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The Sawahlunto Formation is a coal-rich geological formation in the Ombilin Basin, located in central Sumatera. It contains three main coal seams, designated as Seams A, B, and C, with Seam C known for its high-quality coal. The study involved analyzing coal samples from Seam C using petrographic and proximate analysis methods. Petrographic analysis assessed the macroscopic composition and vitrinite reflectance, which are essential for determining the depositional environment and coal rating. Proximate analysis provided additional data necessary for evaluating the coal’s quality. The coal was predominantly composed of vitrinite (79.2%-80.6%) and inertinite (19.2%-20.8%). Plots of the Tissue Preservation Index (TPI) and Gelification Index (GI) indicated a limno-telmatic depositional environment. Vitrinite reflectance ranged from 0.45%-0.51%, classifying the coal as sub-bituminous. The calorific values were 15,117.2 btu/lb and 14,815.94 btu/lb, categorizing the coal as highly volatile bituminous.

Thermal maturity is one of key parameters for source rock evaluation, which affects not only the effectiveness of hydrocarbon generation and the properties of hydrocarbon but also the formation, evolution and extinction of organic matter-hosted pores in shale reservoirs. Thus, accurate assessment of thermal maturity is of great significance to the evaluation of conventional source rocks and gas-bearing properties of unconventional shale reservoirs. However, thermal maturity assessment of the Precambrian–Lower Paleozoic marine shale deposits has been challenging for several decades. On one hand, the widely used gold standard-vitrinite reflectance (VRo) cannot be used for maturity assessment due to the absence of vitrinite in the pre-Devonian strata. On the other hand, traditional organic geochemistry approaches (e.g., Tmax, biomarkers) cannot be used for maturity assessment of marine shale deposits at the high-over matured stages. In this review, four groups of macerals, including solid bitumen, vitrinite-like maceral, zooclasts and liptinite, can be identified in the Precambrian–Lower Paleozoic sediments, and optical characteristics and origins of these macerals have been summarized. Furthermore, various proxies of thermal maturity assessment are systematically investigated, providing potential solutions for accurate assessment of thermal maturity of the Precambrian–Lower Paleozoic high-over matured marine shale reservoirs. However, accurate identification of different macerals (e.g., solid bitumen, vitrinite-like maceral, graptolite) is still challenged, and moreover, the use of optical reflectance of these macerals is hindered due to optical anisotropies and various conversion relationships that converted their reflectance to equivalent vitrinite reflectance (EqVRo). In addition, Raman spectrometry is a rapid and non-destructive approach to evaluate thermal maturity of organic matter, and several Raman parameters, including FWHM-G, RBS and ID/IG ratio, can be used as the reliable maturity indicators, which overcome the influence of optical anisotropy on the optical indicators. Nevertheless, the application of Raman spectroscopy has been limited due to the lack of uniform test standards (e.g., experimental conditions) and peak fitting methods. In a word, every maturity proxy has its suitable range of application and limitations, so a variety of maturity indicators are essential to comprehensively evaluate thermal maturity of organic matter.

Solid organic matter (OM) plays an essential role in the generation, migration, storage, and production of hydrocarbons from economically important shale rock formations. Electron microscopy images have documented spatial heterogeneity in the porosity of OM at nanoscale, and bulk spectroscopy measurements have documented large variation in the chemical composition of OM during petroleum generation. However, information regarding the heterogeneity of OM chemical composition at the nanoscale has been lacking. Here we demonstrate the first application of atomic force microscopy-based infrared spectroscopy (AFM-IR) to measure the chemical and mechanical heterogeneity of OM in shale at the nanoscale, orders of magnitude finer than achievable by traditional chemical imaging tools such as infrared microscopy. We present a combination of optical microscopy and AFM-IR imaging to characterize OM heterogeneity in an artificially matured series of New Albany Shales. The results document the evolution of individual organic macerals with maturation, providing a microscopic picture of the heterogeneous process of petroleum generation. Solid organic matter (OM) plays a key role in the production of hydrocarbons in shale formations, yet information on OM heterogeneity at a nanoscale is lacking. Here, the authors use atomic force microscopy-based infrared spectroscopy to document the evolution of individual organic macerals with maturation.

