Heterogeneity of fill within an incised channel: The Oligocene Grès du Champsaur, SE France

Submarine channel deposits are recognised as the primary heterogeneity within the turbidite systems that host them, with channel fill heterogeneities being considered secondary. An investigation of channel fill heterogeneities was conducted on Oligocene turbidites of the Grès du Champsaur, Hautes Alpes, SE France, which was deposited in a sub-basin within the Alpine peripheral foreland basin. Here, a series of erosional channels are exposed in mountainous terrain. Maximum channel widths are in the order of 1000 m, and depths of the order of 100 m. The channels can be traced along axis for at least 6 km. Mapping of channel facies revealed three orientations of fill heterogeneity expressed as: (1) upward variations in the character of vertically stacked fill sequences, (2) lateral variability, and (3) the occurrence of downstream dipping and stacking units forming the uppermost parts of channel fill. Vertically expressed heterogeneities are thought to have developed via the gradual reduction in the efficiency of channelised flow, leading to a progressive reduction in scour and the deposition of overall thinning and fining upwards sequences. Lateral heterogeneity is related to the development of downstream-oriented elongated scour and fill facies. Downstream-dipping heterogeneity is represented by a series of several metre-scale downstream-dipping low-angle clinoforms thought to be related to final infill of the negative channel bathymetry by a migrating front of sand that would have formed part of a downstream-migrating sheet to channel transition in plan view. Both first and second orders of channel heterogeneity may control the reservoir performance of channel prone turbidite sequences. Preferred-orientation channel facies heterogeneities are thought to affect potential hydrocarbon recovery through their impact on permeability anisotropy within the confines of the channel.