Structural controls on coalbed methane accumulation and high production models in the eastern margin of Ordos Basin, China

Significant progress has been made in coalbed methane (CBM) exploration and development in the eastern margin of the Ordos Basin where nearly 2000 CBM wells have been drilled, achieving a maximum gas production rate of 16000 m3/d by 2013. The geological evolution of the eastern Ordos Basin plays an important role in the CBM formation. This study is focused on the interrelationship between structural geology, gas accumulation and production characteristics of the No. 4 + 5 coal in the Permian Shanxi Formation as well as the No. 8 + 9 coal in the Carboniferous Taiyuan Formation. This research is based on the data collected from CBM production wells and coal samples from coalmines and exploration wells. The results show that thermogenic gas is the dominant CBM source in the study area and there are two significant generation periods, the coalification in the Triassic and the magmatic thermometamorphism during the Yanshan movement. Combining the structure background and hydrogeological conditions, the monoclinic-hydraulic sealing model was proposed as the representative CBM enrichment model. Different types of structures are also classified, and their influence on the CBM accumulation is discussed. Compressional structures formed during the Yanshan movement are conducive to CBM enrichment and retention; however, the tensional structures formed during the Himalaya movement may have led to CBM dissipation. Combining the structural effect on the CBM production with CBM exploration and development practices in the study area, the following three types of high gas production models are summarized: updip of the monocline, the axial part of the anticline or nose structure, and the structural high far from the normal fault.