Secondary Biogenic Gas Research Articles

Gas content, geochemical characteristics and implications of coalbed methane from the Deep Area of Qi'Nan Coalmine in Huaibei Coalfield.

The objective of this work is to investigate the implications of geological influence factors on gas content and geochemical characteristics of deep-buried (> 800m) coalbed methane (CBM) reservoirs. Results show that bituminous coal accounts for the majority, which exhibits similar maturity but differ in maceral and chemical constituents. CBM reservoirs show low porosity, low permeability and moderate temperature, with thickness of 0.85-4.15m. In addition, the total gas content is 4.58-12.33 m3/t (average of 8.83 m3/t). CH4 is the main component with concentration of 92.83-99.22% (average of 96.68%), and the δ13CCH4 and δ13DCH4 is - 53.78‰--44.62‰ (average of - 48.82‰) and - 223.93‰---4.49‰ (average of - 215.37‰), respectively. All CBM samples are the mixtures of thermogenic gases and secondary biogenic gases with CO2 reduction. In addition, gas content characteristic at the critical point of burial depth is the result by positive and negative geological effects. CH4 concentration shows a wide range with the increases of buried depth, while the numerical values of which for the selected samples display complex variation characteristics. Furthermore, the values of δ13CCH4 and δ13DCH4 become heavier with the increases of buried depth. Besides, the above two geochemical parameters are related to Ro, max and reservoir temperature.

Experimental research on biogenic methane generation pathway and microflora constraints based on in-situ gas-water-microbe

Biogenic coalbed methane (CBM), generated through the biodegradation process of coal organic matter, is a relatively clean and environment-friendly unconventional natural gas resource. To investigate the differences in microbial communities (bacteria and archaea) as well as their environmental influencing factors between the Permian Shanxi Formation (P1s) and Taiyuan Formation (P1t), we systematically collected in-situ CBM samples along with coalbed water and microbial samples from a representative biogenic CBM field – the Baode block locked in the Ordos basin in China. The relative content of biogenic CBM was quantitatively calculated while analyzing gas composition, stable isotope, ion content, and high-throughput sequencing analysis to discuss the generation pathway for biogenic CBM and environmental control factors affecting microbial community structure. The results indicate that P1s and P1t are mixed with early thermogenic gas and secondary biogenic gas genetic types within their respective CBMs. The average proportion for biogenic gas within current coal seams is approximately 62.24 % for P1s, while it reaches around 64.37 % for P1t. Water quality type primarily consists of Na-HCO3 type with total dissolved solids (TDS) measuring at approximately 1015.86 mg/L for P1s, whereas P1t exhibits Na-Ca-HCO3-Cl water type with TDS content reaching about 2314.85 mg/L. Combining dominant Methanobacterium species, along with Δ13C(CO2-CH4) ratios and δD(CH4) versus δD(H2O) indexes, represents the biogenic CBM primarily generated through the hydrogenotrophic methanogenesis pathway. The in-situ bacterial communities of P1s are mainly influenced by Ni, pH, and TDS. The archaeal communities exhibit higher sensitivity to environmental factors (such as Co, pH, Ni, and dissolved organic carbon) than bacteria. These findings hold significant implications for understanding the generation process of biogenic CBM in middle and low-rank coal areas.

Mechanisms of secondary biogenic coalbed methane formation in bituminous coal seams: a joint experimental and multi-omics study.

