Geological and geochemical characteristics of the Middle–Lower Jurassic shales in the Kuqa Depression,Tarim Basin: an evaluation of shale gas resources

Middle–Lower Jurassic terrigenous shales constitute a set of significant hydrocarbon source rocks in the Kuqa Depression of the Tarim Basin. Until recently, however, most investigations regarding this set of hydrocarbon source rocks have mainly focused on conventional oil and gas reservoirs, and little research has been conducted on the formation conditions of shale gases. This research, which is based on core samples from nine wells in the Kuqa Depression, investigated the geological, geochemical, mineralogical and porosity characteristics of the shales, analysed the geological and geochemical conditions for the formation of shale gases, and evaluated the shale gas resource potential. The results show that the distribution of the Middle–Lower Jurassic shales is broad, with thicknesses reaching up to 300–500 km. The total organic carbon (TOC) content is relatively high, ranging from 0.2 to 13.5 wt% with a mean of 2.7 wt%. The remaining hydrocarbon generative potential is between 0.1 and 22.34 mg/g, with a large range of variation and a mean value of 3.98 mg/g. It is dominated by type III kerogen with the presence of minor type II₁ kerogen. The vitrinite reflectance values range from 0.517 to 1.572%, indicating the shales are in a mature or highly mature stage. The shales are mainly composed of quartz (19–76%), clay (18–68%) and plagioclase (1–10%) with mean contents of 50.36 wt%, 41.42 wt%, and 3.37 wt%, respectively. The pore spaces are completely dominated by primary porosity, secondary porosity and microfractures. The porosity is less than 10% and is mainly between 0.5 and 4%, and the permeability is generally less than 0.1 mD. These results classify the shale as a low-porosity and ultra-low-permeability reservoir. The porosity has no obvious correlation with the brittle or clay mineral contents, but it is significantly positively correlated with the TOC content. The maximum adsorbed gas content is between 0.82 and 8.52 m³/t with a mean of 3.37 m³/t. In general, the shale gas adsorption content increases with increasing the TOC content, especially when the TOC content is greater than 1.0%. The volumetric method, used to calculate the geological resources of the Middle–Lower Jurassic shales in the Kuqa Depression, shows that the geological resources of the Middle and Lower Jurassic shales reach 667.681 and 988.115 × 10⁹ m³, respectively with good conditions for the formation of shale gas and good prospects for shale gas exploration.     

Study on oil–source correlation by analyzing organic geochemistry characteristics: a case study of the Upper Triassic Yanchang Formation in the south of Ordos Basin,China

In the south of the Ordos Basin, the oil source of the Upper Triassic Yanchang Formation is confused all the time, which affects further exploration. In this study, oil sources from the oil layers of Ch6, Ch8 and Ch9 are all analyzed and confirmed. Through their carbon isotope value and biomarkers, characteristics of crude oils from the Yanchang Formation are analyzed. Then, the oil–source relation is discussed, with the source rocks’ features. Finally, the oil–source relation is calculated through cluster analysis. It is believed that the oils from the Yanchang Formation deposit in a similar redox environment, with weak oxidation–weak reduction, and have all entered maturity stage. Ch9 crude oil is more mature than crude oils from Ch6 and Ch8, and has more advanced plants and fewer algae. Gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) analysis show that crude oils from Ch6 and Ch8 may come from Ch7, and Ch9 crude oil may not. Cluster analysis displays that crude oils from Ch6 and Ch8 have closer squared Euclidean distance with Ch7 source rocks than Ch9 crude oil does, indicating crude oils from Ch6 and Ch8 stem from Ch7 source rocks. And Ch9 crude oil has rather close squared Euclidean distance with Ch9 source rocks, illustrating Ch9 crude oil may be from Ch9 source rocks. This research may provide the theoretical basis for the next exploration deploy in the south of Ordos Basin.     

