Clay Types in Mudrocks of the Upper Devonian Duvernay Formation

Clay types in mudrocks of the Upper Devonian Duvernay Formation Nicholas B. Harris*1, Tian Dong*2,*1 and David L. Bish*3 *1University of Alberta; *2 China University of Geosciences; *3University of Indiana Abstract The conversion of smectite-rich to illite-rich mixed-layer clay is the crux of a suite of mineral reactions in mudrocks, associated with the dissolution of potassium feldspar, albitization of detrital feldspar and the formation of authigenic albite, and the formation of late-stage iron- and magnesium-rich carbonates, clays (chlorite) and quartz cement (e.g. Boles and Franks, 1979, Land et al., 1997). The smectite-to-illite reaction is also critical to the diagenesis and reservoir quality of associated sandstones, providing sources for quartz and late carbonate cements. Control of the smectite-to-illite reaction in mudrocks has been largely attributed to elevated temperatures. Early studies by Hower et al. (1976) of mudrocks in the Tertiary Gulf of Mexico section identified a fairly narrow window between 2800 and 3300 meters in which the reaction takes place. Subsequent studies (for example, Freed and Peacor, 1989) expanded the range of temperatures over which this reaction is observed; the wider range is attributed to variation in the initial rock composition and petrophysical properties of the sediment. We present mineralogical data from the Duvernay Formation in western Canada, an Upper Devonian shale in Alberta that is the focus of shale oil and gas exploration and development. These data, consisting of high-quality x-ray diffractograms of both whole rock and clay fractions, indicate that even at shallow depths and low thermal maturities, the mixed-layer clay that is present is illite-rich and pure illites are the predominant clay in deeper sections. This suggests that either the smectite-rich clay fraction may have never entered the basin, or alternatively, it was converted to illite-rich compositions prior to burial or at shallow burial depth, through processes other than the thermally-driven reaction described for the Gulf of Mexico. The diagenetic minerals commonly associated with the mixed-layer clay conversion (late carbonate cements, quartz cements, chlorite) must have formed in a very different diagenetic setting or may have had an origin unrelated to the mixed-layer clay reaction. Background The Duvernay Formation is a mudstone formation of Frasnian age in the Western Canada Sedimentary Basin (WCSB). It is organic-rich, with present-day total organic carbon (TOC) values commonly in the range of 2 to 4% (Harris et al., 2018), and has sourced oil in conventional pools (hosted largely in coeval reefs of the Leduc Formation); it is also a major unconventional reservoir play for oil, condensate and gas, depending on thermal maturity. A range of depositional facies have been identified on the basis of composition and sedimentary structures (Knapp et al., 2017) associated with distance from Leduc reefs and changes in relative sea level (Knapp et al., 2019). Variations in geomechanical and petrophysical properties are related to mineralogy and thermal maturity (Dong et al., 2018, 2019). The depositional setting for the Duvernay Formation has been conventionally divided into the East Shale Basin and West Shale Basin, separated by a line of reefs termed the Rimbey-Meadowbrook Trend (Figure 1). In a general sense, Duvernay sediments in the East Shale Basin are relatively enriched in carbonate minerals, whereas the Duvernay in the West Shale Basin is enriched in siliciclastic minerals, particularly quartz derived from recrystallization of biogenic silica (Harris et al., 2018). Sediments are identified as autochthonous biogenic material (organic matter and biogenic silica that is now converted to quartz cement), allochthonous carbonate (transported from adjacent Leduc reefs), and allochthonous siliciclastics (clays, silt-size quartz and feldspar). Sources of siliciclastic sediment are incompletely constrained. A source from the north has been postulated (Knapp et al., 2017; Harris et al., 2018), based on the presence of contourite deposits near the base of the Grosmont Platform and south- directed clinoforms in the overlying Ireton Formation. Clastic sediment may also have been derived from the Canadian Shield to the east of the basin, entering the WCSB through gaps in basin-rimming reefs. Concentrations of biogenic sediment are related to relative sea level through bioproductivity, whereas carbonate influx is related to relative sea level through the development or loss of accommodation space on adjacent carbonate platforms (Harris et al., 2018). The WCSB dips southwestward to the Rocky Mountains; as a result, depth to the Duvernay increases from approximately 1500 meters at its eastern limit to