Abstract The Culzean Field is situated 240 km east of Aberdeen in Block 22/25a. The field was discovered in 2008 by well 22/25a-9Z targeting a tilted fault block, encountering lean-gas condensate in the fluvial Triassic Skagerrak (Joanne Sandstone Member) and the Jurassic Pentland formations. The field is high-pressure–high-temperature (HPHT) with initial conditions of 936 Bar (13 575 psi), 176°C (348°F) and charged with a lean-gas condensate. Development of the field was sanctioned in 2015 and is the latest UK HPHT field to be developed with start-up in mid-2019. The field development plan comprises production from six development wells drilled from a wellhead platform with bridge connections to a central process platform with accommodation on an additional bridge-linked utilities and living quarters platform. Gas is exported to shore via the Central Area Transmission System pipeline and produced condensate is exported via the floating storage and offloading vessel ‘Ailsa’. Production is expected to reach a plateau production rate in the order of 100 000 boepd with an overall recovery of up to 300 MMboe.

Abstract The Pierce Field lies 250 km east of Aberdeen, in the UK sector of the East Central Graben. The field comprises twin salt diapirs, forming the trap for oil and free gas in the Paleocene–Eocene Forties Sandstone Member reservoir. The diapirs exerted a strong influence over the sedimentation of the reservoir, with the construction of multistorey sandstone bodies forming a complex reservoir geometry further complicated by a hydrodynamic aquifer. The field currently produces to the Haewene Brim floating production storage and offloading (FPSO) installation, and has undergone several phases of development as the understanding has matured. It was initially developed with six subsea horizontal oil producers tied back to the FPSO, with produced gas reinjected through two gas injectors. In 2004–05, water injection was introduced to South Pierce to provide increased pressure support and improve sweep. To maximize recovery, four additional oil producers were drilled between 2010 and 2016, with the final (third) gas injector drilled in 2010. Production is primarily constrained by topsides gas compression capacity leading to gas/oil ratio optimization being the focus of the current field management strategy. The final phase of field development, included in the original field development plan, involves depressurization of the field with the installation of a gas export line.

Abstract The Stella Field is located in UKCS Block 30/6a, c. 230 km SE of Aberdeen. Stella Field is a four-way, dip-closed and salt-cored dome with a 600 ft gas-condensate column underlain by a 250 ft oil column within the 5–30 ft thick Paleocene Andrew Sandstone Member. Oil is also present in the underlying chalk reservoirs of the Danian Ekofisk and Maastrichtian Tor formations. Following the discovery of the field in 1979 there have been three phases of appraisal followed by the recent development, with first oil in February 2017. The field development has been challenging as the 38-year gap between discovery and first oil illustrates. Principal challenges have included structural undulations and radial faulting combined with a thin primary reservoir, variation in hydrocarbon, compartmentalization and depletion relating to producing fields. These issues have been reviewed following the latest drilling results and ideas on the petroleum geology updated. Block 30/6a, containing the Stella Field, was originally awarded to Shell/Esso in the first Licensing Round in 1964 as part of multi-block licence P011. The current Stella P.011 licence holders are Ithaca Energy (UK) Limited (operator) with 54.66%, Dyas UK Limited with 25.34% and Petrofac GSA Limited with 20%.

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 Mariner Field is located on the East Shetland Platform in UK Block 9/11, 320 km to the NE of Aberdeen. The 9/11-1 well was drilled in 1981, discovering oil in the Paleocene Heimdal Sandstone Member and the Paleocene Maureen Formation. The deeper Maureen Formation was deposited on a submarine slope as canyon fill, and overspill canyon sands and debrites. The Heimdal Sandstone Member was deposited above the Maureen Formation on the same inherited, but younger, submarine slope system as deep-water slope channels, and is encased within suspension-deposited Lista Formation claystones. These slope channel sands have undergone post-depositional remobilization and sand-injection processes, leading to the complex sandstone geometries identified and mapped on seismic data. During a 36-year period, 28 exploration, appraisal and production test wells were drilled in UK Block 9/11, but only now is the field finally undergoing development. The long interval between discovery and development is due to several key issues, including the fact that development of heavy oil is technically and commercially challenging, and that the seismic imaging of the Heimdal Sandstone Member has been difficult or not possible until recently. These challenges are being overcome and the Mariner Field started producing in August 2019.

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 Huntington discoveries are an unusual exploration success in that two oil accumulations were tested in separate syn- and post-rift reservoirs with a single well. The discoveries are located 205 km east of Aberdeen in the East Central Graben some 35 km east of Forties Field in 300 ft of water. The 22/14-5 discovery well, drilled in May 2007, encountered a 122 ft oil column in the Paleocene Forties Sandstone and also a 136 ft oil column in the Upper Jurassic Fulmar Sandstone. Both the Forties and the Fulmar contain high-quality oil, 41 and 39° api gravity, respectively. Aggregate flow rates from the two zones exceeded 11 000 boepd on test. Appraisal drilling of the Forties was completed in late 2007 with first oil targeted for 2011. The Fulmar appraisal programme is currently in progress. The Forties reservoir is a high net to gross sandstone containing stacked channel sequences deposited in a submarine fan system. The Fulmar reservoir also contains a thick sand package deposited in a shallow marine shelf setting. Pre-drill mapping based on reprocessed 3D seismic indicated a structural closure on both horizons at the location tested by the well. At both the Forties and Fulmar targets, however, the oil column height exceeded the pre-drill prognosis. This overview will focus on pre-drill perceptions of the prospect relative to actual drilling results.

Abstract The Jasmine Field is located in blocks 30/06 and 30/07a on the J Ridge, the southeastern extension of the Forties–Montrose High, which separates the eastern and western basins of the UK Central North Sea. The field was discovered in 2006 and is close to two ConocoPhillips producing fields, Jade and Judy, which serve as useful local analogues. The main West Limb structure is a turtle-back faulted anticline NW of the Joanne salt pillow. The primary reservoir is Triassic in age and consists of stacked fluvial sandstones of the Joanne Member of the Skagerrak Formation. The HPHT exploration wells 30/06-6 and geological sidetrack 30/06-6Z discovered a rich gas condensate column of 2300 ft, some 1100 ft deeper than the mapped independent structural closure of the prospect, and achieved good flow rates on test. To appraise the discovery and assess the potential for significant additional volumes in an adjacent downfaulted terrace, a programme comprising a main well and two sidetrack wells was initiated in 2007. Appraisal well 30/06-7 discovered a 550 ft hydrocarbon column in the Northern Terrace with a hydrocarbon–water contact shallower than that observed in the West Limb, thereby proving structural compartmentalization between the two fault blocks. Good flow rates were achieved from a drill stem test in mechanical sidetrack well 30/06-7Z. Sidetracks 30/06-7Y and 30/06-7X were drilled to confirm the northwestern extension of the West Limb discovery and to test the northern extent of the Northern Terrace accumulation, respectively. This programme has reduced volumetric uncertainty but the trapping mechanism and the ultimate extent of the Jasmine accumulation remain unknown. Comprehensive data acquisition throughout the exploration and appraisal phases, including drill stem testing, core recovery and seismic data reprocessing, has facilitated a detailed reservoir characterization programme. Jasmine represents a significant new high pressure/high temperature resource in the mature Central North Sea and is currently undergoing development planning.