Light hydrocarbons composition and carbon isotopes unravel the origin of natural gas in Triassic marine strata of Moxi gas field, central Sichuan Basin

Tight gas is the fastest developing unconventional natural gas resource, becoming the principal part for gas reserves and production growth in China. The Sulige gas field is the largest gas field and also the typical low porosity and low permeability tight sandstone gas field discovered in China, with an annual natural gas output exceeding 300 billion and cumulative output exceeding 290 billion, playing an important role in ensuring national energy provision, helping China’s energy transformation, and promoting green, low-carbon, environmental protection and high-quality development. Based on sample collection and laboratory analysis, natural gas compositions including hydrocarbons, non-hydrocarbons, light hydrocarbons, and noble gases of the Sulige gas field are systematically analyzed, their genetic identifications are identified, and finally gas source originations and contribution proportions are comprehensively discussed from the perspectives of noble gases and hydrocarbon gases. The main achievements are as follows: 1) natural gases in the Sulige gas field are mainly alkane gases, with high methane content, high drying coefficient, low heavy hydrocarbon contents, low non-hydrocarbon gas contents of CO2 and N2, and relatively low noble gas contents. The helium content is relative 2 order of magnitude higher than the atmospheric value, while neon, argon, krypton, and xenon are relatively about 1–2 orders of magnitudes lower than the atmospheric values. 2) The carbon and hydrogen isotopes of alkanes are generally positive sequence distributions with some part inversion. The 3He/4He values are mainly distributed in magnitude of 10−8, the 40Ar/36Ar is ranged from 506 to 1940, the 129Xe is relative loss, and the 132Xe is relative surplus. 3) Natural gases in the Sulige gas field are typical coal-formed gases generated from a humic organic mother material with maturity from high mature to over mature according to C7 and C8 light hydrocarbons and alkane carbon isotopes. Noble gases are typical crustal genesis, mainly originating from the radioactive decay of crustal source materials. 4) The gas source correlations of noble gases and alkane gases and their quantitative evaluations on source contributions show that natural gases in the Sulige gas field are originated from Carboniferous-Permian coal measure source rocks in Ordos Basin, mainly contributed by coals and supplemented by mudstones, accounting for 55–60% and 40–45%, respectively.

To understand the effect of thermochemical sulphate reduction (TSR) on the stable carbon isotopes of light hydrocarbons (LHs) associated with natural gas, 15 gases with varying H2S content from Ordovician reservoir of the Tazhong gas field (TZ-I) in Tarim Basin and Triassic Leikoupo reservoir of the Zhongba gas field (ZB) in Sichuan Basin were collected. Based on the data from molecular components and stable carbon isotope ratios of the C1-C4 alongside the individual LHs (C6-C7) in these gases, the origin of natural gas and the effect of TSR on the stable carbon isotope ratio of individual LHs were studied. The δ13C in ethane (<−28‰), LHs (<−26‰) and the composition distribution characteristic of C6-C7 indicated that the gases were oil-associated gases. Moreover, the gas sourness index, defined as H2S/(H2S+∑Cn) demonstrated that the gases from the TZ-I and ZB gas fields were in the early liquid-hydrocarbon-involved and heavy-hydrocarbon-gas-dominated TSR stages, respectively. The comparison of stable carbon isotope ratios of the LHs between the two gas fields revealed that TSR exhibited a complex effect on the carbon isotope values of LHs, but only little effect on 2-methylpentane (2-MP) and 3-methylpentane (3-MP). The δ13C values of benzene (BEN) and toluene (TOL) were -28.3‰ and -29.4‰ in the TZ-I and -27.7‰ and -28.1‰ in the ZB gas field. The stable carbon isotope ratios of BEN and TOL in ZB gas field exhibited more enriched 13C than those in TZ-I gas field, likely driven by TSR. Meanwhile, cycloalkanes, such as methyl cyclopentane (MCP), cyclohexane (CH), and methylcyclohexane (MCH), enriched 13C with TSR process and displayed a greater trend than aromatic compounds, about 2‰. Therefore, the influence of TSR on the carbon isotopes of individual LHs should be considered while using the stable carbon isotope ratio of cycloalkanes, BEN, and TOL to identify the genetic type and source of marine natural gas, especially at the cross plot (δ13C = −24‰) of coal-derived gas and oil-associated gas.

