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Distribution of Natural Deformation in Organic-Rich Petroleum Systems: Reservoir Implications From Outcrop Analysis, Turonian Second White Specks Formation, Highwood River, Southwest Alberta


Thorough analysis of natural fractures and their intensity in unconventional tight reservoirs is a crucial step in their geomechanical characterization as natural fractures provide essential permeability for hydrocarbons in the subsurface as well as influences hydraulically induced fracture development. Detailed subsurface analysis is challenging since subsurface data is limited in its extent, but outcrops provide useful 3D subsurface analogs. Outcrops of the Second White Specks Formation along the Highwood River in southwest Alberta were divided into three major lithofacies that correspond to geomechanical units: 1) the Jumping Pound Sandstone; 2) interbedded finely laminated siltstones and mudstones; and 3) black organic-rich mudstone. Fracture parameters were recorded from each facies interval at five structurally distinct outcrops using the scanline and circular estimator methods. Results were used to examine the differences in natural fracture characteristics between sedimentary facies at several structural positions. Lithofacies 1 contains conjugate shear fractures that occur at intensities of 4.2–7.4 fractures per meter with average heights of 0.41–0.86 meters. Lithofacies 2 contains extensional fractures that occur at higher intensities of 24-30 fractures per meter with much shorter average heights of 0.04–0.12 meters, the latter being related to the finely interlaminated siltstone-mudstone fabric. Lithofacies 3 contains extensional fractures that occur at intensities of 5.2–8.5 fractures per meter with average heights of 0.44–0.76 meters. Elevated fluid pressures from oil generation from Type II kerogen within the two mudstone facies likely increased pore pressure to the point that promoted the formation of extensional fractures compared to the shear fractures that occur in the overlying Jumping Pound Sandstone. Results from this study suggest the anisotropy and heterogeneity of sedimentary facies characteristics have strong influences on natural deformation in the Second White Specks Formation. These results were combined with high-resolution UAV and GPS mapping to produce a detailed geomechanical characterization of the Second White Specks that highlights the style and intensity of natural deformation along an extensive section of the Highwood River. The observed relationships and distribution of natural deformation give valuable insight into the influence of sedimentary facies on natural deformation in unconventional type reservoir targets.