Tectonostratigraphic evolution, palaeogeography and main petroleum plays of the Nuussuaq Basin: An outcrop analogue for the Cretaceous–Palaeogene rift basins offshore West Greenland

Abstract The onshore Nuussuaq Basin in central West Greenland comprises a complete Albian–Paleocene succession and as such represents the only complete rift succession outcrop analogue for the Cretaceous–Palaeogene offshore frontier basins in West Greenland. The basin fill reflects five tectonostratigraphic phases: 1) Pre-rift, 2) Albian – Early Cenomanian Early Rift, 3) Early Cenomanian – Early Campanian Thermal Subsidence, 4) Early Campanian – Early Paleocene Late Rift, and 5) Early Paleocene – Late Eocene Break-up and Drift. In addition, the succession is divided into nine tectonostratigraphic sequences (TSSs), each representing a specific configuration of depositional elements resulting from tectonic events, which caused major palaeogeographic reconstruction of the basin and therefore critical to the petroleum systems. The sequences are bounded by unconformities or, in one case, by a major flooding phase. The Early Rift Phase in the Nuussuaq Basin was initiated in the Albian with development of half-grabens along N–S directed extensional faults. This phase was characterised by continued growth along extensional faults and embraces three TSSs (TSS1–3). They are characterised by alluvial fan, fan-delta and lacustrine sedimentation, wave- and tidal-dominated deltaic deposition, followed by a major fall in relative sea level and canyon incision. Oil seeps indicate presence of a lacustrine/brackish-water source rock at depth. During the following Late Cenomanian – Early Campanian Thermal Subsidence Phase a regional marine drowning of the basin took place with deposition of organic-rich mudstones, followed by re-establishment of deltaic deposition in the Coniacian–Santonian (TSS4). The Early Campanian – Early Paleocene Late Rift Phase was associated with a change in stress regime, formation of NW–SE directed extensional faults, uplift of highs and a major change in depositional environments from deltaic deposition to deposition from gravity flows in a confined system of slope channels and canyons. During the initial phase, structurally-controlled turbidite channels were established in a NW–SE trending graben-like structure that acted as conduits for sediment transport into the offshore areas (TSS5). This was followed by several phases of major uplift resulting from the rise of the Proto-Icelandic Mantle Plume prior to continental break-up eventually leading to the deposition of two structurally controlled TSSs (TSS6–7) characterised by the formation of submarine and subaerial canyons that acted as conduits for huge amounts of sediments transported into the offshore areas. The Nuussuaq Basin experienced significant subsidence during the initial phase of break-up volcanism with deposition of organic-rich Lower Paleocene marine mudstones blanketing the basin followed by a thick succession of Paleocene–Eocene volcanic rocks referred to as the West Greenland Basalt Province (TSS8–9). Volcanism gradually spread eastwards eventually blocking the connection to the sea stemming up a large lake between the prograding volcanic front and the cratonic mainland to the east. Finally, when the volcanic rocks covered the entire basin, rivers were redirected, and sediments were deposited in the offshore Sisimiut Basin to the south and the Melville Bay Basin to the north. During this latest rifting and break-up phase, the Nuussuaq Basin was aborted and terminated as a failed rift basin. Four main plays have been defined based on the tectonostratigraphic subdivision: An Early Rift Play, a Thermal Subsidence Play, a Late Rift Play and a Break-up and Drift Play. The Early Rift Play has alluvial and estuarine sandstones as reservoir, Albian lacustrine/brackish-water mudstones as source rock and Cenomanian–Turonian mudstones as seal. The Thermal Subsidence Play has deltaic and shallow marine sandstones as reservoirs, Cenomanian–Turonian mudstones as source and Campanian mudstones as seal. The Late Rift Play has incised valley, submarine canyon and turbidite sandstones as reservoirs, and Campanian and Paleocene mudstones as seal and source. The Break-up and Drift Play is subdivided into three sub-plays, including: a Lacustrine Sub-play, an Intra-basaltic Siliciclastic Sub-play and a Fractured Volcanic Sub-play. The Lacustrine Sub-play has fluvio-deltaic sandstones as reservoir and intra-formational lacustrine and delta-plain mudstones as seal. The Intra-basaltic Siliciclastic Sub-play comprise fluvial and lacustrine sandstones as reservoir and volcaniclastic or siliciclastic mudstones as seal. The Fractured Volcanic Sub-play comprises fractured, porous volcanic rocks as reservoir and intra-basaltic mudstones or tight fine-grained volcanic rocks as seal. All three sub-plays require vertical or long-distance migration of hydrocarbons from deeper-seated Cretaceous – Lower Paleocene kitchens. Because the tectonostratigraphic phases recognised in the Nuussuaq Basin can also be applied to the regional seismic mapping offshore Greenland the implications of the Nuussuaq Basin plays can be applied to the frontier offshore basins.