Coal seam microbes, as endogenous drivers of secondary biogenic gas production in coal seams, might be related to methane production in coal seams. In this study, we carried out anaerobic indoor culture experiments of microorganisms from three different depths of bituminous coal seams in Huainan mining area, and revealed the secondary biogas generation mechanism of bituminous coal seams by using the combined analysis of macro-genome and metabolism multi-omics. The results showed that the cumulative mass molar concentrations (Molality) of biomethane production increased with the increase of the coal seam depth in two consecutive cycles. At the genus level, there were significant differences in the bacterial and archaeal community structures corresponding to the three coal seams 1#, 6#, and 9#(p < 0.05). The volatile matter of air-dry basis (Vad) of coal was significantly correlated with differences in genus-level composition of bacteria and archaea, with correlations of R bacterial = 0.368 and R archaeal = 0.463, respectively. Functional gene analysis showed that the relative abundance of methanogenesis increased by 42% before and after anaerobic fermentation cultivation. Meanwhile, a total of 11 classes of carbon metabolism homologues closely related to methanogenesis were detected in the liquid metabolites of coal bed microbes after 60 days of incubation. Finally, the fatty acid, amino acid and carbohydrate synergistic methanogenic metabolic pathway was reconstructed based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. The expression level of mcrA gene within the metabolic pathway of the 1# deep coal sample was significantly higher than that of the other two groups (p < 0.05 for significance), and the efficient expression of mcrA gene at the end of the methanogenic pathway promoted the conversion of bituminous coal organic matter to methane. Therefore, coal matrix compositions may be the key factors causing diversity in microbial community and metabolic function, which might be related to the different methane content in different coal seams.

Geochemical Characteristics and Geological Significance of Coalbed Methane in Suzhou Mining Area of the Huaibei Coalfield.

In this study, gas contents, geochemical features, and origins of coalbed methane (CBM) and their influence factors were investigated on nos. 7 and 8 CBM reservoirs from the Suzhou mining area of the Huaibei coalfield. Results have shown that the selected CBM reservoirs are characterized by various thickness (0.50-9.19 m) and buried depth (619-1226 m), but have relatively better lithology of surrounding rocks. Coal samples have similar maturity (Ro,max = 0.71-1.05%), but show differences in chemical composition and macerals. Gas content of nos. 7 and 8 CBM reservoirs ranges from 6.13-12.25 m3/t, but the value of former is lower than that of later one overall. In addition, CH4 is a predominantly component with a value of 88.23-99.00% (avg. 96.69%), and the heavy hydrocarbon gas (C2+) is 0.00-1.93% (avg. 0.41%). The δ13CCH4 value ranges from -64.54 to -46.36‰ (avg. -53.92‰), and the δ13DCH4 value is -224.36 to -211.75‰ (avg. -219.09‰). Based on the analysis of components and isotopic values, the CBM samples are thermogenic (20.92-71.30%; avg 50.09%) and secondary biogenic gases (28.70-74.49%; avg. 49.91%). Gas content shows changeable characteristics at a buried depth of 300-1300 m, which is affected by buried depth, reservoir temperature and pressure, Mad and vitrinite. However, the CH4 concentration shows no correlation with buried depth. Moreover, the buried depth is significantly positively correlated to δ13CCH4 and δ13DCH4. Based on the relationship between gas content and isotope values, it suggests that δ13CCH4 or δ13DCH4 may have a relationship with the main controlling factors of gas content.

Microbiome of High-Rank Coal Reservoirs in the High-Production Areas of the Southern Qinshui Basin.

To study the distribution features of microorganisms in distinct hydrological areas of the southern Qinshui Basin, C-N-S microorganisms were studied using 16S RNA sequencing, metagenome sequencing and geochemical technologies, showing the high sensitivity of microorganisms to the hydrodynamic dynamics of coal. The hydrodynamic intensity of the #3 coal gradually decreased from the runoff areas to the stagnant areas. The stagnant zones have higher reservoir pressure, methane content, δ13CDIC and TDS and lower SO42-, Fe3+ and NO3- concentrations than the runoff areas. C-N-S-cycling microorganisms, including those engaged in methanogenesis, nitrate respiration, fermentation, nitrate reduction, dark oxidation of sulfur compounds, sulfate respiration, iron respiration, chlorate reduction, aromatic compound degradation, denitrification, ammonification and nitrogen fixation, were more abundant in the stagnant areas. The relative abundance of C-N-S functional genes, including genes related to C metabolism (e.g., mcr, mer, mtr, fwd and mtd), N metabolism (e.g., nifDKH, nirK, narGHI, nosZ, amoB, norC and napAB) and sulfur metabolism (e.g., dsrAB and PAPSS), increased in the stagnant zones, indicating that there was active microbiological C-N-S cycling in the stagnant areas. The degradation and fermentation of terrestrial plant organic carbon and coal seam organic matter could provide substrates for methanogens, while nitrogen fixation and nitrification can provide nitrogen for methanogens, which are all favorable factors for stronger methanogenesis in stagnant areas. The coal in the study area is currently in the secondary biogenic gas generation stage because of the rising of the strata, which recharges atmospheric precipitation. The random forest model shows that the abundance of C-N-S microorganisms and genes could be used to distinguish different hydrological zones in coal reservoirs. Since stagnant zones are usually high-gas-bearing zones and high-production areas of CBM exploration, these microbiological indicators can be used as effective parameters to identify high-production-potential zones. In addition, nitrate respiration and sulfate respiration microorganisms consumed NO3- and SO42-, causing a decrease in the content of these two ions in the stagnant areas.

