Abstract Many of the stratigraphic sequences recognized in North Sea Jurassic well sections correspond to mappable surfaces on seismic sections. Typically, however, sequences are only mappable seismically within individual sub-basins, and seismic correlation between sub-basins, or across highs, is generally impossible without independent control from wells. Particularly prominent seismic sequence boundaries occur at near-base J54 in the Inner Moray Firth (‘Intra-Oxfordian Event’) the Viking Graben (‘Top Heather’ in this area), base J62 (‘Top Heather’, Moray Firth), base J66 (‘Top Lower Hot Shale’, Inner and Outer Moray Firth), base J71 (East Shetland Platform), base J73 (‘Top Siltstone Member’, Moray Firth) and top J70/base K10 (‘Base Cretaceous Unconformity’ (BCU), basin-wide). The BCU is the most frequently mapped seismic horizon in the North Sea Basin in Jurassic–basal Cretaceous studies. This surface, at the base of the Cromer Knoll Group, separates synrift sediments from post-rift successions above and marks a major shift in the tectonic evolution of the North Sea Basin.

Abstract The application of sequence stratigraphic concepts and methods significantly enhances the evaluation of stratigraphic traps. In this chapter, five examples of, as yet undrilled, potential UK North Sea Jurassic combination stratigraphic traps, from the East Shetland Platform, South Viking Graben, Inner Moray Firth and Central Graben, are discussed and the potential application of sequence stratigraphic methods in their evaluation considered.

Abstract The Alma Field (formerly Argyll and then Ardmore) is located within Blocks 30/24 and 30/25 on the western margin of the Central Graben. Hamilton drilled the first discovery well 30/24-1 in 1969 and the field, named ‘Argyll’, became the first UK offshore oilfield when production commenced in 1975. Oil was produced from the Devonian Buchan Formation, Permian Rotliegend and Zechstein groups, and Jurassic Fulmar Formation from 1976 until 1992, when the field was abandoned for economic reasons. In 2002, Tuscan Energy and Acorn Oil & Gas redeveloped the field and renamed it as ‘Ardmore’. A further 5 MMbbl were produced until 2005, when the field was again abandoned due to commercial considerations. In 2011, EnQuest was awarded the licence to redevelop the field and renamed it as ‘Alma’. The field came on stream in October 2015 and has produced oil at an average c. 6000 bopd since start-up. Total ultimate recovery was expected to be about 100 MMbbl. As of end 2005, the field had produced 72.6 MMbbl as Argyll and 5 MMbbl as Ardmore. A further 4.3 MMbbl has been produced from the Alma Field to September 2017 (which includes about 0.5 MMbbl from a long-reach well drilled into the Duncan/Galia Field immediately west of Alma). In January 2020 EnQuest announced that the Alma Field would cease production early. The total production from the three phases of field development will be about 85 MMbbl of oil.

Abstract The Howe and Bardolino fields lie in UK Blocks 22/12a and 22/13a, respectively, on the eastern flank of the Forties–Montrose High. The Howe Field was discovered in 1987 by well 22/12a-1, and Bardolino in 1988 with well 22/13a-1ST. Both share common Jurassic reservoirs, have Upper Jurassic Kimmeridge Clay Formation top seals, require some form of lateral seal and have similar fluids. Howe has been producing relatively dry oil throughout its production life, indicating relatively good connectivity across the field area. In contrast, the Bardolino accumulation is proven to be compartmentalized. Bardolino is likely to be segmented through some fault-related mechanism. In place volumes at the Howe Field are 46.8 MMbbl, with 17 MMbbl produced thus far through a combination of natural aquifer and solution gas cap drive by subsea development well 22/12a-9Z. In place volumes at the Bardolino Field are 11.2 MMbbl, with 1.1 MMbbl produced to date through depletion drive by a subsea development well 22/13a-8. This represents recovery rates of 35% for Howe and 10% for Bardolino to date. In place volumes for the undeveloped Pentland Formation at Howe are 5 MMbbl. In place estimates for the undeveloped Kimmeridge Clay Formation sandstones at Bardolino are 8 MMbbl.

Abstract The Huntington Oil Field is located in Block 22/14b in the Central Graben of the UK Continental Shelf. The reservoir is the Forties Sandstone Member of the Sele Formation, and oil production is from four production wells supported by two water-injection wells, tied back to the Sevan Voyageur FPSO (floating production storage and offloading unit). Initial estimates of oil-in-place were c. 70 MMbbl and the recovery factor at the end of 2017 after 4.5 years of production was 28%, which reflects the weak aquifer and poor pressure support from water injection. The Huntington reservoir is part of a lobate sheet sand system, where low-concentration turbidite sands and linked debrites are preserved between thin mudstones of regional extent. Within the reservoir, three of the thicker mudstone beds can be correlated biostratigraphically on a regional basis. This stacked lobate part of the system sits above a large-scale deep-water Forties channel that is backfilled by a system of vertically aggrading channel storeys. Despite the relatively high net/gross of the reservoir, the thin but laterally extensive mudstones in the upper (lobate) part of the system are effective aquitards and barriers to pressure support from water.

