Accurate identification of natural gas origin is fundamental to the theoretical research on natural gas geosciences and the exploration deployment and resource potential assessment of oil and gas. Since the 1970s, Academician Dai Jinxing has developed a comprehensive system for natural gas origin determination, grounded in geochemical theory and practice, and based on the integrated analysis of stable isotopic compositions, molecular composition, light hydrocarbon fingerprints, and geological context. This paper systematically reviews the core framework established by him and his team according to related references and application results, focusing on the conceptual design and technical pathways of key diagnostic diagrams such as delta C-13(1)-C-1/(C-2+C-3), delta C-13(1)-delta C-13(2)-delta C-13(3), delta C-13(CO2) versus CO2 content, and the C-7 light hydrocarbon ternary plot. We evaluate the applicability and innovation of these tools in distinguishing between oil-type gas, coal-derived gas, microbial gas, and abiogenic gas, as well as in identifying mixed-source gases and multi-stage charging systems. The findings suggest that this identification system has significantly advanced natural gas geochemical interpretation in China, shifting from single-indicator analyses to multi-parameter integration and from qualitative assessments to systematic graphical identification, and has also exerted considerable influence on international research in natural gas geochemistry. The structured overview of the development trajectory of natural gas origin discrimination methodologies provides a technical support for natural gas geological theory and practice and offer a scientific foundation for the academic evaluation and application of related achievements.

Shale gas wastewater from hydraulic fracturing poses significant environmental risks due to its high salinity and complex inorganic composition. This study investigates the behavior of major and trace inorganic constituents across a full-scale treatment train in the Sichuan Basin, China. Despite multi-stage processes including equalization, flocculation, flotation, biological reactors, membrane filtration, and clarification, key inorganic species such as Cl, Na, Br, Sr, Li, and B remained largely persistent in the final effluent with values of 13,760, 8811, 70, 95.9, 26.6, and 60.2 mg/L, respectively. Geochemical tracers including Br/Cl (average: 0.0022 mM/mM), Na/Br (average: 125 mg/mg), and Sr/Ca (average: 0.15 mM/mM) ratios, combined with halide endmember mixing models, revealed that salinity primarily originated from highly evaporated formation brines, with limited evidence for halite dissolution or external contamination. Elevated Sr (average: 89.3 mg/L) and Ca (average: 274 mg/L) levels relative to Mg (average: 32 mg/L) suggest significant water–rock interaction. Environmental risk assessments showed that concentrations of several elements in treated effluent greatly exceeded national and international discharge or reuse standards. These findings underscore the limitations of conventional treatment technologies and highlight the urgent need for advanced processes and regulatory frameworks that address the unique challenges of high-TDS (total dissolved solids) unconventional wastewater.

Different types of natural gas exhibit distinct carbon and hydrogen isotopic compositions, making these isotopic compositions crucial indicators for identifying gas origins. With ongoing advancements in natural gas exploration technology and the increasing volume of exploration data, our understanding of natural gas origins and sources continues to deepen, and how to update and verify the existing data to ensure the applicability of gas genetic diagrams has become crucial. This study comprehensively analyzes the stable carbon and hydrogen isotope characteristics of different genetic types of natural gases in Sichuan, Tarim, Ordos, Turpan-Hami, Songliao, Northern Jiangsu, Sanshui, Qaidam, and Bohai Bay basins in China, together with abiotic gases from the Lost City of the Middle Atlantic Ridge, and the genetic diagrams related to commonly used carbon and hydrogen isotopes are evaluated. The study yields the following four conclusions: (1) The carbon isotopic values of methane (δ13C1), ethane (δ13C2), propane (δ13C3) and butane (δ13C4) of natural gases from China are from −89.4‰ to −11.4‰ (average of −36.6‰), −66.0‰ to −17.5‰ (average of −29.4‰), −49.5‰ to −13.2‰ (average of −27.3‰), −38.5‰ to −16.0‰ (average of −25.6‰), respectively. (2) The hydrogen isotopic values of methane (δD1), ethane (δD2) and propane (δD3) of natural gases from China range from −287‰ to −111‰ (average of −177‰), −249‰ to −94‰ (average of −158‰), and −237‰ to −75‰ (average of −146‰), respectively. (3) The carbon and hydrogen isotopic distribution patterns among methane and its homologues of natural gases in China are mainly in positive order (δ13C1<δ13C2<δ13C3<δ13C4, δD1<δD2<δD3). In most natural gas samples, the fractionation amplitude between methane and ethane is greater than that between ethane and propane (Δ(δ13C2−δ13C1) > Δ(δ13C3−δ13C2), Δ(δD2−δD1) > Δ(δD3−δD2)). (4) The δ13C1–δ13C2–δ13C3, the δ13C1–δD1, δ13C1–C1/C2+3, Δ(δ13C2−δ13C1)–Δ(δ13C3−δ13C2) and Δ(δD2−δD1)–Δ(δD3−δD2) diagrams, can be used to identify the gas origin in many different cases, and the combined application between different charts can enhance the identification effect.

