Abstract: The geochemical characteristics and micro-structure of dark mudstones from the Middle Permian Zhesi Formation in Solun area, Inner Mongolia were studied using field outcrops, samplings, and systematic analyses. Micro-pores (including interparticle, intraparticle and organic pores) and micro-fractures (including interlayer fractures in minerals, fractures between organic matter and minerals, and fractures in organic matter) developed in mudstones in the Zhesi Formation. Organic pores were not well developed in the Zhesi mudstones, mainly due to low TOC content and pore collapse under strong compaction effect. Micro-fractures were well developed thanks to high brittle mineral content. The characteristics of source rocks and reservoirs showed that mudstones in the Zhesi Formation have a good prospect for shale gas exploration. The areas with widespread mudstones rich in organic matter are targets for shale gas in the Upper Paleozoic in Northeast China.