Abstract The Mungo Field is a mature producing asset located in the UK Central North Sea. Discovered in 1989 and brought on production in 1998, it is the largest field within the Eastern Trough Area Project (ETAP). Production occurs via a normally unattended installation and is tied back to the ETAP Central Processing Facility. It is a pierced, four-way dip closure against a salt diapir. Light oil is present within steeply dipping Late Paleocene sandstone and Early Paleocene–Late Cretaceous chalk intervals. The vertical relief of the salt stock is around 1500 m TVDSS and top of the salt canopy lies at about 1350 m TVDSS. The Paleocene sandstones (Forties Sandstone Member of the Sele Formation, Lista Formation and Maureen Formation) make up the primary reservoir and have been extensively developed in three phases since 1998 under water injection and natural depletion. The sandstones were deposited as deep-water turbidite complexes (submarine fans with local channels) on and around the flanks of the rising salt diapir. More recently, successful stimulation of the Chalk Group, coupled with re-evaluation of core and well-log data, has indicated that economic production rates could also be achieved from the underlying fractured chalk reservoir.

Abstract The, as yet undeveloped, heavy-oil fields of the Western Platform contain about 500 MMbbl of oil in place. The fields are reservoired in highly porous and permeable, Middle Eocene, deep-water sandstones of the Tay Sandstone Member, deposited as turbidite flows from a shelf immediately to the west. Oil gravity varies from 19° API in the Harbour Field to 12° API in the northern end of the Pilot Field. The reservoirs are shallow: Pilot and Harbour are at about 2700 ft TVDSS, with the Narwhal, Elke, Blakeney and Feugh discoveries being deeper at about 3300 ft TVDSS. Overall, oil viscosity decreases and API oil gravity increases with depth. To date, the high oil viscosity has precluded development of these discoveries, and many previous operators have considered various development schemes, all based on water flood. The development of the Pilot Field is being planned using either a hot-water-flood, steam-flood or polymer-flood approach, which all have the potential of achieving a very high recovery factor of 35–55%. Steam has been evaluated in most detail and about 240 MMbbl could be recovered should all of these discoveries be steam flooded.

Abstract The Shearwater Field is a high-pressure–high-temperature (HPHT) gas condensate field located 180 km east of Aberdeen in UKCS Blocks 22/30b and 22/30e within the East Central Graben. Shell UK Limited operates the field on behalf of co-venturers Esso Exploration and Production UK Limited and Arco British Limited, via a fixed steel jacket production platform and bridge-linked wellhead jacket in a water depth of 295 ft. Sandstones of the Upper Jurassic Fulmar Formation constitute the primary reservoir upon which the initial field development was sanctioned; however, additional production has been achieved from intra-Heather Formation sandstones, as well as from the Middle Jurassic Pentland Formation. Following first gas in 2000, a series of well failures occurred such that by 2008 production from the main field Fulmar reservoir had ceased. This resulted in a shut-in period for the main field from 2010 before a platform well slot recovery and redevelopment drilling campaign reinstated production from the Fulmar reservoir in 2015. In addition to replacement wells, the redevelopment drilling also included the design and execution of additional wells targeting undeveloped reservoirs and near-field exploration targets, based on the lessons learned during the initial development campaign, resulting in concurrent production from all discovered reservoirs via six active production wells by 2018.

Abstract The Upper Jurassic Wood, Godwin, Shaw and Cayley fields lie in Quadrant 22 on the Forties–Montrose High (FMH), a major intra-basinal high bisecting the Central Graben. The Wood Field was the first to be discovered in 1996 by Amoco. The field was later developed by Talisman Energy in 2007 via a single subsea horizontal producer tied back to the Montrose Alpha Platform. The Cayley, Godwin and Shaw discoveries followed during a drilling campaign carried out by Talisman Energy between 2007 and 2009 and were later developed, with the last field coming online in 2017. The fields are all complex structural and stratigraphic traps with reservoir in the Fulmar Formation. The Fulmar Formation on the FMH records an overall transgression, becoming progressively younger updip, with each field exhibiting a different diagenetic and depositional history in response to the unique evolution of the inter-pod in which they reside. The combined oil in place for the fields is currently estimated at 222 MMboe with an expected ultimate recovery of 84 MMboe. The addition of these reserves has been instrumental in helping to extend the life of the Montrose and Arbroath Platforms beyond 2030.

Abstract The Maria oilfield is located on a fault-bounded terrace in Block 16/29a of the UK sector of the North Sea, at the intersection of the South Viking Graben and the eastern Witch Ground Graben. The field was discovered in December 1993 by the 16/29a-11Y well and was confirmed by two further appraisal wells. The reservoir consists of shoreface sandstones of the Jurassic Fulmar Formation. The Jurassic sandstones, ranging from 100 to 180 ft in thickness, have variable reservoir properties, with porosities ranging from 10 to 18% and permeabilities from 1 to 300 mD. Hydrocarbons are trapped in a truncated rotated fault block, striking NW–SE. The reservoir sequence is sealed by Kimmeridge Clay Formation and Heather Formation claystones. Geochemical analysis suggests that Middle Jurassic Pentland Formation and Upper Jurassic Kimmeridge Clay Formation mudstones have been the source of the Maria hydrocarbons. Estimated recoverable reserves are 10.6 MMbbl and 67 bcf (21.8 MMboe). Two further production wells were drilled in 2018 to access unexploited areas.

