The West Firsby Oilfield exemplifies the classic east Midlands oil play, sourced by early Namurian pro-deltaic shales. Stacked fluvio-deltaic sandstone reservoirs of latest Namurian to early Westphalian 'A' age are structured into a Variscan inversion anticline in the hanging wall of the northeastern boundary fault of the Dinantian-Namurian Gainsborough Trough. The Enterprise West Firsby-1 discovery well, drilled on a 'Top Westphalian 'A'' seismic depth closure, encountered oil-bearing sandstones at three levels. These are thought to represent the earliest Westphalian 'A' Sub-Alton/Crawshaw distributary channel and mouthbar sandstones (Reservoir Zone [RZ] 1), the stacked fluvial channel sands of the Namurian Rough Rock (RZ2) and an underlying high quality mouth-bar sandstone (RZ3). In the hanging wall setting, these units can be correlated over an area far larger than the West Firsby Field.
On acquiring operatorship of the Development Licence (DL 003) in 1991, Tullow Exploration put the WF-1 well into production from RZ2 and 3 disposing of produced water in the earlier-drilled and unsuccessful WF-2 step-out. Between 1992 and 1996, three new production wells (WF-4, -5 and -6) were drilled from the same surface site. The wells principally produce from RZ2 and involve high-angle/horizontal sections in these sandstones, approaching 170 m in length in WF-6. Strong aquifer support, coupled with the injection of produced water into RZ2 of WF-2, promotes early natural flow from such wells at rates up to 800 BOPD, and when this declines jet-pumping can maintain production at rates of several hundred BOPD/well. As of 1st January 1999, the West Firsby Oilfield had produced 0.97 MMBBL of an estimated 13 MMBBL OIIP, maintaining an average annual rate during 1998 of 345 BOPD from four wells. Remaining recoverable reserves are estimated at around 1.03 MMBBL.
Figures & Tables
Memoir 20 is the most comprehensive reference work on the UK’s oil and gas fields available. It updates and substantially extends Memoir 14 (1991), United Kingdom 0il and Gas Fields, one of the Geological Society’s best-selling books. This new edition contains updates on many of the ageing giant fields, as well as entries for fields either undiscovered or undeveloped when Memoir 14 was published.
The book is divided into nine parts covering the major petroleum provinces both offshore and onshore United Kingdom, from the Gas Basin in the southern North Sea to the Viking Graben in the northern North Sea, from the Atlantic Frontier to the Irish Sea and from the Wessex Basin to the East Midlands. Each part contains a reference map showing field locations. The introductory chapters reveal the stories behind the major plays and discoveries therein, and their tectonic and stratigraphic framework. There are two appendices: tabulated field data and a comprehensive list for all of the UK's 300+ oil and gas fields.