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1 Terra Porosa, Independent Consultants, 11839 Cedar Pass Drive, Houston, Texas 77077; terraporosa{at}mindspring.com
2 Baker Atlas, Geoscience, 10205 Westheimer, Houston, Texas 77042; pauline.mollema{at}bakeratlas.com
Marco Antonellini has a degree in geological sciences from the University of Bologna (Italy), an M.S. degree in geology from Michigan State University, and a Ph.D. in quantitative structural geology from Stanford University. He worked for AGIP, Amoco, and as a consultant geologist for Shell. Currently he works as a consultant, and he is interested in faulted and fractured reservoir characterization and fault seal analysis.Pauline Mollema works in the Geomechanics Group of Baker Atlas Geoscience in Houston, where she is involved with the determination of mechanical properties from logs and borehole stability issues. She received a degree in structural geology and hydrology from the University of Utrecht, Netherlands and an M.Sc. degree in geomechanics from Stanford University. Her work at Stanford as well as a few consulting projects was focused on the distribution and geometry of natural fractures and faults and their influence on fluid flow properties.
We have used outcrops of dolomite exposed in the Triassic Sella Group of the Italian Central Dolomites as an analog for subsurface low-porosity faulted and fractured dolomite reservoirs. The Sella Group was mildly deformed at shallow burial depth (21,000 m) in a tectonic strike-slip regime during the Eocene-Miocene Alpine compression that caused the formation of joints and 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 tectonic regimes are intensely jointed when they are mildly deformed. In this type of tectonic regime, in fact, pervasive jointing over a wide area accommodates small strains and is strictly associated with the formation of strike-slip faults. Our observations allow us to recognize different kinds of fault architectures that correspond to different stages of fault development. In addition, theoretical models and microscopic observations were used to estimate the petrophysical properties of the faulted and jointed dolomite. Small-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 high-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-rock, contain high-porosity (10%) breccia and represent areas of preferred fluid flow. Large-offset faults with offsets more than 10 m contain a wide zone of low-porosity (<1%) breccia and form potential permeability barriers. The areas adjacent to the intermediate- and large-offset faults have high permeability (100-3000 md) because of high joint densities. An important implication of the way faults develop in dolomite is the consistent relationship between the orientation of joints and faults: the fault's strikes differ 15-35° from the strikes of the pervasive joint systems. Joint density also increases four to five times in the proximity of the faults. Such relationships can be used to predict the distribution and orientation of joints and faults in subsurface dolomite reservoirs.
The field observations of fractures and the petrophysical model presented provide a key to interpret borehole images in faulted and jointed dolomite and help us to choose the most appropriate tool for the geophysical detection of these structures. This approach will help improve the prediction of sweetspot locations and design the most effective well-bore trajectories.
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