Based on geochemical data from natural gas samples across spring water systems and sedimentary basins, including Songliao, Bohai Bay, Sanshui, Sichuan, Ordos, Tarim and Ying-Qiong, this paper systematically compares the geochemical compositions of abiogenic versus biogenic gases. Emphasis is placed on the diagnostic signatures of abiogenic gases in terms of gas composition, and carbon, hydrogen and helium isotopes. The main findings are as follows. (1) In hydrothermal spring systems, abiogenic alkane gases are extremely scarce. Methane concentrations are typically less than 1%, with almost no detectable C2+ hydrocarbons. The gas is dominantly composed of CO2, while N-2 is the major component in a few samples. (2) Abiogenic alkane gases display distinct isotopic signatures, including enriched methane carbon isotopic compositions (delta C-13(1)>-25 parts per thousand generally), complete carbon isotopic reversal (delta C-13(1)>delta C-13(2)>delta C-13(3)>delta C-13(4)), and enriched helium isotope (R/R-a>0.5, CH4/He-3 <= 10(6) generally). (3) The hydrogen isotopic composition of abiogenic alkane gases may be characterized by a positive sequence (delta D-1delta D-2>delta D-3), or a V-shaped distribution (delta D-1>delta D-2, delta D-2

With the increasing number of shale gas wells being hydraulically fractured with recycled flowback and produced water to reduce the consumption of fresh water, geochemical indicators of hydraulic fracturing flowback fluids (HFFFs) from such wells need to be found to evaluate the potential pollutants caused by shale gas development. To fill this knowledge gap, we analysed 64 HFFF samples from 3 Changning wells (fracked with recycled wastewater) to form a time series and compared them to HFFF samples from 3 Weiyuan wells (fracked with fresh water). We also made comparisons of HFFFs from two types of wells in the Appalachian Basin with reported data. The results indicated that regardless of the fracturing fluid being used, the water‒rock reaction caused by the hydraulic fracturing process resulted in the release of exchangeable phase elements, such as lithium, boron, with relatively depleted δ7Li and δ11B, and strontium with relatively enriched 87Sr/86Sr values on the shale surface, into the HFFFs. The composition of the injected fracturing fluid affected the strength of the water‒rock reaction, but the effect is negligible, as the fracturing process can still form the unique Sr, Li, and B isotope ratios of the HFFF. Thus, the elemental signatures (B/Cl and Sr/Cl) and isotopic fingerprints (δ11B, δ7Li, and 87Sr/86Sr) derived from HFFFs can be used to distinguish between HFFFs from wells hydraulically fractured by either fresh water or recycled wastewater and flowback water from conventional oil and gas wells. The results are important for the environmental evaluation of the large number of wells being fracked with recycled wastewater each year.

