Compartmentalization of an aeolian sandstone by structural heterogeneities: Permo-Triassic Hopeman Sandstone, Moray Firth, Scotland

Many excellent aeolian sandstones suffer a decline of reservoir quality associated with structural compartmentalization by tectonic features. An onshore analogue, the Permo-Triassic Hopeman Sandstone, which crops out along the Southern side of the Moray Firth, Scotland, has been studied due to the difficulty in quantifying tectonic heterogeneities in the sub-surface. A hierarchy of structural flow barriers, with impermeable fault rock, has been observed. These vary from solitary deformation bands, via compound zones of bands to major slip surfaces/faults. Episodic faulting leads to the growth from bands to larger fault zones. Increased cementation of the sandstones is seen adjacent to these tight fault zones, and a zone of enhanced cementation, spatially governed by the main fault in the region, has been mapped. Conjugate extensional systems of deformation bands decrease in density away from major faults. Porosity reduction is related to proximity to faults, with porosity increasing abruptly away from the tectonic barrier; the deformation producing low-quality reservoir and flow barriers. Tectonic features have, intermittently, acted as both fluid conduits and barriers: enhanced cementation of sedimentary features adjacent to 'tight' faults shows migration of diagenetic brines along fractures. The relatively well sorted aeolian Hopeman Sandstones display better developed flow barriers than the poorly-sorted, mixed facies fluviatile Burghead Beds. Interaction of fluid-flow, diagenetic cementation and structural deformation produce flow barriers that compartmentalize the sandstones on all scales.