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The Pingbei area is the main accumulation area of oil and gas in the Xihu Sag. The phase characteristics of oil and gas in this area are complex, and the understanding of their genesis is still unclear. In this paper, based upon discussions of crude oil and natural gas geochemical data, integrated with local geological features, we discuss the sources, migration, and phase state characteristics of oil and gas in the Pingbei area of the Xihu Sag. The study results show that the crude oil and natural gas in the Pingbei area are coal-derived and the oil and gas produced by the humic organic matter during the mature stage. The oil and gas source correlation showed that crude oil and natural gas have good affinity with the Eocene Pinghu Formation (PH) coal-bearing source rocks. Crude oil has the characteristics of near-source accumulation and short-distance migration while natural gas is supplied from a dual source: the coal-derived hydrocarbon rocks of PH in the deep part of the study area, supplemented by the coal-derived hydrocarbon rocks of PH on the bottom of the slope, and adjacent hydrocarbon-bearing sub sag. The distribution characteristics and geochemical migration indices of hydrocarbon show that the oil generated from the hydrocarbon source rocks of PH in the deep Pingbei area mainly migrates vertically along the fault connecting the reservoir and the source rocks to the trap, where it accumulates, while the natural gas exhibits deep and large faults that mainly migrate vertically, supplemented by its lateral migration along the composite transport system composed of faults and sandstone layers in the slope zone. The whole area presents the phase characteristics of "upper oil and lower gas, west oil and east gas". The mechanisms of produced-exhausted and geochromatographic effects (PGE), as well as evaporative fractionation (EF) and phase-controlled migration fractionation (PMF), result in the obvious discrepancy of hydrocarbon's properties on the vertical profiles. [ABSTRACT FROM AUTHOR]

Helium is extensively utilized in aerospace, healthcare, electronics, semiconductors, advanced science, and renewable energy sectors, playing a pivotal role in the advancement of human technology. Presently, helium is primarily extracted from natural gas as an associated resource. The cost of extraction decreases with an increase in the helium concentration within the gases. Therefore, understanding the mechanisms involved in helium generation, migration, and accumulation underground is vital. This article delves into the differential enrichment mechanism of helium in the Jinqiu gas field, situated in the Sichuan Basin of China, by analyzing the molecular and isotopic composition characteristics of the major gases and noble gases, employing gas dissolution and diffusion models. Compared with other Jurassic gas reservoirs in the Sichuan Basin, the Jinqiu gas field contains a significantly higher helium concentration, displaying unique enrichment patterns within its area. The majority of hydrocarbon gases originate from the local underlying Triassic Xujiahe Formation source rocks. Specifically, the western section of the Jinqiu field is primarily supplied by the higher maturity Xujiahe Formation source rocks located in the western Sichuan Basin. Notably, there is no significant contribution of mantle-derived volatile components in the Jinqiu field. Helium and argon are primarily radiogenic, whereas neon primarily originates from atmosphere. The good linear correlation between 4He and 20Ne suggests that their transfer is primarily facilitated through groundwater. The solubility calculation reveals that the helium exsolved from the in-situ pore water of the reservoir, due to stratigraphic uplift during the Himalayan movement, comprises only 0.04%–0.11% of the existing reserves. This suggests that gas desolvation plays a relatively minor role in helium enrichment in the Jinqiu field. Additionally, the calculation results of gas diffusion indicate that diffusion's contribution to helium enrichment in the Jinqiu gas field remains minimal. Finally, our analysis of methane reserve changes suggests that the combined effect of differential loss and dilution of hydrocarbon gases is the primary mechanism for helium's differential enrichment in the Jinqiu field. Notably, methane and carbon dioxide depletion may be a significant factor in helium enrichment within the crustal natural gas system. • Differential loss and dilution jointly cause helium enrichment. • Depletion of CH 4 and CO 2 is important enrichment mechanisms of helium. • Low pressure is a key geological condition for helium-rich gas fields. [ABSTRACT FROM AUTHOR]

