Identification and nanoporosity of macerals in coal by scanning electron microscopy

Abstract Recent applications of scanning electron microscopy (SEM) to shale resource plays at magnifications of 80,000 × have reported nanoporosity in organic matter with limited interpretations of organic matter type. Macerals, inclusive of kerogen and solid bitumen, are recognized and distinguished in reflected white and epifluorescent light in coal and shale samples at magnifications of 200–750 ×. The objectives of this study are to identify macerals by SEM and evaluate which macerals contain primary and secondary nanoporosity. Since coals are organic rich with a better chance of identifying adjacent maceral types than when dispersed in shales, broad ion beam milled samples of humic and sapropelic (boghead and cannel) coals ranging in rank from peat to semianthracite were examined in backscattered electron (BSE) mode at low magnification (≤ 2,500 ×) to identify maceral type. Once identified, macerals were examined at higher magnifications of 1200–75,000 × to assess maceral nanoporosity. Manipulation of the accelerating voltage to 10 kV in BSE mode of a high volatile bituminous humic coal durain lithotype sample revealed a contrast between maceral groups (vitrinite, inertinite, liptinite), with limited identification of individual maceral types. Vitrinite maceral subgroups telovitrinite and detrovitrinite are distinguished based on their relative gray scale appearance compared to other macerals and occurrence as bands or groundmass, respectively. The liptinite macerals alginite, sporinite and cutinite are distinguished based on dark relative gray level and their shape. The liptinite maceral bituminite/amorphinite was recognized by dark relative gray level and occurrence as groundmass in a boghead coal. The inertinite macerals fusinite and semifusinite are recognized by light gray level appearance compared to other macerals and bogen structure but are not distinguishable separately. Macerals dispersed in shale, lacking the subtle contrast of adjacent macerals, are much more difficult to identify. Even though porosity is revealed at high magnification in BSE mode, too high of a magnification (> 15,000 ×) prohibits identification of maceral types. The best approach is to examine samples at a lower magnification (e.g., 650 ×) at 10 kV accelerating voltage in BSE mode to identify the maceral type and then go to a higher magnification at 1–2 kV accelerating voltage to observe nanoporosity. Primary nanoporosity is observed within coal macerals at low rank (peat and subbituminous), but decreases in amount with increasing rank. Primary microporosity occurs as woody cell lumens in semifusinite and fusinite macerals. Secondary nanoporosity develops in post-oil solid bitumen in shale beginning below the peak of the oil window with a lack of nanoporosity at lower thermal maturity. Compared to the abundant nanoporosity of post-oil solid bitumen in shale, only trace amounts of nanoporosity is observed in other macerals in coals of high volatile bituminous rank and higher under the SEM. The emphasis of this study was the identification and nanoporosity of macerals in coal by SEM. The same results may extend to the same macerals in shale. Knowledge of organic matter porosity distribution by maceral type and development by thermal maturity provides insight for coalbed methane, shale gas and tight oil production potential.