Secondary processes on coal deposits change the emission of greenhouse gases

The opencast Bilina coal mine (Czech Republic) produces eight million tons of subbituminous coal per year. To evaluate greenhouse gas emissions from the mine, we measured the chemical and isotopic composition of gases released from fresh coals using the canister method and after coal crushing and milling. The δ13C values of the released methane (CH4) and carbon dioxide (CO2) corresponded to their biogenic origin (δ13C (CH4) −70 ‰ to −55 ‰ vs. VPDB, δ2H − 275 ‰ to −260 ‰ vs. VSMOW, δ13C (CO2) -10 ‰ to −16 ‰ vs. VPDB). The findings revealed that secondary biogenic gas production replaced original coal bed gases and the excavated coal is rich in CO2 (0.615 m3 ton−1), but very low in CH4 (0.09 m3 ton−1). Additionally, most of the secondary CH4 was released before excavation. The loss of CH4 (0.235 m3 ton−1) was calculated back from the isotope fractionation of residual CO2. High CO2 emission increased its atmospheric concentration in the mine (570–2000 ppm).

Composition, Origin, and Accumulation Model of Coalbed Methane in the Panxie Coal Mining Area, Anhui Province, China.

To investigate the geochemical characteristic, genetic types, and accumulation model of coalbed methane (CBM), 16 samples from a burial depth of 621–1494 m were collected in the Panxie Coal Mining Area of Huainan Coalfield. The results indicate that the samples are dominated by methane, and the concentrations are distributed in the range of 73.11–95.42%. The dryness coefficient is 0.77–1.00 (average, 0.93), and the ratio of methane to the sum of ethane and propane (C1/(C2 + C3)) is 3.18–242.64 (average, 36.15). The δ13CCH4 values are distributed in the range of −65.44 to −32.38‰ (average, −45.22‰), the δDCH4 values are in the range of −226.84 to −156.82‰ (average, −182.93‰), and the δ13CCO2 values are in the range of −19.7 to −10.1‰ (average, −15.51‰). CBM samples in the study area are dominated by thermogenic gases, followed by secondary biogenic gases with CO2 reduction. For the percentages of different genetic gases, the distribution range of thermogenic gas is 70.11–97.86%, whereas that of biogenic gas is 58.65–77.86% for five samples from Zhangji, Panyi, Pansan, and Panbei Coalmines. Moreover, desorption-diffusion fractionation and the effect of groundwater dissolution occurred in the Panxie Coal Mining Area, and higher δ13CCH4 values mostly existed in the deeper coal seams. Furthermore, the biogenic gases are more likely to be secondary biogenic gases generated by CO2 reduction on the basis of data comparison, which is related to the flowing water underground. Accumulation models of different genetic types of CBM are correlated with the burial depth of coal seams, location, and type of faults and aquifers.

