Geological Conditions of Natural Gas Accumulation in Ordovician of Lower Paleozoic in Western Ordos Basin

In recent years, the exploration of oil and gas resources in the Western Ordos basin has achieved positive results. In order to deepen the geological understanding, we analyzed the geological conditions of natural gas accumulation of Ordovician in this area. The drilling, logging, and outcrop data were used to investigate the formation characteristics and sedimentary evolution of the study area. The geological conditions for gas accumulation were systematically studied using data from geochemical testing of source rocks and reservoir characteristics. The gas accumulation models were summarized in terms of the configuration relationship among source, reservoir, and cap rocks. The structure of the strata in the study area is complex and the fault system is developed. In the Ordovician, it underwent an incomplete cycle of sea level rise and fall. To the east of the Huianbao fault, the formation is intact, and the thickness shows a trend of increasing from east to west and from north to south. In the study area, the Ordovician source rocks are primarily argillaceous rocks, and the reservoir is a bottom-to-top development of dolomite, limestone and shale. The gas accumulation modes can be mainly classified into three types: self-generating and self-storage at the source, upper and lower storage near the source, and upper and lower storage far from the source. Combined with the geological characteristics of the study area, these accumulation modes can be further concretized. Aiming at the western Ordos Basin, this paper systematically summarizes the natural gas accumulation modes of the Ordovician, which can provide a sufficient geological basis for the subsequent exploration in this area.