Petrophysical Characterization of Gondwana Shales of South Karanpura Coal Field, Jharkhand, India

Abstract

Sedimentary rocks such as shales are the most abundant sedimentary rocks in the earth’s crust. Shales are characterized by preferred particle orientations of platy clay minerals, strong laminations and presence of fissility. In petroleum geology, organic shales behave as both source rocks as well as seal rocks that trap oil and gas. In seismic exploration, shales interface with other rocks to form good reflectors. As a result, seismic and petrophysical properties of shales and the relationships among these properties are important for both exploration and reservoir management. The Gondwana shale of South Karanpura coalfield, Eastern India of Barakar (lower Permian) and Barren-Measures (Middle Permian) formation, majorly characterized by non-marine sedimentary fill and narrow graben structures, are used for petrophysical characterization. Because of thermal maturity and high content of organic matter in shale rock, South Karanpura coalfield is considered as one of the potential shale gas field in Damodar Valley Basin and it is a part of the “Gondwana” basins of Eastern India. This paper discusses various experimental techniques that are applied to shales to obtain velocity, elastic properties structure, minerals composition, texture/fabric and pore types. These techniques include ultrasonic acoustic measurements, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The frequency values used for acoustic measurements are 54 KHz (P-wave) and 250 KHz (S-wave). The resolution of these imaging techniques varies from micrometres to angstroms and the resulting images generally reflect the composition, topography, or combination of both. The estimated P-wave velocity for Barren-Measures and Barakar formation varies from 1463–1890m/s and 2110–3947m/s, respectively while the Shear-wave velocity varies from 898m/s–1232m/s to 1306m/s–2515m/s respectively which suggest that formation is hard and compact in the latter case. XRD and SEM analysis reveals the presence of clay minerals and other minerals, organic matter, texture/topography and pore types in the shale rock sample. The clay minerals identified consist of Kaolinite, Illite while Quartz, Siderite, Muscovite, Orthoclase and Rutile comprises of non-clay minerals in the sample.