Macromolecular transformations for tectonically-deformed high volatile bituminous via HRTEM and XRD analyses

Coalbed methane reservoir properties of tectonically deformed coals (TDCs) vary greatly in comparison to unaltered coals. These differences are attributed, in part, to their macromolecular transformations. The structure of different TDCs at various distances from a fault plane was evaluated using image analysis of aromatic fringes from HRTEM micrographs, and from XRD data. The distribution of fringe length, orientation, stacking, and curvature, were quantified to establish the structural organization. From the fringe length, orientation, and XRD data, two deformation stages were identified a) a weak deformation stage (–cataclastic–schistose–scaly) and b) a strong deformation stage (–wrinkled–mylonitic). In the weak deformation stage, the fringe length distribution was consistent, however the strong deformation coals had longer fringes. The majority (94–98%) of the aromatic fringes were individual layers for both stages. There was a significant improvement in the structural alignments however with strong deformation. The common statement that tectonic stress enhances the stacking of the aromatic clusters (and thus perhaps the graphitization ability) was not observed here. Rather, the enhancement in order was limited to the strong deformation stage. The aromatic fringes of the strong deformation coals had greater curvature extents. The XRD data also supported increased organization (lower d₀₀₂ values) with strong deformation. These observations indicate there was an improvement in the basic structural unit from poorly ordered to more ordered that occurred in the strong deformation stage. These transformations are likely to impact the coalbed methane production and methane behaviors.