MATURITY STUDIES AND SOURCE-ROCK POTENTIAL IN THE SOUTHERN SVERDRUP BASIN, ARCTIC CANADA

The purpose of the study is to better understand the relationship betw een organic matter optical properties and the presence of potentially large oil and gas accumulations in Arctic Canada. The type and thermal maturity of the dispersed organic matter of the Mesozoic formations i n the southern Sverdrup Basin, Melville Island, have been studied usin g organic petrology and Rock-Eval pyrolysis. All types of organic matt er are present in the strata of Mesozoic age. Hydrogen-rich liptinite is dominated by alginite (Botryococcus and Tasmanites), dinoflagellate cysts and amorphous fluorescing matrix. Sporinite, cutinite, resinite and liptodetrinite made up the lesser hydrogen-rich exinite. Vitrinit e reflectance in Cretaceous sediments ranges from 0.36 to 0.65% R(o); in Jurassic sediments it ranges from 0.40 to 1.0% R(o) and in the Tria ssic from 0.45 to 1.30% R(o), showing an overall increase with depth o f burial. Cretaceous sediments of the Deer Bay Formation are thermally immature and contain organic matter of terrestrial origin. The Upper Jurassic shales of the Ringnes Formation contain predominantly organic matter of liptinitic and exinitic origin with a considerable vitrinit ic input. At optimum maturation levels, potential source beds of this formation would have a good hydrocarbon-generating potential. The hydr ocarbon potential, however, would be limited to the generation of gase s due to the leanness of the source rocks. Parts of the Lower Jurassic Jameson Bay Formation are organic-rich and contain a mixed exinitic/v itrinitic organic matter, Botryococcus colonial algae but visible orga nic matter is dominated by high plant remains (mainly spores). The Sch ei Point Group shales and siltstones contain organic matter of almost purely marine origin, whereas the predominantly higher plant-derived o rganic matter found in the Deer Bay, Jameson Bay and partly in the Rin gnes formations have higher TOC. Among the Schei Point Group samples, the Cape Richards and Eden Bay members of the Hoyle Bay Formation are richer in TOC (> 2.0%) than the Murray Harbour Formation (Cape Caledon ia Member). This may reflect differences in the level of maturity or i n the depositional environment (more anoxic conditions for the former) . Regional variations in the level of thermal maturity of Mesozoic sed iments in Sverdrup Basin appear to be 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 o f Sabine Peninsula. However, the regional thermal-maturation pattern o f the Mesozoic is not solely a reflection of the present-day geotherma l gradient, which indicates that anomalous zones of high geothermal gr adient may have existed in the past, at least since when the Mesozoic sediments attained maximum burial depth. The contour pattern of the re gional variation of maturity at the base of numerous Triassic formatio ns is similar to that of the structural contours of the Sverdrup Basin , indicating that present-day maturation levels are largely controlled by basin subsidence.