Biodegradation of coal organic matter associated with the generation of secondary biogenic gas in the Huaibei Coalfield

Although the biodegradation of coal-measure organic matter (OM) provides a crucial material basis for the formation of secondary biogenic gas (SBG), few studies to date have focused on the effect of OM biodegradation on SBG enrichment. Therefore, the compositional characteristics of saturated hydrocarbons and aromatic hydrocarbons in coal and coal-measure mudstone in the Huaibei Coalfield were analyzed to investigate their biodegradation characteristics and the impact on SBG enrichment. The biodegradation analysis of biomarkers showed that all samples had experienced different degrees of biodegradation; however, the degradation of saturated hydrocarbons and aromatic hydrocarbons differed significantly. In saturated hydrocarbons, long-chain n-alkanes showed more extensive degradation than short-chain alkanes, while aromatic compounds with low substituent numbers degraded more rapidly than that with high substituent numbers in the same series. Naphthalene was strongly biodegraded, and the degradation of biphenyl and dibenzofuran was more heavily than that of phenanthrene series compounds. In contrast, phenanthrene and dibenzothiophene compounds showed slight biodegradation. In terms of the biodegradation degree of saturated hydrocarbons, the biodegradation level reached 4 ~ 5 levels, while the biodegradation level of aromatic hydrocarbons was as high as 5 ~ 7 levels. Obviously, the degradation of biomarkers in coal is remarkably different from the phenomenon found in oil. Nevertheless, while microbial degradation provides a sufficient material basis for the generation of SBG, strong biodegradation does not necessarily lead to the best occurrence of SBG. The preservation of SBG is also affected by the characteristics of structural fractures, the preservation conditions of CBM, and so on.

Effects of simulated surface freshwater environment on in situ microorganisms and their methanogenesis after tectonic uplift of a deep coal seam

Secondary biogenic coalbed gas (CBG) is the gas generated by the microbial degradation of coal or wet gas components after freshwater recharge events. However, in situ microorganisms in deep coalbed water with high total dissolved solids (TDS) content may be overlooked. The evolution of the community structure and gene function of in situ microorganisms and their contributions to secondary biogenic CBG are poorly understood. The coalbed water TDS content in the Miquan region of southern Junggar Basin is unusually high (up to 45,000 mg/L), CBG is biogenic, and microorganisms have been detected in coalbed water. This study collected coal and high-TDS coalbed water samples in the Miquan region. Gas production experiments in a simulated freshwater environment (carrying nutrients) were conducted. Microbial community structure and gene function were analyzed using metagenomic sequencing. The gas production capacity of microorganisms in high-TDS coalbed water was significantly enhanced (up to 220.81%) under subculture with deionized water and nutrients. Microbial community analysis indicated that the hydrolytic bacterial group changed from a single genus to a state of synergistic degradation by multiple genera. The abundances of acidogenic bacteria, hydrogen-producing acetogenic bacteria, and methanogens gradually increased. Overall, the relationships between microorganisms were significantly optimized. Gene function analysis showed that most of the functional types were related to metabolism. Low-TDS culture enhanced the metabolic capacity of the microorganisms, thereby improving the fermentation performance. Finally, through combining the succession of the microbial communities, gene function changes, and gas production results, the geological methane-producing process of the in situ microorganisms in deep high-TDS coalbed water against the background of tectonic uplift was recreated.

Review of the Hydrogeological Controls on Coalbed Methane (CBM) and Development Trends

Hydrogeological conditions can control the generation, preservation, enrichment, and production of coalbed methane (CBM) in the field; however, research on these impacts is insufficient, resulting in the limitation of the development of coalbed methane. This paper summarizes the current research status and development trends of the effect of hydrogeology on CBM using methods such as mathematical statistics, literature analysis, well logging, and hydrochemical analysis. The results indicate that it is beneficial for the generation of secondary biogenic gases in low-rank coal seams under the situations like active hydrodynamic conditions with a salinity less than 1000 mg/L, a pH range from 5.9 to 8.8, or a range of oxidation-reduction potential from -540 mV to -590 mV. The abnormally high temperature due to the magmatic-hydrothermal fluids accelerates the metamorphism of coal rocks, leading to the promotion of the generation of thermogenic gases. When the coalbed structural conditions of one area are similar to the depositional conditions in that area, the CBM is accumulated if the conditions of that area meet the following criteria: the water type is NaHCO3, the salinity is greater than 1500 mg/L, the desulfurization coefficient is less than 1, and the sodium-chloride coefficient is less than 10. The stable isotope analysis of CBM well-produced water shows that the δD values in the groundwater shift to the left of the global meteoric water line, indicating that the produced water comes from atmospheric precipitation. In the CBM enrichment zone, the area with a relatively high salinity and a low sodium-chloride coefficient is the high-production area. Based on our study, three high CBM-production patterns are summarized: coalbed structure-hydraulic trapping, fold limb-fracture development, and syncline core-water stagnation. Additionally, four development trends of the control of hydrogeology on CBM are proposed: transformation from qualitative evaluation to quantitative evaluation, from a singular evaluation standard to multiple evaluation standards, from static evaluation to dynamic evaluation, and from pure theoretical research to theoretical guidance on production practices.