Exploration and development of large gas fields is an important way for a country to rapidly develop its natural gas industry. From 1991 to 2020, China discovered 68 new large gas fields, boosting its annual gas output to 1 925×108 m3 in 2020, making it the fourth largest gas-producing country in the world. Based on 1696 molecular components and carbon isotopic composition data of alkane gas in 70 large gas fields in China, the characteristics of carbon isotopic composition of alkane gas in large gas fields in China were obtained. The lightest and average values of δ13C1, δ13C2, δ13C3 and δ13C4 become heavier with increasing carbon number, while the heaviest values of δ13C1, δ13C2, δ13C3 and δ13C4 become lighter with increasing carbon number. The δ13C1 values of large gas fields in China range from −71.2‰ to −11.4‰ (specifically, from −71.2‰ to −56.4‰ for bacterial gas, from −54.4‰ to −21.6‰ for oil-related gas, from −49.3‰ to −18.9‰ for coal-derived gas, and from −35.6‰ to −11.4‰ for abiogenic gas). Based on these data, the δ13C1 chart of large gas fields in China was plotted. Moreover, the δ13C1 values of natural gas in China range from −107.1‰ to −8.9‰, specifically, from −107.1‰ to −55.1‰ for bacterial gas, from −54.4‰ to −21.6‰ for oil-related gas, from −49.3‰ to −13.3‰ for coal-derived gas, and from −36.2‰ to −8.9‰ for abiogenic gas. Based on these data, the δ13C1 chart of natural gas in China was plotted.

Based on an elaboration of the resource potential and annual production of tight sandstone gas and shale gas in the United States and China, this paper reviews the researches on the distribution of tight sandstone gas and shale gas reservoirs, and analyzes the distribution characteristics and genetic types of tight sandstone gas reservoirs. In the United States, the proportion of tight sandstone gas in the total gas production declined from 20%–35% in 2008 to about 8% in 2023, and the shale gas production was 8 310×108 m3 in 2023, about 80% of the total gas production, in contrast to the range of 5%–17% during 2000–2008. In China, the proportion of tight sandstone gas in the total gas production increased from 16% in 2010 to 28% or higher in 2023. China began to produce shale gas in 2012, with the production reaching 250×108 m3 in 2023, about 11% of the total gas production of the country. The distribution of shale gas reservoirs is continuous. According to the fault presence, fault displacement and gas layer thickness, the continuous shale gas reservoirs can be divided into two types: continuity and intermittency. Most previous studies believed that both tight sandstone gas reservoirs and shale gas reservoirs are continuous, but this paper holds that the distribution of tight sandstone gas reservoirs is not continuous. According to the trap types, tight sandstone gas reservoirs can be divided into lithologic, anticlinal, and synclinal reservoirs. The tight sandstone gas is coal-derived in typical basins in China and Egypt, but oil-type gas in typical basins in the United States and Oman.

Guizhou Province of China is rich in shale gas resources. In recent years, important progress has been made in the exploration and development of shale gas in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in Zheng'an block in northern Guizhou, adjacent to Sichuan Basin. Due to the complex geological conditions and fragile ecological environment in the shale gas distribution area, serious environmental pollution risks may exist in shale gas exploitation. Therefore, this paper conducted element, hydrogen and oxygen isotope analyses for shale gas fracturing flowback/produced water of Well AY7-4 in Zheng'an block. The results showed that the contents of K, Na, Ca, Mg and NH+4 in the flowback/produced water of Well AY7-4 were very high, with an average of 118 mg/L, 7616 mg/L, 266 mg/L, 47 mg/L and 76 mg/L, respectively. The content of Na, Ca and Mg is similar to that of shale gas flowback/produced water in Weiyuan, but lower than that in Changning and Fuling. NH+4 content was similar to that in Changning, but much higher than that in Weiyuan. The average Cl content was 11605 mg/L, which was similar to that in Weiyuan, but lower than that in Changning and Fuling. The average Br content is 48 mg/L, slightly lower than that of Weiyuan, and about half of that of Changning and Fuling. Correspondingly, the Br/Cl value of Well AY7-4 is the lowest, reflecting its relatively low Br content. The average content of Li, B and Sr is 17 mg/L, 10 mg/L and 45 mg/L, respectively, which is similar to that of the Weiyuan, Changning and Fuling shale gas fields. The distribution model of Li and Sr is similar to that of Cl, Br and Na, but the B content and B/Cl value of Well AY7-4 are the lowest. The linear relationship between Br and Cl contents and between delta D and delta 18O of flowback/produced water of Well AY7-4, freshwater used for hydraulic fracturing and hydraulic fracturing injected fluid indicates that flowback/produced water of Well AY7-4 is mainly the mixed product between low salinity injected fracturing fluid and high salinity brine retained in shale formation. The hydrogen and oxygen isotopic compositions gradually become heavier over time, indicating that the proportion of high salt end formation brine in flowback/ produced water is increasing. The contents of Cl, Br, Na and NH+4 in the flowback/produced water of Well AY7-4 are far higher than the allowable discharge value, which is a potential environmental pollution risk and cannot be discharged directly. (c) 2024 Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences AND Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KEAI Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

