We investigate the permeability and flow effects of deformation bands in porous granular carbonate rocks in Malta and use results from flow simulations to discuss the practical implications of deformation bands in carbonate and siliciclastic reservoirs rocks in general. Image-and laboratory-based analyses of deformation bands show permeabilities that are 1 -2 orders of magnitude lower than the adjacent host rocks. Small-scale outcrop-based flow models (1 × 1 m) focus on the effect of deformation band on flow at the scale of individual bands. Two-phase flow simulations (water displacing oil) show that at the local scale a decrease in deformation band permeability led to increasing flow complexity, reduced and irregular waterfront propagation and reduction in sweep efficiency. A reduction in host rock permeability is associated with increased sensitivity to deformation bands. In low-permeable host rocks, a single magnitude-order reduction of deformation band permeability significantly delays flow, whereas in higher-permeable host rocks the effect is less pronounced. Hence, in some cases, deformation bands may represent a significant impediment to flow already when they are only 1 -2 orders of magnitude less permeable than host rock. Consequently, deformation bands may have greater practical implications than previously thought, particularly in reservoir rocks with moderate to low host rock permeability.
a b s t r a c tDeformation bands in porous sandstones have been extensively studied for four decades, whereas comparatively less is known about deformation bands in porous carbonate rocks, particularly in extensional settings. Here, we investigate porous grainstones of the Globigerina Limestone Formation in Malta, which contain several types of deformation bands in the hangingwall of the Maghlaq Fault: (i) bed-parallel pure compaction bands (PCB); (ii) pressure solution-dominated compactive shear bands (SCSB) and iii) cataclasis-dominated compactive shear bands (CCSB). Geometric and kinematic analyses show that the bands formed sequentially in the hangingwall of the evolving Maghlaq growth fault. PCBs formed first due to fault-controlled subsidence and vertical loading; a (semi-)tectonic control on PCB formation is thus documented for the first time in an extensional setting. Pressure solution (dominating SCSBs) and cataclasis (dominating CCSBs) appear to have operated separately, and not in concert. Our findings therefore suggest that, in some carbonate rocks, cataclasis within deformation bands may develop irrespective of whether pressure solution processes are involved. We suggest this may be related to stress state, and that whereas pressure solution is a significant facilitator of grain size reduction in contractional settings, grain size reduction within deformation bands in extensional settings is less dependent on pressure solution processes.
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