Abstract The Elgin–Franklin complex contains gas condensates in Upper Jurassic reservoirs in the North Sea Central Graben. Upper parts of the reservoirs contain bitumens, which previous studies have suggested were formed by the thermal cracking of oil as the reservoirs experienced temperatures of >150°C during rapid Plio-Pleistocene subsidence. Bitumen-stained cores contaminated by oil-based drilling muds have been analysed by hydropyrolysis. Asphaltene-bound aliphatic hydrocarbon fractions were dominated by n -hexadecane and n -octadecane originating from fatty acid additives in the muds. Uncontaminated asphaltene-bound aromatic hydrocarbon fractions, however, contained a PAH distribution very similar to normal North Sea oils, suggesting that the bitumens may not have been derived from oil cracking. 1D basin models of well 29/5b-6 and a pseudo-well east of the Elgin–Franklin complex utilize a thermal history derived from the basin's rifting and subsidence histories, combined with the conservation of energy currently not contained in the thermal histories. Vitrinite reflectance values predicted by the conventional kinetic models do not match the measured data. Using the pressure-dependent PresRo ® model, however, a good match was achieved between observed and measured data. The predicted petroleum generation is combined with published diagenetic cement data from the Elgin and Franklin fields to produce a composite model for petroleum generation, diagenetic cement and bitumen formation.

Abstract The northern Dutch offshore is an area that has seen less hydrocarbon exploration activity than other areas of The Netherlands. Acquisition of a new regional 3D seismic dataset allowed further testing and re-evaluation of established geological concepts in this area. It is recognized that the presence and movement of Upper Permian Zechstein evaporites had a major impact on depositional patterns in Mesozoic sediments, structural development and hydrocarbon migration. As such, this study looks specifically at the role of salt tectonics in tectonosedimentary development. To assess this salt tectonic evolution within its structural context, a restoration of the Step Graben and Dutch Central Graben was performed. It follows that depositional patterns are closely linked to the nature of salt structure movement and the timing of regional tectonism. For example, during Late Triassic rifting, salt pillows developed and sedimentation focused away from salt structures into depocentres along regional fault trends. Restoration results show that this interplay between salt movement and tectonism is needed to accommodate the sedimentation patterns associated with the formation of the Step Graben and Central Graben during the Triassic and Jurassic, and later during Late Cretaceous and Cenozoic inversion tectonics.

Abstract In analytical models of salt diapirism, the initial salt-layer thickness and the post-deformation spacing of salt structures are key parameters. Here, 3D seismic data from The Netherlands offshore has enabled these parameters to be measured over large areas which can then be compared with model predictions. Estimates of the original salt-layer thickness were obtained by spatially filtering present thickness, using filters with varied spatial scales that remove local effects. Loss of evaporite minerals by dissolution or erosion during exposure, cannot be ruled out and, as such, thicknesses are minima. Spacing estimates were derived in two dimensions by locating the minimum separation of lines representing ridgelines of diapirs/walls. Because the length scale of spatial filtering was chosen based on the dependent variable, diapir spacing, the results are non-unique. Nevertheless, choosing an apparently optimal filter length of 50 km, a ratio between diapir spacing and original thickness from 12 to 20 is defined. This ratio is greater than has been reported for the pillow province of the UK North Sea Quadrant 44, which is as expected if pillows evolve into diapirs with progressive halokinetic deformation. This work is key to understanding the evolution of salt displacement, a necessity for unlocking remaining hydrocarbon resources.

Abstract The Middle Jurassic–Lower Cretaceous in the eastern Dutch offshore provides excellent examples of sand-rich sediments that locally accumulated in the vicinity of rift basin margins affected by salt tectonics. These types of deposits are often geographically restricted and difficult to identify, but can be valuable targets for hydrocarbon exploration. The distribution, thickness and preservation potential of fluvio-lacustrine, shallow- and deep-marine sediments is discussed to provide new insights into the regional and local tectonostratigraphy of the Dutch Central Graben, the Terschelling Basin and their neighbouring platforms. New sedimentological, geochemical, biostratigraphic, stratigraphic and structural information have been analysed and integrated into a new tectonostratigraphic model for the Callovian Lower Graben Formation, Oxfordian Middle and Upper Graben formations, Early–Middle Volgian Terschelling Sandstone and Noordvaarder members, and the Late Volgian–Early Ryazanian Scruff Greensand Formation. It is demonstrated that salt withdrawal at the basin axis was the primary control on the generation of high accommodation during the Callovian–Early Kimmeridgian. Incised valleys developed on the platforms providing lateral sediment input. During the Late Kimmeridgian–Ryazanian salt migration shifted laterally towards the basin margins, providing accommodation adjacent to active salt bodies and deposition of overthickened sandy strata.