The rapid expansion of shale gas extraction worldwide has raised significant concerns about its impact on water resources. China is expected to undergo a shale revolution following the U.S. Most of the information on water footprint of shale gas exploration and hydraulic fracturing has been focused on the U.S. Here, we addressed this knowledge gap by establishing a comprehensive database of shale gas extraction in China, utilizing operational data from over 90 % of shale gas wells across the country. We present systematic analysis of water usage and flowback and produced water (FP water) production from all the major shale gas fields in China. Between 2012 and 2022, a total of 2740 shale gas wells were hydraulically fractured in China, primarily located in Sichuan and Chongqing Province. About 113 million m(3) water was used for hydraulic fracturing, resulting in a cumulative shale gas production of 116 billion m(3). As of 2022, the annual water use for hydraulic fracturing exceeded 20 million m(3), and the annual FP water production reached 8.56 million m(3). Notably, 80 % similar to 90 % of the FP water has been reused for hydraulic fracturing since 2020, accounting for 29 % to 35 % of the annual water usage for hydraulic fracturing. Water use per well in China varies primarily between 21,730 m(3) to 61,070 m(3) per well, and water use per horizontal length ranges primarily between 20 m(3)/m and 35 m(3)/m. The average ultimate FP water production per well in China was estimated to be 22,460 m(3). The water use intensity (WUI) for shale gas extraction in China mainly ranges from 7 to 25.4 L/GJ, which is significantly higher than that of the U.S. This disparity is largely due to the lower Estimated Ultimate Recovery (EUR) of shale gas wells in China. Despite the considerable water consumption during the hydraulic fracturing process, shale gas has a relatively low water footprint compared to other conventional energy resources in China. The Produced water intensity (PWI) for shale gas extraction in China ranges from 3.9 to 7.3 L/GJ, which is consistent with the previously reported PWI values for shale gas extraction in the U.S. This study predicts water usage and FP production spanning the period 2023 to 2050 under two scenarios to assess the potential impact of shale gas extraction on water resources in the Longmaxi shale region in Sichuan Basin. The first scenario assumed a constant drilling rate, while the second assumed a yearly 10 % increase in drilling rate. With an assumed FP water reuse rate of 85 % for hydraulic fracturing, the estimated annual freshwater consumption for the two scenarios is 10.4 million m(3) and 163 million m(3), respectively. This accounts for only 0.28 parts per thousand and 4.4 parts per thousand of the total annual surface water resources in Sichuan and Chongqing Province. Our findings suggest that freshwater usage for hydraulic fracturing in humid Southern China is small relative to available surface water resources. However, prospective large-scale shale gas extraction in other arid and semi-arid regions may enhance the regional water scarcity. It is necessary to develop new hydraulic fracturing technologies that can use saline groundwater or other types of marginal water, and explore alternative management and treatment strategies for FP water.

