The Lancaster Field: progress in opening the UK's fractured basement play

Abstract

To date, fractured crystalline basement reservoirs (basement) on the UK Continental Shelf (UKCS) have largely been underexplored, despite the fact that numerous indications of hydrocarbons have been reported from basement in wells dating back to the 1970s. As production from the UKCS continues to decline, and with the exploration potential of more traditional plays becoming increasingly mature, the potential of the overlooked and underrated basement play warrants further exploration. Over the last 10 years, Hurricane Energy (Hurricane) have deliberately set out to explore the potential of this untapped resource, focusing on the Rona Ridge trend, West of Shetland. The Lancaster Field has been penetrated by four wells and benefits from a full 3D seismic survey, and, as such, represents Hurricane's most de-risked basement asset. The level of understanding of the Lancaster reservoir is such that Hurricane is now working towards a phased field development. This paper provides a summary of the geology and reservoir characteristics of the Lancaster Discovery, and a description of the technical progress achieved, to date, in de-risking the Lancaster Field.Gold Open Access: This article is published under the terms of the CC-BY 3.0 license.The Lancaster reservoir is the UK's first fractured basement field and was granted field status in September 2015. The field is delimited by four wells, each of which has been the subject of drill stem testing resulting in oil to surface. Estimates of the resource potential at Lancaster are provided by an independent Competent Persons Report (CPR), commissioned as part of the process for the admission of Hurricane Energy (Hurricane) to the AIM market in 2014. Consequently, this volumetric assessment has been subject to the 2007 SPE/AAPG/WPC/SPEE Petroleum Resource Management System (PRMS). Volumetric ranges are presented as Table 1: however, it should be noted that the volumetric assessment was written prior to the 205/21a-6 horizontal development well drilled in 2014, the productivity of which transformed Hurricane's understanding of the Lancaster reservoir productivity potential. This paper builds on previous publications (Slightam 2012;Trice 2014;Bonter 2015;Belaidi et al. 2016) and provides a summary of the progress Hurricane has made in de-risking Lancaster. The paper is split into five sections covering drilling history, reservoir description, fracture porosity, field simulation and technical de-risking. A concluding section on the future plans for the Lancaster Field is also provided. Drilling historyThe Lancaster Field is penetrated by four wells, as summarized in Figure 1. The first well was drilled in 1974 and the most recent well in 2014. The objectives of each well and their findings are summarized in this section.Well 205/21-1A was drilled in 1974 and was targeted at a Mesozoic succession directly overlying the Precambrian Basement. The well serendipitously encountered oil within the basement, with shows observed in open fractures (documented in well-site core de...

Natural fracture propping and earthquake-induced oil migration in fractured basement reservoirs

“…The unaltered condition of the basement cores suggests that the Rona Ridge is little affected by deep subaerial weathering. However, analysis of seismic reflection data and cores from the Lancaster Field (Slightam, 2012;Belaidi et al, 2018) has revealed the presence of mineralized sediment-filled and breccia-filled fissures, several hundred meters deep, and up to several meters wide. These findings are consistent with the sediment-and mineral-filled fracture systems reported here all along the Rona Ridge at depths many hundreds of meters below local top basement.…”

“…1B; Coney et al 1993). Other assets, such as the Lancaster Field and associated discoveries in the southwest Rona Ridge, are hosted almost entirely within fractured basement (Trice, 2014;Belaidi et al, 2018; >18 billion barrels of oil initially in place). CITATION: Holdsworth, R.E., et al, 2019, Natural fracture propping and earthquake-induced oil migration in fractured basement reservoirs: Geology, v. 47, p. 1-5, https:// doi .org /10 .1130 /G46280.1 Geochemical studies of oils suggest a Late Jurassic Kimmeridge Clay source rock (Holmes et al, 1999;Finlay et al, 2011), thought to occur on the downfaulted flank of the ridge in the Faroe-Shetland Basin to the northwest (Ritchie et al, 2011).…”

“…Where oil is present, the main host fractures form the youngest and dominant set of brittle structures seen in basement cores, overprinting a variety of earlier ductile and brittle structures (see Holdsworth et al, 2018). Interpretation of seismic reflection profiles, well data, and oriented cores in Clair and Lancaster suggest a predominance of northeast-southwest fractures parallel to the trend of the ridge (Coney et al, 1993;Pless, 2012;Belaidi et al, 2018). The most continuous sample of these fractures comes from six 10-m-long core sections in the subhorizontal northwest-southeast 206/7a-2 well through the Clair Ridge (drilled by Elf U.K. Ltd. in 1991; Fig.…”

Ecological Role of an Offshore Industry Artificial Structure

“…The observations reported here were made at wells drilled as part of a fractured basement exploration (Trice, 2014) of the Lancaster field, west of Shetland in the North East Atlantic (Figure 1). The Lancaster field is now undergoing development (Belaidi et al, 2018). The protective structure was in an area where the majority of the seabed consists of coarse sandy sediment with areas of hard substratum (boulders) to the north and west (Figure 2).…”

Fractured basement play development on the UK and Norwegian rifted margins

“…Despite the improved porosity associated with fault zones, mapping of porosity using seismic methods is not possible, and areas of potential high porosity are quantified through the mapping of seismic-scale faults and attributing fault zone widths as statistical ranges. Fault zone widths are estimated from the ranges of fault zones established through drilling and wireline data (Belaidi et al 2016). Porosity generation is attributed to a pre-existing fracture network that has been enhanced by a combination of fissuring (tensile failure), partial mineralization due to influx of hydrothermal fluids (e.g.…”