Tectono-sedimentary evolution, paleo-geographic reconstruction and play fairway delineation of the Lower Paleozoic, Ordos Basin

The Lower Paleozoic has been an important target for natural gas exploration in the Ordos Basin. Several suites of gas-bearing sequences, such as weathering crust reservoir on top of the Ordovician, middle assemblage dolomite and sub-salt dolomite, have been discovered successively, which proves that the Lower Paleozoic in the basin is of good exploration and development potentials, and thus paleo-geographic reconstruction there is in urgent need. The study proposes a new idea of play fairway delineation of the Lower Paleozoic in the Ordos Basin from the perspective of tectono-sedimentary evolution and paleo-geographic reconstruction therein. The research shows that Ordos Basin experienced five stages of evolution, namely, the Archean-Proterozoic fault depression-deposition stage, the Cambrian-Ordovician deposition-denudation stage, the Carboniferous-Triassic stable depression-deposition stage, the Jurassic-Cretaceous compressional hydrocarbon accumulation stage and the adjustment and finalization stage from the Late Cretaceous till present. The paleo-geographic pattern of the Lower Paleozoic Ordos Basin underwent the stages of paleo-land reduction, sediment filling, margin rifting and the alternated uplift-depression during the Early Ordovician, which controlled the distribution of quality play fairways. Besides, the latest exploration achievements help us identify four types of quality play fairways in terms of source-reservoir assemblage, spatial distribution and major controlling factors, that is, hydrocarbon accumulation within dolomite of marginal tidal-flat facies, hydrocarbon accumulation within the Ordovician pre-salt layer of intra-platform mound shoal facies, hydrocarbon accumulation within dolomite and shale of marginal tidal-flat and intra-platform mound shoal facies in the west, hydrocarbon accumulation within dolomite of karstification and shoal facies. The play fairways of different types feature separate hydrocarbon play elements and major controlling factors, and thereby targeted exploration strategies are in need.