Methane Carbon Isotope Research Articles

Oil pyrolysis with carbonate minerals: Implications for the thermal stability of deep crude oil

Gold-tube experiments on oil, oil and dolomite, and oil and calcite were conducted to analyse the thermal stability of liquid hydrocarbons in carbonate reservoirs. The pyrolysates were geochemically characterized by gas chromatography and gas chromatography-isotope ratio mass spectrometer. The experimental results show that temperature is the main controlling factor; however, the influence of inorganic minerals cannot be ignored during oil cracking. Dolomite and calcite have been observed to inhibit the generation of methane to some extent, wherein the methane yields were reduced by 42.73 and 33.86 ml/g, respectively. Moreover, carbonate minerals increase the average activation energy of C1-5 generation, leading to an increase in the thermal stability of crude oil. Carbon isotope fractionation of gaseous hydrocarbons during crude oil cracking showed that carbonate minerals promoted methane carbon isotope rollover at EasyRo <1.75%. The extrapolation of kinetic parameters at a constant heating rate of 2 °C/Ma indicated that carbonate minerals will increase the liquid hydrocarbon preservation threshold temperature by 2–10 °C at the same conversion of C1-5. The calculated maximum depth corresponding to crude oil in carbonate reservoirs maintaining the liquid phase (51% oil cracking) can reach 10,000 m. Therefore, the presence of carbonate minerals needs to be considered during the geochemical evaluation indicators to analyse the potential of gas generation during crude oil cracking, the evolution of carbon isotope composition, and the prediction of hydrocarbon phase behaviour.

Generation and geochemical signatures of natural gas from solid bitumen: Implications from closed and stepwise semi-open pyrolysis experiments

Solid bitumen has been acknowledged as an important source of overmature shale gas. However, the gas generation potential, chemical signatures and carbon isotopic compositions of natural gas derived from solid bitumen in an overmature hydrocarbon system are not well-understood with respect to various geological conditions. In this study, a relatively low-maturity solid bitumen from Cambrian reservoirs in the northwest Sichuan Basin was pyrolyzed in both closed and stepwise, semi-open systems. In the closed system, methane generated in the overmature thermal stage is dominated by the secondary cracking of wet gas and cumulative methane carbon isotopic values that are 4.4‰–14.8‰ lighter than the carbon isotope composition of the original solid bitumen. In semi-open systems, the gas is predominantly generated from residual solid bitumen and has wetness (i.e., C2–C5/C1–C5 by volume) values of generally less than 1%. At the same thermal maturity levels, the carbon isotope composition of methane in various semi-open systems is 4.4‰–10.0‰ heavier than those in the closed system and may also be isotopically heavier than the carbon isotope value of the original solid bitumen. The present study also illustrates that the calculated gas yields and methane carbon isotopes within a certain thermal maturity range in the closed system cannot be compared with those from residual solid bitumen as measured in the corresponding semi-open systems. This highlights the need to conduct specific semi-open pyrolysis experiments to investigate the late gas generation potential and geochemical signatures of natural gas from solid bitumen when the geological system is not a closed system. By comparing the above-mentioned results with the geochemical characteristics of Wufeng-Longmaxi shale gas from the Changning and Jiaoshiba gas fields of the Sichuan Basin, southwest China, it is suggested that the shale gas examined in this study is most likely generated in semi-open systems rather than in a closed system. In other words, the early-generated gas has been expelled from the shales, leaving the late-generated gas from residual solid bitumen and/or kerogen as the main gas source in the present hydrocarbon system.

