Synthesis of time-stratigraphic relationships and their impact on hydrocarbon reservoir distribution and performance, Bridport Sand Formation, Wessex Basin, UK
Published:January 01, 2015
Gary J. Hampson, Jenny E. Morris, Howard D. Johnson, 2015. "Synthesis of time-stratigraphic relationships and their impact on hydrocarbon reservoir distribution and performance, Bridport Sand Formation, Wessex Basin, UK", Strata and Time: Probing the Gaps in Our Understanding, D. G. Smith, R. J. Bailey, P. M. Burgess, A. J. Fraser
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The Lower Jurassic Bridport Sand Formation records net deposition in the Wessex Basin, southern UK of a low-energy, siliciclastic shoreface that was dominated by storm-event beds reworked by bioturbation. Shoreface sandstones dip at 2–3° to define (subaerial?) clinoforms that pass distally into a near-horizontal platform, and then steepen again to form steep (2–3°) subaqueous clinoforms in the underlying Down Cliff Clay Member. The overall morphology indicates mud-dominated clinoforms of compound geometry. Compound clinoforms are grouped into progradational sets whose stacking reflects tectonic subsidence and sediment dispersal patterns, and also controls basin-scale reservoir distribution and diachroneity of the formation.
Each shoreface clinoform set consists of an upward-shallowing succession that is several tens of metres thick with a laterally continuous mudstone interval at its base. The successions are punctuated by calcite-cemented concretionary layers of varying lateral continuity, which formed along bioclastic lags at the base of storm-event beds. Concretionary layers thus represent short periods of rapid sediment accumulation, while their distribution likely results from variations in storm-wave climate, relative sea-level, and/or sediment availability. The distribution of impermeable mudstone intervals that bound each clinoform set and concretionary layers along clinoform surfaces controls oil drainage in the Bridport Sand Formation reservoir.
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Strata and Time: Probing the Gaps in Our Understanding
The superposition of stratified rocks is an unmistakable manifestation of the history of sedimentary processes through deep time. However, the relationship between the preserved strata of the rock record and the passage of geological time, indisputable in principle, is unknowable in detail; incompleteness is an essential property of the record. That gaps exist at all scales in sedimentary successions is easily demonstrated from consideration of sediment accumulation rates, and expectations of continuity and completeness at any scale are correspondingly inadvisable. Locating and quantifying the gaps in the record is, however, very much less straightforward. Predictive modelling of strata – essential for their practical exploitation – requires such geohistorical understanding, yet over-simplified assumptions about how time is represented in rock can still lead to inadequate or even false conclusions. The contributions to this volume describe a range of practical studies, theoretical investigations, and numerical experiments in which the nature of the strata–time relationship is explored.