Effective stratigraphical subdivision of the Humber Group and the Late Jurassic evolution of the UK Central Graben

Abstract

New biostratigraphical analysis of the Humber Group sediments from the UK North Sea Central Graben, using integrated palynological and micropalaeontological zonation schemes, allows substage resolution and basin-wide discrimination of subunits of the Heather Formation/Kimmeridge Clay Formation, and associated sandstones. The integrated use of both zonation schemes is particularly important because thermally mature Humber Group sediments buried below 12 000 ft in the Central Graben cannot be dated consistently to substage level using palynology alone. Poor data and the undiagnostic log character of the mudstones and shallow marine sandstones have resulted in a simple lithological scheme being widely applied, with all sandstones assigned to the Fulmar Formation and mudstones to the Heather Formation and the Kimmeridge Clay Formation. Previous generalized usage of the Fulmar Formation for all Upper Jurassic shallow marine sandstones is inappropriate. The Upper Oxfordian Puffin Formation is present in the northern and western Central Graben. The largely Kimmeridgian Fulmar Formation is restricted to the south and east, and the coeval Ula Formation flanks the Jaeren High. These three units are all interpreted as syn-rift sandstones, locally sourced from uplifted and eroded fault block crests within and marginal to the Central Graben. Latest Kimmeridgian or younger shallow marine sandstones are largely confined to platform areas outside the graben, and can be related to regressive phases.

Ankerite Cementation in Deeply Buried Jurassic Sandstone Reservoirs of the Central North Sea

“…1). The reservoir unit is the Oxfordian-age Franklin Sandstone Formation, which is broadly equivalent to the Fulmar Formation in shallower terrace regions of the SCG (Price et al 1993;Howell et al 1996). Samples were collected across ''tight'' carbonate-cemented intervals (Յ5 vol.…”

Disseminated ‘jigsaw piece’ dolomite in Upper Jurassic shelf sandstones, Central North Sea: an example of cement growth during bioturbation?

“…1). The reservoir unit is the 300-m-thick Oxfordian Franklin Sandstone Formation (Lasocki et al, 1999), which is broadly equivalent to the Fulmar Formation in less deeply buried parts of the South Central Graben (Price et al, 1993;Howell et al, 1996). It consists of an aggradational succession of very ®ne-grained marine sandstones, silty sandstones and siltstones, with subordinate mudstones.…”

“…They are not considered to be a suitable source for detrital dolomite with the ®ne grain size and near-marine isotopic composition inferred for Franklin sandstones. Despite the effects of upper Jurassic faulting and halokinesis, regional studies (from seismic data) do not suggest that Permian Zechstein (carbonate) facies were exposed to erosion during the Upper Jurassic (Price et al, 1993). In addition, previous studies of the Fulmar Formation sandstones have noted frequent early diagenetic dolomite, but no abundant detrital dolomites (Johnson et al, 1986;Saigal et al, 1992;Clelland et al, 1993).…”

Sequence stratigraphy of Upper Jurassic reservoir sandstones in the northern part of the Danish Central Trough, North Sea

“…Controlling factors proposed in the literature include the development of stratified water columns in restricted basins (Tyson et al, 1979), a dryer climate during the latter part of the Late Jurassic (Wignall and Ruffell, 1990), and sediment starvation during maximum transgression (Price et al, 1993, but see also Posamentier and James, 1993).…”