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The sealing capacities of shale, roof, and floor strata are essential for the preservation of shale gas. Sealing capacity development and its influence on shale gas composition and organic matter pore development are important but have not been addressed by existing studies. In this paper, the Upper Ordovician Wufeng-the first member of the Lower Silurian Longmaxi Fm. in the Yangtze region is selected as the study object. We dissected the J1 well in the Jiaoshiba block inside the Sichuan Basin and the Y6 well in the Youyang block outside the Sichuan Basin to analyze the gas components and carbon isotopes in recovered shale gas. In this study, two microscope techniques (i.e., FIB-SEM and FIB-HIM), porosity measurement, TOC content analysis, permeability tests, isothermal adsorption measurements, are utilized. The analytical results show that both the roof and floor strata have vertical sealing effects on the organic-rich shale due to their different physical properties compared to the shale. The shale overburden pressure increases with burial depth, and the adsorbed gas increases with increasing TOC content and thickness: all of these can lead to a decrease in permeability and an increase in self-sealing capacity. This paper determined the formation mechanism of the sealing capacity for shale, roof and floor strata. Furthermore, the control patterns of "sealing capacity strength" on the shale gas source and organic matter pore development are clarified. The main findings are as follows: when the shale, roof, and floor strata have a strong sealing capacity, only a small amount of liquid hydrocarbons can be discharged from the shale, but the natural gas formed due to liquid hydrocarbon cracking is abundant. Under such circumstances, shale gas has a mixed origin of liquid hydrocarbon cracked gas and kerogen cracked gas; the carbon isotopes are inverted; abundant pyrobitumen pores with high porosity and good connectivity are developed. When the sealing capacities are weak, the amount of liquid hydrocarbons discharged from the shale are high, and the amount of natural gas is low. The shale gas is mainly derived from kerogen cracking with weak or no carbon isotope inversion, and kerogen pores are small with poor connectivity. These findings will advance the study of shale gas formation mechanisms and provide a scientific basis for selecting shale gas exploration area. • Due to different physical properties, the roof and floor strata provide a seal for the organic-rich shale. • As the burial depth, TOC content, and the shale thickness increase, the shale has a stronger sealing capacity. • Sealing capacity plays an important role in the characteristics of shale gas and organic matter pore development. [ABSTRACT FROM AUTHOR]

• Abrupt high δ13C -CO2-DIC in the hot springs occurred before earthquakes. • The carbon isotopic spikes are caused by fractionation due to tectonic activity. • The δ13C -CO2-DIC of hot springs changed markedly before and after the earthquakes. • The elevated carbon isotopic levels may be a signal for the subsequent earthquakes. Geothermal springs are controlled by deep faults, which are the major causes of earthquakes. A series of geochemical indicators of hot springs, such as carbon dioxide content (CO 2), carbon isotope and noble gas isotope etc., have been used for earthquake monitoring and early warning at home and abroad. Given that previous studies have failed to establish an inevitable link between the carbon isotopic variations of hot springs and seismic activities, in this study, mass spectrometry techniques were used to analyze the carbon isotopes of gaseous CO 2 and noble gas isotope in hot springs, thermal water, and newly formed travertines from western Sichuan before and after an Ms 4.8 earthquake (i.e., 7.14 earthquake). The results showed that the carbon isotope level of the hot spring system increased significantly before the earthquake, i.e., δ13C CO2 = -2‰ to 3.3‰, δ13C DIC = 2.2‰ to 6.6‰ (DIC = dissolved inorganic carbon in geothermal water), R/R A = 0.90–1.83, and δ13C travertine = 1.76‰ to 5.75‰. These indicators decreased markedly 1 week after the earthquake, whereas the helium isotope ratio of R/R A did not exhibit a noticeable change (δ13C CO2 = -3.9‰ to 0.4‰, δ13C DIC = 1.6‰ to 4.1‰, R/R A = 0.68–1.87). Combined with previous thermodynamic calculation and experimental results on carbon isotope fractionation of the CO 2 –calcium carbonate system, we suggest that the high carbon isotope characteristics of hot springs are caused by carbon isotope fractionation following the release of CO 2 from fault carbonate decomposition owing to tectonic activity, not by a mixture of different carbon sources. A high carbon isotopic delivery path in geothermal water, from released CO 2 to DIC and then to travertine, was observed. The new mechanism proposed in this paper reveals the intrinsic relations between the changes of the hot spring geochemical properties and deep tectonic activities, and the variations of these geochemical parameters before and after earthquakes may assist in capturing seismic precursors. A post-peak earthquake warning model has thus been preliminarily proposed, however, long-term monitoring data are required to reduce some uncertainties. [ABSTRACT FROM AUTHOR]

