Porosity characteristics of the Devonian Horn River shale, Canada: Insights from lithofacies classification and shale composition

• We investigate the relationship between porosity and geochemical composition.
• Porosity and shale composition varies among different lithofacies.
• Organic matter is the main factor controlling porosity development.
• Pores associated with organic matter are relatively smaller in diameter.
• Muskwa and Evie shale intervals have relatively higher porosity.

This study evaluates pore systems of the Horn River shale in Western Canada Sedimentary Basin from lithofacies classification of core samples to micro-scale pore structure investigation. Samples from the Middle and Upper Devonian Horn River shale sequence were examined by core description, porosity measurement, SEM, and TEM imaging of ion milled samples, and nitrogen adsorption analysis in order to develop a better understanding of the controls of organic and inorganic rock constituents on porosity development and pore microstructure.Five primary shale lithofacies were identified by hand-core and thin section analyses: massive mudstones, massive mudstones with pyrite streaks, laminated mudstones, bioturbated mudstones and carbonates. Porosity ranges from 0.62% to 12.04% and shows wide variation between different lithofacies. Massive mudstones and pyritic mudstones with high total organic carbon (TOC) content have the highest porosity, whereas bioturbated mudstones and carbonates with low TOC content have the lowest porosity. SEM and TEM images suggest that several kinds of sites for porosity development are present, including organic matter, pyrite framboids, clay platelets, quartz rims, carbonate grains and microfractures. A general positive relationship between TOC and porosity indicates that a large proportion of pores are developed in organic matter. Results from the nitrogen adsorption analysis suggest that samples with more organic matter tend to develop smaller pores. Thus while porosity development is a combined function of organic matter, mineral components, fabric and fractures, it is most affected by organic matter concentration.The Muskwa Formation and the Evie Member have more gas storage capacity as they primarily consist of massive mudstones and pyrite-rich mudstones, showing the best porosity. The Otter Park Member has lower porosity, which may relate to the fact that its lithofacies mainly consists of laminated mudstones and bioturbated mudstones.