Fracture tectonics, fracture porosity evolution and structural diagenesis, Cambrian Eriboll sandstones, Northwestern Scotland
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Cambrian Eriboll Sandstones in Northwestern Scotland contain 5 sets of regional fractures; Sets A through E. Set A strikes north to NNE, Set B strikes NWWNW, Set C strikes NE, Set D strikes east, and Set E strikes north. Crosscutting relationships and orientations with respect to bedding places the relative ages as Set A being the oldest and Set E being the youngest, with sets B through D in between. Based on orientation and intensity patterns I interpret Set A to have formed in a platformal setting before emplacement of the Moine Thrust Zone (MTZ). Sets B and C were likely formed during and shortly after emplacement of the MTZ. Set D may have formed after thrust emplacement, possibly during Late Devonian or later regional extension. Set E was likely caused by late faulting in the Cenozoic. Sets A and D are comparable, in the fact that both are likely formed in a basinal setting, with similar cement patterns and structures. Set A and Set D differ in amount of preserved fracture porosity. Both sets contain characteristic localized cement deposits (synkinematic bridges) having crack seal and lateral growth textures. Such bridges are common in sedimentary basins. Lander et al. (2002) explained bridges as a consequence of greatly differing quartz growth rates on broken (fast) and idiomorphic (slow) quartz surfaces. Bridges form where incremental opening allows growth on some grains to span the fracture between opening steps. Local crossfracture bonding and subsequent rebreaking perpetuates localized fast growth. Set A is the first documented case where former voids between bridges are filled with massive quartz having no crack-seal texture. This pattern indicates that Set A is a basinal system fossilized by protracted burial. Growth rate experimental data (Lander et al., 2002), fluid-inclusion data I collected from synkinematic quartz, burial history, and the 1.6 cm aperture size of the largest fractures allows me to calculate the time needed to fill fractures with slow growing quartz in Sets A and D. Results agree with my fracture timing estimates based on orientation and kinematic compatibility arguments. My observations and sealing estimates also show that fractures can remain open for millions of years in deep basinal settings, where they are potential conduits for fluid flow. Because sets A and D have identical attributes to basinal fracture systems in economically important fractured hydrocarbon systems, such as tight gas sandstones, my fracture observations from the ~200 km long Eriboll Sandstone outcrop belt provide a rare view of regional fracture characteristics typical in the subsurface.