Proximal to distal turbiditic sheet-sand heterogeneities: Characteristics of associated internal channels. Examples from the Trois Evêchés area, Eocene-Oligocene Annot Sandstones (Grès d'Annot), SE France

• Published in: Marine and petroleum geology Vol. 41; pp. 117 - 133
• Main Authors: Etienne, S., Mulder, T., Razin, P., Bez, M., Désaubliaux, G., Joussiaume, R., Tournadour, E.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Kidlington Elsevier Ltd 01-03-2013; Elsevier
• Subjects: Annot Sandstones; Channel-elements; Confined turbidite systems; Earth sciences; Earth, ocean, space; Exact sciences and technology; Hydrocarbons; Marine geology; Sand-rich sheet reservoir heterogeneities; Sedimentary rocks; Stratigraphy; France; Europe; Western Europe; Annot Sandstones; Sand-rich sheet reservoir heterogeneities; Channel-elements; Confined turbidite systems; reservoir rocks; Paleogene; domes; fine-grained materials; reservoirs; sandstone; spatial distribution; Oligocene; Cenozoic; clastic rocks; high resolution; cross-bedding; deep-sea environment; models; outcrops; Tertiary; Eocene; heterogeneity; grains; sedimentary rocks; sand; turbidites; channels; migration; geometry; Phanerozoic
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2012.03.007

The present study brings new insights on the small-scale characteristics of relatively distal turbidite deposits (classically defined as depositional lobes or sheets) from the deep-water clastic slope system of the Annot Sandstones (Grès d'Annot), in the Trois Evêchés area. This “seismic scale” outcrop has been studied in detail to characterize sedimentary geometries and facies associations within and between sheet-like sandstone-bodies. In terms of facies, their internal architecture is dominated by numerous erosional features, variable scale cross-bedding and dewatering structures. Facies range from gravelly sandstones to fine-grained sandstones and their spatial distribution inside sandstone-bodies is provided, allowing us to discuss the spatial changes in flow behaviour. A detailed dataset, made of very high-resolution correlations in thick to thin-bedded sheet-like sandstone-bodies, enables to identify five main types of architectural elements, composed of both channelised and unchannelised depositional bodies. Gravelly-filled channel-elements, laterally stacked channel-elements (interpreted as being related to sinuous channel migrations) and wing-like channel-elements constitute the channelised sheet systems whereas tabular sandstones (sheets sensu stricto), prograding and/or dome-shaped units constitute layered sheet systems. Sinuous channels are represented by the association of erosive-based, laterally stacked bedsets and very coarse-grained, low-angle cross-bedded facies. These latter facies are interpreted as the result of flow deconfinement and overbanks above channel margins. The stratigraphic relationships between these architectural elements permit establishing a conceptual model of the distribution and organisation of the components. This distribution scheme is based on the longitudinal differentiation of gravity processes, which are directly linked with architectural element types. This model is composed of proximal confined units where channelization processes, amalgamation and bypass dominate, to more distal and less confined units where compensation, constructive structures and high deposition rates control sedimentary architectures. The Trois Evêchés sub-basin may be considered as a good example of a channelised lobe to unchannelised lobe transitional environment, which explains the lateral and vertical high variability during its infill. The widespread apparition of residual facies, such as cross-stratified sandstones as well as basal lags, suggests significant bypass. The appearance of both channelised and unchannelised units is consistent with this transitional environment. The provided detailed dataset in this paper displays the associated wide range of architectural elements in these settings, and highlights the significant heterogeneities of sand-rich confined turbidite sheet systems. These heterogeneities are under the seismic resolution and may influence fluid migration in sand-rich deep-water reservoirs.
► The architecture of sand-rich turbidite sheet sandstones has been investigated. ► Different internal channel-elements have been identified. ► Importance of bypass and amalgamation on sheet-sand variability is emphasized. ► A conceptual model of their distribution, based on flow processes, is proposed. ► These elements highlight the associated subseismic scale reservoir heterogeneities.