Abstract During Late Paleocene–Early Eocene times, the modern Rosebank structure was located at the juxtaposition of the easterly advancing Flett volcanic system and the northerly prograding Flett delta. As a result, the Rosebank reservoir sandstones are interstratified with volcanic and volcaniclastic rocks, offering challenges for reservoir imaging, depth prediction and reservoir characterization. These challenges have driven the application of Ocean Bottom Node (OBN) seismic technology. OBN data have yielded improved velocity models for depth conversion, better reservoir definition and key insights to aid the modelling of sand distribution from seismic attributes. Spectral decomposition of the OBN seismic data has facilitated the extraction of distinct volcanic subunits, whilst spectral enhancement has enabled visualization of complex stacking patterns within individual igneous layers. To complement the seismic analysis, detailed geological analogue studies have been undertaken in volcanic provinces such as the Palaeogene volcanic district of SE Greenland and the Columbia River Flood Basalt Province, USA. No single outcrop provides a definitive analogy to Rosebank, but each offers insights that provide an important link to understanding and managing the main subsurface uncertainties associated with field development. Integration of these multiple workflows have improved the reservoir characterization and provided the foundation for the optimization of the field development plan.

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.

Abstract The Clair Field is a giant oilfield located approximately 70 km west of the Shetland Isles, UK. It was discovered in 1977 and brought on stream some 28 years later. Key to unlocking its economic potential was a series of appraisal wells drilled in the early 1990s that identified fractures as the primary production mechanism. Structural geology contributed in several ways to the detailed planning of the development and appraisal wells. In the sandy (Tertiary) tophole section, outcrop analogues and offset wells were used to establish an appropriate standoff from major faults. This was to mitigate the risk of wellbore instability in what is otherwise a relatively benign sequence to drill. The mid-section, Upper Cretaceous mudstone is prone to wellbore instability, believed to be caused by strength anisotropy with respect to bedding. Here, polygonal faulting may contribute directly to wellbore instability. The associated bed rotation also influences anisotropic failure, which depends in part on the wellbore-to-bedding intersection angle. An example is given of how an understanding of the structural evolution of the overburden section impacts well casing placement. Finally, judgement on the nature of the faulted contact between two fault blocks was required for the pressure prognosis of a planned well.