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Sedimentological, petrophysical, and seismic characterization of an Upper Jurassic shoreface-dominated shelf margin (the Boulonnais, northern France)

Abstract:

The integration of high-resolution (25 cm vertical spacing) and continuous quantitative data on grain size, carbonate tent, porosity, density, and P-wave velocity from outcrop, borehole, and core (over 1000 samples) and a real seismic section allows a calibration of seismic sequence stratigraphic principles. This approach has been tested in the Boulonnais, along the English Channel coast in northern France on a succession of Upper Jurassic sharp-based shoreface deposits embedded in offshore marine shales. Statistical analyses of grain-size distributions allow a characterization of hydrodynamic conditions and depositional processes affecting low-inclination shelf margins and create a framework for identifying locations of preferential and enhanced carbonate cementation, which exerts primary control on reservoir and acoustic properties. Stratigraphic surfaces and systems tracts are quantified for physical properties and results in a quantitative mechanical sequence stratigraphic model. The combination of this model with seismic wavelets and a contrast between this synthetic and a real seismic section shows that key bounding sequence stratigraphic surfaces are in fact discontinuous reflectors. Because stratal relationships of sharp-based shoreface deposits are subtle, physical contrasts are generally stratabound, and the deposits are typically not very thick, the geological recordings of sea-level falls and potential reservoirs are easily overlooked or go unseen in seismic sequence stratigraphic analyses. By defining key diagnostic seismic criteria for sharp-based shoreface deposits, this study contributes to the understanding of processes controlling sedimentary fills of epicontinental basins. Copyright © 2006, SEPM (Society for Sedimentary Geology).