Improved global wetland carbon isotopic signatures support post-2006 microbial methane emission increase

Atmospheric concentrations of methane, a powerful greenhouse gas, have strongly increased since 2007. Measurements of stable carbon isotopes of methane can constrain emissions if the isotopic compositions are known; however, isotopic compositions of methane emissions from wetlands are poorly constrained despite their importance. Here, we use a process-based biogeochemistry model to calculate the stable carbon isotopic composition of global wetland methane emissions. We estimate a mean global signature of −61.3 ± 0.7‰ and find that tropical wetland emissions are enriched by ~11‰ relative to boreal wetlands. Our model shows improved resolution of regional, latitudinal and global variations in isotopic composition of wetland emissions. Atmospheric simulation scenarios with the improved wetland isotopic composition suggest that increases in atmospheric methane since 2007 are attributable to rising microbial emissions. Our findings substantially reduce uncertainty in the stable carbon isotopic composition of methane emissions from wetlands and improve understanding of the global methane budget.

Hydrocarbon Accumulation Mechanism of Ordovician in the Halahatang Area, Tarim Basin—Evidence From Organic Geochemistry

As an important oil and gas-bearing area in the Tarim Basin, the Halahatang Area has great potential for resource exploration. However, the research on oil and gas sources and the filling period of the Lower Paleozoic Ordovician is still limited. In this study, the organic geochemical analysis of source rocks, the simulation technology of hydrocarbon generation evolution basin, the distribution characteristics of fluid inclusions, and the measurement of uniform temperature are used and combined with the stable isotope of natural gas and the characteristics of oil-source biomarkers, and the process of oil and gas accumulation is restored. The results show that: ①The Cambrian source rock is the main contributor to Ordovician crude oil in the Halahatang Area, and its crude oil shows the characteristics of lightweight, rich chain hydrocarbons, and poor aromatic hydrocarbons. The composition of C7 compound shows the advantage of n-heptane, which belongs to type I kerogen. The biomarker compounds show the characteristics of high content of long-chain tricyclic terpane and high Ts/Tm values, and the crude oil is considered to be a highly mature sapropelic kerogen source oil. ②The carbon isotopes of methane and ethane of Ordovician natural gas in the Halahatang Area are light (from −35.6 to −29.5‰), showing obvious characteristics of sapropelic gas. ③There are three stages of the oil and gas accumulation process in Ordovician reservoirs in the Halahatang Area. In the middle and late Caledonian periods, the source rock reached the hydrocarbon generation threshold and began to generate oil. The late Hercynian period was the main accumulation period, and the Himalayan period was dominated by dry gas filling. ④The crude oil generated in the Middle-Late Caledonian periods migrated along the faults and conductive layers and finally accumulated in the high parts of the structure in the north. The migration of oil and gas in the late Hercynian period was limited by the bitumen produced by early biodegradation. Since the asphalt plugging condensate gas has almost failed to cause gas invasion from the reservoir, the Ordovician reservoir still maintains the characteristics of crude oil-associated gas, forming an oil and gas reservoir with a gas cap.

Geochemical Characteristics of the Jurassic Alkane Gas in the Muli Depression, South Qilian Basin: Implications for Potential of Light Oil and Condensate

The discovery of gas hydrates in the Muli Depression has made the South Qilian Basin (SQB) a hotspot for oil and gas exploration. Although many studies have been carried out on the gas hydrates, the geochemical characteristics have rarely been reported, which can effectively indicate the genetic type, diffusion, biodegradation, and thermal maturity of natural gas. In this contribution, the molecular composition and carbon isotopes of alkane gases were collected from gas hydrates and free gases in drilled boreholes of the Muli Depression. The results show that the majority of natural gases are oil-type gases, with a little coal-type gases. The natural gases have undergone biodegradation, leading to a decrease in the propane content and an increase in the methane content, with a heavier carbon isotope of propane and a lighter carbon isotope of methane. Some of the natural gases have experienced diffusion, resulting in a decrease in the methane content and an increasing value of Δ(δ13C1-δ13C2). Furthermore, the wet gases from natural gases were derived from the cracking of kerogen and crude oil at the thermal maturity stage. The dry gases in small quantities were produced by biodegradation of the wet gases. We propose that the potential for light oil and condensate exists in the Jurassic Yaojie Formation. Our new findings on the geochemical characteristics of natural gases provide new constraints for the study of petroleum systems and deeply buried sediments in the SQB.

