Petrographical and petrophysical properties of tight siliciclastic rocks from the Ibbenbueren coal mine with regard to mine flooding

With regard to high production costs and the energy transition, German hard coal mining was phased out in 2018. As mine water drainage becomes economically and technically redundant, the mine water rebound may change the local stress state inducing ground movements, microseismicity, and gas anomalies. Geomechanical approaches that attempt to couple subsurface stress changes and surface ground movements require geological and petrophysical information of the subsurface. This study provides petrographic and petrophysical data of Upper Carboniferous (Westphalian B–C), fluvial, tight (most porosities <10 %, most permeabilities <1 mD) siliciclastic rocks from the former Ibbenbueren coal mine in NW Germany where the ground water currently rebounds. Measured compressional (3809–6656 m/s) and shear wave velocities (2126–3723 m/s), and calculated Young’s moduli (31.3–80.9 GPa) vary significantly and are related to Cretaceous maximum burial. Thus, predominantly mechanical compaction and subordinately quartz cementation altered the textural framework of the rocks. Grain size (0.008–1.1 mm) determined by the depositional environment represents the major control on wave velocities and Young’s moduli. Calculated Poisson’s ratios (0.09–0.31) in contrast are a function of the stress history, initial mineralogy, and diagenetic alterations. Therefore, the variations observed in texture, mineralogy, wave velocities, and derived Young’s moduli and Poisson’s ratios are crucial for the poroelastic behavior of the subsurface and need to be considered in case studies, which model post-mining ground movements as a result of mine flooding.