The Hongche fault belt-Chepaizi uplift in the northwestern margin of Junggarbasin has many oil and gas reservoirs,and the physical and chemical properties and geochemical characteristics of crude oil are very complex,so the type and source of crude oil have long been controversial,which directly affects the decision of oil and gas exploration.Based on the summary of typical geochemical characteristics of crude oils and main oil source identification indexes in different ages of Junggar basin,this paper systematically analyzed and studied the geochemical characteristics and sources of the crude oils in Hongche fault belt and Chepaizi uplift,and divided the crude oils in this area into three types with single source and two types with mixed sources.Three types of single source crude oils are derived from the lacustrine source rocks of Permian,coal measure source rocks of the Middle and Lower Jurassic,lacustrine source rocks of the Paleogene Anjihaihe Formation.The two types of mixed crude oils are mixed from the biodegraded heavy oil derived from the Permian lacustrine source rocks,the normal crude oil derived from the Middle and Lower Jurassic coal measure source rocks,and the normal crude oil derived from the Paleogene lacustrine source rocks.The crude oil of the Carboniferous-Cretaceous oil reservoirs in the Hongche fault belt is mainly derived from the Permian lacustrine source rock,and the heavy oil in Chunfeng oilfield in the northeast of Chepaizi uplift is also derived from the Permian lacustrine source rock.Light crude oil in the Neogene Shawan Formation oil reservoir on the east side of Chepaizi Uplift and west side of Hongche fault belt is derived from Paleogene lacustrine source rocks.The heavy oil of the Cretaceous-Paleogene oil reservoirs in the Chunguang oilfield in the middle of Chepaizi uplift is the mixture of the Permian sourced heavy oil and the Jurassic sourced normal crude oil,and the heavy oil of Neogene Shawan Formation oil reservoir is the mixture of the Permian sourced heavy oil and the Neogene sourced normal crude oil.The light crude oil of the Carboniferous-Paleogene oil reservoir in the west of Chepaizi uplift is derived from the Middle-Lower Jurassic coal measure source rocks,while the light crude oil of the Neogene Shawan Formation oil reservoir is derived from the Paleogene lacustrine source rocks.This paper has important reference for the study of hydrocarbon accumulation and regional oil and gas exploration decision in the southern area of northwest margin of Junggar basin.

Sichuan Basin is important commercial shale gas producing basin in China, where shale gas explorations were mainly targeted at the Upper Ordovician Wufeng-Lower Silurian Longmaxi formations with burial depth mostly of 2000 similar to 3500 m. This study performed a detailed investigation about the delta H-2 features of shale gases from the Wufeng-Longmaxi shale with burial depth of <2250 m (Taiyang), 2250 similar to 3500 m (Jiaoshiba) and >3500 m (Weirong). The average delta H-2(1) and delta H-2(2) values are of -135 parts per thousand and -146 parts per thousand, -137 parts per thousand and -157 parts per thousand, -137 parts per thousand and -124 parts per thousand, respectively in Taiyang, Jiaoshiba and Weirong shale gas fields. The average delta H-2(1) values are nearly the same in the three gas fields, while average delta C-13(1) values increase from -35.2 parts per thousand to -28.5 parts per thousand as from Weirong to Taiyang. When wetness >8%, delta H-2(1) values increase with decreasing wetness rapidly implying dominant influences from maturity, when wetness <8%, the delta H-2(1) variation is small and mainly in the range of (-140 +/- 15)parts per thousand, indicating a strong influence from cogenetic water. All gases are characterized by a trend of delta C-13(1) > delta C-13(2) > delta C-13(3), and gases in Taiyang and Jiaoshiba are characterized by delta H-2(1) > delta H-2(2), while gases in Weirong are dominated by delta H-2(1) < delta H-2(2). Generally, with decreasing gas wetness, ethane first become more enriched in D and C-13, then become more depleted in D and C-13, and finally become more enriched in D and C-13 again. The turning point of wetness from rollover to post-rollover zones is 1.2% for the delta C-13(2) while 0.8% for the delta H-2(2). Such delay in delta H-2(2) also indicates the strong influences from the cogenetic water medium.