Isotopic and hydrogeochemical evidence for biogenic gas in Cuervos Formation coal beds, Cesar Rancheria Basin, Colombia

Coal bed methane (CBM) has recently been produced from sub-bituminous to high volatile bituminous low rank coals (vitrinite reflectance 0.39 to 0.53%) from the Paleocene Cuervos Formation in the Cesar Sub-basin, Colombia. Understanding CBM gas origin is vital for designing exploration and production strategies, in order to target either shallow economic accumulations of microbial gas or deeper and thermally mature coal seams. In this context, this study aims to determine whether thermogenic gas, biogenic gas, or a mixture of both gas types have contributed to gas yield in the Cesar Sub-basin; it also discusses the relationship between gas origin and tectonics. For the first time, co-produced waters and gas samples from CBM multi-seam production wells and shallow aquifer water-wells were collected in the Boqueron compartment (Cesar Sub-basin) and were analysed via stable isotope composition and molecular geochemistry. Structural compartmentalisation in the Sub-basin as a result of interplate shortening during the Mid-Paleogene has resulted in slightly different coal ranks, saturation, and gas contents between compartments of the Paleocene Cuervos Formation. This event could have interrupted early thermogenesis and allowed the introduction of meteoric waters carrying bacteria consortia responsible for biogenesis.The produced waters from the CBM wells were HCO3 > Na+ > Cl− type, and ranged in TDS (Total dissolved solids) from 2268 to 6602 mg/L (avg. 3887 mg/L). Recently revised genetic diagrams of δ13C-CH4 versus δ2H-CH4 and additional parameters, such as gas dryness ratio and water chemistry, corroborated biogenesis as the main gas origin. Carbon isotopic differences between carbon dioxide and methane (Δ13CH2O–CH4), as well as those of hydrogen isotopes in water and methane (Δ2HH2O–CH4), also indicated a typical microbial CO2 reduction pathway. The positive carbon isotope composition of dissolved inorganic carbon (δ13C-DIC) not only showed a clear differentiation between waters from CBM wells and those from shallow aquifers, but also confirmed the occurrence of methanogenesis when paired with high alkalinity. In addition, the water quality analysis showed increasing sodium and bicarbonate concentrations with depth, which is typical of CBM production basins. Although biogenic gas was identified, tectonic settings do not seem to explain gas origin occurrence, since secondary biogenesis was expected in an uplifted basin such as Cesar Rancheria. Isotopic data was not decisive in differentiating primary versus secondary biogenesis, since the gas isotope compositions plot close to the boundary between the primary and secondary biogenic gas fields. Further studies on isotopic chemistry are required to refine this interpretation and confirm gas generation in the other two compartments.

Hydrogeochemical Characteristics and Water–Rock Interactions of Coalbed-Produced Water Derived from the Dafosi Biogenic Gas Field in the Southern Margin of Ordos Basin, China

The hydrogeochemical characteristics of coalbed-produced water can provide insights into the sources of ions and water, the groundwater environments, hydrodynamic conditions, and water-rock interactions of depositional basins. To study the water-rock reaction process and reveal whether there is a microbial activity in the groundwater, a case of the Dafosi biogenic gas field was chosen by testing the ionic concentrations and hydrogen and oxygen isotopic compositions of coalbed-produced water and employing R-type cluster and principal component analyses. The results showed that Na+, Cl − , and HCO3- are the principal ions in the coalbed-produced water, while the water type is mainly a Na–Cl. Due to the hydrolysis of HCO3-, the pH in this region was controlled primarily by HCO3-. As the main cation in water, Na+ contributed substantially to the total dissolved solids. Na+ is also related to the exchange between rock-bound Na+ and Ca2+ and Mg2+ in water or surrounding rocks. The coalbed-produced water’s oxygen isotopes displayed a characteristic 18O drift and enrichment, indicating that the 16O isotope in the water was preferentially exchanged with the coal organic matter. Early evaporation is also contributed to the enrichment of TDS (total dissolved solids) and 18O in the water. The central part of the study area, including the Qijia anticline, was affected by the Yanshanian uplift and denudation and subsequently developed a water-conducting fissure zone and was recharged atmospheric precipitation; these conditions were conducive to the formation of secondary biogenic gas.

