DELTA DEPOSITS IN THE SHANXI AND SHANGSHIHEZI FORMATIONS IN THE NORTHEASTERN ORDOS BASIN

Integrated data of sedimentary structures, petrology characteristics, outcrops, and well logs of the Shanxi and Shangshihezi formations are used in the analysis of the Late Paleozoic sedimentary history in the northeastern Ordos Basin. Three phases have been recognized: (1) an initial phase characterized by tide-controlled braided river deltas and continental meandering river deltas; (2) a developmental phase characterized by continental braided river deltas and continental meandering river deltas; and (3) a mature phase characterized by continental anastomosed river deltas. They belong to shallow water deltas charactered by big plains but small fronts, which are consistent with the palaeogeomorphology of Late Paleozoic Ordos intercratonic depression basin that was dominated by broad and gentle slopes dipping to the south. In the early stage of base-level rise, because of profuse sediment supply, distributary channel microfacies and submerged distributary channel microfacies of braided river delta formed a network of sand-bodies which became the most favorable sandstone reservoir.