The character and origin of thick base-of-slope sandstone units of the Peïra Cava outlier, SE France

Abstract

Abstract Many hydrocarbon reservoirs occur within confined turbidite systems in which the depositional pattern of turbidity currents has been strongly influenced by basin-floor topography. In certain settings basin-floor topography may cause the development of anomalously thick (tens of metres) sandstones that are potentially excellent reservoir units. Southern exposures of the Peïra Cava outlier (Eocene-Oligocene; Annot Sandstones) provide well-exposed outcrops of such decametre-thick sandstone bodies. These units are located close to basin margins and downstream from an inferred topographic break-in-slope. Several base-of-slope sandstone bodies are examined that illustrate a common sedimentary theme of a complex basal unit, comprising laterally pinching or inter-fingering debrite and turbidite, abruptly overlain by a single, thick normally graded turbidite deposit. One of these sandstone bodies pinches out laterally over less than several hundred metres and sits within a deep (>20 m) ‘spoon shaped’ erosional scour. The scour is similar to morphological features observed in modern base-of-slope settings recently imaged using high-resolution submarine bathymetric surveys. Several different process interpretations may explain the occurrence of such sandstone bodies including remobilization of newly deposited sediment off basinmargins and enhanced deposition due to flow across a break-in-slope. A submarine channel interpretation is not consistent with the field observations. However, these units do share a number of similar features to channels that could lead to the misinterpretation of reservoir geometry.