Abstract With deep-water basins becoming mature exploration areas, and the number of undrilled structural traps diminishing, stratigraphic traps are growing increasingly important. Onlapping turbidite sands are a common stratigraphic trap type in confined basins; however, onlap architecture can assume a number of different geometrical and facies characteristics as a result of interactions between the turbidity currents and the onlap surface. These variations have important implications for the accumulation and trapping of hydrocarbon reserves, in that the variety affects the quality and recognition of good onlap seal. To investigate stratigraphic traps in turbidite systems, the spectacularly exposed, seismic-scale Montagne de Chalufy onlap section in the Grès d’Annot turbidite system of the French Alps is used to generate various forward synthetic seismic 2D sections. Three parameters are varied independently: input model detail, seismic impedance contrast, and dominant wavelet frequency. Seismic velocity and rock density values are taken from producing reservoirs to represent the observed lithologies in the Grès d’Annot. The three petroleum reservoir scenarios investigated are a Plio-Pleistocene Gulf of Mexico reservoir and Tertiary and Jurassic North Sea reservoirs. Two levels of 2D input model detail are investigated: 1) a detailed model having observed structural and sedimentary complexities to test vertical and lateral seismic resolution, and 2) a simple onlap model without the complexities. Two Ricker wavelet dominant frequencies are chosen, 26 and 50 Hz. Synthetic seismic results show outcrop analogues of turbidite sandstone reservoirs can be usefully converted to forward seismic sections. 2D seismic sections generated from high frequency wavelets allow subsequent geological interpretations that almost replicate detailed input models, although onlap termination or drape was still difficult to resolve. Gross architectures of massive, onlapping, turbidite sandstones may still be resolved in low frequency seismic datasets.

Abstract Turbidite channel systems are a common target in exploring for petroleum reservoirs though their internal architectures can be highly varied. To investigate channel architectures in turbidite systems, the spectacularlyexposed, Ainsa II channel in the Spanish Pyrenees is used to generate forward synthetic seismic 2D sections. Three parameters are varied independently: input model detail, seismic impedance contrast, and dominant wavelet frequency. Representative seismic velocity and rock density values are taken from published reservoir data for observed lithologies. The three petroleum reservoir scenarios investigated are: a Plio-Pleistocene Gulf of Mexico reservoir and Tertiary and Jurassic UK North Sea reservoirs. Four Ricker wavelet dominant frequencies are chosen to generate the seismic models: 26, 52, 78 and 104 Hz. Results show that 2D synthetic seismic images generated from the low frequency wavelets (26 Hz) do not resolve the important heterogeneities within the channel complex. At 52 Hz, frequencies that are typically used for the interpretation of shallow-level, Tertiary targets, an improved seismic resolution of the input model can be achieved, suggesting that the target interval consists of stacked channels, although not all channel units are resolved. At 78 Hz, which is typical of high-frequency seismic acquisition and processing, an interpretation close to the input geological model can be achieved. At 104 Hz, the excellent seismic resolution can be interpreted to reproduce the input model. The results also show that differing impedance contrast values have little effect on the geological interpretation at the high signal/noise ratios used, although impedance values from the UK North Sea Jurassic reservoir analog yield the highest amplitudes in the synthetic sections.

Abstract Part of the Alpine foreland basin, the Grand Coyer remnant lies in a NW-SE-trending syncline and contains turbidity current deposits derived from the south. Palaeotopography of the Marnes Bleues surface is complex but there is generally a NE-dipping confining slope in the west of the area. Palaeocurrent data suggest that aside from localized alteration of flow paths, the mean direction of flow was parallel to this slope. Three sections (Vallon de Fouès & Carton, Laupon and Sommet de la Mole) are used to illustrate the nature of the Marnes Brunes Inférieures—a transitional facies between the Marnes Bleues and the Grès d'Annot. The Marnes Brunes Inférieures is typically thin-bedded, sharp-based and sharp-topped, non-graded, predominantly very fine-grained sands, with planar and ripple lamination, interbedded with thin marls and shale. The finer grained Marnes Brunes Inférieures in the Grand Coyer area is interpreted as a distal and lateral equivalent of the coarser Grès d'Annot, which is generally thought to represent deposition along the main axis of flow. Examples are provided of how these characteristics change away from the onlap slope. Evidence is presented to illustrate the connection between syndepositional structural alteration and the common slumping observed within the Marnes Brunes Inférieures.