Mineralogical & petrophysical characterisation of gas shale : Colorado Group, Western Canada Sedimentary Basin

Abstract

The Colorado Group mudstones within the Western Canada Sedimentary Basin (WCSB), have been characterised for their shale gas potential using a range of mineralogical, geochemical (LECO and Rock-Eval), and petrophysical (lithology, porosity, pore size distributions and microfabric) techniques in order to gain an understanding of the shales’ characteristics as suitable source rocks and reservoirs for shale gas. Semi-quantitative mineralogy of the shales was computed using a chemometric technique developed in this study, combining Attenuated Total Reflection Fourier Transform Infra-red Spectroscopy (ATR-FTIR) and multivariate Partial Least Squares (PLS) analysis. The technique estimates quartz, total clay (illite-smectite, kaolinite, chlorite) and total carbonate (calcite, dolomite) to within 5% absolute of true value and demonstrates potential application for high sample throughput and thus high density sampling strategies for ultra-high resolution or multi-well studies. The mineralogy of the Colorado Group shales shows that clay minerals dominate the composition of each Formation, followed by quartz. Minor amounts of calcite, dolomite and feldspars are also present. Grain size data indicate that the Colorado Group lithology consists of substantial amount of clay-grade plus fine silt materials dominated by grains <10μm deposited as floccules. Pore size distributions are predominantly unimodal with an average mean pore radius (rmean) value of 50nm, although some samples exhibit bimodal pore size distributions reflecting mixture of mudstones and silt size materials. The pore size distributions of the Colorado Group Formations are generally influenced by the relative percentages of clay and silt, as well as by the level of compaction. Clay-rich sediments tend to be unimodal with tight pore size distributions while sediments with both clay-sized and silt materials tend to have broader and occasionally bimodal pore size distributions. The unimodal porosity nature of the Colorado Group formations, and the dominance of clay-sized and silt sediments may promotes the adhesion of gas molecules, which makes these sediments optimal for gas adsorption. Integration of the various data revealed that the Colorado Group depositional system is complex with widely changing seaway conditions showing no simple correlations between mineralogy, grain size and organic facies, or their spatial variation from palaeosource. The presence of sands and reworked shell fragments indicate a dominantly advective transport of sediments within the Colorado Group. Mud and clay cements are observed throughout the Upper Colorado Group, whereas calcareous cements are present only within the Second White Specks Formation, Medicine Hat Member and First White Specks Member. Laminations of calcareous coccoliths within the Second White Specks Formation typically display calcite overgrowths. Calcite overgrowths are also observed within mudstones in the basal part of the Medicine Hat Member. Such differences in appearance show the occurrence of temporal and lateral facies changes; the presence of facies changes is an important factor that can affect shale gas production patterns within a single, seemingly laterally-continuous lithological unit. High TOC (>2wt. %) and Type II kerogen appear to indicate that biogenic gas may be a dominant component of the total gas-in-place in the Colorado Group. The Middle Carlile member is likely the best shale gas target within the Upper Colorado Group. The Carlile Formation and Verger Member also appear to have the highest sorption capacity, based on a dominant clay sized fractions. However, mudstones within all formations and members display moderate to high clay minerals (fluid sensitivity), have poor fraccability and contain clay and mud cements. The presence of muddy siltstone within Middle Carlile member and its high potential for gas adsorption is likely to make the unit a viable target for possible horizontal drilling for gas shale.