Geochemistry and origin of continental natural gas in the western Sichuan basin, China

The origin of natural gas and the hydrocarbon charging history of the Yulin gas field in the Ordos Basin, China

Abstract: The Junggar basin is sandwiched between the Siberian plate, the Kazakhstan plate and the Tarim plate, and is an important part of the Central Asian orogenic belt. Based on the comprehensive analysis of the characteristics of the natural gas composition，carbon isotope, light hydrocarbons and source rocks in the eastern belt around Penyijingxi Sag, Junggar Basin, i.e., our studied area, the genesis and origins of natural gas in this area are discussed. The natural gases in the eastern belt around Penyijingxi Sag, are dominated by alkane gases, and have relatively low contents of heavy hydrocarbons and non-hydrocarbons. Methane is dominant in alkane gas, with volume fraction varies from 70.36% to 93.34%. In non-hydrocarbon gas, the volume fraction of nitrogen varies from 0.69% to 11.95%, and the volume fraction of carbon dioxide varies from 0 to 1.49%. The values of δ13methane(C1), δ13ethane(C2), δ13propane(C3) and δ13butane(C4) of natural gas are in the ranges from −45.57‰ to −31.19‰, −31.69‰ to −24.66‰, −28.76‰ to −23.56‰, −27.96‰ to −23.64‰, respectively. The overall carbon isotopic composition of the alkanes shows a trend as δ13C1 < δ13C2 < δ13C3 < δ13C4, and all δ13C1 values are ≤ -30‰, which are typical of gases of organic origin. The methane and ethane isotopic compositions and the characteristics of light hydrocarbons show that the natural gases in the studied area are dominated by coal-type gas and contain a small amount of oil-type gas. Specifically, the coal-type gas is from the mature to highly mature source rocks of the Lower Urho Formation, and the oil-type gas is from the mature to highly mature source rocks of the Fengcheng Formation. Analysis of gas migration parameters show that, while there was no significant lateral migration of natural gas in the studied area, natural gases once migrated vertically and resulted in the mixing of oil- and coal-type gases as well as the mixing of alkane gases of the same genetic type formed at different stages, which should be the cause of observed reversed carbon isotopic series. The diffusion and migration of carboniferous oil and gas after reservoir formation have led to differences in gas geochemical characteristics among gas wells in this area, which may provide important information for oil and gas exploration in the central Junggar Basin.Keywords: Junggar Basin; geochemistry; natural gas genesis; carbon isotopes; light hydrocarbons

The geochemical characteristics and origin of Ordovician ultra-deep natural gas in the North Shuntuoguole area, Tarim Basin, NW China

Geochemical characteristics of natural gases in the Upper Triassic Xujiahe Formation in the southern Sichuan Basin, SW China

Sichuan Basin is the oil and gas bearing basin that is discovered in China, with the most hydrogen sulfide gas pool and the greatest reserve. In this article, gas pools with high hydrogen sulfide in Sichuan Basin were taken as the object of study; the geochemistry characteristic of natural gas was systematically studied, and the origin of natural gas with hydrogen sulfide in Sichuan Basin was discussed. The hydrogen sulfide content in the hydrogen sulfide gas pools of Sichuan Basin is 0.6%–14.5%; the sulfur isotope values of hydrogen sulfide gas have a close relationship with the sulfur isotope values of anhydrite in the same reservoir, and the sulfur isotope fractionation between the anhydrite and the H2S is 4‰–14‰, showing that the H2S in natural gas and the anhydrite in reservoir have the same sulfur source, the H2S in natural gas is the product of TSR reaction. The relationship among the hydrogen sulfide content in the gas pool, the hydrogen isotope of methane and the sulphur isotope of hydrogen sulfide is not obvious; but the high hydrogen sulfide content corresponds to the heavier ethane carbon isotope composition, and the carbon isotope of carbon dioxide is lower than −10‰, which belongs to the organic origin and has further verified the fact of TSR occurrence in the reservoir. There is the phenomenon of carbon isotopic reversal of methane and ethane in some areas of the Huanglongchang gas field, Wubaiti gas field and Puguang gas field in the Northeast Sichuan as well as Weiyuan gas field in South Sichuan, which is mainly caused by the later period of cracking-based gas-production process of the crude oil. TSR has less influence on these gas pools.

Constraints of molecular and stable isotopic compositions on the origin of natural gas from Middle Triassic reservoirs in the Chuanxi large gas field, Sichuan Basin, SW China