[Display omitted] • The shale gas resource potential of the Longmaxi Formation was evaluated by using mud gas isotope logging technology. • The reservoir properties of shale were evaluated based on gas isotope fractionation effects. • The shale gas in the Changning area belongs to late kerogen cracking gas. • The lower part of the Longmaxi Formation has a better shale gas exploration potential. The Longmaxi Formation in the Sichuan Basin is one of the most important shale gas producing formations in China. Previous studies of shale gas were mostly carried out on based on laboratory analyses and did not provide effective guidelines for shale gas exploration. In this paper, we present a large amount of carbon isotopic data from mud gas isotope logging and rock-cutting headspace gas in wellsites. The δ13C 1 values showed a significant decrease at the bottom of the Longmaxi Formation. The δ13C 1 and δ13C 2 values in the Changning area were observed to be heavier than those in the Weiyuan and Jiaoshiba areas. The second half of the N216H27-3 horizontal well had obvious outgassing and carbon isotope fractionation effects. This indicates that the bottom of the Longmaxi Formation has better shale gas exploration potential. The shale gas of the Longmaxi Formation in the Changning area belongs to late kerogen cracking gas, where exploration prospects are poorer than those in the Weiyuan and Jiaoshiba areas. There were more oil cracking gas resources and better reservoir conditions at the end of the N216H27-3 horizontal well; therefore, drilling should be continued at this site. [ABSTRACT FROM AUTHOR]

The upper Permian Longtan Formation (LTF) shale is a new shale gas exploration and development focus in China, which was formed in a transitional depositional environment. Because of the difference in the sedimentary environment between marine-continental transitional shale and marine shale, the enrichment characteristics and mechanism of organic matter (OM) are very different. Because of a weak research foundation, the enrichment mechanism of OM in the LTF shale in the Sichuan Basin is still unclear. In this paper, shale samples of the upper Permian LTF from the Well F5 were collected, and the abundance, source and sedimentary environment of organic matter were studied by using paleoclimate, detrital input, redox, paleoproductivity and sedimentation rate parameters. The results show that the TOC content of the LTF shale is high (average = 3.04%) and reaches the high to overmature stage (average = 2.46%), indicating a favorable source rock. The LTF shale was deposited in a humid and hot climate, which was conducive to the growth and prosperity of a large number of plants. The source of organic matter was mainly terrestrial plant fragments. The sedimentary environment of the LTF shale was oxic, it had a high sedimentation rate, which reduced the oxidation rate of OM, promoted the rapid deposition of OM, and ensured the preservation of organic matter. In summary, the enrichment of OM of LTF shale in the Sichuan Basin was affected by the sedimentary environment, which was complicated. The main controlling factors were detrital input, redox conditions and sedimentation rate. • The geochemical characteristics of Permian Longtan Formation (PLTF) shale in eastern Sichuan are analyzed. • The sedimentary paleoenvironment of PLTF shale in eastern Sichuan is reconstructed. • The main controlling factors of organic matter enrichment of PLTF in eastern Sichuan are studied for the first time. [ABSTRACT FROM AUTHOR]

The Northwestern Sichuan Basin is a hot spot for natural gas exploration, and in recent years, many wells have obtained high-yield commercial gas flows in the Qixia Formation. However, due to the influence of foreland orogeny, the oil and gas in the Qixia Formation often have the characteristics of multisource filling, multistage accumulation and multistage transformation, which results in an unclear understanding of the gas source and the accumulation process. This paper uses the concept of sub-block research through carbon isotope and biomarkers analyses, to carry out gas-gas, gas-source and bitumen-source rock comparisons in different structural belts to determine the natural gas sources. On this basis, basin simulation and fluid inclusion analysis technology are used to determine the stage of hydrocarbon accumulation, and then the accumulation process is determined. Studies have shown that the gas source of the Qixia Formation are different in the different structural belts: (1) The natural gas from the Qixia Formation in the thrust nappe belt mainly originates from the Qiongzhusi Formation source rock; (2) The natural gas from the Qixia Formation in the hidden fault belt is a mixture of the source rock of the Qixia Formation and the source rock of the Qiongzhusi Formation; and (3) The natural gas from the Qixia Formation in the gentle belt mainly originates from the Middle Permian source rock and the Upper Permian Longtan Formation source rock. The Indosinian, Yanshania, and Himalayan movements all have great influence on the formation of the Qixia Formation reservoir in northwestern Sichuan, and the accumulation process of the different structural belts is quite different, but generally, the accumulation process of the different structural belts have experienced three stages: a primary oil pool stage, gas pool stage and reformation and definition stage. • Through sub-block research, gas source of the foreland thrust belt is studied. • Different gas sources cause differences in carbon isotopes. • Accumulation process of each structural belt is distinct. • The results have implications for exploration in complex tectonic areas. [ABSTRACT FROM AUTHOR]

