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Geological controls on gas sorption capacities and regional gas shale potential of the Lower Cretaceous Buckinghorse Formation, Northeastern British Columbia, Canada Chalmers, Gareth Raymond


The geological controls on methane sorption capacity and the regional gas shale potential have been investigated for the Lower Cretaceous Buckinghorse Formation in Northeastern British Columbia. Geological controls investigated include: (1) total organic carbon (TOC) content; (2) kerogen types; (3) maturity; (4) mineralogy; and (5) moisture content. No published research has evaluated the geological controls on methane capacity of shales. The influence of petrographic composition on methane capacity of coal, particularly liptinite, is still not well understood but is relevant to kerogen types of gas shales. Methane sorption capacity of moisture-equilibrated samples were obtained by using a high pressure, volumetric, gas sorption apparatus. Rock-Eval analysis provided TOC content, kerogen types and maturity data and telovitrinite reflectance and petrographic analyses were performed on coals. Micropore volumes were measured by CO₂ at 0°C and mesopores by N₂ at -196°C on a surface area analyser. Mesopore to macropore volumes were obtained from a mercury porosimeter. Total porosity was determined by helium pycnometry and mercury immersion. Shale mineralogy was determined by X-ray diffraction. TOC content is the primary control on methane sorption capacities for these shales. High maturity samples that are dominated by Type III kerogen have greater methane capacity per unit TOC volume because they contain greater volume of micropores. Liptinite-rich coals store gas in solution while liptinite-poor coals store gas in micropores by physical sorption. Overall rank is the primary influence in methane capacity of coal. Illite is the only mineralogical control because both micro- and mesoporosity increase and illite content also increases with maturity. Moisture has a negative effect on methane capacity of a shale but there is no correlation between methane capacity and equilibrium moisture. In northeastern British Columbia, areas of high gas-in place (GIP) estimates are either high in TOC content (map section 94-P) or reservoir pressure (Liard Basin and 94-I). For the Garbutt-Moosebar-Wilrich formations, the Liard Basin has the greatest GIP due to greater strata thickness. The regional TOC distribution is controlled by elastic input with low TOC at the deformational front and a basinward increase to the northeast.

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