Abstract Reservoirs in the Volund Field are all sandstone intrusions with wings on three sides forming the main reservoir volumes. The southern wing was the target of exploration and appraisal wells, which led to the field development. Identification of three smaller intrusions proves the southern wing to be a composite intrusion, similar to outcrop analogues. Identification from core and borehole logs shows that it comprises sandstone-, mudstone- and mudstone-rich intervals, including mudstone clast breccia. Mudstone clast breccia constitutes a significant missed pay candidate. Breccia is porous and has a sand-supported matrix, which gives it excellent reservoir quality. This may be missed pay using analysis of borehole logs. Well data, largely borehole logs, show consistently uniform sandstone porosity distribution within the intrusions, independent of depth. Significantly, at about 100 m from the depth at which the wing emanates from sills, porosity has a broader spread of values. The spread of values is attributable to mudstone clast breccia and thin-bedded sandstone and mudstone. Porosity derived from borehole logs does not differentiate breccia from siltstone, but inference is possible using calibration of logs with core.

Abstract Identification and exploration drilling of the Volund Field as part of a sand injection complex is the first example of deliberate targeting of sandstone intrusions in oil exploration. Outcrop data were an important element in the process of constraining the uncertainty associated with reservoir presence and connectivity. A strong lobby against the relevance of sand injectites as exploration targets, and significant uncertainty associated with sub-surface sand injectite analogues associated with existing oil fields, combined to discourage and down-grade Volund’s prospectivity. Few outcrop studies provided data of relevance to exploration of sandstone intrusions and original outcrop data were utilized in evaluation of Volund. Sills and saucer-shaped sandstone intrusions are the most common reservoir units observed at outcrop and similar features were identified in the 3D seismic across the Volund Field prospect. A large-scale sand injection origin rather than a depositional origin was proposed. Outcrops of sandstone intrusions demonstrated excellent reservoir quality in composite sandstone bodies that cross-cut depositional bedding. High-quality reservoirs with excellent vertical and lateral connectivity are observed and used to support the prediction of similar quality reservoirs in the Volund prospect.

Abstract The Volund Field lies in the Norwegian sector of the North Sea (Quad 24/9). This field produces from a ‘classic’ large-scale sandstone injection complex located in Lower Eocene strata. The sandstone reservoir has been injected into the lower permeability surrounding mudstones of the Sele and Balder formations and Hordaland Group to create an ‘intrusive trap’. The Volund Field consists of a deeper central unit of stacked sandstone sills, surrounded by shallower, steeper-dipping injected sandstone dykes, which make excellent reservoirs with consistently high porosity and permeability. Many of the steeply-dipping injected dykes appear to have excellent connectivity from the water leg through the oil leg and into the gas cap. The complex was identified on seismic data that exhibit a Class 3 amplitude versus offset (AVO) signature on the far-offset stack reflection seismic volume. The seismic data have been used to successfully locate horizontal production wells. Volund seismic geobodies have been extracted and incorporated into the reservoir geomodel to determine the geometry of the injectite features and to populate sands within the injection complex. Volund Field (estimated mean gross resource of 54 mmboe (million barrels oil equivalent)) is producing oil from four horizontal branches (end December 2012), with one water injector well, and has a common oil–water contact and gas–oil contact.

Abstract Tertiary injectites, which are re-mobilized sandstones, represent commercially attractive targets for near-field, long-reach tie-backs to existing field infrastructure above and adjacent to producing reservoirs. Injectites exhibit exceptional porosity–permeability and productivity, such that discoveries can add incremental reserves and lift production decline in mature fields. Abundant injectites are visible on seismic data in the Tertiary of the Viking Graben, but, as some disappointing wells have established, not all are hydrocarbon-charged. The challenge is to reliably distinguish hydrocarbon-filled high porosity–permeability features from tight or dry reservoirs in a cost-effective way. Regional rock physics analysis of injectite reservoirs, using well data from fields in Norway and the UK, reveals that a combination of elastic attributes can effectively discriminate lithology and hydrocarbon presences in these reservoirs. After pre-stack conditioning, broadband seismic data correlate reliably with wells, giving confidence that pre-stack seismic is faithfully imaging the elastic properties of the subsurface in lower Tertiary target intervals. Informed by rock physics analysis, a combination of broadband seismic elastic attributes is used to predict sand presence and de-risk hydrocarbon presence in reservoirs v. water-wet targets. Hydrocarbon sand distribution predicted from relative acoustic impedance and V P / V S matches to known accumulations and identifies remaining near-field opportunities.

Abstract Observation of basin-scale networks of sandstone intrusions are described from subsurface studies and outcrop locations. Regional scale studies are prevalent in the volume and two new regionally significant subsurface sand injection complexes are described. Higher resolution studies, both outcrop and subsurface, show the small-scale complexity but high level of connectedness of sandstone intrusions. Discordance with bedding at all scales is diagnostic of sandstone intrusions. The propensity of hydraulic fractures to develop and fill with fluidized sand in a broad range of host rocks is demonstrated by examples from metamorphic and magmatic basement, and lignite. Terminology used to describe sandstone intrusions and other elements of sand injection complexes is diverse.

Abstract Field-based studies, be they regional or basin-scale fieldwork, or the examination of outcrop analogues, have been an important component of petroleum system studies in the upstream oil and gas industry for many years. Recent technological advances, founded on traditional field methodologies, whether they are in digital outcrop characterization or developments in processing and computational capabilities, have fuelled a resurgence in such fieldwork. These studies enable the building of realistic models that allow the exploration or production geoscientist to navigate effectively from plate-to-play-to-pore. The interrogation of field data and resulting models provide a more informed means of benchmarking the subsurface, as well as a more realistic view of subsurface uncertainty and the management of associated risk in subsurface description and prediction. This Special Publication records 12 papers given at a conference titled ‘Reducing Subsurface Uncertainty and Risk Through Field-Based Studies’ organized by the Petroleum Group of the Geological Society of London, and held in London from 4 to 6 March 2014. The observations drawn in this introductory paper reflect the authors’ experiences, presentations at the conference and manuscripts within this volume.

Abstract The Balder Field reservoir sandstone has been interpreted as remobilized from a Mesozoic parent bed. This paper seeks to address the questions raised about this unusual origin. Research and broadly analogous processes are reviewed, leading to the proposal that the parent beds may have been fluidized by bedding-parallel retrogressive entrainment of Statfjord Formation sands by a connected larger source of overpressured fluids. These fluids are identified as most likely derived by lateral migration from the Viking Graben, initiated in response to early Eocene basin inversion related to North Atlantic rifting. The event probably involved breaching of the topseal in multiple places over a large section of the Utsira High. The geometry of large sills formed from small breach points and the internal differentiation seen may show that the sills inflated by lateral accretion from a medial active flowing zone of turbulent, transitional or laminar flowing suspension. It is suggested that sequence-stratigraphic and structural context should be considered as additional criteria to discriminate between depositional and intruded sands.