Origin and isotopic fractionation of shale gas from the Shanxi Formation in the southeastern margin of Ordos Basin

The Ordos Basin is abundant in natural gas resources, and the Shanxi Formation with the transitional facies is considered as a potential stratum for shale gas reservoir. However, the origin and geochemical characteristics of the Shanxi Formation shale gas remains unclear. In this study, 22 shale gas samples were collected from the SX-1 well in the southeastern margin of Ordos Basin, and tested for gas components and compositions of stable carbon isotope. Moreover, five rock samples were measured for total organic carbon (TOC), the vitrinite reflectance (Ro), kerogen type, porosity, permeability, and methane adsorption capacity. The shale gas in the Shanxi Formation has a high proportion of methane which is consistent with the high maturity of source rocks. The contradictory relationship between the carbon isotopes of ethane (δ13C2) from the Shanxi Formation shale gas and kerogen types of rock samples may be caused by the complex sedimentary environment, multiple provenance systems, and high maturity. The carbon isotopes of methane (δ13C1), δ13C2 and alkane gas components indicate that the shale gas near coal seams is coal-derived gas, the gas at the bottom of the Shanxi Formation is oil-type gas and the gas in mudstones among the sandstone layer at the top of the Shanxi Formation may be mixed with natural gas migrating from other layers or mixed with gas generated in the later period. Meanwhile, the carbon isotopes of carbon dioxide (δ13CCO2) suggest that the CO2 generated with hydrocarbons in the coal-measures is mainly of organic origin. The carbon isotopic fractionation of methane is commonly observed in shale gas samples. This fractionation is mainly controlled by the diffusion effect which can be promoted by high porosity and permeability. Furthermore, the positive correlation between the TOC values and the degree of fractionations may be attributed to the high TOC values in the samples, which can improve the development of organic pores and increase the content of adsorbed gas.

Riparian Cottonwood Trees and Adjacent River Sediments Have Different Microbial Communities and Produce Methane With Contrasting Carbon Isotope Compositions

AbstractRivers and their adjacent riparian forests are intimately linked by the exchange of water, nutrients, and organic matter. Both riparian cottonwood trees and adjacent river sediments host microbial communities including archeal methanogens, supporting methane production and emission to the atmosphere. Here we combine microbial community and in vitro stable isotope analyses to characterize the drivers of methane cycling in distinct anoxic habitats (river sediments vs. riparian cottonwood stems) associated with the Oldman River, southern Alberta (Canada). We demonstrate that, differences in the chemical characteristics of organic matter support divergent microbial communities that generate methane from distinct metabolic pathways. Organic matter in river sediments had C/N ratios approximately 50‐fold lower than in tree stems and had more diverse dissolved organic components. Contrasting substrate availability between river sediment and tree stems was likely the primary mechanism for the greater microbial diversity in river sediments than in tree stems, the significantly different bacterial communities, and the trend toward differing abundance of methanogen orders. The methane carbon isotope composition (δ13C values) differed for the tree stem (−103.6‰ to −70.6‰) and river sediment (−55.1‰ to −48.4‰) environments, suggesting that methane was primarily produced via CO2‐reduction in tree stems by Methanobacteriales, while river sediments produced more methane through acetate fermentation primarily by Methanosarcinales. This study demonstrates the importance of organic matter quality and microbial community composition in driving metabolic processes contributing to methane production and emission in rivers and adjacent riparian forests.