The Lower Paleozoic Ordovician Wufeng Formation and the Silurian Longmaxi Formation shale in the southeastern part of Sichuan Basin in China are the key strata for shale gas exploration. In this paper, based on a large number of outcrops, cores and natural gamma spectroscopy logs combined with scanning electron microscopy, geochemical parameter testing and mineral component analysis, the geological conditions and exploration potential of shale gas accumulation were studied in this area, and the relationship between structure and shale gas preservation is discussed. The vertical reduction in the U/Th ratio measured in the outcrop sandstone lenses and core horizontal bedding and observed in the energy spectrum logs indicates that the sedimentary water body transformed from a low-energy anoxic reducing environment to a high-energy oxidizing environment. The TI index and the kerogen carbon isotope indicate that the organic matter type of the shale is mainly type I kerogen. The TOC content of high-quality shale is generally above 2.50%, and the R o value is between 2.00% and 3.08%. High organic matter abundance, moderate thermal evolution and a large thickness of high-quality shale ensure an adequate supply of hydrocarbon resources. Multiple types of inorganic pores, primary organic pores and pores formed during hydrocarbon generation provide abundant reservoir space for the enrichment of shale gas. The tectonics have an important influence on the preservation of shale gas. Moderate fracture development, a late tectonic uplift time, small-scale tectonic uplift, and high formation pressure coefficient are conducive to the preservation of shale gas, and the gas content and productivity will be higher. Overall, a structural stability zone with a high formation pressure coefficient is a favorable area for shale gas exploration. • Comprehensive analysis the sedimentary environment based on three scales: outcrop, core and energy spectrum logging. • Later and small-scale tectonic uplift, moderate fracture development, good roof preservation are conducive to gas enrichment. • A structural stability zone with a high formation pressure coefficient is a favorable area for shale gas exploration. [ABSTRACT FROM AUTHOR]

Numerous studies have been conducted on the isotopic geochemical characteristics of shale gas. However, a systematic study of the relationship between the isotopic geochemical characteristics and gas content has not been addressed to date. In this study, we investigated the geochemical and isotopic compositions of shale gas samples, and the gas content of the Lower Silurian Longmaxi marine shale in the Huangjinba area, southern Sichuan Basin, China. In addition, we analyzed the quantitative relationship between the geochemical parameters and the gas content. The Longmaxi shale gas is a dry gas with a low wetness (0.12%–0.67%), dominated by methane (89.49%–99.00%) with a small amount of ethane (0.11%–0.61%). The average values of δ13C methane , δ13C ethane and δ13C propane are −29.8‰, −35.0‰, and −33.2‰, characterized by a partial isotopic reversal (δ13C methane > δ13C propane > δ13C ethane) with a maturity level (Ro ≈ 2.30%–2.51%). The total gas content of the Longmaxi shale varies widely from 0.42 m3/t to 3.88 m3/t. Correlation analyses revealed that the ethane isotope (δ13C ethane) and the magnitude of isotopic reversal (δ13C methane - δ13C ethane) exhibit strong correlations with the total gas content and the free gas content. This study demonstrates that carbon isotopic reversal of shale gas largely depends on the secondary cracking, and thermal maturity and residual liquid hydrocarbons content are important factors affecting the magnitude of carbon isotopic reversal. Based on the quantitative study conducted in this paper, we provide a new proxy for predicting the gas content of high maturity shale using an isotopic method. • Carbon isotopic reversal of shale gas largely depends on the secondary cracking. • Sufficient residual liquid hydrocarbon is necessary for the carbon isotopic reversal. • Wetness of 1.4% in high maturity shale gas indicates the peak of secondary cracking. • δ13C methane –δ13C ethane can be used to predict the gas content of high maturity shale. [ABSTRACT FROM AUTHOR]