The genetic identification of different types of natural gas is notably important for assessment of its sources and exploration potential. The chemical and isotopic (C and H, in particular) ...compositions of natural gas vary significantly due to the complexity of its generation, migration, and accumulation processes. The “coal-type” gas generated from humic matter is generally enriched in 13C as compared to "oil-type" gas generated from sapropelic organic matter. However, gas originating from fresh-brackish water environments is depleted in 13C whereas gas from saline environments is enriched in 13C. Notwithstanding organic precusors and sedimentary environments, both isotope compositions of alkanes tend to become enriched both in 13C and 2H with prograde thermal evolution. Therefore, in addition to thermal maturity, source material is the major controlling factor of carbon isotope compositions, whereas sedimentary environment is predominant in governing hydrogen isotopes. Secondary processes, including thermochemical sulfate reduction (TSR) and diffusion, result in an enrichment of the gases in 13C and 2H due to mass-dependent kinetic isotope effect. Microbial degradation causes a decrease in propane content and an enrichment in 12C and 2H of the residual propane. The abiogenic gases may include methane from deep mantle and high molecular weight hydrocarbons through Fischer-Tropsch type (FTT) synthesis. Methane of mantle origin possesses a narrow range of isotope compositions, although it is still a tall task to determine the exact values. In contrast, isotopes of alkane gases synthesized from FTT processes are in a wide range. In sedimentary basins, the mixing of gases from multiple sources and/or through different secondary processes may pose a challenage to identification of their origins. The detailed assessment is provided here with case studies from major oil and gas basins in China. This review provides identification of misconceptions in genetic types of natural gas using carbon and hydrogen isotopes of alkanes, and sheds insights into using isotope geochemistry as an important diagnostic tool for energy exploration as well.
Classification of natural gas and traditional geochemical methods for genetic identification of natural gas are reviewed.The effect of geological background and secondary alteration on δ13C and δ2H of light alkanes is assessed.New diagnostic tools for genetic classification of natural gas are suggested.
In recent years, theoretical geologic research has made important progress in shale gas exploration and development, and new discoveries, such as the Jiaoshiba, Weiyuan and Changning shale gas ...fields, provide geological cases to define key controls on shale gas accumulation in Wufeng Formation-Longmaxi Formation marine shale strata. A mechanism coupling source and seal development for shale gas enrichment is proposed in this study based on detailed research results of isochronous stratigraphic distribution of source rock and seals and source-seal dynamic evolution. Great thickness, wide distribution and high total organic carbon content (TOC) of sources are key controls on the shale gas content, and storage capacity in organic matter-hosted pores, micro-cracks and bedding fissures is intensively developed. The Wufeng Formation and the lower part of the first member of the Longmaxi Formation (WF2-LM4 graptolitic zone) are favorable sources that feature a slow deposition rate, well graptolitic zone, favorable organic organism type and high TOC. The organic matter type is benefit to the development of organic-matter pores because the planktonic algae tends to generate and expulse large amounts of hydrocarbon and form many organic-matter pores. Both high biogenic quartz and high TOC ensure development of abundant organic matter-hosted pores and result in a three-dimensional network of interconnected organic matter pores that allows shale gas storage and flow. The seal can be divided into direct and indirect seals. The direct seal refers to the middle and upper parts of the first member of the Longmaxi Formation (LM5 and higher graptolitic zone) that are characterized by low TOC and low porosity. The indirect seal refers to the regionally distributed Lower-Middle Triassic gypsum layer(s) and mudstone that maintain regional geopressure systems. The shale gas fields of Jiaoshiba, Fushun-Yongchuan and Changning have high geopressure coefficient factors (1.5, 2.0 and 1.5, respectively). In the absence of the Lower-Middle Triassic gypsum layer(s) and mudstone regional seals, the high-pressure system was destroyed and resulted in low-normal geopressure shale gas reservoirs, for instance, the Pengshui and Zhaotong wells. The static match and dynamic match of source and seal constrain the location of shale gas reservoirs and the degree of shale gas enrichment. The model of a mechanism coupling source and seal for shale gas enrichment provides new ideas and methods for the assessment of shale gas. This model promotes the evaluation of shale gas from the static parameters approach to the source and seal dynamic approach. In the Sichuan Basin and its peripheral areas, the promising areas for shale gas accumulation are those with favorable sources in the WF2-LM4 graptolitic zone and the presence of the Lower-Middle Triassic gypsum layer(s) and mudstone regional seal.