Hydrocarbon migration mechanism in deep Ordovician reservoir of the Halahatang area, Tarim Basin, is investigated based on the analysis of the geological setting and geochemical parameters. The parameters successfully analyzed include oil–gas properties, crude oil maturity [Ts/(Ts + Tm) ratio], gas carbon isotopic ratios, and trace elements (B, V, Ni, and Cu) in crude oil. The results show that hydrocarbons migrate into the Ordovician of the Halahatang area from the Manjiaer Depression and the Halahatang area itself through unconformities and faults. The overall hydrocarbon migration orientation in the area is generally from south to north. The oil and gas migration process determine parameters vary regularly from south to north as follows: oil density, V/Ni ratio, Ni/Cu ratio, and concentrations of V, Ni and (Ni + B) in crude oil regularly increase, whereas Ts/(Ts + Tm), gas/oil ratio, and dryness coefficient gradually decrease, and methane show a trend toward depletion of heavy carbon isotopes. Additionally, there exist obvious anomalies of geochemical parameter values in the Repu3 well block, which indicates that a local fault intersection could provide a favorable hydrocarbon filling point. Based on these findings, further exploration activities in the deep Ordovician reservoir of the Halahatang area should be conducted in the southern part of the Halahatang area along the oil charging pathways, and fault intersection zones should be emphasized. [ABSTRACT FROM AUTHOR]

The Kuche Depression is considered as the most important gas resource potential and gas exploring area with great gas resource potential and prospect in the Tarim Basin. Based on geochemical experimental analyses and comprehensive geological studies, the general geochemical characteristics of molecular and isotope compositions of rare gases as well as hydrocarbon gases and nonhydrocarbon gases are comparatively studied in the Kuche and Southwestern Depressions. Then, their genetic types are separately identified and gas originations are comprehensively discussed. The main results are as follows. (1) Gas fields in the Kuche Depression have a higher methane abundance, accompanied with low N2 and CO2 abundances, but the Akemomu gas field in the Southwestern Depression has a relatively lower average methane abundance, accompanied with high average N2 and CO2 abundances. The helium abundance of natural gases in gas fields from the Kuche Depression general has 1 order of magnitude higher than the air value. Comparatively, it has more than 2 orders of magnitude higher than the atmospheric value in the Akemomu gas field from the Southwestern Depression. The neon, argon, krypton, and xenon abundances in both Kuche and Southwestern Depressions are lower than the corresponding air values. (2) Natural gases from gas fields in the Kuche Depression and the Southwestern Depressions are generally typical coal-formed gases. The rare gases in the Kuche Depression have typical crustal genesis, mainly deriving from the radioactive decay of elements in the crust, while in the Akemomu gas field from the Southwestern Depression, the rare gases have main crustal genesis with a proportion of 92.5%, probably accompanied with a little mantled genetic contribution. (3) Natural gases in the Kuche Depression are generally derived from coal measure source rocks of Jurassic and Triassic, which principally originated from Jurassic in strata period and coals in source rock types. The Jurassic source rocks account for 55%-75% and the Triassic source rocks account for 25%-45% approximately, while coals occupy 68% and mudstones occupy 32% separately. Natural gases from the Akemomu gas field in the Southwestern Depression mainly originated from humic mudstones of marine and continental transitional source rocks of Carboniferous to Permian. [ABSTRACT FROM AUTHOR]

The accumulation mechanism of hydrocarbons in the Yingmai 2 Oilfield of the Tabei Uplift in the Tarim Basin is not well understood. Here, the characteristics and formation of Paleozoic faults in the Yingmai 2 Oilfield are studied with new 3D seismic data and regional geologic data. Inclusion features and geochemical parameters are used to investigate the faults controlling the oil and gas compositions. The results indicate that three fault systems developed in the Yingmai 2 Oilfield: a NNW-trending strike-slip fault system that formed in the middle Caledonian, a NW-trending strike-slip fault system that formed in the Caledonian–late Hercynian and a NE-trending thrust fault system that formed in the late Hercynian–Indosinian, which was associated with volcanic activity. The two strike-slip fault systems were important early hydrocarbon migration pathways in the late Caledonian–early Hercynian, late Hercynian, and Himalayan accumulation periods. The NE-trending thrust faults were sealed by volcanics, and divided the originally uniform oilfield into two accumulation systems during the Himalayan. This finding is supported by the differences in the compositions of the oils and gases in the eastern (YM area) and the western (YG area) Yingmai 2 Oilfield. The oil density, iC 4 /nC 4 , Ph/nC 18 , and V/Ni ratios of the oils in the reservoirs in the YM area are lower than those in the YG area, whereas the TMNr, TeMNr, 4-1/DBT, Pr/Ph, C 29 -diasterane/C 29 -regular sterane ratios, H 2 S content, and the content of Himalayan hydrocarbon inclusions are higher. The hydrocarbon accumulation in the Yingmai 2 Oilfield is attributed to multiple episodes of hydrocarbon charging and suggests that the YM area has greater exploration potential than the YG area. [ABSTRACT FROM AUTHOR]

