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Is Siltstone Geomechanics on the Mixing Line Between Sandstone and Shale? Example From the Western Canada Sedimentary Basin Montney Formation


The Lower Triassic Montney Formation, one of the largest and most prolific reservoirs in Canada, is a unique tight oil and gas reservoir, composed dominantly of siltstone. Most of the production in the Montney is through horizontal hydraulically fractured wells, where completion and production designs are strongly influenced by the natural fracture system in the rock and its elastic properties. Fracture system development, whether natural or stimulated is a function of the organic and mineralogical composition of the rocks. Tight sandstones, supported by a framework of hard grains, would commonly respond well to hydraulic fracturing. Shales on the other hand, typically contain more soft clays and organic matter, and should respond in a more ductile way to stimulation. Montney siltstone, contains hard grains and moderate amounts of organic matter, also relatively high quantities of clay minerals (up to 30 wt%). Since siltstone reservoirs are understudies, the relationship between rock composition and rock properties is not well established. We evaluate the mechanical characteristics of the Montney Formation in three different locations across the basin; distal, central, and proximal. Dynamic Young's Modulus (YM), Poisson's Ratio (PR) and brittleness are derived from diapole sonic and density well-logs, and where possible are also calibrated to static data. In addition, the hardness of core samples was measured. Mechanical profiles are then compared with rock composition measured by QEMSCAN (inorganic) and LECO-TOC (organic). Several facies were identified suggesting several depositional conditions such as tidal flat, off shore, and deep water turbidite deposits. Dynamic YM is twice as high as the static YM, whereas the dynamic PR is similar to the static data. Brittleness calculated from well-logs averages at 400 - much higher than typical shales. Changes in the vertical profiles of YM and PR can be correlated to facies transition and compositional variations. Statistical analysis indicates that the control on hardness in some of the facies in the Montney lies in the organic matter, as is the case for shale. In other facies, soft minerals or organic matter seems to have little to no control on the geomechanical properties. This suggests that geomechanical behavior of siltstone is intermediate between endmember sandstone and shale. It important to recognize which facies has the appropriate elastic characteristic in order to insure successful completion.