The Rock-Eval thermal maturation parameter (T_max) 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 T_max 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 T_max suppression over part of its thickness. A consistent relationship emerges from calculating a linear regression of T_max 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), T_max is suppressed by about 1°C for every increase in HI of 50 mg hydrocarbon/g TOC. Thus T_max suppression in the Second White Speckled Shale can be corrected by extrapolating all T_max values to a constant HI of 100 to 150 mg hydrocarbon/g TOC, values typical of type III kerogen.