Three depositional models interpreting the Late Ordovician deep-water gravity flow systems in the Tarim Basin, Western China

The Tarim Basin, located in Western China, is a large sedimentary basin with a long geological evolution history stretching from the Sinian (Ediacaran) to the Neogene period. An enormous deep-water gravity flow system, up to 3,000-5,000m in thickness, developed during the Late Ordovician in the central and northern parts of the basin, with the great potential for forming large lithologic-stratigraphic oil/gas traps. However, the poorly-understood reservoir condition has caused oil/gas exploration to fall to a standstill. Based on several seismic surveys, well profiles, and outcrop data, this study focuses on sequence-depositional systems, paleo-geomorphology, and paleo-geography of the Late Ordovician reservoir rocks. The goal is to probe into the relationship between different types of deep-water gravity flow systems and the paleo-geomorphology, taking the tectonic development into account. Three different depositional models are applied to interpret the deep-water gravity flow systems preserved in the Upper Ordovician succession. The latter are distributed in the Tabei, Tazhong, and Tadong areas underneath the great deserts, respectively. During the Late Ordovician, the Tazhong area seemed to be a debris-flow-dominated setting with shallow, sandy channelized systems controlled by multiple slopes. The Tabei area was a debris-flow-dominated setting with few U-shaped deep incised ditches filled with muddy debris, and the Tadong area a typical submarine turbidite-dominated fan system in that time. These depositional models also attempt to predict the favourable reservoir sand bodies and lithologic-stratigraphic oil/gas traps with the Upper Ordovician successions in the subsurface of the Tarim Basin.