SOLID BITUMEN REFLECTANCE AND ROCK-EVAL T(MAX) AS MATURATION INDEXES - AN EXAMPLE FROM THE NORDEGG MEMBER, WESTERN CANADA SEDIMENTARY BASIN

Marine, organic-rich rock units commonly contain little vitrinite for vitrinite reflectance (VR(o)) measurement, the most commonly used meth od of assessing thermal maturity. This is true of the Lower Jurassic ' 'Nordegg Member'', a type I/II, sulphur-rich source rock from the West ern Canada Sedimentary Basin. This study examines the advantages and p itfalls associated with the use of Rock-Eval T(max) and solid bitumen reflectance (BR(o)) to determine maturity in the ''Nordegg''. Vitrinit e reflectance data from Cretaceous coals and known coalification gradi ents in the study area are used to extrapolate VR(o) values for the '' Nordegg''. T(max) increases non-linearly with respect to both BR. and extrapolated VR(o) values. A sharp increase in the reflectance of both solid bitumen and vitrinite occurs between T(max) 440-450-degrees-C, and is coincident with a pronounced decrease in Hydrogen Index values and the loss of solid bitumen and tetalginite fluorescence over the sa me narrow T(max) interval. This T(max) range is interpreted as the mai n zone of hydrocarbon generation in the ''Nordegg'', and corresponds t o extrapolated VR. values of 0.55-0.8%. The moderate to high sulphur c ontents in the kerogen played a significant role in determining the bo undaries of the ''Nordegg'' oil window. A linear relationship between BR. and extrapolated VR(o), as proposed elsewhere, is not true for the ''Nordegg''. BR(o) increases with respect to extrapolated VR. accordi ng to Jacob's (1985) formula (VR(o) = 0.618 x (BR(o)) +0.40) up to VR( o) almost-equal-to 0.72% (BR(o) almost-equal-to 0.52%). Beyond this po int, BR(o) increases sharply relative to extrapolated VR(o), according to the relationship VR(o) = 0.277 x (BR(o)) + 0.57 (R2 = 0.91). The b reak in the BR(o) - VR(o) curve at approximately 0.72% VR(o) is though t to signify the peak of hydrocarbon generation and represents a previ ously unrecognized coalification jump in the solid bitumen analogous t o the first coalification jump of liptinites.