more than 5000 meters near the deformation front. Thermal maturities show a corresponding increase, from immature in the east and northeast to dry gas window in the southwest (Figure 1). Figure 1. Location map for well discussed in this presentation, with thermal maturities calculated from Tmax data. Roequiv is calculated from an equation in Jarvie (2015). Figure modified from Harris et al. (2018). Aims and Objectives We examine mudrocks in the Duvernay Formation to characterize the origin of clay minerals and to test whether the critical influence on clay mineralogy was provenance or temperature and whether other controls might have been important. We consider whether the illitic clay mineral assemblage was primarily controlled by variations in sediment provenance or the transformation of mixed layer illite smectite to illite, quartz and albite during burial and if the latter, the conditions under which that transformation took place. Materials and methods Samples were obtained from five long drill cores across the WCSB (Figure 1, Table 1 – Appendix) that represent a range of maturities from immature (Esso Redwater) to dry gas window (Encana Cecilia). The cores were previously logged for facies analysis (Knapp et al., 2017) and characterized for geomechanical and petrophysical properties (Dong et al., 2018, 2019). Samples described here were part of a larger set of geochemistry samples reported in Harris et al. (2018). Samples consisted of 10 cm long slabs cut from the back of cores that were subsequently split vertically into aliquots for multiple analyses. Estimations of thermal maturity were derived from the Tmax parameter of Rock-eval pyrolysis analysis. Thirteen x-ray diffraction samples were analyzed at the University of Indiana Department of Earth and Atmospheric Sciences under the supervision of Dr. David Bish. Samples were crushed and ground into fine powder and then were analyzed by a Bruker D8 Advance X-ray diffractometer equipped with a Sol-X solid-state detector and a Cu X-ray tube. Bulk mineralogical analysis was performed on random mounts of whole-rock powders, scanned from 2-70o at a rate of approximately 0.5 degree/minute. Oriented mounts of separated clay-size fraction (< 2 μm) were scanned from 2-20o both after air drying and after exposure to ethylene glycol vapor. Rietveld refinements with Bruker AXS TOPAS software were used to determine the absolute concentrations of each mineral. Results Mineralogical results are summarized in Table 2 (appendix). In terms of bulk composition, West Shale Basin wells are distinguished by elevated quartz, K-feldspar and clay contents and the East Shale Basin by higher calcite content. Plagioclase and mica contents is slightly elevated in the West Shale Basin samples. The concentrations of other minor phases, including dolomite, ankerite and pyrite, are not systematically different between the two phases. Clay contents range from 2 to 13% in East Shale Basin wells and from 0 to 37% in West Shale Basin wells and consist primarily of illite with minor chlorite (Figure 2). One sample from the lowest maturity well, the Esso Redwater, contains a highly illitic mixed-layer clay, with an expandability of approximately 5%. Figure 2. Representative x-ray diffraction analysis of clay separates from Duvernay Formation. Discussion The presence of low expandability, highly illitic mixed-layer clay in a low thermal maturity sample and illite clay in remaining samples (Figure 2) suggests that mixed-layer smectitic clays may have never entered the basin during Duvernay deposition. Studies of the smectite-to-illite conversion demonstrate that at low thermal maturities, consistent with maturation levels in the Esso Redwater core, any mixed-layer clays entering the basin should have retained their smectitic character, at least according to reaction models based on Gulf of Mexico. We identify two possible models that explain our data: (1) the illite clay was detrital in origin, probably in the form of discrete, finely divided muscovite, and no smecite-rich mixed-layer clay was ever present; or (2) original mixed-layer clays had converted to highly illitic clays before burial or at relatively shallow burial depths through non-thermally controlled processes. The presence of detrital illite is consistent with a siliciclastic sediment origin from the Canadian Shield and is probably consistent with a northern source as well, although that source is more poorly constrained. Two mechanisms exist for illitizing mixed-layer clays under surficial or shallow burial conditions. Kim et al. (2019) demonstrated in geologically young sediments of the Nankai Trough that microbially induced reduction of Fe (III) in smectite promotes illitization at temperatures much lower than through purely abiotic

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