Regional maturity and source-rock potential of palaeozoic and mesozoic strata, Melville Island, Arctic Canada

Abstract

The thermal maturity and source-rock potential of the Palaeozoic and Mesozoic sediments in Melville Island, Arctic Canada were studied using organic petrology and Rock-Eval pyrolysis. A total of 2,000 polished whole-rock samples were examined and their phytoclast reflectances (% Ro random) measured. In addition, selected samples Were analyzed qualitatively and quantitatively using ultraviolet excitation. Hydrogen-rich organic matter was dominated by alginite (Botryococcus and Iasmanites), dinoflagellate cysts and amorphous fluorescing matrix. Sporinite, cutinite, resinite and liptodetrinite formed the lesser hydrogen-rich organic matter. Vitrinite reflectance in Cretaceous sediments ranged from Ro = 0.36 to 0.65%; in Jurassic sediments it ranged from Ro = 0.40 to 1.0% and in Triassic sediments from Ro = 0.45 to 1.30%. Vitrinite showed an overall increase in %Ro with increasing depth of burial but Variations do occur, possibly due to the effects of mineral matrix (lithology) and liptinite Content. It was observed that when total liptinite percentage was high (20-30%), reflectance was lowered by a magnitude of 0.1 to 0.15%. In addition, the lowering of the reflectance, which was genuine and not experimental, was due to: 1) the effect of bitumen impregnation (staining); and 2) differences in the type of organic matter (hydrogen-rich Vitrinite of marine origin). A large number of shale samples exceeded the worldwide average total organic carbon (TOC) content of 1.05 wt% with the Jurassic-Cretaceous shales and siltstones having higher values than those of Triassic age. The Triassic Schei Point Group shales and siltstones contained organic matter of marine origin, whereas the predominantly Plant-derived organic matter present in the Jameson Bay, Ringnes and Deer Bay formations had a higher TOC. Among the Schei Point Group sediments, the Cape Richards and Eden Bay Members of the Hoyle Bay Formation were richer in TOC (>2.0 wt%) than the Murray Harbour Formation (Cape Caledonia Member), which may reflect differences in the level Of maturity or in the depositional environment (more anoxic conditions for the former). Higher average TOC contents (> 3.0 wt%) were reported in the Cape Richards Member in drill hole Hecla C-32, as well as in the Eden Bay Member in drill holes Hecla C-32, Roche Paint J-43 and North Sabine H-49, all located in Sabine Peninsula. The Schei Point Group sediments contained mainly liptinitic organic matter of marine origin, were Occasionally bitumen stained and have high potential for the generation of liquid hYdrocarbons. Organic matter in the Palaeozoic strata of the Franklinian miogeosyncline was represented mainly by bitumen in the Silurian and Devonian. Different bitumen types Were identified depending on their morphology, reflectance range and association with the enclosing mineral matrix. Thermal maturity of the Palaeozoic strata, based on vitrinite reflectance calculated from bitumen reflectance was high, an indication that the strata Were in the overmature stage of hydrocarbon generation and that only dry gas should be expected. There is petrological evidence that hydrocarbons were generated and Illigrated through the sediments. Regional variations in the level of thermal maturity of Mesozoic sediments in the Sverdrup Basin are a function of burial depth. The Mesozoic formations thicken towards the basin centre (N-NE direction), reflecting the general pattern of increasing thermal maturity north of the Sabine Peninsula. In addition, periods of high heat flow Illost likely existed during rifting episodes from Carboniferous to Early Tertiary. The contour pattern of the regional variation of maturity at the base ofnumerous Triassic formations is similar to that of the structural contours of the Sverdrup Basin, indicating that present-day maturation levels are largely controlled by basin subsidence.