EDITORIAL article Front. Earth Sci., 15 September 2022Sec. Geochemistry https://doi.org/10.3389/feart.2022.1013940

Shale gas wastewater (SGW) disposal is a major challenge in the areas in central China due to its increasing volume associated with intensification of shale gas exploration and its high levels of contaminants. In the Fuling shale gas field of Sichuan Basin, a small amount of SGW originated from the flowback and produced water (FPW) is treated and then discharged to a local stream. This study investigated the inorganic water geochemistry and Sr isotopic composition of the FPW in Fuling shale gas field, the SGW effluent that is generated in the treatment facility, and the quality of a local river after the disposal of treated SGW. The data generated in this study reveals that FPW generate after several years of shale gas operation maintain the original geochemical fingerprints detected in early stages of FPW generation, and consistent with the FPW composition detected in other shale gas fields in Sichuan Basin. We show that reuse of saline FPW for hydraulic fracturing can generate an inverse salinity trend, where the salinity of FPW decreases with time, reflecting the increase of the contribution of formation water with lower salinity. The treatment of the FPW results in ~40 % reduction of the salts by dilution with freshwater and selective (80-90 %) removal of some of the inorganic contaminants. The original geochemical fingerprints of the FPW from Fuling shale gas field was not modified during FPW treatment, reinforcing the applicability of these tracers for detecting SGW in the environment. Discharge of treated SGW effluent to a local river causes a major 200-fold dilution and reduction of all contaminants levels below drinking water and ecological standards. Overall, this study emphasizes the importance of water quality monitoring of treated SGW and the overall measures needed to protect public health and the environment in areas of shale gas development.

Natural gases from the Taiyang (shallow), Jiaoshiba (middle), and Weirong (deep) shale gas fields in the southern Sichuan Basin were analyzed for molecular and stable carbon isotopic compositions to investigate the geochemical characteristics and gas origins. All the gases belong to shale gas from the Upper Ordovician–Lower Silurian shale and are dominated by methane with gas wetness generally less than 0.83%. The δ13C1 values are −28.5‰, −30.3‰, and −35.2‰ in Taiyang, Jiaoshiba, and Weirong shale gas fields, respectively. The extremely high thermal maturity is the controlling factor for the enrichment of 13C in methane, with a minor contribution from the heavy carbon isotope of the organic matter in the Ordovician Wufeng Formation. Fischer–Tropsch-type synthesis of hydrocarbon gas from CO2 and H2 contributes to the increase of wet gas, which results in the offset from the δ13C1∼wetness linear trend in the Taiyang and Jiaoshiba gas fields. Methane, ethane, and propane in the Taiyang shale gas field have increasing δ13C values with increasing burial depth, which is mainly caused by diffusive migration. All gases are characterized by a complete carbon isotopic reversal trend (δ13C1 > δ13C2 > δ13C3), and it is mainly caused by the reversible free-radical reactions with the conversion from alkane to alkyl groups, with some contribution from the Fischer–Tropsch-type synthesis. The results of this study will improve our understanding of the geochemical characteristics of shale gases from different burial depths and have important implications for future shale gas exploration in the deep and shallow layers.

This research analyzed the composition and hydrocarbon isotope geochemical characteristics of ultra-deep natural gas (buried deeper than 6000 m) in more than 130 wells in the Tarim Basin. Characteristics of the ultra-deep natural gas components in the Tarim Basin are as follows: natural gas in the Kuqa sandstone reservoir is developed as dry gas, with a high methane content (an average of 95.53%) and a low ethane content (an average of only 1.39%). The ultra-deep gas of the marine craton carbonate reservoirs in the North Tarim-Central Tarim-East Tarim area is developed as wet gas. It is mainly composed of methane ranging from 56.1% to 98.8%, with an average content of 76.36% and also contains a small amount of ethane amounting to 6.74%. CO 2 and H 2 S contents of ultra-deep condensate gas in the Central Tarim area are high, with maximum values of 24.2% and 23.1%, respectively. Isotopic characteristics are as follows: δ 13 C 1 values of Kuqa natural gas are distributed between −36‰ and −25.3‰, with an average of −28.2‰, δ 13 C 2 values are distributed between −26.2‰ and −13.8‰, with an average of −18.3‰. Most gas samples in Kuqa area are developed with the reversal of carbon isotopic series as δ 13 C 2 > δ 13 C 3 , and the δ 13 C CO2 values are distributed between −19.5‰ and −10.3‰. The δ 13 C 1 values of ultra-deep gas in marine craton carbonate rocks are distributed between −54.4‰ and −33.3‰, with an average of −42.6‰. δ 13 C 2 values are distributed between −41.1‰ and −29.4‰, with an average of −33.8‰, and δ 13 C CO2 values are distributed between −28.2‰–0.6‰. According to the identification chart, it can be seen that the ultra-deep gas in the sandstone reservoir of the Kuqa foreland is mainly coal-derived gas in the high over-mature stage, and the reversal of carbon isotopic series may be caused by the mixing of natural gas of the same type and different sources. The natural gas in carbonate reservoirs of marine craton is mainly oil-type gas with complex sources, including both kerogen cracking gas and oil cracking gas. CO 2 can be formed by organic and inorganic genesis. Inorganic CO 2 is most likely to be generated by the dissolution of carbonate rocks under acid formation water.

