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Abstract Adsorption and desorption of coal to methane are substantially related to coal facies, referring to the production potential of a coalbed methane (CBM) well. Based on analyses of the stratified channel samples from coal seams No. 2, No. 4 and No. 9 in the Xiqu Drainage Area, Gujiao CBM Block, North China, the relationship between coal facies and adsorption-desorption behaviors was first proposed and discussed. Using the coal facies diagrams suggested by previous investigators, four types of coal facies were distinguished in the coal seams, including the upper delta plain wet forest swamp (I), the lower delta plain fen (II), the lower delta plain marsh (III) and the back barrier low moor (IV). The upper delta plain wet forest swamp occurred dominantly, developing at the bottom and top of all the coal seams. Both the ash yield and groundwater index (GWI) tend to increase from coal facies I to IV, whereas the volatile compounds, fixed carbon, organic sulfur, total sulfur, micropore volume and specific surface area, tissue preservation index (TPI) and vegetation index (VI) decreased, which indicates changes in the vegetation types, water supplements and disturbance, acid-base properties and redox conditions in the paleomires. Paleomire evolution accompanied the rise and drop of the sea level, with a transgression or regression process that could be represented by the coal facies sequence. Coal seams No. 2, No. 4 and No. 9 were discriminated into two, three and two evolving stages, respectively, and these stages were characterized by complete cycles, except stage 4-I from the lower delta plain marsh. Mudstone partings in the coal seams deposited during the highest sea level. Collectively, paleomire environments, especially the variation of vegetation types and mire water condition, result in adsorption and desorption divergences among different coal facies. Arborescent plants decrease from coal facies I to IV as the hydrodynamic intensity increases, leading to corresponding changes in botanical tissue preservation, medium oxidizability, mineral content and, ultimately, adsorption and desorption behaviors. In view of the gas content and desorption capacity of the coal layers in the same coal seam, the layers from the upper delta plain wet forest swamp may have the best potential for CBM drainage, followed by those from the lower delta plain fen and marsh, while those from the back barrier low moor may have the worst potential.

Abstract The Yanchang Formation in the Ordos Basin is the most important petroleum play not only for conventional oil and gas accumulations, but also for newly emerging shale oil and tight gas resources. The molecular characterization of the basinwide source rocks predicts three groups of generative petroleum types: Paraffinic High Wax Oil, mixed base (Paraffinic-Naphthenic-Aromatic) Low Wax Oil, and Gas and Condensate. Supplementary to previous work, 68 samples including the crude oils, source bitumens and reservoir extracts from the Yanchang petroleum play are analyzed. The distribution of two terpane classes (eight tricyclic terpanes and eight pentacyclic terpanes) are determined with subsequent simultaneous RQ-mode factor analysis for a composite data set of these samples alongside 216 published crude oils worldwide with known facies descriptions. Thermal maturity has been evaluated as a consistent distribution at first using a combined method of a maturity-related biomarker [Ts / (Ts + Tm)] and aromatic parameters (Methyldibenzothiophene Ratios) to alleviate the maturity differences effect when discussing geochemical characterization. The R-mode factor analysis consists of the first two factors that are describing 45 present of the cumulative total variance in the data set, and presents a sample grouping pattern in Q-mode factor analysis which is determined by different contributions of terpane associations, i.e., the tricyclic C21 coupled with pentacyclic C26, C27, C28 and C30, in the same factor space. Three terpane associations, the C26 and C28 terpanes, the C21 and C30 terpanes and the C27 pentacyclic terpenes, are respectively responsible for discriminating crude oil, reservoir extracts and source bitumens in RQ-mode factor analysis. Molecular compositions further address more detailed interrelationships among three sample groups that crude oils and reservoir extracts are sharing close genetic relationships both in depositional environment typing and C27-C28-C29 sterane distribution. Samples from source rocks vary much significantly. A mixing process which occurs after oils has been expelled from host source rocks into carrier units during accumulation. In addition, the migration-contamination of C29 sterols when oils are cross through the Chang 7–2 unit along migration pathways might also explain this lack of correlation between source rocks and oil-reservoir system.

Abstract Carbon geo-sequestration (CGS) and recovery enhancement with carbon dioxide injection (CO2-ECBM) have brought increasing focus on the CO2 and water wettability of coal. The CO2 and water contact angles measured using existing conventional methods, such as the pendent drop tilting plate technique and the captive-bubble technique, show low reproducibility due to coal heterogeneity and operational complexity. In this study, a novel NMR-based approach was developed to evaluate the CO2 and water wettability of coal. The experimental results of nine bituminous and anthracite coals show that water wettability can be linearly correlated with the changes of the T2 spectra peak positions. Based on the measured contact angle from the profile of the water adhering to the coal powder disc and the T2 from the NMR of coal powder, we proposed a linear formulation to evaluate the contact angle using the change of T2g of P3. Using this method, we analyzed the CO2-water wettability characteristics of coal. The results show that CO2 reduces the water wettability of coal. Low temperature and/or high CO2 pressure can enhance the CO2 wettability of coal. The change of water-coal wetting behavior with injection of CO2, is resulted by three factors: change of CO2 adsorption capacity of coal, change of interfacial tension, and dissolution of CO2 in water. This study makes it possible to evaluate changes in the water and CO2 wettability of coal, which is essential for evaluating the fluid-interaction mechanisms during the process of carbon geo-sequestration and enhanced coalbed methane recovery with carbon dioxide injection.

