Fluorescence properties of Palaeozoic oil-prone alginite in relation to hydrocarbon generation, Williston Basin, Saskatchewan, Canada

Abstract

Alginite macerals, the oil-prone microscopic components of hydrocarbon source rocks, fluoresce in the visible region when irradiated with ultraviolet light. The fluorescence properties ( λ max and red/green quotient) of coccoidal, unicellular and filamentous alginite macerals were investigated in Palaeozoic potential hydrocarbon source rocks, northern Williston Basin, Saskatchewan, Canada. This study documents the variation in fluorescence properties of alginite during thermal maturation and the onset of petroleum generation. Values of λ max (nm) of disseminated, coccoidal Gloeocapsomorpha alginite from Upper Cambrian siliciclastics and Upper Ordovician carbonates show different magnitudes of red shift with increasing thermal maturity. Gloeocapsomorpha prisca disseminated alginite and G. prisca stromatolitic alginite from Upper Ordovician kukersites exhibit unique fluorescence properties with increasing thermal maturity. The former undergoes a greater red shift in fluorescence than the latter over an 1800 m change in depth of burial. Devonian and Mississippian unicellular Prasinophyte Leiosphaeridia and Tasmanites alginite show a minimal change in fluorescence properties ( λ max 450–500 nm) between vitrinite reflectance values of 0.35 and 0.65%Ro. Beyond 0.65%Ro vitrinite reflectance, Prasinophyte alginites show a marked red shift to a λ max of 560–600 nm. This phenomenon is coincident with peak hydrocarbon generation from a Type II filamentous alginite-bituminite kerogen within Middle Devonian carbonate source rocks. A comparison of the fluorescence properties of Palaeozoic Prasinophytes with the fluorescence properties of Mesozoic Prasinophytes reveals that the latter undergo a substantially greater red shift in λ max with increasing thermal maturity relative to the former. This conspicuous difference is interpreted as a reflection of the well documented, distinct difference in species and abundance of Leiosphaeridia and Tasmanites alginite between the geological eras.