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The effect of shale composition on the gas sorption potential of organic-rich mudrocks in the Western Canadian sedimentary basin Ramos, Sharleen

Abstract

The gas sorption capacity of six organic-rich mudstones found throughout the Phanerozoic in the Western Canadian Sedimentary Basin has been measured through 66 high-pressure methane sorption isotherms at 30°C. The shales vary in total organic carbon and maturity (Tmax). Variations of gas sorption potential with shale composition, organic matter abundance, kerogen types, composition, geochemistry, and maturity have been investigated. For the combined data set, the amount of total organic carbon is strongly related to methane sorption capacity (r² = 0.78). Organic-rich shales show better correlation between total organic carbon and methane sorption capacity and higher sorption capacities than organic-lean shales. Low-TOC shales are influenced by mineralogical differences (more clay for sorption) and pore/moisture/maturity relationships. Gas sorption capacity increases with maturation because there is decreased competition of organic (eg. vitrinite) and rnineral matter (eg. clays) sites for sorption from moisture because of reduced moisture contents with depth. Increased microporosity of the organic matter is also associated with higher maturity shales as seen by the flattening of the isotherm curve. Methane sorption capacity and TOC abundance varies with organic matter type, nature, HI, kerogen isotopic composition and depositional environment. Difficulties exist when attempting to determine if methane sorption capacity varies with organic matter composition. However, it is noted that isolating samples of similar TOC and maturity, samples with more vitrinite (Type III) have more sorption capacity. The shales sampled in this study vary in inorganic and organic compositions, organic matter abundances, kerogen types, maturity, porosity, and permeability, which vary vertically and laterally throughout a shale sequence. Therefore, the spatial variability of these variables should be considered in exploration programs for natural gas from shale strata. Future work on gas shale targets includes detailed sequence stratigraphy, paleoenvironmental analysis, and structural analysis. Core, well log signatures (eg. gamma ray, density, sonic and resistivity), and sorption data can be correlated with total organic carbon and compositional variations (using mineralogy and kerogen isotopes) to identify varying composition and gas potential throughout a target. Kerogen can be isolated to measure differences in sorption of varying organic matter compositions. The porosity available to methane (free gas porosity) and pore structures also should be researched.

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