近10年(2012—2021年)中国天然气工业快速发展,主要是:①近10年共产气15105.2×108 m3,是前10年(2002—2011年)总产气量6468.38×108 m3的2.3倍;②天然气储量猛增,近10年新增探明天然气地质储量84499.58×108 m3,是前10年总储量55696.57×108 m3的1.5倍;③页岩气勘探开发开花结硕果,近10年发现并探明7个页岩气田,新增探明页岩气地质储量2.74×1012 m3,累计生产页岩气920×108 m3;④年产百亿方大气田前10年2个,近10年为3个;⑤长输气管道四通八达,近10年建成长输气管道60766.5 km,是1949—2011年间建成管道长度的2.6倍.

The Sichuan Basin, covering an area of 180 × 103 km2, has the following advantages in natural gas geology: The sedimentary rocks are 6,000–12,000 m thick with high maturity of source rocks, and nine sets of primary gas source rocks are developed in the basin with a gas–oil ratio of 80:1, and thus it is a gas basin. The remaining recoverable reserves of conventional and unconventional natural gas are up to 13.6404 × 1012 m3. Multiple gas-bearing systems are developed with 25 conventional and tight oil and gas producing layers and 135 discovered gas fields, and the total proved geological reserves and cumulative production of natural gas by the end of 2019 were 5.7966 × 1012 m3 and 648.8 × 109 m3, respectively. The CO2 components and the correlation with relevant parameters for 243 samples from 22 gas fields indicate that CO2 in the Sichuan Basin display the following two characteristics: (1) Relatively low CO2 content of 0.02%–22.90% with an average of 2.96%, which guaranteed the commerciality of natural gas exploration and production; (2) cratonic CO2, which is characterized by low CO2 contents (<5%) and low R/Ra ratios (<0.24). According to the δ13CCO2 values and the relationship with R/Ra, δ13C1, CO2 contents, and wetness coefficient (W) for 263 gas samples, the δ13CCO2 values display three characteristics: (1) The highest δ13CCO2 value (10.4‰) in China is found in the Fuling shale gas field, which extends the interval values from previous −39‰–7‰ to −39‰–10.4‰. (2) The δ13CCO2 values can be applied to identify the CO2 origin of natural gas in the Sichuan Basin: type A, organic origin from thermal decomposition of organic matter, with an average δ13CCO2 value of −12.8‰ and average wetness coefficient of 7.8% for 44 samples; type B, organic origin from thermal cracking of organic matter, with an average δ13CCO2 value of −15.7‰ and average wetness coefficient of 1.30% for 34 samples; type C, inorganic origin from thermal decomposition or organic acid dissolution of carbonate rocks or minerals, with an average δ13CCO2 value of −1.8‰ and average wetness coefficient of 0.85% for 175 samples. (3) δ13CCO2>δ13CCH4. This is a common characteristic shared by all geological age (from Z2dn to J2s) gas reservoirs and various gas types (coal-derived gas, oil-associated gas, and shale gas).

