seismicity risk. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'.

Seismic and aseismic weakening effects in transtension: field and microstructural observations on the mechanics and architecture of a large fault zone in SE Tibet

Fault-zone surveying and microstructural analyses focus on a transtensionally strained section of a >100 km-long, 5–15 km-wide, active, sinistral strike-slip W-E fault — the Damxung-Jiali Shear Zone (DJSZ). Deformation fabric superposition, palaeostress and neotectonic measurements reveal a progressive dominance of the transtension coaxial component. Vertical flattening and broadening of the DJSZ results in a 10–20 km-wide pull-apart depression with reciprocal emergence of flanking highlands that expose abandoned fault-zone domains that are hundreds of metres by tens of km in size. Polylithological fault rocks preserve a suite of frictional and viscous deformation to c. 350 °C. Amongst the fault strands, intensity and distribution of grain size reduction from comminution, widespread solution transfer, intragranular plasticity and recrystallization is heterogeneous. A regular pattern of pseudotachylite-coated surfaces short-cutting strong volcanic mylonite domains between weak creeping calcite domains indicates seismic with aseismic strain, and is interpreted as velocity-weakening evidence. Re-brecciation of solution-transfer accommodation-assisted S-C-cataclasite domains document switches from frictional to viscous strain and back again. The co-location of brittle and plastic constituent behaviour for varied fault lithologies plus our interpretations on deformation temperatures suggests major overlap for the frictional and viscous depth ranges. This indicates a very broad and strongly velocity- and fluid-dependent ‘brittle-ductile’ transition.