Dissolution characteristics of deep?buried dolostone in the Member 5 of Ordovician Majiagou Formation in southern Ordos Basin

The different types of deep-buried dissolution process in the Member 5 of Ordovician Majiagou Formation in the southern Ordos Basin and its influence on the reservoir properties are studied in this paper. It shows that three types of mechanisms include organic acid fluid, hydrothermal fluid and TSR are identified through studies of core observation, thin section analysis, inclusion temperature, trace elements and rare earth elements. It is found that the dissolution of organic acid fluid causes the stratified dissolution pores, film-like asphaltene and ring-like asphaltene, while hydrothermal fluid causes the non-selective dissolution pores without petrofabric, veins of pyrites, massive pyrites and the association of hypothermal minerals. Four occurrence models of dissolution include, (1) the deep-buried dissolution of low-temperature hydrothermal fluid occurs in Zhiluo Period of Middle Jurassic; (2) the deep-buried dissolution of organic acid fluid started from the late Middle Jurassic followed by the first hydrocarbon injection; (3) the deep-buried dissolution of middle-high temperature hydrothermal fluid occurred in the Dongsheng Period of Early Cretaceous; (4) TSR occurred at the end of Early Cretaceous with the second larger hydrocarbon injection. Both organic acid fluid and hydrothermal fluid can improve the porosity and permeability of reservoir, but the hydrothermal fluid is more effective than that of organic acid fluid.     

Silurian-Devonian and Jurassic Thermal Events in the Ordos Basin, China: Indications from K-Ar Dating on Illites

In the Ordos basin, two distinct thermal events of different ages have been identified for the first time by means of K-Ar dating combined with illite crystallinity analysis. For the Late Triassic and Late Permian samples, the K-Ar ages of the < 0.2μm fractions (159-173 Ma) reflect an illitization age related to the Yanshanian movement and indicate a short thermal event in the Middle Jurassic; the K-Ar ages of the <2 μm fractions (210-308 Ma) are interpreted as mixed ages of detrital material and authigenic illites. The K-Ar ages of both < 0.2μm and < 2μm fractions of a Middle Cambrian sample (368 Ma and 419 Ma) correspond to the ages of the metamorphism and earliest granite intrusion in the northern Caledonian Qinling fold zone (380-420 Ma) and show a thermal event during Silurian-Devonian time.     

Coexistence and inherence of diverse energy resources in the Ordos Basin, China

1 Introduction In recent years rapid economic developmentbadly needs energy resources. Exploration andexploitation of more kinds of energy resources shouldbe intensified to meet this need. According to presentliterature, oil/gas fields, coal fields as we…     

Geochronology,Mineralogy, and Geochemistry of the Tonsteins from the Permo–Carboniferous Benxi Formation,Ordos Basin,North China Craton

Tonstein layers are found worldwide in the Permo–Carboniferous coal-bearing strata. This study investigates the geochronology, mineralogy, and geochemistry of four tonstein samples from the Permo–Carboniferous Benxi Formation, Ordos Basin, North China Craton(NCC). The typical features of the studied tonsteins include thin beds, lateral continuity, angular quartz grains, and euhedral zircons with similar U-Pb ages, indicating a significant pyroclastic origin. In addition, the tonstein samples hav...     

Middle Jurassic–Early Cretaceous tectonic evolution of the Bayanhushuo area,southern Great Xing’an Range,NE China: constraints from zircon U–Pb geochronological and geochemical data of volcanic and subvolcanic rocks