•The mechanism of source and seal coupling for shale gas enrichment was analyzed.•The WF2-LM4 graptolitic zone is a favorable source for shale gas enrichment.•Spatiotemporal evolution of direct and indirect seals is critical for shale gas reservoirs.•Static-dynamic match of source and seal controls the location and enrichment of shale gas.
As two essential controls on the formation of organic matter (OM)-enriched shales, both paleoproductivity and preservation models remain controversial for the Wufeng and Longmaxi shales in the ...Sichuan Basin, southwest China. This study provides geochemical parameters as proxies for the paleoproductivity characteristics (Ba, P/Al, and (Ni+Cu)/Al), bottom water redox status (U/Th, V/(V+Ni), V/Cr, and Mo), and terrigenous clastic flux (Th and Al) of the Wufeng–Longmaxi Formations in the Sichuan Basin, a back-deep basin in a foreland basin system in southwest China. Combining these data with total organic carbon (TOC) content and sedimentation rate data obtained from graptolite zones allowed us to discuss the validity of these popular indices and to identify the main factors that controlled OM enrichment in the formations. TOC contents exceed 4 wt% in the upper Wufeng Formation to the bottom part of the Longmaxi Formation (LM1-3), which are the most OM-rich intervals. The average P/Al ratio for well JY2 in the Jiaoshiba area is 0.009, which is lower than those for well YY1 (0.028) in the Yongchuan area and well DY1 (0.02) in the Dingshan area, indicating the lowest productivity levels were in the Jiaoshiba area. Based on the redox proxies, the samples from well DY1 reflect less reducing bottom water conditions than those from wells JY2 and YY1. The Th and Al indices increase upwards, indicating that the terrigenous clastic input increased, and that those in the Yongchuan area were lower than those in the Jiaoshiba and Dingshan areas. The sedimentation rate was too low to dilute the OM in the Sichuan Basin, especially during the depositional time of the OM-rich intervals, such that the redox status of the water became more critical. Given the more strongly reducing conditions of the sedimentary environment and the high thermal maturity of the OM in this formation, the Ba concentrations appeared anomalously low and became inaccurate as a paleoproductivity index, as well as the validity of V, V/Cr, and V/(V+Ni) anomalously decreased as redox indices.
•Geochemical study on the Wufeng–Longmaxi Formations in the Sichuan Basin was conducted.•Geochemical data are provided as proxies for paleoproductivity characteristics.•Total organic carbon and sedimentation rates for graptolite zones were obtained.•Validities of Ba and V are discussed.
The Eocene is an enigmatic and interesting period due to the critical thermal events that occurred during the gradual global cooling, which have been demonstrated in the marine realm but remain ...poorly reported in terrestrial basins. In this study, a continuous ~404-m interval of the Middle Eocene terrestrial succession in Bohai Bay Basin, eastern China, was recovered from Well Fanye-1 in the Shengli Oilfield. This interval provides a unique record that can be used to better clarify the terrestrial sedimentary responses to astronomically forced climate changes, especially the Middle Eocene Climatic Optimum (MECO). Here, a high-resolution cyclostratigraphic framework from the lower third member (Es3l) to the upper fourth chun member (Es4cu) of the Shahejie Formation (Middle Eocene) was constructed by using magnetic susceptibility (MS) as a palaeoenvironmental proxy, in association with new paleomagnetic data. Power spectral analysis of the MS series reveal that the wavelength ratios of stratigraphic cycles are ~43.1–25:15.3–7.5:4.5–3:2.2–1.4, consistent with the period ratios of astronomical cycles. Moreover, a high-resolution astronomical time scale (ATS) for Well Fanye-1 was established by tuning the MS curves to the astronomical solution in combination with the magnetic stratigraphic framework. The method for establishing a high-resolution ATS with magnetostratigraphy and cyclostratigraphy was well applied in Well Fanye-1, making it possible to recalculate the ages in the Jiyang Depression. Finally, the terrestrial record across the MECO event was constructed and characterized based on the widespread “interbedded” argillaceous limestone. The resulting record is consistent with the time frame based on marine records and highlights the apparently primary role of the MECO event in shaping Asian paleoenvironments.
