International Journal of Coal Geology
International Journal of Coal Geology
5 years ago

Geochemical and petrophysical characteristics of Permian shale gas reservoirs of Raniganj Basin, West Bengal, India

Thick shale horizons of Permian age emerged as the potential source of gas through primary investigation, and resulted into the first Indian pilot-scale gas recovery demonstration project in the Raniganj basin. However, the relation of geochemical properties with shale pore matrix, porosity and permeability mechanism are yet to be evaluated for better dealing the phenomena of diffusion, transport and gas recovery. In this study, the analyses like proximate, Rock-Eval, TOC, VRo, low pressure N2 sorption, porosity, permeability, SEM-EDX and image processing have been carried out. The values of S1, S2, TOC and Tmax indicated fair to excellent source rock potential of the shales having type III/IV kerogens prone to thermal gas genesis. The low pressure N2 desorption curve of Raniganj Formation shales shows the Type H3 hysteresis comprising slit-shaped pores. The Barren Measures shales have Type H2 and H3 hysteresis patterns specifying to ink bottle-shaped pores and slit-shaped pores. Whereas, the oldest Barakar Formation shales are having ink bottle-shaped pores caused due to blocking effect. The slit-shaped pores favour the pore network and characteristically excellent for the flow of gas. The results of SEM-EDX are indicating alteration stability in the order of O<C<Si<Al<Fe<K<Na<Ca following the trend of least to strong weathering (Barakar<Barren Measures<Raniganj). The siliciclastic facies (Al-Si-Fe) signifying massive and laminated shale beds deposited under fluvio-lacustrine palaeoenvironment favouring alteration and accumulation of K-feldspar and aluminous minerals to clay. The increasing Tmax values with a centric decrease in porosity and permeability, specifying the role of devolatilization, disintegration and blocking of pore spaces/openings is a function of the thermal gradient. The linear evolution of multipoint BET surface area with increasing porosity suggests that the porosity values from 2.0 to 6.5% mainly corresponds to pore size of 3.0–11.0nm. Similarly, the inverse relationship between average pore size and porosity attributed to a greater contribution of smaller pores in total porosity. The pore network model derived through the SEM image processing has shown two types of connectivity - i) various pore sizes of diverse pore throats with dual opening directions, and ii) interlinked large and small pores obeying similar normal distributions. The Barren Measures shales have shown higher pore connectivity than the Raniganj and Barakar Formation shales.

Publisher URL: www.sciencedirect.com/science

DOI: S0166516217306754

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