Design of High Precision Temperature and Pressure Closed-Loop Control System for Methane Carbon Isotope Ratio Measurement by Laser Absorption Spectroscopy

In order to achieve the accurate measurement of gas stable isotope, according to the structure of gas cavity and the characteristics of optical elements, a constant temperature and pressure gas isotope measurement experimental system based on tunable diode laser absorption spectroscopy (TDLAS) technology is designed, which can achieve high-precision and stable control of temperature and pressure in the box. The hardware circuit system including high-precision constant-current source, two-channel temperature sampling module, conditioning circuit and pressure control circuit is designed in the system. The corresponding system closed-loop control software is developed to ensure the stability of the temperature and pressure of the optical cavity. Through the experimental verification, the temperature is controlled at 35ºC and the pressure is controlled at 150 mbar in the present work. The measurement results verified the stability of the temperature and pressure system, as follows: the optimal temperature fluctuation control is ±0.005ºC and the pressure stability is ±0.1 mbar. Then, in our experiment, the CH

Progressively released gases from fluid inclusions reveal new insights on W-Sn mineralization of the Yaogangxian tungsten deposit, South China

The gentle stepwise crushing technique permits progressive extraction of gases from large, irregular, secondary fluid inclusions (SFIs) along micro-cracks to small, regular, liquid-rich primary fluid inclusions (PFIs). Previous studies have verified this general gas release pattern in different hydrothermal minerals based on the agreements between 40Ar/39Ar ages of the released gases from different crushing stages and the most likely corresponding geological events dated with other methods; and based on the excellent agreements between gas chemistry measured by QMS (quadrupole mass spectrometer) during stepwise crushing and the expected characteristics of SFIs and PFIs in the same samples acquired using Raman spectroscopy. In this study, we applied this QMS-based gentle stepwise crushing technique on ore and gangue minerals from the world-class Yaogangxian tungsten deposit in South China. Gases released from PFIs in cassiterite, wolframite, and quartz from ore-bearing veins show high CH4, N2, C3H8, and low CO2, C4H10 contents, low CO2/CH4 (≪1) and high N2/Ar (327–861 for ore minerals and 60.7–943 for quartzs from ore veins) ratios, along with multiple organic gaseous species. Their compositions resemble those of gases released from PFIs in quartzs from the Yaogangxian granites which represent magmatic-hydrothermal fluids that exsolved from the granitic magma. However, compared with these exsolved magmatic-hydrothermal fluids, ore mineral PFIs have relatively high He contents which indicate contributions from non-magmatic crustal fluids. This is consistent with high Ca2+, low F− concentrations, and low K+/Na+ (0.21–0.32) ratios observed in fluids in the quartz samples, as well as their relatively low water hydrogen isotopes (δD-H2O = −102 to −59‰). These chemical characteristics, coupled with their magmatic sulfur (δ34S-molybdenite and arsenopyrite = −0.2–1.4‰), methane hydrogen (δD-CH4 = −58 to −42‰), and fluid oxygen (δ18O-H2O = 3.9–5.1‰) isotopic compositions, indicate both magmatic and non-magmatic crustal components in the mineralizing fluids. Based on these observations and the low methane carbon isotope values (δ13C-CH4) in the studied quartz samples, we propose that fluid-rock interactions between the exsolved magmatic-hydrothermal fluids and the metasedimentary wall rocks played an important role in promoting W-Sn precipitation in this ore deposit. This study demonstrates the power of combining QMS-based stepwise crushing technique with traditional geochemical techniques in investigating ore-forming processes.

New approaches to distinguish shale-sourced and coal-sourced gases in petroleum systems

Current approaches relating thermogenic gases to either shale source rocks (predominantly type II kerogen) or coal source rocks (predominantly type III kerogen) are not reliable and not globally applicable. This is because these mostly empirical approaches were developed using small poorly-constrained datasets from limited locations. The evaluation of a large global dataset of molecular and isotopic properties of gases from unconventional shale and coal reservoirs suggests that two genetic diagrams based on stable carbon isotopes of methane and ethane, δ13C-C2H6versus δ13C-CH4 and δ13C-CH4versus Δ(δ13C-C2H6 – δ13C-CH4), provide the best separation of shale-sourced and coal-sourced gases. Newly designated genetic fields and shale/coal separation lines on these diagrams were tested and validated using data from five petroleum systems with, likely, only shale (class B and A organofacies) source rocks (the Maracaibo Basin in Venezuela, the Guajira Basin in Colombia and the Rub Al Khali Basin in Iran) and only coal (class F organofacies) source rocks (the Southern Permian Basin in Germany and the Sichuan Basin in China). The practical usefulness of this new approach to gas-source correlations was demonstrated in two case studies from petroleum systems with debated source rock organofacies (the Mozambique Basin in Mozambique and the Indus Basin in Pakistan). These better constrained and more reliable diagrams with genetic fields and shale/coal separation lines represent a new tool for the evaluation of petroleum systems.

