Challenges of terrestrial shale gas exploration and development from Chang 7 shale in the Ordos Basin

There has been great success of marine shale gas exploration and production from Lower Silurian Longmaxi Formation (called Longmaxi shale) in Sichuan Basin, China, while whether commercial gas production could be obtained from lacustrine shales is still a question. The 7th Member of Triassic Yanchang Formation (called Chang 7 shale), which is the exploration target for the only national lacustrine shales gas pilot, was investigated to identify the key issues on the accumulation and development of terrestrial shale gas accumulation. It is considered that there are many differences in terrestrial and marine shales including depositional environment and provenance, distribution characteristics, organic matter types, mineral composition, and pore types. So evaluation standards of the two shales should not be the same. The Longmaxi shale was deposited in deep shelf, widely distributed, stable, mature–overmature (in gas window), presence of plenty micro-pores, brittle, and overpressured, which make it a good condition for shale gas accumulation. The key factors that affect the production of the Longmaxi shale concluded that the shale is rich in organic matter, high thermal evolution, brittle, and overpressured. The Chang 7 shale in Ordos Basin was deposited in deep lake, relative limited distribution, in low stage of thermal evolution (in oil window), high clay mineral content, and conducive to shale gas adsorption. On the basis of comparing the geological features of the two shales, it is found that the shale is richer in the organic matter content than in the Longmaxi shale, while the shale is not mature enough to produce more gas, and the strata pressure is relatively lower which means the shale gas was not preserved very well either. So through this study, it is believed that the shale gas formation condition in the Chang 7 shale may not be as good as in the Longmaxi shale, the absorbed gas may be the future of this shale, and the shale oil should be the focus of petroleum exploration and development in this region.