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Improving Burial and Thermal History Modeling Based on Geochemistry and Progressive Clay Mineralogical Transformation in Devonian Shales — Examples From the Duvernay and Muskwa Formations in Western Canada

Abstract

The Muskwa and Duvernay Formations are contemporaneously deposited stratigraphic units of the late Devonian (Frasnian) in the Western Canadian Sedimentary Basin. Both units are composed of organic-rich and fine-grained sediments deposited along the western shelf of the North American Craton within adjacent basins, north (Muskwa) and south (Duvernay) of the Peace River Arch. A new dataset of mostly cuttings samples from >1400 wells across the two formations has been established that contains both geochemical and mineralogical details. Regional mapping of total organic content, thermal maturity and mineralogical compositions reveals details of sediment origin, transport and diagenetic alterations. In addition, clay mineralogy of selected wells illustrates a progressive clay mineral structural transformation associated with burial depth. In the Muswka Formation, smectitic clays are common in shales in the northeastern part of the basin buried to ~900 m (~3000 ft) depth, but are absent in rocks at ~1300 m (~4300 ft) depth where mixed-layer smectite-illite clay minerals are present. In rocks further west at >1600 m (>5200 ft) depth, mixed layer clay minerals are absent and only illite with high crystallinity is observed. Organic geochemical data of the same samples illustrate that thermal maturity also increases towards the west but depth/TMax details show that maturity is not only linked to burial depth but also to underlying basement residual heat flow. In the Duvernay Formation, burial depths are >1200 m (~4000 ft) and therefore both thermal maturity and clay mineral structures are different than in the Muskwa. Clay mineral analysis in these rocks illustrates the absence of smectites and the dominance of thin illite/poorly structured illite without swelling capacity. Geochemical analysis shows a complex thermal maturity of the organic matter with immature material (Tmax <430°C) in the shallow (~1400-2400 m, 4500-7800 ft depth) eastern part of the basin and high maturity (Tmax >470°C) along the deeper (>3300 m, 11,000 ft) western deformation front. This presentation provides insights into regional geochemical and mineralogical changes across both the Muskwa and Duvernay Formation and provides details of clay mineralogical structural changes (XRD) and organic thermal maturity (Tmax, TOC). Clay mineralogy in both basins allows interpretations about possible original detrital influx, paleoocean currents and paleotopographic reliefs. Maturity trends and clay mineral transformation at local and regional scales provide specific basin model parameters and thermal maturity information associated with both local basement heat flow and regional burial depth. The data improved both burial and diagenetic parameters and thus the resulting burial models.