Origin of the Gas Hydrate and Free Gas in the Qilian Permafrost, Northwest China: Evidence from Molecular Composition and Carbon Isotopes

AbstractThe Qilian permafrost of the South Qilian Basin (SQB) has become a research focus since gas hydrates were discovered in 2009. Although many works from different perspectives have been conducted in this area, the origin of gas from gas hydrate is still controversial. Molecular composition and carbon isotope of 190 samples related to gas hydrates collected from 11 boreholes allowed exploration of genetic type, thermal maturity, biodegradation, as well as gas‐source correlation of alkane gases from gas hydrates and free gases. Results indicate that alkane gases biodegraded after the formation of natural gas. According to differences in carbon isotopes of methane and their congeners (CH4, C2H6, C3H8), the thermal maturity (vitrinite reflectance, VRo) of most alkane gases ranges from 0.6% to 1.5%, indicating a mature to high mature stage. The thermal maturity VRo of a small part of alkane gas (in boreholes DK5 and DK6) is higher than 1.3%, indicating a high mature stage. Alkane gases were mainly produced by secondary cracking, consisting of crude oil‐cracking gases and wet gases cracking to dry gases. Genetic types of alkane gases are primarily oil‐type gases generated from shales and mudstones in the upper Yaojie Formation of Jurassic, with less coal‐type gases originated from the mudstones in the Triassic Galedesi Formation and the lower Yaojie Formation of Jurassic. Carbon dioxides associated with alkanes from gas hydrates and free gases indicate the thermal decomposition and biodegradation of organic matter. The origins of natural gases from gas hydrates and free gases shed light on the evaluation of petroleum resource potential, deeply buried sediments, and petroleum resource exploration in the SQB.

Impacts of gas pressure on carbon isotope fractionation during methane degassing—An experimental study on shales from Wufeng and Longmaxi Formations in southeast Sichuan, China

Methane carbon isotope fractionation (CIF) during shale gas desorption provides a new method to evaluating in-situ gas content and the ratios of adsorbed and free gas as well as predicting the production status of shale gas wells. Considering anisotropic pressures in shale gas in-situ reservoirs, five gas pressure values were selected for this study to perform methane isotherm adsorption-desorption experiments for four shale samples from the Wufeng and Longmaxi Formations in the southeastern Sichuan Basin, and the characteristics of carbon isotope fractionation during methane desorption processes were investigated. Results show that: (1) under different gas pressures, CIF of the in-methane transport process shows four stages: stable stage (I), lighter stage (II), heavier stage (III), and another lighter stage (IV); (2) CIF during methane desorption is impacted by multiple factors including the total organic carbon (TOC) content, pore structure, gas pressure, and adsorbed gas ratio, while material composition and pore structure largely determine the characteristics of isotopic fractionation in methane desorption, which are extremely important control factors; and (3) gas pressure has an obvious effect on CIF characteristics by changing the proportion of adsorbed gas, especially in stages II and IV, and the sample with developed organic matter (OM) had a stronger response. The lower the gas pressure, the earlier the desorption process corresponding to the start of significant isotope fractionation. The higher the ratio of adsorbed gas, the larger the range of isotope fractionation. This demonstrates the significance of adsorption-desorption for CIF in shale gas degassing. Furthermore, we used a technique integrating the Amoco curve fit (ACF) method and CIF to calculate in-situ gas content and the dynamic change in adsorbed and free gas ratios during gas degassing.

