Solid bitumen as a determinant of reservoir quality in an unconventional tight gas siltstone play

In this study of the Triassic Montney tight gas siltstone play in the Western Canadian Sedimentary Basin petrophysical measurements of drill-core samples (porosity, pore throat size, water saturation and grain size) are integrated with Rock-Eval TOC data, organic petrography observations and SEM imaging to show that reservoir quality in the gas window is strongly influenced by the pervasive presence of pore-occluding solid bitumen (and pyrobitumen at higher thermal maturity). The solid bitumen formed as a pore-filling liquid oil phase that was diagenetically and thermally degraded with further burial and increase in temperature. The proportion of solid bitumen filling the intergranular paleopore network can be expressed as bitumen saturation, and this attribute is found to be the dominant control on pore throat size and absolute permeability. The samples with low bitumen saturation and large pore throat radius (> 0.01 μm) have water saturations that generally increase as pore throat size diminishes, a relationship consistent with capillary theory for conventional water wet conditions. The samples with high bitumen saturation and small pore throat radius (< 0.01 μm), on the other hand, have abnormally low water saturation, a condition inconsistent with capillary theory for conventional water wet rocks. The coincidence of small pore throat size, low water saturation and high bitumen saturation is attributed to the presence of well-connected nanopores within the devolatilized, solid bitumen and the hydrophobic nature of the bitumen. Siltstones in economic portions of the Montney tight gas fairway have porosity mostly in the range of 3 to 7%. The results of this study show that reservoir quality in this economically key porosity range is influenced more strongly by bitumen saturation than by conventional determinants of porosity and permeability such as grain size, sorting, clay content and cementation. The concept of pore-occluding solid bitumen as an important negative control of reservoir quality elucidated here for Montney siltstones likely has application to the technical and economic evaluation of other tight gas plays particularly those in indirect basin-centered gas accumulations.