Reservoir architecture of the cook formation on the gullfaks field based on sequence stratigraphic concepts

Abstract

The late Pliensbachian to early Toarcian Cook Formation is a secondary reservoir unit in the Gullfaks field, northern North Sea. In preparing for the main phase of production from this unit, a reservoir model based on the concepts of sequence stratigraphy has been established. This model is constrained by core, log and biostratigraphical data from 65 wells, and shows that the succession studied contains elements of two sequences formed in a ramp-setting. The lower fine-grained part of the succession (Cook-1 and Cook-2) was formed during relative sea-level highstand, and consists of stacked parasequences containing offshore to lower shoreface deposits supplied from distant source areas to the southeast and southwest. A minor unconformity, interpreted as a marine downshift surface, is present near the top of Cook-2. This surface marks the initiation of relative sea-level fall (falling stage systems tract), and is overlain by a thin heterolithic succession formed in a tidal shoreface setting. In proximal parts, this succession is usually absent due to erosion during the subsequent maximum regression phase. The transition from Cook-2 to the main reservoir unit, Cook-3, is interpreted as a sequence-bounding unconformity (sensu EXXON), and is associated with a major change in the drainage pattern. A relative fall in sea-level, possibly accentuated by tectonic uplift, led to bypass and erosion in the proximal (northeastern) parts of the field, and a depocentre was created in the central to southwestern parts of the Gullfaks field. Marginal marine deposits formed in a tide-influenced deltaic setting overlie the sequence boundary, and constitute the lowstand wedge systems tract which gradually onlaps the sequence boundary. This phase of onlap caused significant thickness variation within Cook-3. The first major flooding surface above the sequence boundary is associated with an increased rate of onlap and a change from overall progradation to overall retrogradation. A sandstone-dominated transgressive systems tract (TST) commences at this surface, and consists of tide- and fluvial-influenced delta front to delta plain deposits contained within two to three weakly backstepping parasequences. Continued high rate of sediment supply enabled these intra-TST regressive units to extend a considerable distance basinwards, and ravinement is associated with at least one of the intra-TST flooding surfaces. A major flooding event terminated the accumulation of deltaic sandstones, and initiated deposition of the Drake Formation mudstones in the study area. The reservoir architecture of the Cook Formation is constrained by a framework consisting of the sequence stratigraphical bounding surfaces and systems tracts. For intra-reservoir correlation purposes, six main types of bounding surfaces have been denned: (1) sequence bounding unconformity, (2) marine downshift surface, (3) transgressive surface, (4) ravinement surface, (5) maximum flooding "surface", and (6) minor flooding surfaces. The most significant flow-barriers in the reservoir are associated with these surfaces. The revised reservoir zonation is derived from the sequence stratigraphical framework, and is based mostly on the recognition of parasequences. Palaeogeographical maps constructed for each systems tract and reservoir zone illustrate the lateral distribution of large-scale flow-units. The combination of these maps and the correlation structure has produced a series of reservoir quality cross-sections which highlight the location of high-permeability zones that may be targeted by dedicated wells.