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For identifying the occurrence and extent of thermochemical sulfate reduction (TSR) reaction of natural gas and better understanding the chemical and carbon isotopic variations in natural gas reservoirs, high-pressure hydro-pyrolysis with a special designed apparatus was performed using natural gas and various amounts of MgSO4·7H2O at up to 360 °C. The yields, chemical and isotopic compositions of the gases produced during TSR and thermal cracking were measured. As the extent of TSR reaction increased, the concentrations of CH4, CO2 and H2S increased in a nonlinear way, while those of C2H6 and C3H8 decreased. According to the variation of gas content, the TSR reaction of alkane gases can be divided into an uncatalyzed and a catalyzed stage, which is different from previous studies that treated the TSR reaction of alkane gases as a non-autocatalytic reduction process. As the concentration of MgSO4·7H2O increased, the rate of TSR reaction with hydrocarbon gases increased. The concentrations of HSO4− and volume of aqueous phase could be responsible for the different TSR reaction rates in the catalyzed stage. The co-variation of ln(C1/C2) and ln(C2/C3) could be related to the TSR reaction of alkane gases. Our study provides clues for understanding the compositional variations in natural conditions. [ABSTRACT FROM AUTHOR]

Carbon isotopic anomaly occurs in the central gas field of the Ordos Basin (δ13C 1 > δ13C 2), and natural gas is mainly stored in the Ordovician Majiagou Formation dolomite (OMFD). Inorganic geochemical characteristics of the OMFD, including oxygen, carbon, strontium isotopes, order degree, and rare earth elements (REEs) were measured. Organic geochemical parameters, including natural gas and source rock, were also investigated. The causes of the carbon isotopic anomaly were studied based on the combination of inorganic and organic geochemical characteristics. The δ18O of the OMFD ranges from −11.9 to −5.8‰ with an average −8.4‰. Some δ18O values are more depleted than the average value of the Ordovician seawater, indicating that the dolomite is high-temperature burial dolomite, which is consistent with the fluid inclusion. The 87Sr/86Sr varies from 0.7088 to 0.7369, which is higher than the contemporaneous seawater. This suggests that the OMFD is high-temperature burial dolomite. The order degree changes from 0.72 to 0.98 with an average value of 0.89, which also indicates that the OMFD may have formed in the burial diagenesis stage at higher temperature. The REEs distribution patterns illustrate that the OMFD was influenced by the high-temperature hydrothermal alteration in the later period. The δ13C of some OMFD are more depleted than the Ordovician seawater, indicating that there was organic carbon mixing in the process of dolomitization. Inorganic and organic geochemical characteristics confirm that carbon isotopic anomaly of natural gas is the result of high temperature and mixing of the OMFD organic carbon. • Inorganic and organic geochemistry are combined to study carbon isotope anomaly. • Inorganic parameters of the OMFD has been comprehensively investigated. • High-temperature effect and organic carbon mixing lead to carbon isotope anomaly. [ABSTRACT FROM AUTHOR]

By using the methods of hydrocarbon generation kinetics and carbon isotope kinetics, combined with geological background of natural gas pool formation, the generation and accumulation of natural gas from Yinan 2 gas pool was studied in Kuqa Depression of the Tarim Basin. Natural gas of Yinan 2 gas pool is mainly derived from Middle and Lower Jurassic coal-bearing source rocks, and generally belongs to long time-accumulated gas. It is suggested that Yinan 2 gas is chiefly accumulated in the last 5 Ma, its Ro ranges from 1.25% to 1.95%, and the loss rate of natural gas is about 25%–30%. This work not only complements and reduces the deficiency of formation model of natural gas pools which traditionally depends on the matching relationships between source rock, reservoir, cap rock, and trap, but also is a useful reference in the study of other gas pools. [ABSTRACT FROM AUTHOR]