Variation characteristic and mechanism of carbon isotope composition of coalbed methane under different conditions and its tracing significance

Coalbed gas (CBG) is a form of new energy with a large resource potential. The δ13C1 is an important indicator in trace CBG origins and thermal evolution degrees. However, regarding CBG δ13C1 as a genetic tracer still exists uncertainty. Here, we systematically studied the distribution characteristics, variation mechanisms and applications of the δ13C1 of CBG samples produced under different conditions. The study shows CBG δ13C1 are highly variable and complex. The δ13C1 are from −79.5‰ to −10.1‰, and the correlation between δ13C1 and Ro is week. The δ13C1 of the production gas and coal core desorption gas are stable and could be used as an effective genetic tracer. The δ13C1 of the mining coal desorption gas are significantly higher than the theoretical δ13C1, however, the δ13C1 of the drained coalmine gas are lower, suggesting that the mining coal desorption gas represents the residual CBG, however, the drained coalmine gas represents the first desorbed CBG. Therefore, their δ13C1 cannot be used as genetic tracer, but have great potential in tracing the CBG prevention condition. The mixture of secondary biogenic gas (SBG) and thermogenic gas makes δ13C1 high variability. It cannot be directly used as tracer, but is of great significant in study the mixing ratios of the two kinds of gases. The main reasons for the high variability and complexity of CBG δ13C1 are the isotope desorption fractionation and the mixing of SBG and thermogenic gas. This study provides a new scientific basis for geochemical tracing and resource evaluation of CBG.

Subsurface Microbial Invasion Affects the Microbial Community of Coal Seams

Coal seam microorganisms have attracted much attention for their special functions that are closely related to secondary biogenic gas formation, but the influence of subsurface microbial invasion on coal microorganisms remains poorly understood. In this study, subsurface microorganisms from soils at different depths were used to infect coal samples, and the abundance and composition of the microbial community were analyzed by quantitative polymerase chain reaction (PCR) and high-throughput sequencing. The results showed that methane production from the coal samples treated with soil extractions did not increase significantly nor did the abundance and transcriptional activity of archaeal 16S rRNA and mcrA genes. In contrast, two soil extractions significantly improved the abundance and transcriptional activity of the coal bacteria. There was no significant difference in the effects of different soil extractions on bacterial abundance and transcriptional activity, but differences in the effects on α- and β-bacterial diversity between the coal samples treated by different soil extractions were found. Soil extraction from the shallow layer soil far away from the coal mine increased the bacterial α-diversity of the coal samples and changed the bacterial community composition. In addition, this soil extraction increased various bacterial groups in the coal sample and changed the predictive metagenome functional contents of the coal samples from the bacterial sequence data. In summary, this study provides basic microbial information for subsurface microbial invasion of coal seams and helps increase understanding of the source of microorganisms in in situ coal seams and the changes in the microbial community during subsurface microbial migration.

Occurrence of fluids in high dip angled coal measures: Geological and geochemical assessments for southern Junggar Basin, China