This study presents new zircon U–Pb geochronology, geochemistry, and zircon Hf isotopic data of volcanic and subvolcanic rocks that crop out in the Bayanhushuo area of the southern Great Xing’an Range (GXR) of NE China. These data provide insights into the tectonic evolution of this area during the late Mesozoic and constrain the evolution of the Mongol–Okhotsk Ocean. Combining these new ages with previously published data suggests that the late Mesozoic volcanism occurred in two distinct episodes: Early–Middle Jurassic (176–173 Ma) and Late Jurassic–Early Cretaceous (151–138 Ma). The Early–Middle Jurassic dacite porphyry belongs to high-K calc-alkaline series, showing the features of I-type igneous rock. This unit has zircon ε_Hf(t) values from +4.06 to +11.62 that yield two-stage model ages (T_DM2) from 959 to 481 Ma. The geochemistry of the dacite porphyry is indicative of formation in a volcanic arc tectonic setting, and it is derived from a primary magma generated by the partial melting of juvenile mafic crustal material. The Late Jurassic–Early Cretaceous volcanic rocks belong to high-K calc-alkaline or shoshonite series and have A₂-type affinities. These volcanics have ε_Hf(t) and T_DM2 values from +5.00 to +8.93 and from 879 to 627 Ma, respectively. The geochemistry of these Late Jurassic–Early Cretaceous volcanic rocks is indicative of formation in a post-collisional extensional environment, and they formed from primary magmas generated by the partial melting of juvenile mafic lower crust. The discovery of late Mesozoic volcanic and subvolcanic rocks within the southern GXR indicates that this region was in volcanic arc and extensional tectonic settings during the Early–Middle Jurassic and the Late Jurassic–Early Cretaceous, respectively. This indicates that the Mongol–Okhotsk oceanic plate was undergoing subduction during the Early–Middle Jurassic, and this ocean adjacent to the GXR may have closed by the Late Middle Jurassic–Early Late Jurassic.     

Compositional characteristics and geochemical significance of light hydrocarbons for crude oils in Tarim Basin, China

Light hydrocarbons, especially C7 isoalkanes, cyclopentanes, cyclohexanes and n-heptane in 540 oils from the Tarim Basin were studied systematically. The results suggested the K1 values [ （2-MH＋ 2,3-DMP）/（3-MH ＋ 2,4-DMP） ] of crude oils vary from a lowest value of 0.78, to a highest value of 1.54, but the K1 values of the majority of oil samples （80%） range from 0.90 to 1.20. However, the absolute contents of （ 2-methylhexane ＋ 2,3-dimethylpentane） and （ 3-methylhexane ＋ 2,4-dimethylpentane） in the total light hydrocarbons （ C4-C7 ） show a similar variation trend. The variation of K1 values of crude oils can suggest a typical sedimentary environment for source rocks. Therefore, according to the compositional characteristics of C7 light hydrocarbons in light of the steady-state catalytic hypothesis, nonmarine oils from the Yingmaili area and marine oils from the Tazhong uplift in the Tarim Basin were genetically classified.     

Novel C26 tetracyclic degraded bicadinanes in oil sands from the Haitoubei–Maichen Sag of the Beibuwan Basin,South China Sea

Three C₂₆ tetracyclic terpanes were detected in the saturated hydrocarbon fractions of oil sand samples especially rich in bicadinanes W, T and R from the Oligocene Huizhou Formation in the XW-1 well in the Haitoubei–Maichen Sag of the Beibuwan Basin. Their mass spectra show molecular ions of the three tetracyclic terpanes at m/z 358, and their key fragment ions at m/z 343 (M⁺-15) and m/z 315 due to loss of an isopropyl group. These characteristics are very similar to that of C₃₀ cis–cis–trans (W) and trans–trans–trans (T) bicadinanes. In addition, the relative abundances of the C₂₆ tetracyclic terpanes show an excellent positive correlation with the C₃₀ bicadinanes W and T in 27 oil sand samples. Therefore, the three C₂₆ tetracyclic terpanes are tentatively identified as degraded bicadinanes, which like bicadinanes, may indicate input from dammar resins and can be used to discriminate the origin of crude oils.     

