Factors Influencing Coal Bed Methane Reservoir in Southeastern Edge of Ordos Basin, China

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Abstract
The southeastern edge of Ordos Basin is rich in coal bed methane (CBM) resource (more than 30.8 × 109 m3). The study indicates that rank of coal, maceral composition, depositional environment, thermal evolution history and hydrodynamic are the mainly factors influencing CBM reservoir characterization. The coals in the research area are high rank (1.7% ∼2.4% Ro) vitrinite-rich coals, which is favorable for methane sorption capacity. The peak gas generation occurred form Jurassic to Early Cretaceous period when the Permo-Pennsylvanian coals were deeply buried (about 3500 m) and were subjected to temperatures >160°C. At northeastern part of study area, the coal seam is exposed at the surface and this leads to CBM desorption as lower reservoir pressure. In the meantime, strong groundwater runoff also intensifies CBM loss. There are two patterns of hydrogeological characteristics of coal bed methane-controlled in study area, that is, the hydraulic migration-escape (shallow area in the east) and the hydraulic jam-up (deep area in the west). The former leads to the coal bed methane escape and is unfavorable for accumulation, conversely, the hydraulic jam-up process are favorable for coal bed methane accumulation.

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