Deep sour gases are found in the Ordovician carbonate reservoir of the Tazhong Uplift in the Tarim Basin, northwest China. All sour gases in this field are dominated by gas hydrocarbons (C 1 –C 4 ) with varying amounts of H 2 S. Most of the gases in the eastern Tazhong are characterized by high gas dryness coefficients (>0.95), 13 C-enriched methane (δ 13 C 1 ranging from −44.6‰ to −35.8‰), and small difference between values of the δ 13 C 2 and δ 13 C 1 (δ 13 C 2 - 1 ). In contrast, the gases in the western Tazhong are characterized by relatively low gas dryness coefficients (mostly < 0.90), 13 C-depleted methane (δ 13 C 1 ranging from −54.9‰ to −35.7‰), and high values of δ 13 C 2 - 1 , indicating relatively lower maturity. Gas compositions, carbon isotope ratios, and diamantane data together with thermal pyrolysis experiments using sealed gold tubes suggest that the Ordovician gases in the Tazhong Uplift belong to oil cracking gases mixed with high-maturity 13 C-enriched dry gases, which originated from the deeper paleo-reservoirs cracking. The H 2 S (0.0008%–23.1000%) is attributed to thermochemical sulfate reduction (TSR); in contrast to typical TSR-altered gases, the natural gases in the western Tazhong are characterized by low dryness coefficients, relatively 13 C-depleted methane, and high δ 13 C 2 - 1 , suggesting a low degree of TSR alteration. Observations suggest that the H 2 S contents are generally high in the western Tazhong, with positive correlation to Mg 2+ concentrations and total dissolved solids (TDS) in the Ordovician formation water, indicating that TSR has entered early period of SO 4 2− contact ion pairs (CIPs) oxidation. Compared with the eastern Tazhong, the Mg 2+ concentrations and TDS in the western Tazhong are higher and form more CIPs that accelerate TSR and produce H 2 S. In addition, based on the analysis of the Cambrian formation water and the Cambrian gypsiferous salt, TSR might have also exist in the deeper Cambrian reservoirs, and the TSR-altered gases with high content of H 2 S migrated along faults to Ordovician reservoirs. [ABSTRACT FROM AUTHOR]

