A braidplain facies model for the Westphalian B Coal Measures of north-east England

Incised valley fill sandstone bodies in Upper Carboniferous fluvio–deltaic strata: recognition and reservoir characterization of Southern North Sea analogues

Multistorey fluvial sandstone bodies in Upper Carboniferous fluvio–deltaic strata are proven reservoirs in the Southern North Sea. Several of these bodies have been interpreted as incised valley fills. Analogous valley fill sandstone bodies have been studied in outcrop and subsurface datasets from Upper Carboniferous successions in the onshore UK and Germany. Using techniques and tools applicable to Southern North Sea datasets, we review the diagnostic criteria of these valley fills: (1) stratigraphic context, characterized by deep erosion into underlying strata, lateral correlation to an interfluve surface and intimate association with an initial flooding surface in overlying strata; (2) basinward facies shift at the base of the sandstone body; (3) erosion of underlying time markers; and (4) distinctive internal architecture, which reflects increasing accommodation space during valley filling.

Several features of onshore valley fill sandstones have been characterized in order to constrain the properties of analogous valley fill reservoirs in the Southern North Sea. (1) Valley geometry and extent. Studied onshore valley fills generally have a uniform thickness of 20–25 m, thickening locally to 30–45 m, and widths of 5–25 km. A small number of valley fills, associated with turbidite-fronted deltas, thicken to 50–80m in the 2–5 km adjacent to their mouths. Sheet-like fluvial sandstones overlying sequence boundaries have thicknesses comparable to conventional valley fills, but exceed 35–70 km in width. (2) Valley connectivity; typically, valley fills occur as discrete sandstone bodies, but they may amalgamate laterally, to form sheet-like sandstones, and vertically, producing thick (up to 200 m) sand-prone stratigraphic packages. The internal geometry of these sand-prone packages is complex. (3) Valley fill character. Valley fills generally lack an overall fining-upwards trend and are dominated by coarse- to medium-grained sandstone of low-sinuosity (braided?) fluvial facies throughout. Where present, fine-grained facies are preferentially preserved in the upper part of a valley fill. (4) Detrital mineralogy and provenance. At least some valley fill sandstones exhibit a different composition and provenance to surrounding strata. Such valley fill sandstones are likely to be anomalously feldspathic in the Southern North Sea.

Regional correlation of Westphalian sandbodies onshore UK: implications for reservoirs in the Southern North Sea

An extensive and closely-spaced seismic and well dataset from coal and hydrocarbon exploration in the East Pennine Coalfield and the East Midlands Oilfield has enabled the detailed subsurface correlation and mapping of major Westphalian A to C sandbodies. Maximum flooding surfaces are represented by marine bands which are faunal concentrate condensed horizons. The large fluvial sandbodies do not show characteristics of incized valley fills. The remoteness of the study area from the sea during deposition meant that eustatic changes are less significant than other factors. The interplay of tectonics, climate and autocyclic processes was responsible for exceedingly complex sequences in terms of both lateral and vertical organization, and it is difficult to isolate the relative importance of the various mechanisms controlling sedimentation.

Closely spaced onshore borehole data and outcrop studies have been used to model reservoir characteristics and continuity offshore in the Southern North Sea, where well spacing is far greater. Architectural variability and 3D geometry of onshore sandbodies can be used as an analogue for Southern North Sea reservoirs. Geometric data from 20 mapped onshore sandbodies illustrates a relationship between sandbody width and thickness, with 90% of channel sandbodies being less than 25 km in width and less than 40 m in thickness. A remarkable similarity exists between the sand distribution onshore UK and Westphalian reservoir intervals in the Southern North Sea. Sandbodies onshore and offshore show comparable thicknesses (up to 100 m) and widths (up to 30 km). Additionally, the sandbodies do not display incised bases, as is also typical of all the main Westphalian reservoir targets in the Southern North Sea.