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Lower Cretaceous gas shales in northeastern British Columbia, Part I: geological controls on methane sorption capacity

Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada

R. Marc Bustin

Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada

The geological controls on methane sorption capacity for the Lower Cretaceous Buckinghorse Formation and equivalent strata in northeastern British Columbia, Canada have been investigated. The methane sorption capacity ranges between 0.04 to 1.89 cm³/g at 6 MPa (3.2 to 60.4 scf/ton at 870 PSIA) and the corresponding total organic carbon (TOC) content is between 0.5 and 17 wt%. Equilibrium moisture content is between 1.5 and 11 wt% and the organic maturity measured by T_max ranges between 416°C (immature) and 476°C (overmature).

TOC content is the most significant control on methane sorption capacity, however, other important factors include the kerogen type, maturity and clay content, in particular the abundance of illite. A positive correlation exists between the TOC content and methane capacity. Samples with higher surface area have higher methane sorption capacities. The micro- and mesoporous surface area increase with TOC and illite content. On a per unit TOC volume basis, type II/III and III kerogens have higher methane sorption capacity compared to types I and II because of their higher micropore volumes. Micropore volume on a per unit TOC volume basis increases with maturity for all kerogen types. Across the study area there is a decrease in TOC concentration with increasing maturity which, in part, is attributed to hydrocarbon generation but also coincidently reflects the different depositional environments. The amount of illite also increases with maturity through the process of illitization. No correlation exists between moisture content and methane capacity. Samples with high moisture content can have high methane capacities which indicate water and methane molecules occupy different sorption sites.