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1 Mobil North Sea Limited, 3 Clements Inn, London WC2A 2EB, UK
2 Department of Earth Sciences, University of Leeds, Leeds, UK
The Sherwood Sandstone Group (SSG), of Early Triassic Scythian age, contains attractive fluvial and aeolian sandstone reservoirs in the basins between Ireland and Britain (Morecambe Bay gas field, 6.5TCF) and in southern England (Wytch Farm oil field, 300 MMBBL). While the SSG is very thick in the northern portions of the Mesozoic basin system, it is thinner and generally less well preserved in the southern portions of the Mesozoic basin system which are underlain by the Carboniferous aged Variscan fold-thrust belt. Previously it has been suggested that the SSG was deposited during a time of rifting, and consequently its variable presence was attributed to wells penetrating positions within fault blocks that were high and devoid of syn-rift deposition. Data examined in this study from the East Irish Sea Basin and from the South Celtic Sea Basin, however, suggest that the SSG was deposited in palaeotopographic lows, the positions of which were controlled by erosion around massifs and resistant thrust belt hangingwalls that formed the palaeotopography. Furthermore, the evidence presented suggests that rifting occurred during the deposition of the Upper and Middle Triassic Mercia Mudstone Group (MMG), which is often defined as a post-rift succession.
Successful prediction of the areas of SSG deposition depends on the ability to reconstruct the Early Triassic palaeotopography as it was controlled by the Carboniferous thrust belt morphology. A reconstruction of the Variscan thrust belt is presented, which is based on onshore surface geology, thrusts and transfer faults from published onshore mapping, published offshore thrust plane reflector mapping, gravity data showing lineaments and indications of granite plutons, and a set of observations from well and seismic data. Some thrusts are postulated by observations of reactivation during Mesozoic extension.
A set of predictive tools has been developed for identifying the presence of the SSG. Determination of the presence of the SSG on seismic reflection data is often difficult. Where the data quality is poor, we postulate that there is a better chance of finding the SSG where halites are found in the overlying MMG. This empirical relationship is believed to occur because rifts developed over SSG foreland and intermontane valleys were near sea-level, allowing marine incursions and salt precipitation. By contrast, rifts that developed in high topographic areas were above sea-level and not subject to marine incursion. Additionally, the degree of deformation seen in the pre-Triassic section on seismic data can be used to predict the position of the Variscan Front and thus the topographically low, sand-prone areas overlying the Carboniferous foreland basin. A palaeogeographic map is presented, incorporating the analysis of all of these datasets and utilizing a comparison to the topography of the exposed Ouachita Mountains of the southern USA.
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