To identify the reasons for more depleted ethane carbon isotopic value (MD-δ13C 2) in the Southeastern gas field (SEGF), fluid inclusions samples from northern and the SEGF were collected. Homogenization temperature and salinity of fluid inclusions, components and carbon isotopic values of gases released from the crushed inclusions were tested. Geochemical fingerprints information of producing gas in wells and primary gas in inclusions were contrasted. The comparison suggests that geochemical parameters of primary and producing gas in the NGF are similar, whereas those in the SEGF are completely different. The ethane carbon isotopic values of producing gas became more depleted compared with the primary gas, whereas the methane carbon isotopic values are not much different from the primary gas. This indicates that the secondary effect caused the more depleted ethane carbon isotopic values. There was one more period of high temperature fluid filling (140–160 °C) in the SEGF in comparison with the NGF. Higher thermal maturity is the key inducement of the abnormal MD-δ13C 2. • Geochemical parameters comparison of primary gas with producing gas was conducted. • More depleted ethane carbon isotopic values were studied based on geochemistry. • Fluid inclusions and natural gas parameters were combined to study the mechanism. • Higher thermal maturity is the key inducement of the abnormal carbon isotopic values. [ABSTRACT FROM AUTHOR]

Improved understanding of the hydrogen sulfide (H 2 S) origins is of significant importance for petroleum exploration, exploitation and refinement due to it being highly toxic and corrosive. H 2 S was discovered from the Qianjiang Formation (Eq) in the northern area and Xinggouzui Formation (Ex) in the southern area of the Qianjiang Depression, Jianghan Basin, China. The sulfur isotopic compositions of H 2 S, water-dissolved sulfates and oils and chemical compositions of volatile organic sulfur compounds (VOSCs) in natural gas as well as molecular and stable carbon isotopic compositions of oil and gas samples were measured to decipher the specific generation pathways of H 2 S. The conventional wisdom relays on bacteria sulfate reduction (BSR) for producing H 2 S because of the current low thermal regime (<80 °C). Crude oils, H 2 S and water-dissolved sulfates are progressively enriched in 34S. The δ 34S values of H 2 S are around 12.0–32.0‰ lower relative to that of the dissolved sulfates and 0.8–24.0‰ greater relative to that of crude oils. This indicates that the current major contribution of H 2 S is actually related to BSR according to the S isotopic fractionation levels in BSR. The secondary contribution of H 2 S is attributed to be thermogenic mainly by thermal cracking alteration of organic matters (TCA). This clue is provided by the distribution patterns of VOSCs in natural gas rather than S isotope of crude oils, H 2 S and water-dissolved sulfates due to the limited input of TCA-associated H 2 S. The Eq TCA-associated gases are significantly enriched in thiols, sulfides, and alklyated thiophenes, the Ex BSR-associated gases contain mainly thiols and a little bit sulfides and alkylated thiophenes. The relevant early-matured Eq oils are also rich in labile organosulfur compounds, favoring the generation of VOSCs by TCA. VOSCs detected on the artificially pyrolyzed gaseous products of Eq rock and oil both show the identical distribution patterns as those of Eq natural gas samples, supporting the occurrence of TCA-associated H 2 S in the Eq reservoirs. The positive correlation between i -C 4 / n -C 4 and H 2 S concentration for Eq gases indicates the catalytic effects of clays, promoting the generation by non-biological processes within the low thermal regimes. This study presents a novel method to clarify the origins of H 2 S by VOSCs in natural gas and improves our understanding of H 2 S generation pathways within low-temperature regime in nature. • The origins of H 2 S in the Qianjiang Depression investigated. • Bacteria sulfate reduction (BSR) makes the major contribution. • Thermal cracking alteration of organic matters (TCA) has the minor contribution. • BSR-associated gases are depleted in volatile organic sulfur compounds. • TCA-associated gases are enriched in volatile organic sulfur compounds. [ABSTRACT FROM AUTHOR]

