Significance of three reservoir profiles for the risk exploration in Ordos Basin

The Ordos Basin went through six main stages of geological evolution, developed three sets of soure rocks, two unconformities, several sets of reservoirs and caprocks, and had three typical petroleum systems, i.e. Lower Paleozoic Ordovician gas reservoirs, Upper Paleozoic Carboniferous-Permian gas reservoirs, and Mesozoic Triassic-Jurassic oil reservoirs. Three well-tie profiles of oil and gas reservoirs in the basin are analyzed, which are the Wuzhong-Mizhi profile, Guyuan-Yichuan profile, and Baotou-Chengcheng profile. The size and distribution of the oil and gas reservoirs are controlled by the spatial distribution of soure rocks. Mesozoic oil reservoirs are located inside or near hydrocarbon depressions, where oil and gas had a short secondary migration distance. Paleozoic gas in the basin, after having a large-scale short migration in the range of source rocks, eventually gathered in the effective traps of the Ordovician weathering crust, and in the low permeability effective sand traps inside or near Carboniferous-Permian source rocks. The future risk exploration targets are: (1) for Mesozoic oil exploration, it will extend northward and explore unconventional reservoirs in source rocks; (2) for Upper Paleozoic gas exploration, it will extend southward and explore marine clastic gas reserviors; (3) for Lower Paleozoic gas exploration, it will search for lithologic-structural traps of carbonate weathering crust in the north-central and southeastern Yishan slope and those of carbonate reef flat.