Solid bitumen reflectance and Rock-Eval Tmax as maturation indices: an example from the “Nordegg Member”, Western Canada Sedimentary Basin

Abstract

Marine, organic-rich rock units commonly contain little for vitrinite reflectance (V R 0) measurement, the most commoly used method of assessing thermal maturity. This is true of the Lower Jurassic “Nordegg Member”, a type I/II, sulphur-rich source rock from the Western Canada Sedimentary Basin. This study examines the advantages and pitfalls associated with the use of Rock-Eval T max and solid bitumen reflectance (B R 0) to determined maturity in the “Nordegg”. Vitrinite reflectance data from Cretaceous coals and known coalification gradients in the study area are used to extrapolate V R 0 values for the “Nordegg”. T max increases non-linearly with respect to both B R 0 and extrapolated V R 0 values. A sharp increase in the reflectaance of both solid bitumen and vitrinite occurs between T max 440–450°C, and is coincident with a pronounced decrease in Hydrogen Index values and the loss of solid bitumen and telalginite fluorescence over the same narrow T max interval. This T max range is interpreted as the main zone of hydrocarbon generation in the “Nordegg”, and corresponds to extrapolated V R 0 values of 0.55–0.85%. The moderate to high sulphur contents in the kerogen played a significant role in determining the boundaries of the “Nordegg” oil window. A linear relationship between B R 0 and extrapolated V R 0, as proposed elsewhere, is not true for the “Nordegg”. B R 0 increases with respect to extrapolated V R 0 according to Jacob's (1985) formula (V R 0=0.618×(B R 0)+0.40) up to V R 0≈0.72% (B R 0≈0.52%). Beyond this point, B R 0 increases sharply relative to extrapolated V R 0, according to the relatioship V R 0 = 0.277 × (B R 0) + 0.57 ( R 2 = 0.91). The break in the B R 0−V R 0 curve at ∼0.72%V R 0 is thought to signifiy the peak of hydrocarbon generation and represents a previously unrecognized coalification jump in the solid bitumen analogous to the first coalification jump of liptinites.