The Junggar Basin, Xinjiang has abundant coalbed methane (CBM) resources, and most of the CBM is found in the coal reservoirs with high dip angle (>50°), where the generation, accumulation and extraction of CBM significantly differs with flat coal reservoirs. This study clarifies the effect of high dip angle on the fluid pressure field, in situ stress field, permeability and gas bearing capacity of CBM reservoir in the southern Junggar Basin. The studies show that the high dip angle results in the fluid pressure differentiation of CBM reservoirs at 800–1000 m. The shallow opened fluid pressure system is in an ultralow-normal pressure state, while the deep closed system is characterized by overpressure. The self-sealing property of high dip CBM reservoirs can prevent the gas escaping upward along the inclined bedding. The larger the dip angle, the higher the gas column sealed by the self-physical property. The overburden compaction is reduced for dip formation, which causes the in situ stress of inclined coal seam to decrease but the anisotropy to increase. Furthermore, the high dip angle decreases the gas content per meter of coal seam, and diversifies the gas genesis, which causes the secondary biogenic gas to generate in the shallow seam under the effect of active hydrodynamics, and the thermogenic gas to be better accumulated in the deep coal seam. Due to the desorption hysteresis of the coal seam in downdip direction, the gas production curve of high dip coal seam has obvious bimodal characteristics, and the second peak is significantly higher than the first.

Geochemical characteristics, origin of the Chang 8 oil and natural gas in the southwestern Ordos Basin, China

Petroleum exploration in the Ordos Basin gradually expanded to the edge of the basin, but encountered difficulties on the southwestern edge. Compared with mature exploration areas inside the basin, the nature of reservoir fluids in the southwestern area is more complex. In order to better understand the oil/gas system in this area, this study conducted a detailed research on the geochemical characteristics of potential source rocks and reservoir fluids in this area.The results show that although the Chang 7 oil group in the study area developed thicker mudstone and has a high abundance of organic matter, the low degree of thermal evolution limits its hydrocarbon production. The Chang 8 oil in the study area is the result of lateral migration of products from mature source rocks inside the basin, which can be explained by the similar biomarker characteristics and gradually changing nitrogen compounds between them. Anaerobic biodegradation is an important factor that complicates the fluid properties in the study area. It not only thickens part of the oil, but also produces secondary biogenic natural gas, mainly methane.

Structure- and hydrology-controlled isotopic coupling and heterogeneity of coalbed gases and co-produced water in the Yanchuannan block, southeastern Ordos Basin

Gas and water geochemistry in the No.2 coal reservoir of the Yanchuannan CBM block in the southeastern Ordos basin reflects a complex interaction of tectonic, hydrodynamic, thermogenic, and biogenic processes. Stable isotope and component analyses of gas and water samples from the No. 2 coal seam of 17 CBM wells were performed to determine the correlations between stable isotopes of gases and co-produced water, and to understand the controls of structure and hydrology on isotope distribution. The results show that the co-produced water continuously ranges from brackish, isotopically lighter Na-HCO3/Na-HCO3-Cl water to hypersaline, isotopically heavier Na-Cl/Na-Ca-Cl brine. Based on the hydrochemical features, fault development, and burial depth of coal seam, four structure-controlled hydrodynamic zones are distinguished, including strong through-flow, medium through-flow, weak through-flow, and stagnant zones. The gas isotope analysis shows that thermogenic methane is the dominant source of CBM, and secondary biogenic gases only exist in the strong through-flow zone. δ2H-CH4 is ultimately inherited from formation water, and more 2H can be transferred during the formation of thermogenic gases. δ13C-CH4 and δ13C-CO2 are mainly controlled by thermal maturity and gas genesis respectively, and there is a negative correlation between them. The stable isotopes of coalbed gases and water show strong spatial heterogeneity with the changing hydrodynamic strength. Owing to the influence of water recharge, gas leakage, and secondary biogas formation, the strong through-flow zone is characterized by the most negative δ13C-CH4, δ2H-CH4, δ18O-H2O, δ2H-H2O and the most positive δ13C-CO2, Δ2H-(H2O-CH4) and Δ13C-(CO2-CH4). However, the stagnant zone has maintained stable in a long geological period, which leads to the high concentration of original connate water and mass preservation of coalbed gases, and hence its isotopic feature is completely opposite to the strong through-flow zone. This study confirms the significant controls of hydrogeological conditions on the migration of water and gas and the application of isotopic indicators in the exploration of deep CBM.

