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The origin and nature of seismic reflections of sharp-based shoreface deposits (upper Jurassic Siliciclastics, northern France)

Abstract:

In order to advance understanding of the relationship between geological properties and their physical expression in reflection images, this study has focused expertise in reflection geophysics, petrophysics and sedimentology on the same geological object, in this case a succession of Upper Jurassic sharp-based shoreface deposits embedded in offshore marine shales in northern France. This integrated approach to determine firstly the origin and nature of seismic reflections (calibration) and secondly to provide a means of extracting geological information from seismic imagery (inverse calibration) was built on the following analytical steps. Firstly, detailed and extensive petrophysical analyses of outcrop (plug) samples, continuous core and sonic well logs, in combination with a quantification of mineralogical and textural properties, allowed a direct conversion of acoustic properties (impedance) into sedimentological properties, resulting in a quantitative physical sequence stratigraphic model. Secondly, the integration of scale-dependent acoustic measurements, ranging from 0.01 m and 320 kHz on cores up to the wavelength of field seismic data was established using an averaging algorithm (an effective-medium-theory type) as an upscaling approach. This alternative to a VSP or check shot allows an optimized depth-time conversion and hence determination of the origin of the seismic reflections with previously unattainable accuracy. Finally, the shape and scale dependence of impedance contrasts were integrated into so-called singularity parameters that directly link depositional changes with information from seismic reflections: depositional changes in the shallow-water domain are generally characterized by step functions, whereas those in more distal depositional environments are represented by spiky functions. This approach allows the recognition of the associated reflection events and, vice versa, it provides a unique opportunity to extract the character of impedance changes, and thus changes in depositional environment, from seismic reflection records in general. This integrated and multiscale characterization of sharp-based shoreface deposits calibrates the typical reflection patterns for such sedimentary units. These include continuous high-amplitude smooth and flat tops, discontinuous sharp basal reflections with variable amplitude, and complex sigmoidal high-amplitude reflections within the compound shoreface deposits. In addition, the results of this study, by detailing the effects of scale and frequency on impedance changes, improve the identification of similar deposits in subsurface seismic data and the extraction of maximum amounts of geological information beyond seismic resolution. © 2006 European Association of Geoscientists & Engineers.