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 o ccur in association with other deep-water sediments - the massive sand faci es association. Following careful examination of some 70 examples of massiv e sands drawn from deep-water successions of all ages and lithologies, we a re confident that the shroud of mystery surrounding these deposits can be l ifted, their origin and nature can be explained and their importance as hyd rocarbon reservoirs call 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 amalgamati on 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-d ensity turbidity currents (HDTs). Post-depositional liquefaction and sand i njection can significantly affect either type. They generally occur as part of a thicker sand-dominated sequence or sand body (sand/shale ratios 7:1 t o >9:1) fed from a clean sand and/or gravel-rich source. The variety of sca les and geometries is dependent upon the depositional setting: chutes, scou rs, flow-slides and lobe sheets on smaller-scale fan-deltas; channel, ribbo n and tongue-like bodies on open slopes and proximal fans; lobate and lenso id bodies more distally; and trough/basin sand bodies that are broadly lens oid to tabular. The process/facies type and depositional setting profoundly affect both the internal architecture and external geometry of DWMS bodies . (C) 2000 Elsevier Science Ltd. All rights reserved.