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Facies-Controlled Fracture Stratigraphy in Organic-Rich Unconventional Petroleum Systems: Implications From Outcrop and Core Analysis of the Turonian Second White Specks Formation, Southwest Alberta


Detailed fracture analysis in unconventional reservoirs is critical to their geomechanical characterization as natural fractures can provide essential permeability for hydrocarbons and influence hydraulically induced fractures. Comprehensive characterization of subsurface fracture networks is challenging since subsurface data is limited in extent and resolution. However, outcrops provide useful 3D subsurface analogs that can be used in conjunction with core-derived fracture data to elucidate the interaction between sedimentary facies, geomechanical stratigraphy, and natural fracture geometry. Core and equivalent outcrops of the Second White Specks Formation in southwest Alberta were subdivided according to lithofacies boundaries and geomechanical stratigraphy. Mechanical character was defined using rebound values derived from core and outcrop. Fracture parameters were recorded from each facies interval in core, and in outcrop at three distinct structural positions. Results were used to examine the differences in natural fracture geometry between sedimentary facies in core, and at various structural positions in outcropping analogs. In core, calcite-filled opening-mode fractures are well-developed throughout the succession; with the highest intensities occurring in zones dominated by thin, competent sandstone beds that restrict fracture height growth. In outcrop, fracture geometry varies markedly depending on lithofacies. Consistent variations in fracture intensity, height, and orientation between lithofacies are observed at all structural positions. Systematic variations in fracture orientation indicate distinct fracturing mechanisms between facies, with extension fractures occurring in organic-rich mudstone facies and conjugate shear fractures occurring in competent sandstone facies. Elevated fluid pressures from oil generation from Type II kerogen within the mudstone facies likely increased pore pressures to levels that promoted the formation of extensional fractures. The results suggest lithofacies and the resulting geomechanical stratigraphy strongly influence fracture geometry. Thus, accurate fracture prediction and modelling in unconventional reservoirs must consider both large- and small-scale influences on natural fracture geometry. The observed relationships and character of natural fractures give valuable insight into the influence of sedimentary facies on natural fracture geometry in unconventional reservoir targets.