The mechanisms by which fluids modify the mineralogy and geochemistry of fault zones and the role of rock–fluid interactions in fault weakening are subjects of debate. We assessed the mineralogical and geochemical variations in fault rocks in the Shaba area of the Yingxiu–Beichuan surface rupture zone of the Mw 7.9 Wenchuan earthquake using analyses of their mineralogical compositions, major elements, and the microstructural characteristics of wall rocks, damage zones, and oriented fault gouge samples from the principal slip zones. The elemental and mineralogical compositions show a certain regularity across the fault zone from the wall rocks to the fault gouge so that (1) Al 2 O 3 , Fe 2 O 3 T, and K 2 O contents increase and exhibit significant enrichments in the fault gouge; (2) Na 2 O and P 2 O 5 contents decrease gradually and exhibit significant depletions in the fault gouge; (3) quartz, feldspar, and carbonate mineral contents decline; and (4) clay mineral contents increase dramatically. Isocon analysis indicates that mass losses in the Shaba fault zone occur with losses in the damage zone < the fault gouge. The mechanisms of material loss and transformation in the fault zone are complex, and mechanical fracturing, dehydration reactions, and fluid–rock interactions are important factors in changing and controlling the material compositions and fault zone evolution, with material losses and transformation especially important within the fault core. Moreover, due to the damage zone having a higher permeability than the fault core, it was conducive to hydrothermal upwelling, fluid–rock interactions, and fracture healing. • Mineral assemblages and major element contents exhibit variations across the fault zone. • Enrichment/Depletion of elements and mass balance transfers are recorded in the fault zone. • Fault rocks exhibit numerous features for fluid-rock interactions. • Material loss occurred in the fault zone varies with time and space. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new hydrocarbon generation and expulsion model was established. • Mudstones and Upper-Middle Eocene source rocks contributed most to petroleum system. • The new model provides an efficient method for identifying oil-gas sources. The widely developed Tertiary coal-bearing intervals in the Xihu Sag, East China Sea Shelf Basin in offshore eastern China, are hosted in the Middle-Upper Eocene Pinghu Formation (PHF) and Oligocene Huagang Formation (HGF). Based on field data and geochemical features, a new model for the hydrocarbon generation and expulsion is developed using Rock-Eval pyrolysis that was applied to quantify the potentials of coaly source rocks of the PHF and HGF throughout the Xihu Sag. The results suggest the coaly source rocks of the HGF and PHF are primarily composed of type II/III kerogen, and begin to expel hydrocarbon at 0.68 %R o and 0.71 %R o , respectively. Several hydrocarbon expulsion centers are located in the western slope zone for coals and the central anticline zone for mudstones, with the highest intensity of 3.5 × 106 and 24 × 106 t/km2 for coals and mudstones, respectively. Among all source rocks, the coals and mudstones contribute 2.5% and 97.5%, respectively, of the total expelled hydrocarbon, whereas the source rocks of the HGF and PHF provide 19% and 81%, respectively, of the total hydrocarbon. The study also shows that the natural gas discovered in the north of the central anticline zone and the north of the western subsag have mixed sources from the HGF and PHF while the light oil and condensate accumulated in other zones are primarily derived from the source rocks of the PHF. The model used in the Xihu Sag provides an efficient method for evaluating source rocks and identifying oil-gas sources. [ABSTRACT FROM AUTHOR]

With wells Puguang 2 and 6 in the Puguang gas field as the research objects by choosing organic carbon as characterization reservoir carbon (indirectly reflecting the bitumen contents), this paper discusses the relationship between bitumen contents and lithologic characteristics, reservoir characteristics, and calculates the volume of paleooil and oil cracked gas. The study shows that the organic carbon contents of the reservoirs are related to lithologic characteristics. Due to variations in the particle size or structure of rocks, the porosity of the reservoirs varies, furthermore, it will influence the contents of organic carbon. And the relationship between these two parameters is positive. Based on the analysis, the relationship between total organic carbon (TOC) and the contents of reservoir bitumen also appears to be positive. The volume of paleo-oil and cracked gas of the Changxing-Feixian'guan Formation are respectively 495 million tons and 360.4 billion m through the calculation of reservoir's organic carbon. Therefore, the relationship between carbon-bearing rate and characteristics of the reservoirs can be used to indirectly evaluate reservoir types. Besides, the carbon-bearing rate of the reservoirs also reflects the contents of reservoir bitumen, paleo-oil as well as natural gas. It makes a new approach for the study of the types of reservoirs and the amounts of paleo-oil and oil cracked gas in northeastern Sichuan Basin. [ABSTRACT FROM AUTHOR]

Study indicates that the elements rich in marine floor hydrothermal fluids have a positive influence, not only on total organic carbon (TOC), but also on the abundance of SiO2 in some Middle-Upper Proterozoic strata. The TOC bears a better positive correlation with abundance of SiO2. By the correlation analysis between organic matter and sea-floor hot fluid characterized trace elements such as P, Cu, Zn, Ni, Ba, etc in marine carbonate rocks, we conclude that organic matter enrichment of carbonate rocks were related to marine floor hydrothermal activity in a part of layers. Some thin-layered cherts, which were thought to be the indicators of hydrothermal activity, frequently occurring in Gaoyuzhuang, Wumishan, Tieling and Xiamaling formation support our conclusion. With the increasing of trace elements in marine carbonate rocks, the TOC and Fe/Ti ratio is creasing. In contrast, the Al/(Al+Fe+Mn) ratio is usually decreasing. [ABSTRACT FROM AUTHOR]

