Fracture Pattern Heterogeneity in Km-Scale Reservoir-Analog Outcrops

Kira Diaz Tushman, Stephen E. Laubach, and Randall Marrett
The University of Texas at Austin, Austin, TX

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. Using km-scale outcrops, we document patterns of fracture heterogeneity on interwell scales. 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. 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. 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.