Abstract Matrix compressibility and pore properties (pore size distribution) of a rank range of coals was investigated using mercury intrusion porosimetry (MIP) on coal cores with the pore size distribution also being determined using low temperature at 77 K nitrogen adsorption/desorption isotherms for crushed samples. The coal matrix compressibility is significant when the pressure of MIP is from 0.0074–35 MPa. Mathematical models were developed (based on MIP and nitrogen adsorption/desorption isotherms) to establish the porosity/pore size distribution relationships with matrix compressibility. For coal ranks, the matrix compressibility was between 0.24 × 10−4 to 13.56 × 10−4 MPa−1, and had a negative exponential relationship with the vitrinite reflectance (Ro,m%). Lignites have the maximum matrix compressibility due to their structural open structure having limitied compaction during coalification. In addition to the pore structure relationship the composition, moisture, and ash yields impacts on compressibility were also examined. Inertinite-rich coals however had a low matrix compressibility across the rank range, which may be due to the interinhibitive relationships between the mesopores, macropores and minerals. The wetting action of high moisture (water molecules) weakens the link between the coal particles of the lignites and the subbituminous coals, which causes abnormally high compressibility. Observations here relate to hydrofracturing or CO2 injection behaviors during enhancing coalbed methane (CBM) recovery.

Abstract Vitrinite reflectance (VRo in %) is used routinely to quantify thermal maturity in sedimentary basins. Reflectance and fluorescence of other macerals such as solid bitumen and amorphous organic matter (AOM) can provide an independent means to assess thermal maturity and hydrocarbon generation potential. However, similarity in petrographic characteristics of these macerals, solid bitumen and vitrinite in particular, often causes difficulties with their identification and, consequently, reflectance can be measured on misidentified particles, making reported VRo values unreliable. The purpose of this study is to compare reflectance values of various macerals in early mature shales and to evaluate the implications of misidentifying solid bitumen and vitrinite for assessing thermal maturity. To address this issue, 15 organic-matter-rich samples from the Middle Devonian/Lower Mississippian New Albany Shale from a corehole in Daviess County, Indiana, were selected. These samples were chosen because they had distinct and easily identified AOM, solid bitumen, vitrinite, and inertinite particles, allowing for statistically relevant comparisons. VRo values (0.57 to 0.65%) of the studied sample suite cover the early mature stage, and expressed no trend with depth. In comparison, reflectance values of solid bitumen (BRo) and AOM (AOMRo) from the same samples are lower, and range from 0.44 to 0.52% and 0.27 to 0.31%, respectively. These differences are accompanied by corresponding differences in chemistry of macerals as demonstrated by the micro-FTIR technique. Specifically, compared to vitrinite and inertinite, solid bitumen shows lower aromaticity, and compared to AOM and alginite, it exhibits shorter aliphatic chains. Reflectance was observed to vary systematically; samples having higher VRo also feature elevated solid bitumen and AOM reflectance values. The relationship between vitrinite and solid bitumen can be expressed by the following equation: vitrinite reflectance equivalent (VRoE in %) = (0.83 × BRo) + 0.22, whereas for vitrinite and AOM, VRoE = (0.84 × AOMRo) + 0.38. Statistical evaluation of the differences in reflectance values caused by maceral misidentification indicates that in extreme cases, when a petrographer cannot distinguish between vitrinite and solid bitumen, the reflectance can be shifted by 0.06–0.09%. For this set of samples, such a difference could shift maturity assessment from early mature to immature. In more common cases, when the analyst can distinguish between the macerals but has difficulties with their overlapping reflectance interval, the reflectance difference that results from misidentification is only within a 0.0–0.02% range. Therefore, if the level of uncertainty in maceral identification can be determined, the calculation of VRoE values from measured BRo, can reduce inaccuracy in VRo values for interpreting the thermal histories of sedimentary basins, which, in turn, is essential for the assessment of oil and gas resources and for building a successful exploration model.

