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Helium gas is a scarce but important strategic resource, which is usually associated with natural gas. Presently, only one extra-large helium-rich gas field has been found in China: the Hetianhe Gas Field in the Tarim Basin. This paper reports a new example, the Dongsheng Gas Field (DGF) in the Ordos Basin. In this study, 92 natural gas samples from the DGF were analyzed for helium content and isotope composition using isotope mass spectrometry. The natural gas samples were found to have an average helium content of 0.133%, with 65 (70.7%) of the samples having a helium content of 0.1% or more. Based on the proven natural gas reserves of the DGF, the proven geological helium reserves were calculated to be 1.96×108 m3, suggesting that it represents the first extra-large helium-rich natural gas reservoir to be hosted in tight sandstone in China. The 3He/4He ratios of 5 natural gas samples from the DGF are within the range of 3.03×10−8−3.44×10−8. Therefore, the helium in the gas field is thought to be of typical crustal origin and to have formed in the granitic basement that is rich in uranium and thorium. The accumulation of helium-rich natural gas was controlled by regional tectonic activities. Activity along the fault connecting the reservoir with the basement caused release of the helium gas, which entered the overlying strata along the fault and accumulated with conventional hydrocarbon gas. Based on the structural background and the distribution of helium source rocks in the Ordos Basin, the main helium source rocks with high exploration potential are located in deep strata within the north and middle parts of the basin. [ABSTRACT FROM AUTHOR]

• The TTG, sanukitoids, and plagioclase-amphibolite gneiss were formed at ∼ 2.51–2.46 Ga. • The NOB might be in a transition stage from a compressional regime to an extensional regime during ∼ 2.51–2.46 Ga. • The NOB is in a long-term active continental margin setting during ∼ 2.51–1.95 Ga. The most poorly exposed and least understood in the North China Craton (NCC) lies largely hidden beneath the Ordos Basin, severely limiting the cognition of the Precambrian tectonic evolution in the western NCC and the tectonic division. Diverse orthogneiss samples collected from four boreholes drilled into the basement of the north side of Hangjinqi Fault Zone (HFZ) in the North Ordos Basin (NOB) are investigated by integrating zircon U-Pb dating, Hf-Nd isotopes, and whole-rock geochemical analyses. The TTG gneiss, quartz-monzonite gneiss, granodiorite gneiss, and amphibolite were emplaced at ca. 2.51–2.46 Ga, of which the amphibolite also recorded the metamorphic age of ca. 1.95 Ga. The TTG gneisses display low MgO, Mg#, Cr, and Ni, high HREE and Rb/Sr, enrichment of LILEs and depletion of Nb, Ta, and Ti, and positive εNd(t) (2.04–4.06) and εHf(t) (3.04–7.59) values, suggesting that they were mainly originated from partial melting of the tectonically thickened mafic island-arc crust. In contrast, the quartz monzonite gneisses and granodiorite gneisses show the geochemical features of Archean sanukitoids with higher MgO, Mg#, Cr, and Ni, and similar ε Nd (t) (1.34–3.82) and ε Hf (t) values (3.94–7.32), suggesting that they were derived from the partial melting of the mantle metasomatized by fluids and melts of the subduction. The amphibolites exhibit a mixed characteristic of E-MORB and arc-like geochemical affinities resembling the Mariana back-arc basin, which was derived from the partial melting of spinel lherzolite. The arc and arc-back lithological assemblages suggest that the north margin of the NOB might be in a geodynamic transition period from a compressional regime correlated with plate subduction to an extensional regime correlated with slab rollback during ca. 2.51–2.46 Ga. Combined with previous analytical results, the northern margin of the NOB was most likely in a long-term active continental margin setting during 2.51–1.95 Ga and involved in the collision event between the Yinshan Block and Ordos Block at ca. 1.95 Ga. [ABSTRACT FROM AUTHOR]

This study analyzes the structural evolution of the Hangjinqi area in the northern Ordos Basin, based on seismic sections and exploration well data. This area shows excellent examples of the relationship between tectonics and sedimentation. Three tectonic sequences separated by four erosional surfaces have been recognized overlying a paleoslope surface (T9) in this area: (1) The Late Carboniferous-Permian and Triassic tectonic sequence (TS-1); sedimentation of the lowest part of this succession is strongly controlled by the Hangjinqi Fault Zone (HFZ), whereas the rest of the sequence indicates much less fault control; (2) the Middle Jurassic tectonic sequence (TS-2); (3) the Late Early Cretaceous tectonic sequence (TS-3); and the TS-2 and TS-3 sequences are more extensive than the TS-1 sequence throughout the Hangjinqi area. The structural framework displayed by the TS-2 sequence is very different from that displayed by the TS-3 sequence. More specifically, the general structure beneath the unconformity (T3) between the Late Jurassic strata and the Early Cretaceous strata consists of an E–W trending transtensional fault system. This resulted from a multi-stage dextral phase, which was caused by oblique subduction of the Paleo-Pacific Plate under the Eurasian Plate and dextral movement of the HFZ, and a subsequent inversion event, which caused significant reverse movement along previous transtensional structures. The study area, which underlies the unconformity (T2) between the Late Jurassic strata and the Early Cretaceous strata, has been deformed since the Late Cretaceous, when tectonic inversion occurred, possibly due to changes in the strike-slip motion, from dextral-to sinistral, of the HFZ. Late Cretaceous erosion was responsible for an important and unequal denudation. The timing of the transition from extension to uplift cannot be determined due to continuous erosion, but this uplift finally marked the end of the structural evolution of the Hanjinqi area and even the Ordos Basin. [ABSTRACT FROM AUTHOR]

The genetic types, source precursors and filling pattern of natural gas in the Upper Carboniferous Taiyuan Formation, Lower Permian Shanxi Formation and Lower Shihezi Formation gas reservoirs of Daniudi gas field were investigated using chemical composition as well as carbon and hydrogen isotopic compositions. Geochemical analysis of natural gases in 25 representative wells shows that natural gas in the Daniudi gas field is composed predominantly of hydrocarbons with a dryness coefficient of 0.884–0.978. The carbon isotopic values of ethane and propane are higher than −28‰ and −25‰, respectively, and the hydrogen isotopic values of methane are lower than −180‰, indicating that natural gas in the Daniudi field is a typical coal-type gas, derived mainly from humic organic matter in the transitional facies of the Carboniferous–Permian age. Hydrogen isotopic values of CH 4 and H 2 display a good positive correlation, suggesting that both were controlled by thermal maturity. When the mixing of ethane generated from mudstone and coal with the same kerogen type and similar thermal maturity occurred, the carbon isotopic values of ethane barely reflect the thermal maturity. Although the fractionation of hydrogen isotopes of ethane is significantly higher than that of carbon, hydrogen isotopic values of ethane in natural gas reservoirs evidently are not related to thermal maturity. The Daniudi natural gas reservoirs represent both self-sourced and near-source accumulations. The natural gas accumulations in the Late Triassic–Early Jurassic periods are mainly of the self-sourced type, while accumulations in the Late Jurassic–Early Cretaceous period comprise both self-sourced and near-source patterns, and the natural gas reservoirs formed after the Late Cretaceous period are mainly of the near-source type. [ABSTRACT FROM AUTHOR]