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AAPG Geoscience Technology Workshop

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Impact of Deformation Bands and Discrete Fractures on Carbonate Reservoirs

Abstract

Carbonate reservoirs in the Eastern Mediterranean are increasingly being recognized to be associate to both tight and porous carbonates, in which porosity, permeability pathways and/or impermeable fluid barriers are controlled by the interaction between fracture networks and host rock matrix. Contrasting deformation mechanisms considerably modify the porosity and permeability of tight and porous carbonate rocks in opposite ways influencing fluid flow behavior and reservoir characteristics. Deformation bands occurring within porous media usually create barriers to flow with significant reservoir anisotropy, whereas fractures within low-porosity, tight carbonates are known to contribute in enhancing permeability. Deformation bands (DBs) are strain-localization tabular zones of mm- to cm-thick, which accumulate low-displacement shear and/or volumetric deformation in porous rocks and sediments. They are well known in sandstones but in the last decade several studies demonstrated that DBs enucleates also in porous carbonate rocks. In contrast, fractures, joints and faults accommodate deformation in tight, low-porosity rocks with a broader range of displacement and configuration, defining discrete mechanical discontinuities made by distinct paired surfaces of separation. Their characterization from outcrops analogues allow to understand the nature and evolution of fracture networks providing also key insights about the impact of these deformation mechanisms on reservoir properties and performance. The features of these contrasting fracture networks within porous and tight carbonates and their spatial relationships have been analyzed on the Matera structural high in southern Italy. This horst feature is located within the foredeep-foreland system of the Southern Apennines, in which the extensionally faulted Cretaceous shallow-water tight limestones of the Apulian Platform are unconformably overlain by Plio-Pleistocene porous calcarenites (Calcarenite di Gravina Fm.). Deformation bands appear as light-coloured, mm- to cm-thick structures forming protruding ridges within the litho-bioclastic calcarenites. They are mostly compactive shear bands also associated with some pressure-solution; they are oriented at high angles to bedding and widely distributed across the study area with different patterns. Pure compaction bands also occur but are less diffuse. Compaction is evident as a significant reduction of pore space within the bands (up to one order of magnitude in reduction), relative to the porous host rock (porosity in the range of 20-30%) with also a transition zone with intermediate porosity. Grain reorganization and pressure solution mainly accommodate strain localization. Grain crushing and micro transgranular fractures are also suggesting some cataclasis. Their spatial arrangement of DBs spans from single bands with along-strike splays, lenses and/or relay structures, zone of bands, conjugate sets, swarms, to anastomosed patterns or more organized networks. On the other hand, outcrop- and large-scale extensional faults, sub-vertical throughgoing fracture corridors, fracture sets and sub-horizontal stylolites characterize the tight Cretaceous carbonates. Fracture sets are with dm-to-m spacing and are organized in roughly perpendicular sets with mutual cross-cutting relations. Some of them are also locally associated to paleokarst dissolution features. These different fracture networks have similar spatial distributions and are vertically stacked reflecting the stratigraphic overlay of the porous carbonates onto the tight limestones. The field characterization of these two contrasting fracture networks may help to understand their impact on reservoir performances similar vertically coupled porous-tight carbonates in the subsurface.