Mud volcanoes, as important migration channels for gases from deep to surface, are used to research chemical interactions at depth and assess petroleum accumulation. To study the post-genetic processes that change initial compositional and isotopic characteristics of reservoir gases, a large number of geochemical data of gases from mud volcanoes and adjacent oilfields in the Junggar Basin have been selected and analyzed. Compared with the homologous reservoir gases, the seeping gases performed in mud volcanoes generally have the following characteristics: (1) dryness coefficients (C 1 /C 1-4) significantly increase; (2) methane tends to become depleted in 13C; (3) propane tends to become enriched in 13C. Through geochemical and microbial analyses, it is proved that the reservoir gases belong to thermogenic coal-gases, which are originated from the Lower-Middle Jurassic source rocks. After discharging from reservoirs, the gases migrated and diffused upward to the mud volcanoes, and methane anaerobic oxidation might occur between reservoir and surface. However, due to the migration advection and effusion, it reveals the obvious isotopic and molecular fractionation with the dry coefficient increasing (loss of C 2+ alkanes) and methane carbon isotope lightening, which covers the weak anaerobic oxidation of methane. In addition, oxidation by ferric (Fe3+) - bearing minerals may also lead to the significant depletion of 13C in methane, and iron-bearing minerals have been reduced. Propane tends to become enriched in 13C, mainly results from that sulfate-reducing bacteria prefers to use propane as growth substrates rather than ethane, and preferentially oxidizes propane. The results including the gas geneses and its secondary post-genetic processes may offer important insight into the evaluation of the related hydrocarbon resources, especially the unexplored regions. • Both mud volcano gases and reservoir gases are from the Lower-Middle Jurassic source. • Molecular and isotopic composition anomalies of mud volcano gases are observed. • Post-genetic processes caused the molecular and isotopic composition anomalies. • Analyzing relationship of seep vs. reservoir gases contributes to evaluation of petroleum systems. [ABSTRACT FROM AUTHOR]

The carbon isotopic composition of sedimentary organic matter (δ13C OM) is notably important for the genetic identification of source rock and its products. The sapropelic organic matter (mostly plankton with high hydrogen index) is generally depleted in 13C as compared to humic organic matter (mainly higher plants with lower hydrogen index). However, some lacustrine sediments contain sapropelic source rocks with anomalous 13C-enriched isotopic ratios, especially reported in freshwater lakes. The purpose of this study is to discuss the geneses of lacustrine sedimentary organic matter characterized by anomalous 13C-enriched ratios in saline and freshwater sediments. To this end, we integrate the detailed organic geochemical data (including Rock-Eval analysis data, total organic carbon contents, total organic carbon isotopic compositions, biomarkers, and trace element concentrations) of 64 saline lake sediments and 39 freshwater lake sediments obtained from the Paleogene Shahejie Formation of Dongpu sag and Qikou sag, separately, in Bohai Bay Basin of eastern China. Both sags exist samples with high hydrogen index values (HI > 400 mg/g) and high total organic carbon contents (TOC > 2%), but anomalous 13C-enriched isotopic ratios (up to −21.1‰). In Qikou sag, the δ13C OM values of sapropelic kerogen correlate positively with TOC contents. The anomalous enrichment of 13C in organic matter is associated with extremely high productivity of algae, such as algae bloom. In Dongpu sag, the enrichment of 13C in organic matter is attributed to the significant increase of drought and salinity. High salinity and aridity climate may lead to the abundance of C 4 plants and Chlorobium as well as the enrichment of 13C in plankton. • Sapropelic source rocks with anomalous 13C-enriched isotopic ratios have been found in both freshwater and saline lakes. • Two models are proposed to explain the geneses of organic matter with anomalous 13C-enriched isotopic ratios. • The application of δ13C OM values in the genetic identification of lacustrine source rock and its products may be restricted. [ABSTRACT FROM AUTHOR]