This study investigates the structural and sorption properties of coal maceral groups from Poland. Seven fractions with varying maceral compositions were obtained by gravity separation, and their maceral proportions were determined using an automated classification method based on artificial neural networks. The samples were analyzed by microscopy, low-pressure gas adsorption, and methane adsorption and diffusion measurements. Results showed that a higher vitrinite content was associated with greater CH4 adsorption capacity, while inertinite-rich fractions exhibited lower values. The estimated adsorption capacity of pure vitrinite reached 14.3 cm3/g at 1.5 MPa, nearly double that of pure inertinite (6.78 cm3/g). Diffusion analysis revealed that fractions with lower vitrinite content demonstrated significantly higher diffusion coefficients, highlighting the key role of maceral composition in methane storage and transport in coal.

Organic matter (OM) type critically controls the hydrocarbon generation potential and organic pore development in black shales. However, maceral variation in lacustrine shales and its control on hydrocarbon generation potential and organic pore development are not yet well understood. In this study, 15 Chang 7 Member shale samples of the Yanchang Formation, Ordos Basin, were investigated with organic petrography, Rock-Eval pyrolysis, and a scanning electron microscope to study the maceral composition, hydrocarbon generation potential, and organic pores in this black shale succession. The results show that the studied shales are in the oil window (Ro~0.70%). OM belongs to Type I and Type III kerogen, as demonstrated by Rock-Eval pyrolysis. Macerals in the Chang 7 Member shales are composed of amorphous OM, alginite, sporinite, liptodetrinite, vitrinite, inertinite, and solid bitumen. Amorphous OM and alginite are major hydrocarbon-generating macerals, and their content determines the hydrocarbon potential of shales. Secondary organic pores were not observed in the studied Chang 7 Member shales due to either a low thermal maturity or a dominance of terrigenous OM. Maceral variation can affect the reliability of using Rock-Eval Tmax as a thermal maturity indicator. This study provides important insights into maceral control on hydrocarbon generation and organic pore development in black shales, calling for a critical evaluation of OM in black shale successions with organic petrography.

Deep coalbed methane (CBM, commonly accepted as >1500 m) has enormous exploration and development potential, whereas the commercial development of deep CBM exploration areas wordwide has been quite limited. The Linxing area, with coals buried approximately 2000 m deep, shows great development potential. Based on a basic geological analysis of structural and hydrodynamic conditions, combining field tests of reservoir temperature and pressure and indoor measurements of maceral composition, proximate analysis, thermal maturity, porosity and permeability, the factors controlling deep CBM accumulations were discussed. The results show that the present burial depth of the No. 8 + 9 coal seam, mainly between 1698 and 2158 m, exhibits a high reservoir temperature (45.0–64.0 °C) and pressure (15.6–18.8 MPa), except for the uplift area caused by the Zijinshan magma event (with coal depth approximately 1000 m). The maximum vitrinite reflectance (Ro,max) of the coal varies from 1.06% to 1.47%, while the magma-influenced areas reach 3.58% with a relatively high ash content of 31.3% (air-dry basis). The gas content calculated by field desorption tests shows a wide range from 7.18 to 21.64 m3/t. The key factors controlling methane accumulation are concluded from regional geological condition variations. The north area is mainly controlled by structural conditions and the high gas content area located in the syncline zones. The center area is dominated by the Zijinshan magma, with relatively high thermal maturity and a high gas content of as much as 14.5 m3/t. The south area is developed with gentle structural variations, and the gas content is mainly influenced by the regional faults. Furthermore, the groundwater activity in the eastern section is stronger than that in the west, and the hydrodynamic stagnant areas in the western are more beneficial for gas accumulation. The coals vary from 3.35% to 6.50% in porosity and 0.08 to 5.70 mD in permeability; thus, hydrofracturing considering high temperature and pressure should be applied carefully in future reservoir engineering, and the co-production of gas from adjacent tight sandstones also should be evaluated.

The pore evolution history varies with different types of macerals. The pore characteristics of the Huolinhe lignite of the Erlian Basin were examined under laboratory thermal maturation conditions to study the correlation of coal rank and macerals on pore evolution. Mercury intrusion porosimetry, low-pressure CO 2 adsorption, light microscopy, and scanning electron microscopy (SEM) were used to characterize the pores of macerals developed during different stages of pyrolysis. The results suggest that evolution of pores can be divided into three stages: maximum reflectivity of vitrinite ( R o, max ) < 0.55% (first stage), 0.55% <  R o, max  < 1.70% (second stage) and R o, max  > 1.70% (third stage). The proportion of pore area of macerals in vitrinite decreased at R o, max  < 0.55%, and then increased with the R o, max in the range of 0.55–3.35%, while the collodetrinite had the maximum proportion of pore area (4–38%). The volume and specific surface area of different pore sizes first decreased and then increased with increase in coal rank. Moreover, most of the gas pores were mesopores, whereas the plant tissue pores and mold pores were macropores. The SEM analysis indicated that the generation of gas pores was dominant in collodetrinite. Therefore, the content of collodetrinite had a significant influence on the content of coalbed methane and pore structure of the coal reservoir.

