Reservoir architecture of the upper Sherwood Sandstone, Wytch Farm field, southern England

Abstract

Abstract The Sherwood Sandstone Group reservoir in the Wytch Farm field comprises a c. 150 m thick succession of arkosic sandstones deposited in a variety of fluvial, lacustrine and aeolian depositional systems. These systems show at least three orders of facies variability, which are interpreted to be the depositional response to climatic changes. These comprise a first-order evolutionary trend over the entire Sherwood Sandstone Group from perennial braidplain to ephemeral sheetflood systems to ephemeral lacustrine conditions. This trend culminated in deposition of the Mercia Mudstone Group, and reflects a long-term waning of sand supply and increasing ‘flashiness’ of the fluvial system. This trend is further subdivided into second-order cycles defined by five areally widespread floodplain and lacustrine deposits containing minimal development of fluvial sandstones. These represent widespread, episodic reductions in fluvial sediment supply and rising base level during more ‘humid’ climatic conditions. These horizons form the basis for the reservoir layering scheme. Each floodplain episode is increasingly more mud-rich upwards through the Sherwood section, and the sand-rich fluvial packages between become systematically more ephemeral in character. Third-order cycles are defined by thin (<2 m), but alreally widespread floodplain and lacustrine horizons which are most readily identifiable in the upper half of the Sherwood section. The sandstones between these cycles are composed of aeolian and sheetflood deposits, but are incised by coarse-grained multistorey-multilateral channel deposits. The incisions are interpreted to be the result of fluvial erosion during dry climatic conditions when lake levels fell and the alluvial plain was devegetated. These incised fluvial deposits form the principal producing intervals in the upper part of the reservoir, particularly in the eastern part of the field. Higher frequency stratigraphic cycles are locally expressed by variations in ephemeral lake levels, palaeosol development and episodic development of wind-blown sand patches. At outcrop, the stratigraphically equivalent Otter Sandstone Formation ( c. 100 km to the west) shows comparable evolutionary patterns, albeit with a subtly different facies make-up. The recognition of a hierarchy of climatically driven cycles within the reservoir permits high-resolution correlation and the recognition of subtle, but important, changes in sandbody geometry and connectivity within successive cycles.