Temperature and Pressure Insensitive Spectroscopic Method for Measuring 13CH4 during Oil and Natural Gas Drilling Operations

A novel rugged two-section driving NIR TDLAS scheme was implemented to reduce temperature and pressure sensitivity of methane carbon isotope measurement during oil and natural gas drilling operations. Isotope spectra line groups with same lower energy levels were selected to derive the concentration of 13CH4 and 12CH4. Dynamic pressure linewidth broadening was introduced in the absorbance curve fitting. Various uncontrollable factors such as spectra shift, stretching, and baseline trending were incorporated in the comprehensive multi-peak fitting. The results showed that the sensitivity of isotope ratios to temperature and pressure variation was greatly suppressed. The δ13CH4 uncertainty in the temperature test was 2.8‰ with fitted δ13CH4-T slope of 0.021‰/°C in 25 ± 5°C range. The δ13CH4 uncertainty in the pressure test was 1.4‰ with fitted δ13CH4-P slope of &amp;lt;0.001‰/mbar from 400 to 1000 mbar. The limit standard deviation of long-term test was 0.15‰ (@135 s). The results indicate that our scheme is a promising solution for δ13CH4 measurement during drilling operations due to the enhanced robustness.

Geochemical characteristics and source analysis of natural gas in the saline lacustrine basin in the Western Qaidam basin

This paper analyzed the geochemical characteristics and the sources of natural gas in the western Qaidam Basin based on isotopes and gas composition. The natural gas in the western Qaidam Basin is mainly comprised of hydrocarbon gases, including sapropelic, humic and mixed gas. In the southwestern Qaidam Basin, the main gas is sapropel gas, with some mixed gas and a small amount of humic gas. In contrast, the northwestern Qaidam Basin contains mainly humic gas, some mixed gas, and a small amount of sapropel gas. The dryness coefficient varies between 0.6 and 0.95. The carbon isotopes of methane, ethane, and propane are distributed between −32.9‰ ~ −51.4‰, −34.2‰ ~ −22.4‰, −30‰ ~ −17.4‰, respectively. The calculated Ro value of the natural gas is between 0.5% and 1.87%. In general, the natural gas shows the characteristics of high dryness coefficient, heavy carbon isotopes, and high thermal maturity. The gas-source correlation makes it clear that the main hydrocarbon source rocks in the western Qaidam Basin are the lower Ganchaigou (E3), the upper Ganchaigou (N1) and the lower Youshashan formations (N21). The gas-producing layers in the northwestern Qaidam Basin are primarily from the lower Ganchaigou (E3), the lower Youshashan (N21), and the upper Youshashan formations (N22). The primary gas source rocks are the lower Ganchaigou (E3), the upper Ganchaigou (N1), and the lower Youshashan formations (N21). The main gas-producing layers in the southwestern Qaidam Basin are the lower Ganchaigou (E3), the upper Ganchaigou (N1), the lower Youshashan (N21) and the upper Youshashan formations (N22), with the main gas source rocks being the lower Ganchaigou Formation (E3).

Reasons for carbon isotope rollover in the Yan'an gas field of the southern Ordos Basin, China: Evidence from the geochemical comparison of gas from fluid inclusions with wells

Samples for inclusion analysis were collected to investigate reasons for carbon isotope rollover in the Yan'an gas field (YAGF). Homogenization temperature of fluid inclusions, constituents and carbon isotope of gases in fluid inclusions were measured. Geochemical parameters of gas in wells and fluid inclusions in the northern gas fields (NGF) and YAGF were compared. The results show that geochemical characteristics of well and fluid inclusions gas in the NGF are almost the same, suggesting that it was originated from one set of source rocks. The direct reason for carbon isotope rollover in the YAGF is the depleted ethane carbon isotopes, while the methane carbon isotopes do not change much compared with the primary gas. There was a period of high temperature fluid filling and accumulation (140–160 °C) in the YAGF. The higher thermal maturity has disparate effects on methane and ethane. Higher thermal maturity is an important inducement of carbon isotope rollover. Mixture of oil-derived gas generated from the marine Ordovician Majiagou Formation will also result in carbon isotope rollover.

Differentiation of Carbon Isotope Composition and Stratabound Mechanism of Gas Desorption in Shallow-Buried Low-Rank Multiple Coal Seams: Case Study of Well DE-A, Northeast Inner Mongolia

The desorbed gas composition and δ13C(CH4) and δD(CH4) of five coal groups in Well DE-A of the Labudalin Basin in northeastern Inner Mongolia show that the δ13C(CH4) and δD(CH4) are heavy, and significantly heavier in some layers, whereas the overall δ13C(CO2) is abnormally light. Based on coalfield exploration data, this study analyzes the characteristics and control factors of methane isotope alienation of shallow-buried low-rank multiple coal seams in a small coal-bearing depression. The maximum buried depth of the coal seam in the Derbu coalfield is less than 600 m, and the coal rank is sub-bituminous coal. The carbon isotope composition (δ13C(CH4)) of CBM varies from − 56.3 to − 39.8‰, wherein the arithmetic average is − 48.72‰. Compared with coal seams of the same metamorphic degree, the δ13C(CH4) in Derbu is relatively heavy, wherein different coal groups and different coal seams of the same coal group have different characteristics, and the δ13C(CO2) is relatively light. Studies have shown that the existence of thermogenic gas, anaerobic oxidation of methane microorganisms, and desorption-fractionation of methane isotopes in Well DE-A are causes for the heavier δ13C(CH4) and δD(CH4). Note that the thermal cause is the basis for the heavier δ13C(CH4) of the gas sample, while the carbon isotope fractionation of methane and oxidation of microorganisms further aggravate the δ13C(CH4). The preservation conditions, such as lithofacies combination sequence and hydrogeological conditions, of different coal formations are also important factors influencing δ13C(CH4) differentiation, and the superposition and coupling of various reasons make this differentiation more obvious. By comparing the geological characteristics of Well DE-A with the vertical variation characteristics of carbon isotopes in the desorbed methane of the Surat Basin, it was concluded that the vertical variation of the desorbed gas sample in shallow-buried low-rank multiple coal seams has complex characteristics. In addition to maturity, local horizons are more obviously controlled by lithologic association and hydrological characteristics. Especially for small coal-bearing depressions or coal-bearing strata near outcrops with shallow-buried depth, the horizontal distance between the outcrop line of different coal formations and target well locations is obviously short. Thus, surface water and surface microorganisms can migrate along the short high permeability layers (coal seam or sandstone layer), and influence the coal-bearing seam. Meanwhile, different groundwater dynamic conditions also result in the transformation of desorbed gas in different coal groups, revealing variation in the methane carbon isotopes.

Study on the characteristics of in-situ gas desorption from the Wufeng - Longmaxi shale in the Sichuan Basin

Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation shale in the Sichuan Basin and its periphery is an important horizon for shale gas exploration and development in my country. The rapid on-site desorption instrument independently developed by Sinopec Wuxi Institute of Petroleum Geology was used to test the desorption gas volume, lost gas volume, total gas content, natural gas components of desorbed gas and carbon isotope of methane in typical shale gas wells in this area. Data analysis shows that the carbon isotope value and characteristics of the first sample of methane can predict shale gas enrichment layers, and the total gas content and desorbed natural gas components can identify shale gas sweet spots; shale gas components and carbon isotope values can jointly identify shale gas Genesis type, kerogen carbon isotope characteristics and shale gas carbon isotope characteristics can be used as effective means for gas source comparison; shale gas composition characteristics can distinguish the preservation conditions of the area, which are indicative for the study of the degree of shale gas loss; The combined application of total gas content and initial dissipation index can better identify the sweet spots of shale gas, and avoid single-factor identification of sweet spots; the carbon isotope value of methane shows obvious fractionation effect with desorption time, which can predict shale gas production stage and residual gas content.

Study on characteristics and source of tight gas in the Shahezi formation, Xujiaweizi Fault Depression, Songliao Basin, China

The Xujiaweizi fault depression is rich in natural gas resources in Lower Cretaceous and a major fault depression of the deep strata in northern Songliao Basin. In this study, the tight gas is coaliferous gas in the K1sh Formation of the Xujiaweizi Fault Depression. The methane carbon isotope value of tight gas is controlled by mature evolution. The cause of reversed sequence and partially reversed sequence of gas components carbon isotope is carbon isotope fractionation with hydrocarbon generation evolution of high-overmature stage source rock. The carbon isotope values of the low content components are obviously lighter with the increase of buried depth at the late stage of mature evolution. The carbon isotope values of adsorbed gas show similar characteristics, whether mudstone, carbargilite or coal in the same well. The tight gas reservoir is self-generation and self-accumulation gas reservoir by vertical diffusion and short distance migration in the K1sh Formation of the Xujiaweizi Fault Depression. It is important to study on the geochemical characteristics and the genesis of tight gas for guiding the exploration and deployment in the future.

Improved sampling technique to collect natural gas from hydrate-bearing pressure cores

High quality gas compositional data are an important factor in interpreting the genetic source of natural gas hosted in hydrate-bearing sediments and other subsurface systems. In order to accurately characterize the composition of gas samples degassed from hydrate-bearing pressure cores, one must use a reproducible sampling technique that minimizes artifacts of the sampling process. Herein, we review sediment core degassing techniques and compare data obtained from a commonly used degassing approach, which we term the standard quantitative degassing (SQD) technique, to our newly developed modified quantitative degassing (MQD) method designed to minimize atmospheric contamination and gas-water interactions. The SQD method allows sample gas to interact with water in a bubbling chamber, which we hypothesize could alter the gas composition following mixing with water or dissolved gases in the bubbling chamber. Whereas, the MQD method allows for the collection of sample gas prior to the bubbling chamber. To compare the SQD and MQD methods, we performed a side-by-side comparison of noble (He, Ne, Ar, Kr, and Xe), major (H2, N2, O2, and CO2), and hydrocarbon (CH4, C2H6, C3H8, i-C4H10, C4H10, i-C5H12, C5H12) gas concentrations and select isotopic compositions obtained using both sample collection techniques. Gas samples were collected from hydrate-bearing pressure cores recovered and maintained under hydrate stable conditions from the northern Gulf of Mexico during the UT-GOM2-1 Expedition. The MQD method displayed significantly lower concentrations of atmospheric gases, higher proportions of hydrocarbon gases, lower ratios of C1/C2+, and heavier stable carbon and hydrogen isotopes of methane than the SQD method. These results demonstrate that the MQD method reduced air contamination and minimized alteration of the hydrocarbon gases. Therefore, we conclude this method may be important for future work that seeks to determine the composition of natural gas from pressure cores using quantitative degassing experiments, especially those seeking to measure major (e.g., N2) and noble gases.

Influence of methane diffusion on geochemical characteristics of natural gas: a case study of the Shiqiantan area in Junggar Basin, China

ABSTRACT The geochemical study of natural gas has made a great contribution to oil and gas exploration. However, the diffusion of natural gas components will affect the determination accuracy of natural gas origin. This study supplemented the organic carbon content, rock pyrolysis, gas component composition, and component carbon isotope analysis and testing of the source rocks and natural gas samples. The results indicated that the methane and ethane carbon isotope values of the natural gas in the Shiqiantan area are heavy. It should be originated from Carboniferous source rocks. When the maturity and genesis index of natural gas are highly matched with source rocks, the abnormal drying coefficient of natural gas may be related to the methane diffusion.