Abstract Deposition of the Callovian–Ryazanian Humber Group of the UK Central Graben occurred during rifting and long-term relative sea-level rise, which acted to suppress the formation of eustatically forced Exxon-type sequence boundaries. The superposition of highly variable halokinetically controlled subsidence means that classic, passive-margin derived sequence stratigraphic models are not appropriate to describe stratigraphic evolution in this rift setting. The sequence stratigraphy of the Humber Group has been re-evaluated using a transgressive–regressive sequence model, where maximum regressive surfaces are employed as sequence bounding surfaces. The Humber Group comprises two megasequences which reveal distinct phases of evolution of the basin. The latest Callovian–Kimmeridgian megasequence comprises a conformable sequence stack which lacks significant internal unconformities and records progressive marine flooding and overall backstepping onto the basin flanks during a phase of active rifting. The Volgian–Ryazanian megasequence is condensed and highly fragmentary due to punctuation by a number of unconformities which are consistently recognizable throughout the basin. The onset of this change in architectural style corresponds to the oldest unconformity at the base of the Volgian–Ryazanian succession, termed the Base Volgian–Ryazanian Unconformity, of latest Kimmeridgian to earliest Volgian age. The patterns of erosion of the Callovian–Kimmeridgian megasequence and the intra Volgian–Ryazanian unconformities record the effects of dramatic redistribution of underlying salt accompanied by probable uplift of the Forties–Montrose High and J Ridge, resulting in major modification of the basin morphology and the severing of possible earlier links with the Fisher Bank Basin. The kinematics of this event are equivocal, but it is possible that restricted Volgian–Ryazanian depocentres resulted from localized salt collapse rather than basement extension. Widespread erosion of Callovian–Kimmeridgian Humber Group sediments may have occurred in some areas where Volgian–Ryazanian Kimmeridge Clay deposits now overlie pre-Jurassic strata, and exploration models must incorporate the effects of Volgian reconfiguration in order to accurately predict reservoir distribution.

Abstract The Scott Field straddles Blocks 15/21 and 15/22 on the southern flanks of the Witch Ground Graben in the Outer Moray Firth Basin, UKCS. The oil field is developed in the highly productive Upper Jurassic Humber Group sandstones of Oxfordian to Kimmeridgian age. The field was discovered in 1983, sanctioned in 1990, and produced first oil in 1993. The field structure, effectively a large southwards tilted fault block, is compartmentalized into a series of four main pressure isolated fault blocks by mid to late Jurassic faulting. The Kimmeridge Clay Formation provides both the top seal and the source of the trapped hydrocarbons. Fluid contact, overpressure and compositional trends suggest that the trap was filled primarily from the north. Some trap-defining faults were already active during the deposition of the reservoir intervals. Well data indicate that the development of accommodation space was technically controlled during this period, with subsidence occurring more rapidly in the western areas of the field. The Scott Field reservoir consists of two major sand packages, the Scott Sandstone Member and the Piper Sandstone Member, bounded above and below by marine flooding surfaces. The late Oxfordian Scott Sandstone Member consists of a westwards prograding marine shoreface sandstone overlain by aggradational and retrogradational back-barrier deposits. Above this, the Mid Shale is a regionally extensive flooding event separating the Scott Sandstone Member from the overlying Piper Sandstone Member. The early Kimmeridgian Piper Sandstone Member consists of stacked mass flow sandstones, overlain by a shoreface/back-barrier system. Lateral facies changes and thickness variations significantly affect reservoir distribution in both Scott and Piper intervals. The best reservoir quality occurs within the coarsest grained, highest energy facies, particularly the shoreface and proximal washover deposits. At the crest of the field, 10400 ft TVDss, multi-Darcy permeabilities and porosities of 20% are common. However, reservoir quality declines progressively downflank due to increased quartz cementation and compaction. The Scott Field currently produces from 23 wells supported by 20 water injectors. Current modelling is aimed at targeting bypassed oil to increase ultimate recovery. The field has presently produced 300 MMSTB of oil from forecast reserves of 440 MMSTB with an estimated ultimate recovery factor of c. 46%.

Abstract An organic carbon (C org ) and sulphur (S) storage inventory for Holocene sediments in the Humber Estuary is established; sources of organic matter and their variation over time are identified, and with chronological control, the importance of estuarine sediments as C org and S stores is demonstrated. Humber Holocene sediments are grouped into seven widespread environmental facies with statistically significant geochemical data sets: (1) oak-hazel fenwood (OHF); (2) alder carr (AC), appearing as peats in core; (3) river channel muds or sands (Rcm/s); (4) high saltmarsh (HSM); (5) low saltmarsh (LSM); (6) intertidal mudflat (ITMF); and (7) a sandy facies (S). Carbon, nitrogen and sulphur (CNS) abundances show that these facies have diagnostic geochemical signatures and δ 13 C values for bulk organic matter exhibit a range of average values: −28‰ (terrestrial peats), −27‰ (HSM), and −24.5‰ (ITMF) reflecting the up core transition from terrestrial peats through saltmarshes to more open marine mudflat environments as regional sea-level rose. Chronology and average sedimentation rates are partly constrained by radiocarbon dates; palaeomagnetic techniques helped define discrete sediment packages and discontinuities (time gaps). Although the Humber Holocene sediment record is not continuous, long-term sedimentation rates (about 1 mm a −1 ) show that sediment accretion kept pace with regional sea-level rise between 6 and 2 cal. ka BP . This sedimentation rate, combined with core evidence to allow a geographic reconstruction of the palaeo-Humber (3–2cal. ka BP ), is used to calculate storage values for C org and S in the various environments of the palaeo-Humber. Comparison of the C org and S sedimentation and storage terms for the palaeo-Humber with modern values highlights the impacts of reclamation and commercial/urban development in the estuary in the last 300 years. OHF and AC peats, which were the largest C org and S stores in the palaeo-estuary, are now absent (reclaimed), while saltmarshes are no longer widespread. Conservative calculations show a net decrease in C org deposition from about 3.2 × 10 5 tonne in the palaeo-estuary to no more than 2.5 × 10 3 tonne today, a >99% reduction in potential C org storage capacity. The total modern yearly S deposition is approximately 2% of its value 2ka ago. Removal of saltmarsh and associated brackish-freshwater wetland suggests that suspended sediment and associated C org and S are currently bypassing former (Holocene) storage areas and may be impacting North Sea biogeochemical cycling.