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

Abstract

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.