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•A cyclostratigraphic analysis of a middle Eocene lacustrine succession from the Bohai Bay Basin in eastern China was accomplished.•The method to establish an ATS with magnetostratigraphy and cyclostratigraphy is well applied in Well Fanye-1.•The terrestrial record across the MECO event was constructed and characterized based on the “interbedded” argillaceous limestone.
•Volcanic SO2 and H2S caused seawater entering into euxinic environment in the Sinian peroid.•Positive relationships between TOC and element concentrations suggest contribution of Volcanic ...activities.•Parameters of V/Cr, Th/U indicate high TOC shales were developed in anoxic condition.•Volcanic/hydrothermal activities enhanced both productivity and preservation of organic matters.
Volcanic eruptions and related submarine hydrothermal activities were very extensive during the Precambrian periods in the Yangtze craton in southern China. How these geological events affected redox chemistry changes linked to the enhancement of mudstone/shale hydrocarbon source rocks in ancient strata is an important concern for oil and gas exploration in the Precambrian strata. Samples were collected from two Sinian (Ediacaran) outcrops in the northern Sichuan Basin, the Zhaiziyan and Yanhe outcrops, which revealed Doushantuo Formation (Z1ds) shales approximately 30.5 m in thickness, and shales in the third member of the Deying Formation (Z2dn3) approximately 30.0 m in thickness. TOC contents of the black carbonaceous shales in the Doushantuo Formation of the Zhaiziyan outcrop are mostly greater than 5.0% (up to 12.9%). Positive correlations were observed between TOC contents and trace and rare earth element concentrations in the shales: samples with high concentrations of trace elements such as V, Ni, Cu, Zr, Rb, and Pb and total rare earth element (∑REE) concentrations had relatively high TOC contents. The shales are rich in volcanic ash interlayers and thin siliceous layers and have positive Eu anomalies, indicating that volcanic activities and related hydrothermal activities significantly impacted the development of black shale source rocks. In the rifting background of the Sinian cratonal marginal, extensive volcanic activities brought rich nutrients to the seawater, which may have promoted the breeding of cyanobacteria and multicellular algae. At the same time, the volcanic activities released large amounts of H2S and SO2, which could cause the seawater to become a euxinic environment in which the bottom seawater was sulfurized and anoxic. Massive amounts of organic matters formed in the surface seawater entered the anoxic bottom seawater and were well preserved in the shale sediments. Therefore, under the influence of volcanic activities, ancient Sinian organic-rich high-quality source rocks were widely developed and had a large hydrocarbon supply potential for the Sinian dolomite reservoirs.
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•The Jurassic and Upper Triassic coal-type gas is from primary cracking of kerogen.•Natural gas in the Leikoupo Formation is free of H2S and mainly coal-type gas.•H2S-bearing gas in ...marine strata is secondary oil-cracking gas and altered by TSR.•The Permian and Lower Triassic gas is derived from the Upper Permian source rocks.
