Influence of organic and inorganic content on fractal dimensions of Barakar and Barren Measures shale gas reservoirs of Raniganj basin, India

The plot of ln(ln(P/P0)) versus ln(V) have shown three distinct straight line sections within the whole relative pressure range (0.0000-1.0000), further denoted as Region I (P/P0 = 0.0002-0.0090; D1), Region II (P/P0 = 0.0090-0.3000; D2) and Region III (P/P0 = 0.3000-1.0000; D3) and the linear fitting equations were obtained with different slopes. The values of D1, D2, D3, signifying the complexity of micro-, meso- and macropores, providing supplementary sites for gas adsorption. Fractal dimensions have shown a positive correlation with clay content, whereas negative correlation with total organic content indicates that inorganic content plays a vital role in the rugged surface formation useful for gas storage. The positive linear correlation of fractal dimensions (D1 and D2) with Langmuir volume accentuated that smaller pores (micro and meso) contains ideal rugged surfaces suitable for gas adsorption due to heterogeneity, irregular pore surfaces, complex pore openings and structures. Furthermore, D3 shown negligible negative correlation with VL specifies the larger pore size do not provide sites for adsorption space, because of the altered smooth surfaces formed during diagenesis. An empirical method for estimation of sorption capacity (ESC) has been proposed taking into account of the positive and negative influence of the fractal dimensions, clay, minerals and total organic content. The strong positive linear relationship of Langmuir volume (VL) with an empirically estimated sorption capacity (ESC) (R2 = 0.86) and about 90% curves match, signifies the proposed empirical formula can be used as an indirect method for estimation of sorption capacity of shale samples.