• δ15N bulk of mudstones from the Chang-7 3 Member is higher than that of Chang-7 1,2. • Nitrogen isotopic composition of mudstone is related to depositional redox condition. • Nitrogen isotopic composition can be used to evaluate depositional redox condition. In this study, we report new analyses of stable nitrogen isotopic compositions (δ15N) of the black mudstones from the Chang-7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin, China, to examine the possible linkage of such isotopic compositions with the depositional environment. These mudstones were deposited mainly under suboxic bottom water column conditions. The core descriptions and trace element concentrations differ between samples obtained from the Chang-7 3 and Chang-7 1,2 members, situated at the bottom and top of the Chang-7 Member, respectively, such that the depositional condition of the former implies a more reduced water regime. The bulk rock nitrogen isotope (δ15N bulk) values of the samples obtained from the Chang-7 3 Member are higher than those of the Chang-7 1,2 Member. No significant differences were identified in the organic carbon isotopes (δ13C org), total organic carbon (TOC), or total nitrogen (TN) values between the two units. The results of δ15N bulk cross-plotting with the TOC, TN, and carbon to nitrogen atomic ratio (C/N) show an absence of significant modification of the primary nitrogen isotopic compositions from post-depositional processes. Given the depositional history of these units, we suggest that the sedimentary redox environment can have a significant influence on δ15N bulk. Denitrification, which occurs mainly in suboxic conditions, plays an important role in such an environment, resulting in 15N enrichment during the original deposition. Thus, owing to its sensitivity to sedimentary redox environments, the δ15N bulk index can be an effective tool for evaluating depositional redox conditions, particularly for further subdivision of suboxic environments. [ABSTRACT FROM AUTHOR]

Paleogene coals and organic rich mudstones in Xihu sag, East China Sea Shelf basin in offshore eastern China were analysed to evaluate the hydrocarbon generation potential based on organic petrographic and organic geochemical methods. The results reveal that the source rocks are dominated by Type III-II2 kerogens deposited in sub-oxic to oxic environments as coal-bearing source rocks. The total organic carbon (TOC) content values for the samples range from 0.31 wt% to 75.68 wt%, suggesting fair to excellent hydrocarbon generation potential, which is also consistent with the Hydrogen Index (HI) and extractable organic matter (EOM) content results. Thermal maturity of organic matter in Western Depression Group (WDG) area are mainly in early thermal maturity stage (Rr ranges from 0.57% to 0.76%), while Central Uplift Group (CUG) area samples have relatively higher thermal maturity (Rr ranges from 0.72% to 1.41%). The characteristics of Paleogene coal and organic rich mudstones reveal that the potential source rocks in both WDG and CUG area have fair to excellent hydrocarbon generative potential, but vary in hydrocarbon phase. WDG source rocks are expected to generate both commercial liquid hydrocarbons and gaseous hydrocarbons, which is also implied by the presence of significant liptinite and exsudatinite macerals. On the other hand, source rocks for CUG area exhibit a dominant gaseous hydrocarbon generation and some oil hydrocarbon generation potential. The data exhibited in this study illustrate source rock characteristics and provide evidence for hydrocarbon generation potential of Paleogene coal and organic rich mudstones in Xihu sag, in offshore eastern China. • The source rocks characteristics of Paleogene coals and organic rich mudstones are investigated. • Source rocks from Western Depression Group area have liquid generation potential. • Mainly gas and minor liquid generating potential for the source rocks from Central Uplift Group area. [ABSTRACT FROM AUTHOR]