Abstract Two sets of lacustrine source rocks in the Junggar Basin and Ordos Basin have been analysed by organic geochemical approach and organic petrographic technique. One of these source rocks, the Junggar shales (Lucaogou Formation and Hongyanchi Formation), was deposited in a hydrologically closed, brackish lake under a sub-humid climate in higher latitude (39°–43° N), and consisted of organic-rich profundal lake shales (Lucaogou Formation) and lean fluvial-lacustrine lake margin mudstones. The other one, the Chang 7 Shale, was deposited in a hydrologically open, freshwater lake under a humid climate in a 31° N palaeolatitude, and was characterized by high concentration of alginite-rich organic matter. The profundal Lucaogou Shale and Chang 7–3 Shale possess abundant total organic carbon, high generative potential S2 and high Hydrogen Indices, which rank as the best prolific source rocks in both basins followed by Chang 7-1 Shale. Hongyanchi Formation and Chang 7-2 are much leaner than their counterparts, and contain relatively small portion of alginite but more terrigenous macerals. This nature of organic matter makes them less prolific and becoming gas-prone potential. These differences in petroleum potential mirror depositional environment changes in source rocks formation. Potential source rocks in both basins have the potential for generating high-wax oil but the brackish system clearly contains more PNA oil. The kerogen structures revealed by pyrolysis gas chromatography allowexplaining this trend. The Lucaogou, Chang 7-3 and Chang 7-1 shales enriching in long and moderate alkyl chains share a nature of high wax oil generation-trend. In contrast, the Hongyanchi and Chang 7-2 shales appears to be dominated by short to moderated chain length distribution and higher overall aromaticity to generate more PNA oil, even gas and condensate. Thermal liability was shown to be linked to kerogen structure, in particular the presence of alicyclic and oxygen-containing moieties, and these in turn are correlate with total organic carbon content.

Nanoindentation is a valuable tool, which enables insights into the material properties of natural, highly inhomogeneous composite materials such as shales and organic matter-rich rocks. However, the inherent complexity of these rocks and its constituents complicates the extraction of representative material parameters such as the reduced elastic modulus (Er) and hardness (H) for organic matter (OM) via nanoindentation. The present study aims to extract the representative H and Er values for OM within an over-mature sample set (1.33-2.23%Rr) from the Chinese Songliao Basin and evaluate influencing factors of the resulting parameters. This was realized by means of high-speed nanoindentation mapping in combination with comprehensive optical and high resolution imaging methods. The average Er and H values for the different particles range from 3.86 +/- 0.17 to 7.52 +/- 3.80 GPa and from 0.36 +/- 0.02 to 0.64 +/- 0.09 GPa, respectively. The results were subsequently processed by the unsupervised machine learning algorithm k-means clustering in order to evaluate representative Er and H results. The post-processing suggests that inherent heterogeneity of OM is responsible for considerable data scattering. In fact, surrounding, underlying and inherent mineral matter lead to confinement effects and enhanced Er values, whereas cracks and pores are responsible for a lowered stiffness. Adjusted for these influencing factors, a declining trend with increasing maturity (up to 1.96%Rr) could be observed for Er, with average values calculated from representative clusters ranging from 5.88 +/- 0.37 down to 4.07 +/- 0.32 GPa. Er slightly increases again between 2.00 and 2.23%Rr (up to 4.85 +/- 0.35 GPa). No clear relationship of H with thermal maturity was observed. The enhanced accuracy archived by a large data set facilitated machine learning approach not only improves further modelling attempts but also allows insights of impacting geological processes on the material parameter and general understanding of mechanical behavior of OM in rock formations. Thus, the presented multimethod approach promotes a fast and reliable assessment of representative material parameters from organic rock constituents.

This paper investigates the mineralogy of coal and non-coal samples from the Haerjiao exploration area in the Jimunai depression, Xinjiang Uygur Autonomous Region, northwestern China, using X-ray diffraction (XRD), optical microscopy and scanning electron microscopy with an energy dispersive X-ray spectrometer (SEM-EDX). The minerals in the anthracite proximal to the igneous intrusions are represented by the occurrence of chlorite, muscovite, and illite, which are absent in the semianthracite distal to the igneous intrusions. Kaolinite, the dominant mineral in the semianthracite, decreases in abundance or even disappears in the anthracite. Muscovite and illite in the anthracite are interpreted to result from the alteration of the kaolinite precursor and possibly formed by the interaction of kaolinite and elements (e.g., K, Fe, and Mg) from igneous hydrothermal solutions. The formation of chlorite is the result of the transformation of kaolinite due to the interaction of kaolinite with Fe-Mg-rich solutions during the diagenetic stage and the direct epigenetic precipitation from Fe-Mg-rich igneous siliceous hydrothermal fluids. Calcite, ankerite, and sulfide minerals resulted from epigenetic precipitation from hydrothermal fluids. Cleat/fracture mineralization indicates that the Fe-Mg-rich hydrothermal fluid from which chlorite precipitated was introduced earlier than the Ca-rich fluid from which calcite precipitated. The injection of hydrothermal solutions that formed sulfide minerals occurred at the latest stage. © 2019 This research was funded by the National Science Foundation for Young Scientists of China (No. 41602176), National Key R&D Plan of China (no. 2016YFA0602002), and Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education (TPR-2018-16). The authors would like to express sincere thanks to the Xinjiang Bureau of Prospecting and Development of Geology and Mineral Resources for assistance with sampling and acknowledge Institute of Environmental Assessment and Water Research, CSIC, Spain for assistance with the sample analysis. Peer reviewed