Fracture Pattern Evolution During Diagenesis: Fracture History of Cambrian Eriboll Group, Northwestern Scotland

Kira Diaz Tushman¹ and Stephen E. Laubach^2
1BP, Houston, TX
²Bureau of Economic Geology, University of Texas at Austin, Auston, TX

Cambrian Eriboll Sandstones in Northwestern Scotland contain two sets of regional fractures; Set A (striking N to NNE) and Set C (crosscut by A and striking EW). Based on orientation and intensity patterns, Set A is interpreted as having formed in a platformal setting before emplacement of the Moine Thrust Zone (MTZ). Set C likely formed in the late Devonian after thrust emplacement, during later regional extension. Set A and Set C differ in the 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 them as a consequence of greatly differing quartz growth rates on broken (fast) and idiomorphic (slow) quartz surfaces. 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 collected from synkinematic quartz, burial history and the 1.6 cm aperture size of the largest fractures allows the calculation of the time needed to fill fractures in Sets A and C with slow growing quartz. Observations of cement fill and sealing estimates 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 C have identical attributes to several moderately buried basinal fracture systems in tight gas basin, fracture observations from the ~150 km long Eriboll outcrop belt provide a rare view of regional fracture characteristics typical of the subsurface.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas