Cambrian sedimentary evolution and natural gas accumulation conditions in Ordos Basin

Based on the authors’ years research on Cambrian in Ordos Basin, and combined with field profile observation and drilling data analysis in recent years, the stratigraphic division, sedimentary system types, sedimentary evolution characteristics and nature gas accumulation condition of the Cambrian were studied. The research shows that there are three types of sedimentary systems in Cambrian: coastal tidal flat, carbonate platform and platform margin, and the Cambrian of Ordos Basin is composed of epicontinental deposits on a stable continental margin. The tectonic paleogeographic pattern of the carbonate platform is represented by inner restricted tidal flat, outer open platform, marginal oolitic beach and front slope from inside to outside. And the sedimentary evolution experienced a complete evolution cycle, including Early Cambrian coastal tidal flat, Middle Cambrian restricted to open platform and Late Cambrian restricted to evaporation platform. The result suggests that the deep-water deposits are not developed in the Cambrian of Ordos Basin, and the hydrocarbon source rocks are usually poorly developed. The Huaiyuan Movement during the Late Cambrian to Early Ordovician resulted in the formation of the Central Paleouplift, which affected the sedimentary pattern of the Late Cambrian and controlled the dolomitization of the Middle-Upper Cambrian and the development of weathered crust karst reservoirs.