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ABSTRACT

The Triassic Sherwood Sandstone Group reservoir in the Wytch Farm Field is a c. 150m thick red bed succession deposited in a variety of fluvial, lacustrine and aeolian depositional systems within an actively extensional basin. These systems show at least three orders of facies cyclicity which are interpreted to be the depositional response to base level and climatic changes. The first-order evolutionary trend spans the entire Sherwood Sandstone Group and marks a change from perennial braidplain to ephemeral sheetflood systems to ephemeral lacustrine conditions, culminating in the deposition of playa mudrocks of the overlying Mercia Mudstone Group. The Mercia mudstone shows evidence of marine incursions (several hundred meters above the Sherwood). The first order trend reflects a long-term waning of sand supply across the Wytch Farm area and increasing ‘flashiness’ of the fluvial system coupled with a long term base level rise. The first order facies trend can be subdivided into second-order cycles within the Sherwood by four areally widespread, heavily rooted floodplain and ephemeral lacustrine deposits containing only minor fluvial sandstones. The floodplain and lacustrine deposits represent widespread reductions in sand flux combined with rising base level during more ‘humid’ climatic conditions. These horizons form the basis for the reservoir layering scheme because they are field-wide, have unique core and wireline log expressions and bound fluvial intervals of fundamentally different depositional character. Higher frequency third order cycles are defined by thin (<2m), but areally 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 eroded by coarse grained multistorey/multilateral channel deposits infilling incisional topography. 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. The interfluves to the incisions are locally marked by deep rhizocretionary calcrete precipitated around the roots of phreatophyte-like plants which attempted to reach the lowered water table. The infilling fluvial deposits form the principal producing intervals in the upper part of the reservoir. At outcrop, the stratigraphically equivalent Otter Sandstone Formation (c. 100km to the west) shows a comparable punctuated evolutionary pattern, albeit with a subtly different facies make-up. The recognition of a hierarchy of facies cycles within the reservoir permits high resolution correlation and the recognition of subtle, but important changes in sandbody geometry and connectivity within successive cycles.

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