Abstract

Direct methods for fracture detection using P-wave seismic data typically require an azimuthal analysis of the reflected wavefield. However, conventional data acquisition practices often lack sufficient azimuthal coverage for proper application of these techniques. In such cases, alternative methods become necessary. We investigated the use of subsurface properties estimated from seismic data under isotropic assumptions to delineate fracture systems in the Second White Speckled Shale of Alberta, Canada. We implemented two methodologies for fracture detection. (1) Using seismic attributes sensitive to the structure of the seismic image, structural changes such as folds were identified, from which the associated fracture systems can be inferred. (2) For fractures not directly correlated to such structural changes, analysis of the effective elastic properties of fractured media proved useful. In particular, failure criteria and effective-medium theories were used to investigate fracture phenomena and their corresponding seismic response. Using standard isotropic inversion techniques, estimates of reservoir elastic properties were derived. Subsequently, an interpretation of these results was conducted through consideration of anisotropic models. Specifically, low values of Poisson’s ratio were interpreted as more favorable conditions for fracturing and low values of Young’s modulus and vertical P-wave velocity were interpreted as direct indicators for the presence of fractures. The structural analysis identified a subtle fold where fracturing in its vicinity can be inferred. Furthermore, investigation into the elastic properties of fractured media revealed locations on the flanks of the fold that were likely to be fractured, providing an indication of the lateral extent of fracturing that was not possible from structural attributes alone. The combined interpretation of these results suggested the existence of a contractional fault-bend fold, where an area at the crest of the fold did not appear to contain fractures, corresponding to the undeformed zone as predicted by structural models of fault-bend folding.

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