Integrated Analysis on Geochemical and Petrophysical Variation in the Horn River Shale, Middle and Upper Devonian, Northeastern British Columbia, Canada

Abstract

Samples from the Middle and Upper Devonian Horn River shale, a major gas shale play in northeastern British Columbia, were assessed with a variety of techniques to understand controls on petrophysical properties, including porosity, permeability and pore system dimensions. Samples were examined by ICP-MS, Rock-Eval pyrolysis, helium porosimetry, pulse decay permeability analysis, SEM imaging of ion milled samples, nitrogen adsorption measurements and mercury injection analysis; and the results were compared to rock composition. The Evie, Otter Park and Muskwa Formations exhibit a range of organic and inorganic compositions, suggesting that reservoir quality should be stratigraphically constrained. The Evie Formation is carbonate-rich, while the Otter Park and Muskwa Formations are clay-rich and quartz-rich, respectively. The Evie has moderate organic matter concentration. The Otter Park is characterized by the lowest organic matter enrichment, while the Muskwa has the highest TOC content. Porosity ranges from 2.4 to 10.8% and is slightly elevated in the Evie Formation. A positive correlation is observed between porosity, organic matter abundance and quartz content, probably indicating that organic matter content is a primary factor controlling porosity development. A similar correlation between TOC and quartz indicates the latter is mainly biogenic in origin. Negative correlations are observed between porosity and the carbonate and clay content. SEM images suggest that in Horn river mudstones, several kinds of sites are provided for porosity development, including organic matter, pyrite framboids, clay plates, quartz rims and fractures. Results from nitrogen adsorption and mercury injection analysis suggest that pore size distribution includes micropores, mesopores and macropores, ranging from 1 nm to 1000 nm. Pores more than 10 nm dominate in terms of volume. Permeability ranges from 3.4 to 33 nanodarcy. Sea level fluctuation has direct impacts on geochemical composition and indirect influence on the petrophysical propreties. Evie, Otter Park and Muskwa formations are assigned to HST, TST, and LST, respectively. Shale successions deposited during rising sea level stage (Evie and Muskwa formation) have higher porosity than intervals deposited during falling sea level stage (Otter Park formation), possibly because rising sea level rising is more favorable for organic matter accumulation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015