T–R sequence stratigraphy, facies analysis and reservoir distribution in the uppermost Triassic–Lower Jurassic succession, western Sverdrup Basin, Arctic Canada

Abstract

Widespread porous and permeable sandstones of the Heiberg Formation/Group (uppermost Triassic-Lower Jurassic) are host to oil and/or gas in sixteen fields in the western Sverdrup Basin of the Canadian Arctic Islands. The sandstones are part of three, third-order, T–R (transgressive–regressive) sequences which are of Rhaetian–Hettangian, Sinemurian and Pliensbachian–Toarcian age. The general depositional/tectonic setting for the three T–R sequences was a marine shelf undergoing relatively low subsidence. Sediment influx to the shelf was generally low and increased eastward toward a deltaic input centre located in the south-central portion of the basin. Each T–R sequence is bounded by subaerial unconformities (usually modified by shoreface erosion) on the basin margin and by correlative transgressive surfaces farther seaward. A maximum flooding surface within each sequence separates a lower transgressive systems tract from an overlying regressive systems tract. The transgressive systems tract thickens eastward in each sequence and consists of a variety of facies including massive to cross-bedded sandstone (inner shelf), rippled to burrowed sandstone (mid-shelf), glauconitic sandstone, siltstone and shale (outer shelf) and oolitic ironstone (starved inner shelf). The predominant facies recognized in the regressive systems tracts are burrowed, grey shale and siltstone (offshore shelf), rippled to burrowed siltstone and sandstone (lower shoreface-mid shelf), cross-bedded to massive sandstone (upper shore face-inner shelf) and carbonaceous shale and siltstone with a highly variable sandstone content (lagoon, coastal plain). In the Rhaetian–Hettangian sequence a distinctive restricted marine facies association (foreshore-offshore shelf) is widespread and is characterized by an almost total lack of burrowing and by red and green coloration of shale–siltstone units. Oil and gas are structurally trapped almost exclusively in the uppermost porous sandstones of the succession beneath thick argillaceous deposits of the Jameson Bay Formation. The stratigraphic position of such sandstones varies across the study area. Regressive shoreface–foreshore sandstones of the Rhaetian–Hettangian sequence form the main reservoirs along the southwest basin margin. Farther northeast the regressive inner shelf to coastal plain sandstones of the Sinemurian sequence and the transgressive inner shelf sandstones of the Pliensbachian–Toarcian sequence are the main reservoirs. Comparison of the sequence bounding events with the Haq et al. (1988) curve reveals both matches and mismatches. This result, plus the occurrence of major changes in the amount and location of sediment input and in subsidence pattern from one sequence to the next, suggest that regional tectonics was the main factor controlling sequence development.