Abstract The Bacchus Field, discovered in 2004, is a small borderline high-pressure–high-temperature (HPHT) oil field 6.8 km east of the Forties Alpha Platform. The reservoir is Fulmar Sandstone with a rotated fault-block trap. The reservoir is typically thin (10–50 m) and difficult to image seismically. Compartmentalization was anticipated due to significant in-field faulting. The Bacchus development decision was made when considerable geological uncertainty remained. The key risk-mitigation strategies employed during the development of Bacchus were to drill long horizontal wells, contacting multiple reservoir compartments, while maintaining a flexible development plan. The ability to react to unexpected results was facilitated by optimizing the development data-acquisition programme. Drilling risk and cost were minimized by exploiting existing well control for landing development wells, combined with pilot drilling in untested parts of the reservoir. Development wells were designed to be geometrically robust, minimizing the requirement for geo-steering. This ensured low wellbore tortuosity that did not compromise the completions. Bacchus was successfully developed despite the final distribution of reserves being radically different from the pre-development perception. It is argued that maintaining a flexible development plan was far more effective in maximizing the value of the Bacchus development than more extensive pre-development appraisal or modelling.

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 Forties Field, discovered by BP in 1970, is the largest oilfield on the UK Continental Shelf. It is trapped in a simple four-way dip closure, with a Paleocene turbidite sandstone reservoir. The Forties Field originally contained between 4.2 and 5 billion bbl of oil, with 2.75 billion bbl produced to June 2017. Production has been supported by water injection and the influx of a regional aquifer. The original development contained equally spaced producers with peripheral injectors. As the field matured, production was concentrated in the crestal parts of the field with injectors tending to be moved upflank. With the development of seismic lithology prediction and fluid detection, together with 4D seismic technology, it became possible in the late 1990s to target bypassed oil in unexpected locations throughout the field. In 2003, BP sold the field to Apache who were able to rejuvenate production, adding over 170 MMbbl oil reserves, with an extended drilling campaign targeting bypassed pay identified using seismic technologies. Production at the Forties facility has been further enhanced by the development of four satellite oilfields, Bacchus (Jurassic reservoir), Brimmond, Maule and Tonto (Eocene reservoirs), together with Aviat (Pleistocene reservoir) produced for fuel gas supply.

Abstract The Forties Field, the largest oilfield in the UK North Sea, has been a prolific producer since its initial development. With an initial plateau rate of 500 000 bopd the field had produced some 2500 mmbo and the field rate had declined to 41 000 bopd by 2003 when the operatorship changed from BP to Apache. From 2004 to 2012, over 100 bypassed pay targets were drilled with a success rate of 75%, establishing a late life plateau of 50 000–60 000 bopd. The Forties reservoir is provided by Paleocene turbidites of the Forties Sandstone Member of the Sele Formation, deposited in a channelized proximal area of the Forties Fan. In this paper, the reservoir architecture is described, and bypassed pay examples are discussed in the context of the reservoir architecture and the production history. Bypassed pay is shown to occur in both the high net to gross channel axes and the heterogeneous wing deposits. Oil is trapped by subseismic channel architecture and subtle faulting. The occurrence of bypassed pay at a particular location is also shown to be dependent on the continually evolving pattern of injection and production within the field.

ABSTRACT Sixteen oil and gas fields have been discovered and developed along the western margin of the South Viking Graben in Quadrant 16 of the United Kingdom Continental Shelf. Late Jurassic extension created the graben, and submarine fan conglomerates and sandstones along its margin form most of the fields’ reservoirs. In the early 1970s, 2-D seismic was able to identify structures beneath the Base Cretaceous unconformity, which became the targets for initial drilling. The first well was Shell’s 16/8-1 in 1972, drilled toward the graben center. This well found hydrocarbons in interbedded sandstones and shales later developed as the Kingfisher field. Drilling in the more proximal Brae Formation conglomerates began in 1974 when 16/7-1 discovered the North Brae gas condensate field. However, an appraisal well to the south found an oil column, and this subsequently became Central Brae field. In 1976, drilling on another submarine fan two blocks to the south discovered the Thelma field. However, the key to developing the area was the discovery of the world-class South Brae oil field in 1978. This was rapidly appraised in the next two years and the Brae A platform was installed, with first oil produced in 1983. Meanwhile, the compulsorily relinquished portions of Blocks 16/7 and 16/8 were awarded to BP and Conoco, respectively, who discovered the Miller field extending across the block boundary in 1982. A further four platforms have been installed in the area: Brae B on North Brae, onstream in 1988; Miller in 1992; and East Brae and Tiffany in 1993. A further 12 fields have been developed by subsea tieback or by extended reach drilling. A billion barrels of oil and 7 tcf of gas have been produced from these fields.

