Sequence stratigraphic control on Middle and Upper Jurassic reservoir distribution within the UK Central North Sea

Abstract

An integrated sequence stratigraphic analysis, utilizing seismic data, well-log cross-sections, biostratigraphic information, and core descriptions, provides a chronostratigraphic framework to analyse and predict the distribution of reservoir sandstones within the Middle to Upper Jurassic section of the UK Central North Sea.The Callovian (Middle Jurassic) through Ryazanian (lowermost Cretaceous) section in the UK Central North Sea forms a depositional wedge that thins towards the western graben margin. This wedge is bounded by surfaces traditionally interpreted as the Middle Cimmerian and Late Cimmerian unconformities. Marginal marine (Fulmar) sandstones are present within this wedge, and can be subdivided into informal lower and upper Fulmar members. Each member has a unique spatial and temporal distribution within the basin that can be related to primary depositional palaeogeography, post-depositional erosion, and basin tectonics.The lower member of the Fulmar Formation is Callovian to Early Oxfordian in age, and is restricted to the Central Graben. This member overlies Middle Jurassic strata of the Pentland Formation, and is conformably overlain by Oxfordian-age strata of the Kimmeridge Clay. The upper member of the Fulmar Formation is Early Kimmeridgian in age, and is distributed along the western graben margin. The upper member is conformably overlain by Kimmeridgian-age strata of Kimmeridge Clay, and typically overlies Triassic deposits of the Skagerrak Formation.Post-depositional erosion, association with relative falls in sea-level and accentuated by basement and salt tectonism, is the major factor that modifies primary depositional patterns within the Fulmar Formation. Truncation associated with unconformities interpreted as the 142 Ma, 138 Ma, and 136 Ma sequence boundaries affect the distribution, fades patterns, and thickness of the upper member of the Fulmar. These unconformities, as well as the 155.5 Ma, 150.5 Ma and 144 Ma sequence boundaries, may also affect the lower member of the Fulmar.