The chemical compositions and stable carbon and hydrogen isotopic compositions of natural gases in different strata of the Yuanba gas field in the Sichuan Basin were analyzed to investigate their genetic types and source. Natural gases from the Permian Changxing and Triassic Feixianguan formations have dryness coefficients higher than 0.995 and contain H2S in most gas samples, having a positive correlation between the H2S and CO2 contents. The identification of gas origin and gas-source correlation from this study indicate that natural gas in the Ziliujing Formation and most gas samples from the Xujiahe Formation are coal-type gas produced from primary cracking of kerogen, and they are self-sourced reservoirs in relatively independent systems. The natural gases in the 1st and 2nd members of the Xujiahe Formation were mixed with minor amounts of oil-type gas from argillaceous carbonate rocks in the Leikoupo Formation. In addition to the self-accumulated oil-type gas in the Leikoupo Formation, natural gas mainly came from the overlying or lateral source rocks in the Xujiahe Formation. The H2S-bearing natural gases in the Changxing and Feixianguan formations were produced by the cracking of crude oil from source rocks in the underlying Wujiaping Formation, with partial contributions from certain local source rocks in the Permian Dalong Formation. The H2S-bearing natural gas has undergone the thermochemical sulfate reduction (TSR) which were dominated by heavy hydrocarbons. The high δ13C values of CO2 in these gas reservoirs mainly result from the interaction between acidic fluids and carbonate rocks in the reservoirs.
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•Lacustrine shale has strong heterogeneity in terms of its mineral composition.•Seven types of lithofacies developed in the Jiyang Depression.•Three types of lithofacies assemblages ...are present in the study area.•The Type C lithofacies assemblage is the most favorable exploration target.
There are multiple sets of lacustrine source rocks in the Paleogene of the Jiyang Depression, Bohai Bay Basin, Eastern China, which are main targets in the terrestrial shale oil exploration. Compared with marine shales, lacustrine fine-grained rocks are more complex in their mineralogical compositions, sedimentary structures and vertical stacking patterns. Therefore, it is of great theoretical and practical significance to carry out petrological characterization and lithofacies classification in lacustrine fine-grained rocks. Based on observations in cores and thin sections as well as elemental and organic geochemical analyses, this study systematically analyzes petrological characteristics of Eocene lacustrine fine-grained sedimentary rocks in the Jiyang Depression from perspectives of mineralogical compositions, sedimentary structures and organic matter abundance. Three end members (carbonate minerals, clay minerals and terrigenous clastic minerals), together with organic matter abundance and sedimentary structure, are employed to establish a lithofacies classification scheme that is suitable for fine-grained rocks in the Jiyang Depression, Eastern China. A total of 7 lithofacies types are recognized, such as the organic matter-rich laminated calcareous mudstone lithofacies and the moderately organic matter-rich layered calcareous mudstone lithofacies. Through analysis of lithofacies from the bottom to the top of the drilling well, sixth-order sequence units are found to have good correspondence to lithofacies assemblages. Based on consideration of magnetic susceptibility, mineral content, and geochemical proxies, these sixth-order sequence units are divided into three types of lithofacies assemblages, namely the Type A lithofacies assemblage corresponding to the dominantly rising base level, Type B lithofacies assemblage corresponding to the dominantly falling base level, and Type C lithofacies assemblage with the isostatic base level. Among them, the Type C lithofacies assemblage is the most favorable for shale oil exploration and development. Results of this study are expected to not only deepen understanding of lithofacies and its genesis and origin of lacustrine fine-grained rocks, but also provide a theoretical basis for shale oil exploration and development.
•Stromatolites and thrombolites constitute major proportion of ancient Ediacaran gas reservoirs.•Buildup of early reservoir spaces was dominated by cavities in microbialite framework and karst ...vugs.•Petroleum charge and pyrobitumen coat were beneficial for porosity preservation in later burial.•Acidic fluid conditions related to presence of H2S and CO2 linked to petroleum crack and TSR prevent cementation.
