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Mineralogical and Petrophysical Characterization of the Reservoir Facies of Doig Formation in British Columbia, Triassic of Western Canada Sedimentary Basin

Abstract

The Triassic section of the Western Canada Sedimentary Basin is the richest interval within the basin, in terms of volume of oil per volume of rock. The Lower and Middle Triassic, Doig Formation has been historically known for the limited production from its conventional reservoirs in British Columbia (BC) and Alberta (AB). More recently, the Doig has received attention for its unconventional potential as a gas and natural gas liquids play, with estimates of total gas in-place ranging from 40 to 200 Tcf. The Doig has been informally subdivided into a lower Phosphate Zone (DPZ), and an upper siltstone interval. While the source-rock reservoir potential of the Doig is undoubtedly large, so are the uncertainties related to the spatial and stratigraphic distribution of its reservoir facies and properties. This study is an investigation of the reservoir facies of the Doig Formation in BC and characterizes its variability in porosity, pore throat size distribution, mineralogy and organic content. A grid of 15 cored wells covering its entire extent in BC, was selected. Eight reservoir facies are recognized, based on lithofacies, mineralogy by X ray diffraction (XRD), total organic carbon by Rock-Eval pyrolysis, porosity and pore size distribution by helium pycnometry and mercury intrusion, and permeability by pressure pulse-decay. In contrast to many shale reservoirs, the Doig is characterized mineralogically by a low clay content, and higher quartz and carbonate, especially dolomite. The DPZ is more clay and organic rich than the upper Doig, showing overall lower porosity and smaller average pore throat radii. In spite of these generalizations, facies stacking patterns confer a high degree of internal heterogeneity within these two major zones. Coarser and clay poor facies are characterized by median pore throat sizes three to four times larger than the finer clay rich facies. Pore throat size has a high degree of correlation with matrix permeability, which spans four orders of magnitude, from 10-7 and 10-4 md. Additional heterogeneity is caused by superimposing diagenetic features, such as authigenic pyrite, apatite, calcite veins and clay-smeared fractures, and may not be defined at the core or well log scales.