The Ordos Basin, which is situated in the north part of China, is one of the largest basins in terms of natural gas resources in China. Natural gas accumulation from marine source rocks in Ordovician was found recently in the west margin of Ordos basin. The purpose of this paper is to describe the geological conditions of gas accumulation in the Ordovician in the west margin of Ordos basin and the identification of the Petroleum System. The marine source rocks, occurred in the Kelimoli and Wulalike formations of Ordovician which consist of shale, mudstone and marlstone, are mainly distributed in the west margin of Ordos basin where sedimentary environments are deep ramp faces. The thickness of strata ranges from 20m to 200m, with the TOC ranges from 0.19% to 4.55%. The kerogens are dominated by type I or II and the thermal maturity is mostly in the range of 1.4 to 2.8% Ro. Limestone bedding karst cavity reservoir in Kelimoli formation constituted the main space for the gas accumulation. The bedding karst cavity reservoir, with the average porosity between 4% to 6% and the permeability between 0.1×10-3μm2 and 2.0×10-3μm2, is the main gas reservoir. The limestone bedding karst cavity reservoir mainly distributed in the west with the thickness between 60m and 170m and is interbedded with the source rock while the dolomite reservoir mainly distributed in the east. The overlying coal-bearing strata which are wide continuous distributed prepared the good cap rock with high quality. The Kelimoli limestone is the main reservoir rock. The Wulalike mudstone is not only the source rock but also good local cap rock. The overburden rock includes the late Carboniferous, Permian and all the Mesozoic and Cenozoic. Lithology lateral change in Kelimoli formation is the main reasons to control the traps,and structure movements are the minor reasons for traps. The vitrinite reflectance values and the homogenization temperature of fluid inclusions suggest that the onset of the oil generation started in the late Jurassic and gas in the early Cretaceous.


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