Abstract A novel 4D methodology by targeting softening responses, a decrease in impedance over time, is described from the Forties Field. It is demonstrated that these anomalies identified areas where oil had re-saturated previously swept rock. When first observed in some of the older vintages of 4D data, softening responses were dismissed as side lobes of underlying water-swept reservoirs. With improved 4D data quality, softening responses were seen to be clearly responding to an acoustic softening effect isolated in the reservoir. Pressure measurements ruled out hypotheses of gas exsolution or geomechanically induced dilatation of the overlying shale. At the time of writing 22 re-saturation targets had been drilled at Forties with an average water saturation ( S w ) of 28%, close to virgin conditions. These results indicated that a previously swept sand can be re-saturated to near initial oil conditions. The driving mechanism behind the re-saturation is understood to be related to changes in both production offtake and water injection. The resulting complex hydrodynamic interactions appeared to be moving banks of oil around the field. From 2011 to year-end of 2015 re-saturation targets produced 14.6 MMBO and were a key target type at the Forties Field.

Abstract The Brechin Field was discovered by Paladin Resources in 2004. The reservoir is the Paleocene Forties Sandstone Member, with oil trapped in a small dip closure. The prospect was identified as a bright anomaly on a fluid volume processed as part of a 4D survey. However, the anomaly was not consistent with rock physics modelling, and an earlier processed version of the baseline 3D survey showed the opposite response to the 4D datasets. It was established that the fluid anomaly on the 4D data resulted from constructive interference between reflections from the oil–water contact and from top reservoir. Thus, both versions of the data were responding to the presence of hydrocarbons. The discovery well, 22/23a-7, encountered 137 ft of gross oil-bearing reservoir. Without testing or further appraisal, the field was developed in 2005 with a single production well tieback to the Arbroath platform, via the production manifold at Arkwright Field. Brechin is now operated by Repsol Sinopec Resources and cumulative production to mid-2018 was 4.4 MMbbl. Seismic attributes and well-developed reservoir understanding from neighbouring analogue fields permitted the necessary de-risking of hydrocarbon presence, recovery and volumetric uncertainty, to permit the development of an otherwise economically marginal resource.

Abstract This paper updates the earlier account of the Forties Field detailed in Geological Society Memoir 14 ( Wills 1991 ), and gives a brief description of the Brimmond Field, a small Eocene accumulation overlying Forties (Fig. 1 ).

Abstract The Eocene age Brimmond Sand Fairway is situated along the north-eastern flank of the Paleocene Forties Field (UKCNS blocks 21/10 and 22/6). Located along the western margin of this Brimmond Fairway are well imaged remobilized sands that form the reservoir interval for the Maule and Tonto Fields and, along with deep-water channels, the Brimmond Field. These Eocene Brimmond sandstones are encased in the Horda Shale which provides the sealing lithology. The interpretation of these remobilized and injected sands is driven from geometries derived from 3D seismic and historic logging of thin sandstones in the Eocene interval. Conical shape features with sills and steep dykes are mapped, with seismic evidence of injection along active faults and fractures. The developments of the Brimmond, Maule and Tonto Fields has been successful due to impressive seismic imaging with inversion and Direct Hydrocarbon Indicator (DHI) volumes allowing the identification of hydrocarbon bearing remobilized sandstones, along with 4D data imaging un-swept areas.

Abstract The search for a gas source near to Apache's Forties Field in the North Sea was motivated by the prediction of an ever-increasing fuel gas shortfall as the field oil rate declined. The Central North Sea is well known for a large number of shallow gas hazards in the Pleistocene section that have historically caused blowouts during exploration and development. These gas accumulations typically show up as small bright anomalies on seismic data. In 2009, a large gas anomaly was identified to the east of Forties, and the Aviat Field was discovered in 2010 when exploration well 22/7-5 was drilled. The Aviat Field reservoir is interpreted to be a subaqueous glacial outwash fan, consisting of silt-grade, rock flour material, deposited in front of a grounded ice sheet in some 400 m of water. Aviat sits on an overcompacted silty mudstone that was deformed by this ice sheet – the Crenulate Marker. The distribution of this horizon implies that the Early Pleistocene ice sheet covered at least the northern half of the UK North Sea. Although the Aviat reservoir is thin (2–9 m thick), the well tests, pressure profiles and geophysical response demonstrate that the reservoir is well connected, extensive (over 35 km 2 ) with high deliverability (up to 18 MMscfd achieved). Aviat was sanctioned in 2014 for development as a fuel gas supply for the Forties Field, with first gas achieved in July 2016.