In order to have a better understanding of the geochemical characteristics of gases from deep depths, gases from the clastic sandstone reservoirs in the Dabei and Keshen gas fields in the Kuqa depression, Tarim Basin, and gases from the marine carbonate reservoirs (Ordovician and Cambrian) in the craton area of Tarim Basin and Sichuan Basin (Yuanba, Longgang, Puguang gas fields) are investigated based on the molecular composition, stable carbon and hydrogen isotopes. Deep gas, either from the clastic sandstone reservoirs or from the marine carbonate reservoirs, is dominated by alkane gas. Gases from Kuqa depression and Sichuan Basin are dry gas, with high gas dryness coefficient, 0.976 and 0.999, respectively. Deep gas from the craton area in Tarim Basin includes both dry and wet gases. N2 and CO2 are the common non-hydrocarbon components in the deep gas. Gases from the continental sandstone reservoirs have no H2S, while gases from the marine carbonate reservoirs often have H2S. The relatively high δ13C2 value in the Kuqa depression indicates the gas was generated from humic type III kerogen, while the relatively low δ13C2 value in the craton area of Tarim Basin indicates most of the gas was generated from the marine sapropelic organic matter. Deep gas in Sichuan Basin, which has medium δ13C2 value, was generated from both humic type III and sapropelic type II organic matter. Carbon isotopic anomaly such as partial carbon isotopic reversal or relatively heavy carbon isotope is common in the deep gas, which is caused by secondary alteration. Gases from the Dabei gas field have a mean δ2H1 value of –156‰, while gases from the craton area of Tarim Basin, and Yuanba and Puguang gas fields in Sichuan Basin have relatively heavier δ2H1 value, i.e., average at −130 and −122‰, respectively. The abnormally heavier δ2H1 value in Dabei gas field is due to the high thermal maturity and possible saline depositional environment of the source rocks. This study performed a comprehensive comparison of the geochemical characteristics of the deep gases with different origins, which may provide a hint for future exploration of deep gas in the world.

Geochemical characteristics and origin of natural gas from Wufeng-Longmaxi shales of the Fuling gas field, Sichuan Basin (China)

Tracing of natural gas migration by light hydrocarbons: A case study of the Dongsheng gas field in the Ordos Basin, NW China

Gas source of the Middle Jurassic Shaximiao Formation in the Zhongjiang large gas field of Western Sichuan Depression: Constraints from geochemical characteristics of light hydrocarbons

The study of noble gases and reactive gases (i.e., CO2, N2 and CH4) in crustal reservoirs can help better understand the origin, migration and accumulation processes of crustal fluids in the subsurface environment, which provide further insights into fluid dynamics underground. This PhD thesis develops noble gas isotopes as geochemical tools in crustal fluid studies. Study sites have been selected to cover different types of geofluids, including geothermal fluids in Krafla, Iceland and natural gases in the Sichuan Basin, China. Following a short introduction to the research background, objectives and thesis layout are presented in Chapter 1 and Chapter 2 gives a literature review on the application of noble gases as powerful geochemical tools in hydrothermal and hydrocarbon systems. Chapter 3 describes a noble gas extraction and purification system, interfaced to a multi-collector NGX noble gas mass spectrometer (Isotopx), which is constructed particularly for this study. The detailed description of each section in sample preparation system, as well as gas sampling, extraction, purification and separation protocols is provided. Temperatures of 50 K and 95 K can be used as the optimal releasing temperatures for He and Ne on the cryotrap in sample prepline. The temperature of 210 K is tested to be the optimal temperature for releasing Ar and Kr but keeping Xe being trapped onto the charcoal trap in the prepline. Chapter 4 characterizes noble gas and stable isotope data of hydrothermal fluid system in Krafla, Iceland. Stable isotope results indicate that CO2 in the samples is considered to be magmatic in origin, with δ13C (CO2) ranging between -7.99 and -3.86 ‰. Modelling results show that processes of boiling and steam separation have occurred during the circulation of geothermal fluids in the shallow crust in the Krafla field. Air addition, possibly introduced by groundwater re-injection, has had a significant effect on the geochemical signatures of Krafla geothermal fluids as well. Chapter 5 focuses on using noble gas isotopes to trace the interaction between hydrocarbon and groundwater systems in the central Sichuan Basin, China. Twenty-six natural gas samples were collected from the Anyue and Weiyuan gas fields in the central Sichuan basin, China for stable isotope and noble gas isotope determination. Noble gases together with stable carbon isotopic parameter (Δδ 13C1-2) can be an effective tool to differentiate the occurrence of two distinct genetic groups of natural gases in the central Sichuan basin. Elemental fractionations of noble gases in samples from the western area (coal-derived gas) can be well explained by solubility-controlled Rayleigh fractionation model with relatively less effect of mass-dependent fractionation process. However, noble gas elemental and isotopic compositions in samples from the eastern area can be interpreted as a mixing effect of solubility-controlled and mass-dependent fractionation processes. This thesis further develops noble gases as a versatile tool in the study of crustal fluids. It is the first-time noble gas isotopes are used in understanding fluid dynamics in natural gas reservoirs in China. This work has demonstrated a huge potential to apply the noble gas tool in basin studies in China and Asian geology in general.

Carbon isotope reversal and its relationship with natural gas origins in the Jingbian gas field, Ordos Basin, China