Regional Subthrust Fracture Arrays in Outcrop: Guide to Attributes of Tight Gas Sandstones
Cambrian Eriboll Group Sandstones beneath the Moine Thrust Zone (MTZ), NW Scotland, contain arrays of opening-mode fractures that range in size from microfractures having lengths of microns to macrofractures having trace lengths of more than 100 m. Diagenetic, size scaling, and intensity patterns of these fractures match those found in horizontal cores of tight gas sandstones in the Rocky Mountain region and elsewhere. In this field example, fractures can be divided into at least three regionally extensive sets based on crosscutting relations. From oldest to youngest these sets strike N to NNE, WNW, and WNW to NE. Microfractures and associated macroscopic opening-mode fractures are sealed or locally lined with authigenic quartz that crosscutting relations and crack-seal texture suggests is in part contemporaneous with pore-filling quartz cement in the rock mass. Cumulative apertures along a line of observation record strains of as much as 4.9 percent for the oldest, N to NNE-striking fracture sets. Based on increased abundance near the fault zone, some WNW-striking fractures may be associated with WNW emplacement of the MTZ. Some WNW- to ENE-striking fractures, which are youngest based on crosscutting relations, are locally bridged by quartz containing crack-seal texture but otherwise retain porosity in fractures having apertures >0.1 mm. Residual porosity in fractures implies that after they formed fractures cooled to less than a quartz accumulation threshold of about 80ºC. These field observations show that porous opening-mode fracture arrays can persist for great lengths of time in the subsurface. Cement precipitation is the primary cause of fracture porosity destruction. Large fractures have great lateral persistence and large apertures, showing that they likely have significant impact on fluid producibility. Fluid-flow modeling shows that such fracture arrays could have effects comparable to those observed in the Rockies.