The exploration history of Bohai Sea reveals that Liaodong Bay is relatively rich in natural gas, which is mainly gas condensate; however, the source rocks in Liaodong Bay share a medium maturity level with values of %Ro being generally < 1.3. This study was undertaken to better understand gas potential of Paleogene oil-prone source rocks (II) in Liaodong Bay, Bohai Sea. Three immature Paleogene source rocks were selected to complete gold tube pyrolysis experiments. The experiment results reveal that the secondary gas from oil cracking can be neglected until the cumulative yields of C 6 –C 14 compounds reach the maximum (corresponding to %Ro ∼ 1.3). Above %Ro ∼ 1.3%, oil starts to crack to wet gases substantially. Based on comparative analysis of pyrolysis results and source rock geochemical parameters, we constructed a hydrocarbon generation model, especially for hydrocarbon gas, and validated the calculated %Ro values for pyrolysis conditions. The C 1 –C 5 yields for source rocks at ∼1.3 %Ro in gold tube pyrolysis approximate the maximum gas yield for source rock in open system pyrolysis, indicating the majority of primary gas from kerogen cracking has been generated at ∼1.3 %Ro, with its yields being 156 ml/g TOC, 100 ml/g TOC and 117 ml/g TOC for Ed 3 , Es 1 and Es 3 source rocks, respectively. This suggests that source rocks with %Ro < 1.3 are of good primary gas generation potential. The extrapolation of kinetic parameters of C 1 –C 5 shows that source rocks are generally at the stage of primary gas in Liaodong Bay, where proved gases are primary gas with estimated values of %Ro < 1.3. [ABSTRACT FROM AUTHOR]

Highlights: [•] Natural gases in reservoir are similar to gases from fluid inclusions. [•] Gases in fluid inclusions vary from gases generated from low maturity coals. [•] Gas charging seems to be earlier than quartz cementation with gaseous fluid inclusions. [•] Late CO2 at dry gas phase plays an important role in sealing reservoir sandstone. [Copyright &y& Elsevier]

Abstract: Compound-specific carbon and hydrogen isotopic compositions of three natural gas round robins were calibrated by ten laboratories carrying out more than 800 measurements including both on-line and off-line methods. Two-point calibrations were performed with international measurement standards for hydrogen isotope ratios (VSMOW and SLAP) and carbon isotope ratios (NBS 19 and L-SVEC CO2). The consensus δ 13C values and uncertainties were derived from the Maximum Likelihood Estimation (MLE) based on off-line measurements; the consensus δ 2H values and uncertainties were derived from MLE of both off-line and on-line measurements, taking the bias of on-line measurements into account. The calibrated consensus values in ‰ relative to VSMOW and VPDB are: NG1 (coal-related gas): Methane: δ 2HVSMOW =−185.1‰±1.2‰, δ 13CVPDB =−34.18‰±0.10‰ Ethane: δ 2HVSMOW =−156.3‰±1.8‰, δ 13CVPDB =−24.66‰±0.11‰ Propane: δ 2HVSMOW =−143.6‰±3.3‰, δ 13CVPDB =−22.21‰±0.11‰ i-Butane: δ 13CVPDB =−21.62‰±0.12‰ n-Butane: δ 13CVPDB =−21.74‰±0.13‰ CO2: δ 13CVPDB =−5.00‰±0.12‰ NG2 (biogas): Methane: δ 2HVSMOW =−237.0‰±1.2‰, δ 13CVPDB =−68.89‰±0.12‰ NG3 (oil-related gas): Methane: δ 2HVSMOW =−167.6‰±1.0‰, δ 13CVPDB =−43.61‰±0.09‰ Ethane: δ 2HVSMOW =−164.1‰±2.4‰, δ 13CVPDB =−40.24‰±0.10‰ Propane: δ 2HVSMOW =−138.4‰±3.0‰, δ 13CVPDB =−33.79‰±0.09‰ All of the assigned values are traceable to the international carbon isotope standard of VPDB and hydrogen isotope standard of VSMOW. [Copyright &y& Elsevier]

Abstract: Permian coal and Triassic mudstone from the Ordos Basin were pyrolyzed in a closed system using a gold tube technique. Carbon and hydrogen isotopes of the gases generated from pyrolysis were compared to Mesozoic gases in the basin to interpret the origin, maturity and any mixing of gases. Maturation trends for thermogenic methane from both coal and lacustrine kerogens in our experiment were found to be independent of heating rate, allowing their use for determination of gas provenance. Gases from a tectonically stable area like the Shanbei slope are derived mainly from Yanchang lacustrine kerogen, and gases in tectonically active areas consist of mixtures of coal-derived gases and oil-associated methane from deeply buried formations, as well as oil-associated gases and biogenic gases from shallow depth. The thermal maturity of the C2 and C3 gases is estimated to cover an equivalent vitrinite reflectance range from 0.7% to 1.2% R o, whereas C1 gas exhibits a wide maturity variation ranging from 0.5% to 1.5% R o and implying significant mixing of Mesozoic methane in the basin. [Copyright &y& Elsevier]