The ancient Ediacaran Dengying Formation consists of microbialites with unusually high porosity that represent a large proportion of the Weiyuan-Anyue gas reservoirs in southwestern China’s Sichuan Basin. However, a clear model for the development and preservation of these microbialite reservoirs has yet not been determined. We used samples from outcrops and borehole cores to characterize these microbialites and propose a model for early development and subsequent reservation of their porosity. Primary porosity in the microbialite framework was mainly represented by fenestrate vugs and elongated cavities between microbial layers that were formed during initial sedimentation as well as secondary karst vugs and caves formed by meteoric karst resulting from subsequent tectonic uplift; these features represent the early development of the microbialite reservoirs. Subsequent filling of vugs by early-stage radial dolomite (RD) and late-stage saddle dolomite (SD) occurred during the deep burial stage, with δ18O and homogenization temperature patterns suggesting continuous cementation. However, most vugs were not cemented due to combination of (1) a rigid dolomitized framework, (2) a closed burial system restricting the flow of mineralized fluid, (3) the presence of petroleum or pyrobitumen, and (4) acidic fluid conditions related to high concentrations of H2S and CO2. The third and fourth factors were linked to presence of hydrocarbons and had made big contribution to porosity retention during deep burial diagenetic stage.
Global climate changes urge prompt energy transition for less carbon emissions, from traditional fossil fuels to renewable and sustainable clean energy. However, in reality, the world's energy ...majority cannot make U-turn immediately to renewables or clean energy due to the immature technology readiness, insufficient resource availability and unstable energy supply. In the next few decades, the fossil fuels, particularly oil and gas, will continue acting as the primary energy sector. Thus, instead of absolutely abandoning fossil fuel and seeking for impractical carbon mitigation technologies, to decarbonise the oil and gas will be definitely feasible and contribute more to net-zero transitions. This study, initially put eyes on the oil and gas decarbonization, critically reviewing the oil and gas resources, technologies, policies, and their futures toward net-zero. Basically, the status of oil and gas resources from different global regions, including the details of reserves, productions, consumptions, are summarized and analyzed. Moreover, the oil and gas technologies are categorized as gas, thermal and non-thermal, new recovery methods, each of which is specifically discussed in the applicable reservoir, mechanism, features and examples. Then, the global carbon emissions are reviewed in perspectives of emissions from fuel types and world regions as well as mitigations policies. Accordingly, the carbon mitigation approaches, specially in the oil and gas industry, are collected and listed from enterprise managements and technology renovations. Lastly, based on all the information and analyses and assisted with IEA energy outlook report, we provide a potential pathway for the oil and gas towards carbon neutral. This paper provides comprehensive overview on the oil and gas pathway to net-zero, which will not only technically guide the oil and gas decarbonisations, also be of interest to wide-range readers who are not experts but intend to understand the energy transitions.
•The first comprehensive overview on the oil and gas pathway to net-zero.•Oil and gas resources, technologies, policies, and their futures toward net-zero initially reviewed.•Solid qualitative and quantitative analyses fill the knowledge gap of oil and gas decarbonization.
•Hydrothermal activity and TSR co-exist in the Sinian Dengying Formation reservoir.•Hydrothermal fluids provide necessary high temperature condition for TSR to proceed.•Coupled alterations of ...hydrothermal activity and TSR enhanced porosity development and preservation.
To better understand the fluid activities and their roles in the formation of petroleum reservoirs, dolomite was thoroughly studied in the ancient Sinian Dengying Formation in Sichuan Basin, southern China. The high homogenization temperatures (above 190°C) of fluid inclusions, positive Eu anomaly, and O and Sr isotopic values of coarse crystalline pore-filling dolomite (FD) in dissolution pore spaces have suggested that the FDs are of hydrothermal origin. There are also other minerals (pyrite), organic compounds (pyrobitumen) and gas components (H2S, CO2 and CH4) in pore spaces. The sulfur isotope compositions of pyrite and H2S, along with the carbon isotopes of CO2 have shown the involvement of thermochemical sulfate reduction (TSR). Hydrothermal activities during burial diagenesis of the Dengying Formation provided high temperature environment necessary for TSR to proceed. The presence of CO2 and H2S in both processes enhanced fluid acidities and consequently porosity and permeability of the Dengying Formation. Emplacement of petroleum and acidic fluids helped to maintain pre-existing reservoir spaces during deep burial stage. Overall, the coupled hydrothermal activities and TSR provided favorable environments to allow for maintaining high quality reservoirs.