Zircon U–Pb Geochronology of Baiyingaolao Formation from the Hadayingzi Region in the southern Great Xing’an Range, Northeast China

Blueschists in the Yilan area, Heilongjiang Province, are known as important parts of the Heilongjiang Complex. However, their metamorphic evolution history is still unclear. Petrographical, and mineral chemical investigations on the blueschists from the Yilan area are carried out in this study. P‐T pseudosections are calculated in the system Na₂O‐CaO‐K₂O‐MnO‐FeO‐MgO‐Al₂O₃‐SiO₂‐H₂O‐TiO₂‐O (Fe₂O₃) (NCKMnFMASHTO) with the PERPLE_X software for three blueschist samples. Peak metamorphic conditions are estimated to be 500‐525°C, 1.14‐1.2 GPa for garnet‐barroisite schist (g‐brs‐wnc‐act‐phn‐chl‐ep‐ab‐stlp); 480°C, 1.4 GPa for chlorite‐epidote glaucophane schist (gl‐brs‐wnc‐act‐chl‐ep‐stlp) and 450‐500°C, 0.95‐1.2 GPa for phengite‐glaucophane schist (gl‐wnc‐phn‐ab‐ep‐chl‐sph), which indicate that they all underwent peak metamorphism in the limit epidote‐blueschist facies conditions. Combined with the previously reported geochemical and geochronological data, the clockwise P‐T paths of the blueschists are constructed. The formation of blueschists in the study area marks the young oceanic crust subduction, and represents the final closure of the Paleo‐Asian Ocean and the beginning of the Paleo‐Pacific tectonic system. The P‐T pseudosection recorded the P‐T increasing process before the metamorphic peak and that the maximum temperature was coincident with the maximum pressure in the process, which indicates that the epidote‐blueschist facies conditions implemented in the process of subduction, rather than exhumation. The retrograde trajectory nearly coincidently retraced the prograde trajectory, which represents the similar geothermal gradient (11‐14°C/km) in the subduction and the relatively slow exhumation process.     

The Shishugou Fauna of the Middle–Late Jurassic Transition Period in the Junggar Basin of Western China

The Middle–Late Jurassic transition period is a critical period for the evolution of terrestrial vertebrates, but the global fossil record from this time is relatively poor. The Shishugou Fauna of this period has recently produced significant fossil remains of dinosaurs and other vertebrate groups, some representing the earliest known members of several dinosaurian groups and other vertebrate groups and some representing the best-known specimens of their group. These discoveries are significant for our understanding of the origin and evolution of several vertebrate lineages. Radiometric dating indicates that the fauna is aged approximately 159–164 Ma. Comparisons with other similarly-aged terrestrial faunas such as Shaximiao and Yanliao show both taxonomic similarities and differences between these faunas and indicate that the Junggar deposits might have preserved the most complete vertebrate fossil record for a Middle–Late Jurassic Laurasian terrestrial fauna.     

Kalligrammatid Lacewings from the Upper Jurassic Daohugou Formation in Inner Mongolia, China

A new species, referable to a new genus, is erected, and named the Sinokalligramma jurassicum gen. et sp. nov. It is the second finding of kalligrammatids in the Daohugou Formation. The origin and migration of the family Kalligrammatidae are discussed. The geological age and stratigraphic correlation of the Daohugou and Karabastau Formations are briefly reviewed and reassessed.     

Tooth Plates of Ceratodus (Dipnoi, Ceratodontidae) from the Upper Jurassic Shaximiao Formation of Guang’an, Sichuan Province, China

Ceratodus is a form genus of ceratodontid lungfishes. Until now, only 14 specimens have been reported from the Mesozoic of China. Detailed measurements and characteristic comparisons based on 15 well-preserved lungfish tooth plates of Ceratodus type, uncovered from the upper member of the Upper Jurassic Shaximiao Formation at Luojiashan, Qianfeng District, Guang''an, Sichuan Province in 2019, allow us to establish a new species, Ceratodus guanganensis sp. nov., on the basis of its small swollen irregular triangular tooth plate with four low and swollen radial crests on the occlusal surface without denticles. Other specimens from the same locality can be referred to C. szechuanensis and C. youngi. This is the first discovery of Ceratodus in the Guang''an region and represents the fifth Ceratodus fossil site in the Sichuan Basin. The discovery indicates that the climate was hot and arid in the Sichuan Basin during the Late Jurassic, and reveals a tectonic paleogeographic relationship between the Yangtze block in southwestern China and the Kola Basin in northern Thailand.     

