Deep-water massive sands: nature, origin and hydrocarbon implications

Deep-water massive sands (DWMS) are here defined as very thick (>1 m) sand beds or units that are devoid of primary sedimentary structures and that occur in association with other deep-water sediments — the massive sand facies association. Following careful examination of some 70 examples of massive sands drawn from deep-water successions of all ages and lithologies, we are confident that the shroud of mystery surrounding these deposits can be lifted, their origin and nature can be explained and their importance as hydrocarbon reservoirs can be brought sharply into focus. Besides their very thick bedding and structureless aspect, key features of DWMSs are the common presence of water escape structures, subtle amalgamation surfaces and shale clasts. Typically they show poor to moderate sorting and compositional immaturity. The two key processes involved in their long-distance transport and emplacement are sandy debris flows (SDFs) and high-density turbidity currents (HDTs). Post-depositional liquefaction and sand injection can significantly affect either type. They generally occur as part of a thicker sand-dominated sequence or sand body (sand/shale ratios 7:1 to >9:1) fed from a clean sand and/or gravel-rich source. The variety of scales and geometries is dependent upon the depositional setting: chutes, scours, flow-slides and lobe sheets on smaller-scale fan-deltas; channel, ribbon and tongue-like bodies on open slopes and proximal fans; lobate and lensoid bodies more distally; and trough/basin sand bodies that are broadly lensoid to tabular. The process/facies type and depositional setting profoundly affect both the internal architecture and external geometry of DWMS bodies.