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The Influence of Oil-Based Drilling Muds on S2 and Tmax Pyrolysis


In the Western Canadian Sedimentary Basin (WCSB) a number of shale, mudstone and tight siltstone reservoirs are targets for exploration. The majority of these formations are overlain by clay-rich units that contain various amounts of mixed-layer illite/smectite or smectite clay minerals. Swelling clays can pose challenges to the success of drilling a well. In recent years, oil-based mud systems have been shown to improve wellbore stability, however, these mud systems contaminate core and cuttings sample material, affecting the measurement of free hydrocarbons and often influencing thermal maturity determinations. Here we present new source rock analysis data of archived rock samples. The samples were collected from multiple formations in the WCSB. Wells were selected so that maturity ranging from immature, oil, condensate and dry-gas windows are included. Analysis of the samples was performed as followed: 1) as-received; 2) as-received and solvent-extracted; 3) exposed to oil-based drilling mud for 8 hrs; 4) exposed to oil-based drilling mud for 4 weeks; 5) exposed to oil-based drilling mud for 8 hrs and solvent-extracted; and 6) exposed to oil-based drilling mud for 4 weeks and solvent-extracted. The results are compared and possible reasons for the differences are discussed.