Middle Ordovician Bioturbation Structures from Southeastern Margin of Ordos Basin and Their Environmentary Interpretation

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Late Jurassic–Early Cretaceous tectonic evolution of the Great Xing’an Range: geochronological and geochemical evidence from granitoids and volcanic rocks in the Erguna Block,NE China

ABSTRACT

The late Mesozoic magmatic record within the Erguna Block is critical to evaluate the tectonic history and geodynamic evolution of the Great Xing’an Range, NE China. Here, we provide geochronological and geochemical data on Late Jurassic–Early Cretaceous plutonic-volcanic rocks in the northern Erguna Block and discuss their origin within a regional tectonic framework. Late Mesozoic magmatism in the Erguna Block can be divided into two major periods: Late Jurassic (162–150 Ma) and Early Cretaceous (140–125 Ma). Late Jurassic quartz monzonite and dacite show adakite characteristics such as high Al₂O₃, high Sr, and steeply fractionated REE patterns. Contemporary granitoids and rhyolites are also characterized by strong enrichment of light rare earth elements (LREE) and significant depletion in heavy rare earth elements (HREE), but with more pronounced negative Eu anomalies. Early Cretaceous trachytes and monzoporphyries exhibit moderate LREE enrichment and relatively flat HREE distributions. Coeval granites and rhyolites have transitional signatures between A-type and fractionated I-type felsic rocks. Both Late Jurassic and Early Cretaceous rocks have distinctive negative Nb, Ta, and Ti anomalies, and positive zircon ε_Hf(t) values, suggesting that these magmas were derived from partial melting of Meso-Neoproterozoic accreted lower crust, although melting occurred at a variety of crustal levels. The transition from adakite to non-adakite magmatism reflects continued crustal thinning from Late Jurassic to Early Cretaceous. Our data, together with recently reported isotopic data for plutonic and volcanic rocks, as well as geochemical data, in NE China, suggest that Late Jurassic–Early Cretaceous magmatism in the Erguna Block was possibly induced by post-collisional extension after closure of the Mongol-Okhotsk Ocean.     

Characteristics and Formation Mechanism of the Geothermal Field in the North China Downfaulted Basin

The geothermal field is mainly controlled by the regional tectonic framework characterized by alternationsof uplifted and depressed basement. and exhibits a similar zoned distribution of temperatures. In the upliftedarea the geothermal gradient (G) and terrestrial heat flow value(q) of the Cenozoic sedimentary cover are rela-tively high, with G=3.5-5.0℃/100m and q=63-84mW/m~2; whereas in the depressions they are rela-tively low, with G=2.7-3.5℃/100m and q=46-59mW/m~2. In the whole region, G=3.58℃/100m and q=61.5±13.4nW/m~2, indicating a comparatively high geothermal background and the presence of localgeothermal anomalies. A comparison of the results of mathematical simulation of the geothermal field with themeasured values shows a good agrecment between them. The geothermal difference between various tectonicunits is caused chiefly by the lateral and vertical variation of thermal properties of shallow crustal rocks. Thisphenomenon can be regarded as the result of redistribution of relatively uniform heat flows from the deep crustin the surficial part of the crust in the process of their upward conduction.     

Sporopollen Assemblages from the Cretaceous Yimin Formation of the Hailar Basin, Inner Mongolia, China

Three sporopollen assemblages are recognized for the first time from the Cretaceous Yimin Formation in the Hailar Basin of eastern Inner Mongolia. They are （in ascending order）： the lmpardecispora-Aequitriradites- Clawttipollenites assemblage; the Triporoletes-Pilosisporites-Asteropollis assemblage; and the Appendicisporites- Asteropollis-Tricolpites assemblage, distributed in Members 1, 2 and 3 of the Yimin Formation respectively. Recognition of this biostratigraphic sequence is very important for the division and correlation of the Yimin Formation in the basin. Meanwhile, the age of the Yimin Formation is considered to be Barremian to Early Albian based on the palynological data.