China is rich in shale gas resources, which are mainly distributed in densely populated southern marine carbonate areas. The geological and surface conditions are complex, the ecological environment is fragile, and water resources are scarce or unevenly distributed. Therefore, large-scale fracturing mining has high pressure of water resources utilization and serious risk of water environment pollution. In this paper, the geochemical characteristics of traditional and non-traditional stable isotopes such as hydrogen, oxygen, boron, lithium, strontium in shale gas hydraulic fracturing flowback/produced water (FP water) were comprehensively analyzed. The results show that, the hydrogen and oxygen isotopic composition of the FP water in Sichuan Basin, China has similar evolution trend with the produced water of conventional wells from Cambrian, Permian and Triassic Xujiahe, Jialingjiang and Leikoupo formations, but different from that of the Sinian conventional produced water. It indicates that the FP water in Sichuan Basin is a mixture of fracturing injected fluid and formation brine retained in Silurian shale. The saline end member is close to the formation water of Cambrian, but with higher δ11B values. The FP water in Sichuan Basin has δ11B values close to that of the Marcellus FP water, and both are derived from the evaporated seawater. The δ11B values of FP water in Sichuan Basin have overlap with that of the conventional produced water from different strata, so it can not be precisely distinguished. However, the δ11B and B/Cl values of the FP water in Sichuan Basin can be clearly distinguished from the river and the FP water from non-marine facies shale in Qaidam Basin. The FP water in Sichuan Basin has slightly higher δ7Li values than that of the Marcellus FP water, but has overlap with that of the Yangtze River. The average value of 87Sr/86Sr of FP water is 0.7197 in Weiyuan and 0.7193 in Changning, which is much higher than that of the produced water from conventional wells in different formations. This is because the Silurian shale is affected by terrestrial siliceous sediments and the underlying Sr-rich fluids. As a result, 87Sr/86Sr values measured in the Silurian strata in southern Sichuan Basin are high, which makes 87Sr/86Sr values become an effective index to distinguish shale gas fracturing FP water from conventional gas well produced water and shallow groundwater in Sichuan Basin.

Oilfield water contains valuable information on the origin, migration, and geochemical evolution of fluids in sedimentary basins. Jiuquan Basin is one of the richest oil basins in China and holds large potential for future tight oil exploration. We use a wide range of geochemical and isotopic tracers to evaluate the origin and reconstruct the migration of oilfield water across Jiuquan Basin, including major (Ca, Mg, Na, K, NH4, Cl, SO4, Br, HCO3) and minor (B, Li, Ba, Sr, Rb) elements, water isotopes (δ18O, δ2H), the isotopes of carbon (δ13C-DIC), boron, (δ11B), and strontium (87Sr/86Sr). We show that the oilfield water was co-generated with the original hydrocarbons in the deep Qingxi sub-basin in the western part of the basin, and were derived from blending of two distinctive sources (1) deep-source brine that originated from relicts of evaporated seawater; and (2) geothermal water that underwent intensive water-rock interactions with the Lower Cretaceous Xiagou Formation, characterized by high DIC, Li+ , B, and SO4 concentrations and distinctive δ18O, δ13C-DIC, and 87Sr/86Sr, which are consistent with the composition of the source rocks of Xiagou Formation. The distinctive geothermal signature was detected in the Yaerxia oilfield water in the eastern side of the Qingxi sub-basin, suggesting eastward co-migration of the geothermal water and crude oil to the shallow geological trap. Further eastward migration of the saline formation water into less saline environment in the central (Laojunmiao) and eastern (Shiyougou) fields caused base-exchange reactions, adsorption, and sulfate reduction that resulted in a progressive reduction in the overall salinity, Na+, NH4+, Li+, B, SO42−, and 87Sr/86Sr, coupled with increasing Ca2+, Mg2+, Sr2+, and δ11B. Later dilution of Laojunmiao oil field caused B desorption, oxidation of organic matter, and secondary methanogenesis. The integration of multiple geochemical tracers provides systematic geochemical criteria's for reconstructing the origin and evolution of the oilfield water in Jiuquan Basin and the ability to distinguish between the original composition and secondary modification of the geochemistry of the oilfield water.