A natural analog for a fractured and faulted reservoir in dolomite: Triassic Sella Group, northern Italy

We have used outcrops of dolomite exposed in the Triassic Sella Group of th e Italian Central Dolomites as an analog for subsurface low-porosity faulte d and fractured dolomite reservoirs. The: Sella Group was mildly deformed a t shallow burial depth (21,000 m) in a tectonic strike-slip regime during t he Eocene-Miocene Alpine compression that caused the formation of joints an d strike-slip faults. Because the matrix porosity in the dolomites is low ( <5%) and poorly connected, joints and faults are essential to connect vugs and to provide permeability. Field observations of the Sella Group explain why many dolomite reservoirs and aquifers in strike-slip/compressive tecton ic regimes are intensely jointed when they are mildly deformed, In this typ e of tectonic regime, in fact, pervasive jointing over a wide area accommod ates small strains and is strictly associated with the formation of strike- slip faults. Our observations allow us to recognize different kinds of faul t architectures that correspond to different stages of fault development. I n addition, theoretical models and microscopic observations were used to es timate the petrophysical properties of the faulted and jointed dolomite, Sm all-offset faults (offsets up to 30 mm), characterized by en echelon arrays of joints and pockets or seams of breccia up to 10 mm wide, form. areas of high permeability (100-3000 md) due to the presence of many joints and hig h-porosity breccia. Faults with 1-10 m offsets, characterized by a breccia zone (1-2 m in width) and associated with high joint density in the wall-ro ck, contain high-porosity (10%) breccia and represent areas of preferred fl uid flow, Large-offset faults with offsets more than 10 m contain a wide zo ne of low-porosity (<1%) breccia and form potential permeability barriers, The areas adjacent to the intermediate- and large-offset faults have high p ermeability (100-3000 rnd) because of high joint densities. An important im plication of the way faults develop in dolomite is the consistent relations hip between the orientation of joints and faults: the fault's strikes diffe r 15-35 degrees from the strikes of the pervasive joint systems. Joint dens ity also increases four to five times in the proximity of the faults. Such relationships can be used to predict the distribution and orientation of jo ints and faults in subsurface dolomite reservoirs. The field observations of fractures and the petrophysical model presented p rovide a key to interpret borehole images in faulted and jointed dolomite a nd help us to choose the most appropriate tool for the geophysical detectio n of these structures. This approach will help improve the prediction of sw eetspot locations and design the most effective well-bore trajectories.