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 80ÂșC.
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.