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Outcrop-Scale Variations in Petrophysical Properties of Faulted Carbonates: Exploring the Relative Influence of Lithology, Fault Displacement and Juxtaposition

Dave Healy¹, Emma Michie¹, Tom Haines¹, Joyce Neilson¹, Ian Alsop¹, Nick Timms², and Moyra Wilson²
¹University of Aberdeen, Aberdeen, UK
²Curtin University, Perth, Australia

Carbonate reservoirs are highly heterogeneous in their internal fabric and structure, due in part to the variety of depositional & diagenetic processes that affect these rocks. This intrinsic heterogeneity presents major challenges when characterising the physical properties of a reservoir, either from seismic or well data. An additional factor in many regions is due to the effects of fractures. Variations in fracture patterns, or damage, around faults, provide an extra source of heterogeneity in fractured carbonate reservoirs. Fault damage zones are often the main conduits for fluid-flow, whereas fault cores tend to be sealing and act as barriers to across-fault flow. High strains in the fault core tend to obliterate depositional & diagenetic textures, which consequently have little influence on fluid flow. In contrast, lower strains in fault damage zones mean that the effects of depositional & diagenetic textures are more apparent. Porosity and permeability will be controlled by the combination of original textures and fracture patterns in the fault damage zone. This contribution explores the relative effects of original lithology, fault displacement and juxtaposition on the key petrophysical properties (porosity, permeability and seismic velocities) of faulted carbonate reservoirs based on outcrop analogues in Malta. Our eventual aim is to provide a quantitative understanding of: a) the ways in which extrinsic brittle damage around faults combines with intrinsic lithological variation of different carbonate facies, and b) how these combinations are expressed in the petrophysical attributes.

Outcrops in Malta were chosen for this study because of their limited degree of diagenesis and an overall low degree of tectonic strain. This improves the chances of being able to measure the effects of initial sedimentary facies on the pore systems, and to systematically measure and quantify the fault related deformation at accessible outcrops. The faulted rocks in our study are Oligo-Miocene carbonates that were deposited offshore Africa in a relatively low energy area of the palaeo-Mediterranean. There is only minor evidence of syn-sedimentary tectonic movements, with minor fault-controlled thickness changes and the development of sparse neptunian dykes (Meulenkamp and Sissingh, 2003). The faults measured in our study were formed in the Pliocene, with the Tyrrhenian Basin opening to the north and rifting in the Pantellaria Rift to the south, and related uplift of the northern rift flank (i.e. present-day Malta and Gozo; Hill and Hayward, 1988). The faulted sediments we have analysed in Malta comprise the Lower Coralline Limestone composed of grainstones and bioclastic packstones, and the Globerigina Limestone which is divided into the Lower (bioclastic wackestones/packstones), Middle (micritic) and Upper (micritic) Members. Each of these is separated by a hardground with several intervening firmgrounds. The limestones are capped by the Blue Clay, a Mid Miocene deep water clastic unit deposited in up to 150 m water (Pedley, 1975).

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012