Upper Palaeozoic gas accumulations of the Yimeng Uplift, Ordos Basin

This paper analyzes the Upper Palaeozoic gas accumulations and conditions of the study area by using the definition and method of petroleum system, and summarizes the natural gas accumulation pattern. The Upper Palaeozoic natural gas in the Yimeng Uplift is mainly from source rocks of Taiyuan and Shanxi Formations in the southern part and Wushenqi area. Braided channel sandbodies of alluvial fan, river channel sandbodies and distributary channel sandbodies of delta plain in the Shanxi and Xiashihezi Formations are the most favorable reservoirs. The thick mudstone layers of the Shangshihezi and Shiqianfeng Formations which cover the whole region constitute the regional seal rock, and there are also many local seal rocks. Oil source, reservoir and seal rocks form a good relationship in time and space and they form two types of assemblage which are the self-generating and self-preserving assemblage in the southern part of the Yimeng Uplift and the lower-generating and upper-preserving assemblage in the southern and northern parts. Natural gas migrated to the north through the migration pathways composed by advantageous sandbody, unconformity, faults and cracks. Structural traps and the distal accumulation pattern dominate in the northern part, while the southern part is characterized by lithologic traps and the proximal accumulation pattern.