Tectonically deformed coal: Focus on microstructures & implications for basin evolution

Abstract

Traditionally, the Gondwana basins of India were believed to have formed primarily through extensional tectonics that created rift basins. However, recent models propose that strike-slip movements along deep-seated crustal fractures also contributed to their development. This article investigates this theory through a comprehensive structural analysis of the Bokaro Basin, located in the Damodar Valley of eastern India. This study deciphers tectonic history and evolutionary mechanisms of the Bokaro Basin by analyzing its coal samples, especially their structures—a relatively unexplored aspect in Indian coal research. We investigate maceral types, deformation manifestations, and fracture patterns at various scales. This led us to refine the structural classification of (Indian) coals. Deformation imprints were identified using remote sensing for regional features, geological fieldwork for megascopic observations, and petrographic analysis for micro-scale studies. The study utilized ArcGIS for lithology and fault mapping in conjunction with fieldwork in the highly deformed southwestern Bokaro Basin. Petrographic analysis revealed coal macerals and deformation signs. This research confirms that the Bokaro Basin's tectonic evolution involved uneven fault distribution, with significant rift and pull-apart mechanism. Coal seams show diverse deformation patterns, from ductile in fusinite to brittle in collotelinite, indicating multiple deformation stages under varying stress regimes. The East Bokaro Basin, with deeper seams, shows stronger deformation than the West. Shear stress, alongside extension, played a role in shaping the basin, with sigmoidal tension gashes from the coals of Barakar Formation, confirming active shearing through or afterwards coalification during or/and after the Early Permian time. The structural classification aids in coal bed methane (CBM) reservoir characterization, highlighting promising potential in cataclastic/tectonically deformed coal reservoirs. Tectonic deformation has affected the coals from the Bokaro Basin, resulting in structures that range from blocky to cataclastic, with some instances of granulation.