Influence Of Organic Matrix And Cations On Bio-Methane Yield With Anthracite Methanogenic Consortium

Organic compounds fermentation of coal has been used to generate secondary biogenic gas and enhance gas reservoirs in coal bed. To enhance the bio-degradation process, culture nutrition plays an important role in remediating the nutritional deficiency of the coal seam. The influence of bio-methane yield with organic inputs and cation concentrations was examined. Research of organic matrix influence revealed that the traditional organic material except yeast extract should forbid, and the input of yeast extract should limit at 1.00g/L also. Further, the study demonstrated that the ion concentration of sodium, potassium, magnesium, calcium and ammonia nitrogen also influenced methane and carbon dioxide yields. And the optimize concentrations for Ca2+, K+, Na+, Mg2+ were 5.1, 1.7, 23 and 1.3 mmol/L. The Mg2+ was particularly sensitive in inhibiting CH4 metabolism processes largely for gas-coal methanogenic consortium.

Differences in CH4 and C2H6 carbon isotopic compositions from open and closed pores in coal: Implications for understanding the two-stage δ13C shift during canister desorption

Canister desorption of freshly drilled coal cores is routinely employed for exploratory assessments of coalbed gas. Coals are molecular sieves containing both open and closed pores. However, little attention has been paid to the differences between the carbon isotopic compositions of CH4 and C2H6 from open and closed pores in coal and their influence on isotopic fractionation during desorption. In this study, canister desorption and pulverization–degassing experiments were conducted to study the carbon isotopic shifts of CH4 and C2H6 during desorption. The δ13C values of CH4 and C2H6 desorbed from three coal samples from the Zhina Coalfield in southwest China displayed a two-stage shift over time during canister desorption (that is, an isotopic decrease followed by an increase), ranging from −38.4 to −30.6‰ and − 32.1 to −28.5‰, respectively. The δ13C values of CH4 and C2H6 desorbed from the coal samples during pulverization ranged from −29.3 to −24.1‰ and − 31.5 to −27.2‰, respectively, approaching the isotopic ranges of coal-derived gas. Low-pressure N2 adsorption measurements showed that many closed pores in coal were opened during progressive pulverization. Gases trapped in closed pores may be considered to be of their original nature, unaffected by subsequent geological processes (i.e., mixing of secondary biogenic gas). Thus, CH4 and C2H6 were more depleted in 13C in open pores than in closed pores. Some open pores on the surface of the coal core were previously closed when in situ but were opened during drilling. Therefore, the initial gas released from the cores was a mixture of gases desorbed from the open and closed pores. The 13C-enriched CH4 and C2H6 desorbed from the pores on the coal core surfaces may migrate into canister headspaces preferentially, thereby providing insight into the two-stage δ13C shift observed during canister desorption.

Geochemical characteristics of tight sandstone gas and hydrocarbon charging history of Linxing area in Ordos Basin, China

Abstract Genetic type, source, charging history and reasons for secondary biogenic transformation of tight sandstone gas in the Linxing area in China's Ordos Basin have been studied through a combined analysis of gas compositions, isotopes of alkane and carbon dioxide, fluid inclusions and basin simulation. Tight sandstone gas in this area is predominated by methane, and the dryness coefficient is between 0.864 and 0.997 (average = 0.969). The δ13C1 values of methane and ethane in this area range from −44.5‰ to −33.2‰ (average = −39.4‰) and from −27.5‰ to −24.4‰ (average = −25.9‰), respectively. The non-hydrocarbon gases are nitrogen, followed by carbon dioxide and argon, and δ13C of carbon dioxide ranges from −14.6‰ to 8.6‰. The homogenization temperatures of gas and gas-liquid inclusions in P2s and C3t in Linxing range from 80 °C to 170 °C, which are concentrated at 120–130 °C. The identification of gas origin indicates that tight sandstone gas is coal-type gas, which is mainly due to thermal evolution, followed by the mixing of secondary biogenic gas. Geological conditions and gas geochemical characteristics suggest that tight sandstone gas was produced by coal seam and carbonaceous shale in Upper Paleozoic in the Linxing area. The homogenization temperature of inclusion reflects the continuous accumulation process of tight sandstone gas, and the Cretaceous was the main period for gas charging. The monoclinal structure, maturity of source rocks and activity of underground water in this area are the important factors for secondary biogenic gas formation.