Pore geometrical complexity and fractal facets of Permian shales and coals from Auranga Basin, Jharkhand, India

Abstract

The pore system is a significant factor for the hydrocarbon generation, storage and production. Several studies had been carried earlier by distinguished researchers on pore system characterization, where little information regarding a thermally immature basin has been discussed so far. To understand these, total fifty-one samples including 41-shales and 10-coals are taken for study from Barakar (L.Sakmarian-Kungurian) Formation of Auranga basin to investigate the pore characteristics of a low mature substance. This work provides information regarding a low mature basin having oil generation potential. For this, authors have carried low pressure N2 sorption, FE-SEM/EDX with rock eval pyrolysis, total organic carbon (TOC), ash yield, vitrinite reflectance and clay content. The low-pressure N2 sorption: BET (Brunauer-Emmett-Teller) and BJH (Barrett-Joyner-Halenda) are employed to analyze the pore size, area, geometry and its distribution. Shale samples have shown variation in the specific surface areas (BET) and pore volume from 7.43 to 30.36 m2/g and 0.019–0.069 cm3/g respectively; whereas coal samples exhibits these properties ranging from 3.13 to 17.2 m2/g and 0.08–0.31 cm3/g respectively. Here, two kinds of desorption curve have been observed: (a) sub-hysteresis types H2′ where rapid desorption (lacking a plateau at high pressure) indicating pipette shaped pore (b) hysteresis type H3 having slow rate of desorption suggesting slit shaped pore. The subtype H2′ i.e. lacking the plateau at high pressure has been distinguished under H2 hysteresis. The dominance of mesopores to macropores are deduced from BJH and presence of micropores were also observed in few samples from t-plot method. The Type II isotherms are observed dominantly in shales and few coal samples (27-shales; 3-coals) whereas Type IV isotherms (13 shales; 7 coals) are mainly noticed in coals and in limited number of shales. Moreover, the total organic carbon (TOC) content of the shale and coal samples ranges from 1.35 to 29.42 wt% and 32.38–63.46 wt% respectively. Tmax [temperature under S2 to release maximum amount of pyrolyzate from the kerogen under rock eval pyrolysis (REP)] range from 409 to 468 °C and 420–426 °C of shales and coals respectively indicating of immature to early mature stage of the sample. The TOC normalized-BET (BET*) in relation to ash yield exhibits the significance of mineral matter in the shales for pore formation. The relation of pyrolysis parameters (S1and S2) with BET* gives the indication of bitumen retention in the pore spaces of organic matter, which reduces their surface area in coals. Fractal geometry of the samples were also studied. The surface fractal dimensions viz. D1 (P/Po = 0.0–0.5) and D2 (P/Po = 0.5–1.0) both are calculated for the basin. The D1 (pore surface) varies from 1.9888 to 2.5530 and 1.8190–2.4430 for shales and coals respectively pointing towards surface heterogeneity and ruggedness of the surface favorable for increasing the adsorption capability. However, D2 (pore structure) for the shales and coals are placed in the range of 2.570–2.759 and 2.6150–2.7530 respectively indicating large heterogeneity of the pore structure causing high capillary condensation that reduces the adsorption ability. The FE-SEM with EDX study supports the analysis of pore structure, characteristics and fractal behaviour of shales and coals.