Fracturing and fluid flow in a sub-décollement sandstone; or, a leak in the basement

Abstract

Crack-seal texture within fracture cements in the Triassic El Alamar Formation, NE Mexico, shows that the fractures opened during precipitation of quartz cements; later, overlapping calcite cements further occluded pore space. Previous workers defined four systematic fracture sets, A (oldest) to D (youngest), with relative timing constrained by crosscutting relationships. Quartz fluid inclusion homogenization temperatures are higher within Set B (148 ± 20°C) than in Set C (105 ± 12°C). These data and previous burial history modelling are consistent with Set C forming during exhumation. Fluid inclusions in Set C quartz have higher salinity than those in Set B (22.9 v. 14.2 wt% NaCl equivalent, respectively), and Set C quartz cement is more enriched in 18 O (20.2 v. 18.7‰ VSMOW). Under most assumptions about the true temperature during fracture opening, the burial duration, the amount of cement precipitated and fluid-flow patterns, it appears that the fracture fluid became depleted in 18 O and enriched in 13 C. This isotopic evolution, combined with increasing salinity, suggests that throughout fracture opening there was a gravity-driven influx of fluid from upsection Jurassic evaporites, which form a regional décollement. Fracture opening amid downward fluid motion suggests that fracturing was driven by external stresses such as tectonic stretching or unloading, rather than increases in fluid pressure.

Exfoliation microcracks in building granite. Implications for anisotropy

“…In Alpedrete granite exfoliation microcracks were predominantly intracrystalline, straight and not always interconnected. Those features distinguished them from the open microcracks found at depths of hundreds of metres, attributable to tectonic forces and so meaningful in hydrocarbon and water prospecting (Hooker et al, 2015).…”

The Role of Chemistry in Fracture Pattern Development and Opportunities to Advance Interpretations of Geological Materials

“…(a) East Texas, Becker et al (). (b) Northern Piceance Basin, after Fall et al (); (c) Southern Piceance Basin; Fall et al (); (d) Green River Basin, Laubach et al (); (e) Mexico, Hooker et al (). Curves show burial history of various units (primarily Mesozoic sandstones); gray and black bars on curves mark durations of fracture growth from reconstructed fluid inclusion assemblage trapping temperatures sequenced by crack seal texture reconstruction.…”

“…P, porosity; Br, bridge (all images). Hooker et al (). (b) Quartz bridges and quartz rinds, fracture face, Cretaceous sandstone, Colorado.…”

“…Opening‐mode fractures in the Earth form under a state of effective tension that can be reached by either increasing pressure from within or reducing pressure from without. Crack seal textures are interpreted as evidence for cyclic changes in fluid pressure and/or far‐field stress (e.g., Fall et al, ; Fisher & Brantley, ; Hooker et al, ). With the additional constraint of timing information, Fall et al () showed that fractures in a Colorado basin were a response to increasing pressure from methane catagenesis, sourced from interbedded coals and marine shales.…”

“…With the additional constraint of timing information, Fall et al () showed that fractures in a Colorado basin were a response to increasing pressure from methane catagenesis, sourced from interbedded coals and marine shales. For similar sandstones in Mexico, Hooker et al () concluded that external, tectonic stresses drove fracture development, with fluids driven by gravity passively filling opening fractures. Using timing information from a quartz‐cemented sandstone in Wyoming, Laubach et al () found elevated pore fluid pressure, thermoelastic contraction, and local folding in different burial depth and structural settings.…”