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The Rock-Eval thermal maturation parameter (Tmax) may occasionally be suppressed relative to surrounding rock units. This is commonly assumed to result from either a bitumen contribution to the S2 peak or the presence of sulfur-rich (and hence thermally labile) kerogen. The phenomenon is frequently coincident with a high natural gamma-ray signature and elevated Rock-Eval hydrogen index (HI) and total organic carbon (TOC) contents. Determination of thermal maturity of such units relies either on the interpolation of Tmax from surrounding units or the direct estimation of the extent of suppression on a given sample. The latter may be required if the available samples are restricted to conventional or sidewall-core material representative of imited stratigraphic intervals.
The Second White Speckled Shale (part of the Cretaceous Colorado Group) in the Alberta basin displays Tmax suppression over part of its thickness. A consistent relationship emerges from calculating a linear regression of Tmax as a function of either TOC or HI for several wells at different levels of thermal maturity. Below values of about 453°C (late oil window), Tmax is suppressed by about 1°C for every increase in HI of 50 mg hydrocarbon/g TOC. Thus Tmax suppression in the Second White Speckled Shale can be corrected by extrapolating all Tmax values to a constant HI of 100 to 150 mg hydrocarbon/g TOC, values typical of type III kerogen.
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