Maturity characterization and ultrasonic velocities of coals

Abstract

Coals have been studied extensively to understand their chemical and thermodynamic properties. Yet, surprisingly very few data exist on their maturation and elastic properties relationship. In this paper, we present the relationships between rank metamorphism and ultrasonic velocity of coals. Seven coals maturation from several basins in world was assessed by Source Rock Analyzer. The total organic carbon (TOC) range values show that the organic matter is predominantly gas prone (mostly type III). The vitrinite reflectances suggest wide rank of maturation from lignite to semi‐anthracite. Laboratory ultrasonic velocity measurements of the 7 dry coals exhibit different dynamic elastic properties which correlate to coal rank and applied confining pressure. Compressional and shear wave velocities increase as function of coal rank increases. Nevertheless, pressure dependence on coal elastic properties exist for confining pressure under 5 MPa. This sensitivity is affected by the cleats presence. Based on geochemistry — velocity cross plot elastic anisotropy may occur at high maturity coal rank due to the increasing TOC or decreasing hydrogen index (HI).