Sand Body Dimensions and Infill Sequences of Stable, Humid-Climate Delta Plain Channels

Abstract

Sediments deposited in delta plain settings are common in the Jurassic sequences of the Norwegian continental shelf (e.g. the Ness Formation). The most prominent reservoir units in this type of deposit are distributary channel sand bodies. These mainly appear to be laterally discontinuous in a direction normal to the palaeoslope, and probably reflect cut-and-fill processes in stable distributaries. Based on well-data alone, it is difficult to determine vital reservoir parameters such as the geometry and interconnectedness of these channel sand bodies. Studies of well-exposed possible analogues like the Saltwick Formation of Yorkshire (England) and the Aspelintoppen Formation of Spitsbergen were therefore undertaken. This study indicates that single-storey sand bodies deposited in stable cut-and-fill channels have ribbon-like geometries, whereas multistorey channel bodies of the same type tend to display more sheet-like dimensions. A curve relating the width and thickness of these sand bodies has been constructed. Humid climate cut-and-fill sand bodies deposited in stabilized distributaries can be recognized by an infill sequence that, when complete, reflects three stages of infilling (active, partly active and passive phase fills). Active phase infills frequently have the best reservoir quality at the top, owing to high-discharge events prior to (partial) abandonment. In addition to palaeo-river morphology, sandbody dimensions and the degree of interconnectedness are governed by the pattern of differential subsidence along depositional strike, and by the ratio between sediment supply and accommodation.