Structural model and fracture analyses for a major gas emplacement in Devonian sandstones of the Subandes
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The fold and thrust belt of the Subandean Ranges (central and southern Bolivia, and northern Argentina) contains both gas and condensate production and reserves in Devonian quartzose sandstones within deep structures. Reservoir sandstones present values of permeability close to 0.01 mD, implying that reservoir drainage must be controlled by a fracture system that enhances permeability. Hydrocarbon production in naturally fractured reservoirs is affected by fracture quality (degree of openness), spatial arrangement, size distribution (including aperture, height and length), fracture abundance, and arrangement with respect to stratigraphic and macro-structural features. Systematic study of fractures in the subsurface is complicated by the small probability that a well will intersect sufficient fractures for direct analysis of their attributes. Because of this fracture data obtained from logging and coring must be complemented with alternative methodologies. In my study I performed a workflow that includes geologic mapping of outcrop analogs of subsurface units, fracture characterization in outcrops and thin sections, the construction of kinematic structural model using algorithms for 3D analysis, petrographic description of composition and diagenesis, and statistical multivariate analyses in order to define how structural, lithologic and diagenetic features affect fracture distribution. From the construction of a structural model and the analyses of its properties, I generated semi-quantitative models of fracture attributes based on classic fold-related fracture concepts. This model was tested with direct fracture observations from core and outcrop, coupled with microstructural imaging using SEM-CL, to document fracture attributes. The models all show high curvature and strain values homogeneously distributed along the azimuth and close to the hinge of the anticline, which implies this domain should be most fractured. On the other hand, microfracture studies reveal that although highest strain values are found in the hinge, low strains also are found along the hinge even for samples with similar lithologies. The study of macro and microfractures in outcrop and core samples allowed me to clearly identify two opening mode fracture sets for the Devonian sandstones. These present an orthogonal arrangement and variable cross cutting relations. The dominant set (defined as Set I) has a WNW strike and is perpendicular to the structural trend of the Subandean Ranges; the subordinate set (defines as Set II) has a NNE strike, and is parallel to the previously mentioned structural trend. Set I has higher strain accumulation, log-normal spacing distribution, and is strongly controlled by the primary quartz content of the rocks. Set II also has a log-normal spacing distribution, and presents structural control.