To reveal the forming process of organic matter pores in shales, an experiment combining thermal heating and scanning electron microscopy (SEM) was conducted on an oil shale sample with a vitrinite reflectance value of 0.46% from the Huadian Formation in the Huadian Basin, northeastern China. The heating temperatures were from 417.8 °C to 700.8 °C, and the corresponding Easy%Ro values were between 1.00% and 3.70%. Four pieces of macerals in the SEM images, including vitrinite (one piece), funginite (one piece), and solid bitumen (two pieces), were observed during the whole heating process. The results showed that organic pores started to appear and increased in all the studied macerals. Each piece of maceral had two rapid growth points of organic matter pores. During heating, organic pores were initially isolated and then became connected. Among the three types of macerals, solid bitumen was more porous, which may be related to the fact that solid bitumen was more easily thermally degraded. Funginite had more pores than vitrinite at all the heating temperatures. Cracks were observed in vitrinite and funginite during heating, and the vitrinite had more cracks, which may be attributed to its stiffness and brittleness. Almost all the organic matter pores were irregular in this study, but bubble-like or sponge-like organic pores have been reported in natural shales. The difference in shapes of organic matter pores may be derived from our experimental system as it cannot consider pressure. These results provide some implications for the mechanism of formation of organic matter pores.

Maceral compositions take a great role in coalbed methane adsorption. Two controversial viewpoints coexist on the effect of maceral compositions to coalbed methane adsorption. One is vitrinite has better adsorption capacity than inertinite and the other is inertinite has enhanced adsorption capacity than vitrinite. In order to clarify this issue, a series of coal samples were collected and highly purified vitrinite and inertinite concentrates were gained by heavy-fluid flotation and centrifugal separation. Isothermal adsorption experiments of methane were performed to these concentrates with equilibrium moisture and their ultimate adsorption volume were obtained finally. The results show that the adsorption capacity of vitrinite is weaker and the capacity of inertinite is stronger for low-rank coal. For high-rank coal, the adsorption capacity of vitrinite is stronger and the capacity of inertinite is weaker. Along with the increase of coal rank, the adsorption capacity of vitrinite rises gradually and the adsorption capacity of inertinite declines little by little. This result shows that the adsorption capacity of coal to methane not only relates to contents of vitrinite and inertinite, but also relates to metamorphic grade of the coal, because with the increase of metamorphism of coal, molecular structure, functional group and pore characteristic of vitrinite and inertinite change gradually, which results in tremendous changes in the adsorption capacity of coal.

The Xihu Sag in the East China Sea Shelf Basin contains abundant oil and gas reserves and is a focus for hydrocarbon exploration and development. Source rocks are mainly coals and coalmeasures mudstones in the Paleogene Pinghu and Huagang formations. Samples from the Pinghu Formation in the Xihu Sag were collected for petrology, total organic carbon, and Rock-Eval analysis for the purpose of investigating macerals component and their contributions to hydrocarbon generation potential. The coaly source rocks from the Pinghu Formation are dominated by vitrinite (average 86.18%) but have an obviously elevated content of liptinite (average 12.59%) and a much lower amount of inertinite (average 1.23%). Liptinite of the samples is mainly composed of resinite, with a small amount of cutinite, sporinite and alginate in descending order. TOC values are 37.55%–65.58% (average 49.16%). Effective HI values are 167–281 mg HC/g TOC (average 223.5 mg HC/g TOC), suggesting the organic matter is type II kerogen. Relatively high HI values and macerals components suggest that the coaly source rocks can generate both oil and gas. Although the liptinite in the coaly source rocks has a content lower than vitrinite values, it makes a significant contribution to both total hydrocarbon and liquid hydrocarbon generation. The contributions of vitrinite, liptinite and inertinite to the total hydrocarbon generation approximately are 63.21%, 36.46% and 0.33%, respectively. The contributions of vitrinite and liptinite to the liquid hydrocarbon generation are approximately 40.95% and 59.05%, respectively. These results demonstrate that the coaly source rocks are dominated by vitrinite macerals with a relatively higher content of liptinite macerals, especially resinite, and these source rocks are more prone to both total hydrocarbon and liquid hydrocarbon generation. Paleogene coaly source rocks from other parts of the world should be considered for their oil-prone nature.