Preliminary evaluation of gas content of the No. 2 coal seam in the Yanchuannan area, southeast Ordos basin, China

Gas contents are highly variable in coalbed methane (CBM) reservoirs of the Yanchuannan (YCN) area of the southeast (SE) Ordos Basin, China. We used diverse geologic data derived from more than five years of exploration to provide insight into the origin of this variability and the consequences of gas content on reservoir performance. Major factors affecting gas content variability include gas generation, migration, trapping and preservation. Gas generation affects gas content variability on the scale of the total resource, whereas gas migration influences the inhomogeneous redistribution of gas content on a regional or local scale. Gas trapping and preservation affect the “as-observed” content. The potential for high gas content is controlled directly by the composite result of gas generation, migration, trapping and preservation. CBM in the YCN area is produced from the relatively thick seam (~2.09 m and 8.05 m, with an average of 5.97 m) that is distributed through 450-1200 m of the stratigraphic section. Gas content tends to be structurally and hydrodynamically controlled in the order of simple structure (folds and small faults)>complex structure (large regional faults) and groundwater stagnant zones>runoff zones. Coal samples in the YCN area typically have Langmuir volumes between 31.86 and 46.51 cm3/g, which correlates with coal rank. Reservoir heterogeneity including coal composition, pore structure and matrix moisture content may contribute to the heterogeneous gas content. Gas content is generally high where hydrodynamic trapping of gases occurs and may be anomalously low in areas of active recharge with downward flow potential and/or convergent flow where there is no mechanism for entrapment. In the YCN area, the most favorable area for CBM exploration and development is in the center block (block B), where great coal thickness, moderate burial depth, favorable hydrodynamics and an anticlinal trap coincide to yield high gas contents.