• A series of short lanostanes are first identified in alkaline lacustrine sediments. • The carbon number of lanostanes is expanded to include C 23. • The short chain lanostanes and long chain lanostanes have common biogenic precursors. • Cyanobacteria may be a possible biological source of lanostanes. Highly mature organic-rich source rocks in the Early Permian Fengcheng Formation, Mahu Sag, Junggar Basin, northwestern China, were deposited in an alkaline lacustrine environment. Analysis of extracts of eight core samples by gas chromatography–mass spectrometry detected a series of short-chain lanostanes (C 23 –C 25) lanostanes. The relative abundance of short-chain lanostanes has a strong positive correlation with those of long chain (C 30 –C 32) homologues, indicating a possible common biological origin and genesis. Compound specific isotopic analyses of phytane, steranes, hopanes and C 30 lanostane from the core extracts reflect input from primary photosynthetic producers including cyanobacteria. Abundant lamalginite was observed in the samples from the study area. A well-defined positive relationship is also evident between the relative abundance of lanostanes and the ratios of 7-+8-methyl heptadecanes/ C max and 2-methyl hopanes/C 30 -hopanes. Given that 7-+8-methyl heptadecanes and 2-methyl hopanes are considered to be diagnostic molecular biomarkers for organic matter derived from cyanobacteria, it is possible that cyanobacteria may serve as a possible biological precursor for lanostanes in the Fengcheng Formation. The influence of maturity on the lanostanes is considered to be minor, as no significant relationship is found between lanostanes and thermal maturity parameters, although this may be due to a limited maturity range of the samples. The co-existence of high contents of phytane, β-carotanes, and gammacerane, and their correlations in abundance with lanostanes, indicates a strongly reducing sedimentary environment. [ABSTRACT FROM AUTHOR]

The Eopalaeozoic was a pivotal transitional period following the Neoproterozoic glaciation that witnessed dramatic changes in global ocean environments. The black rock series were extensively sedimented, preserving the original seawater signature. However, due to the scarcity of geological records, the palaeo-environment and organic matter (OM) enrichment in the Tarim Basin, China, are poorly constrained. Herein, Eopalaeozoic shales from typical outcrops in the Kalpin block, northwestern Tarim Basin, have been subjected to integrated organic and inorganic geochemistry research. In the study area, Eopalaeozoic black rock series can be found in the Yuertusi Formation (YUF), Saergan Formation (SAF) and Yingan Formation (YIF) from bottom to top. Although the maturity is moderate (~1.2%), the OM abundance varies greatly vertically. Microscopic observations and biomarkers revealed that the OM source was dominated by diverse planktonic algae. The chemical alteration index indicates a transition from cold and dry climates with low chemical weathering to warm and humid climates with moderate chemical weathering, and subsequently back to cold and dry climates. The stratified water column and euxinic–suboxic conditions are revealed by redox proxies (Mo, enrichment factors, Th/U, Mo U covariation pattern, pristane/phytane and gammacerane index). The YUF was deposited in a euxinic environment. Productivity proxies (biogenic Ba, Cu/Al, Ni/Al and carbon isotopes) show that primary productivity was superelevated in the lower YUF and moderate in the other formations. Primary productivity and preservation conditions were heavily influenced by complementary hydrothermal activities and upwelling currents. The hydrothermal activities and upwelling currents provide abundant nutrients and highly reductive environments, creating enormous possibilities for the formation of excellent source rocks with considerable hydrocarbon geological significance. • Three sets of Eopalaeozoic black rock series, northwestern Tarim Basin were systematically researched. • Integrated organic and inorganic geochemistry analyses were conducted for palaeo-environment interpretation. • Palaeo-environmental evolution and organic matter enrichment differed tremendously vertically. • Hydrothermal activities and upwelling currents favours the formation of high-quality source rocks. [ABSTRACT FROM AUTHOR]

• A distinct oil group was discovered in the Dongying Depression. • Related source rock of the distinct oil group has abundant bacteria/algae inputs. • Norcholestanes and triaromatic steroids are effectively used in oil classification. • This oil group has a higher abundance of nordiacholestanes than norcholestanes. • This oil group contains extremely high abundances of 4-methyltriaromatic steroids. The lower third and upper fourth members of the Eocene Shahejie Formation (lower Es3 and upper Es4) are known as two sets of effective source rocks in the Dongying Depression, Bohai Bay Basin. In this study, the saturated and aromatic steroids of 20 crude oil samples from the Dongying Depression were systematically investigated using gas chromatography–mass spectrometry (GC–MS) and GC–MS metastable reaction monitoring. Based on the distribution patterns of the steranes, norcholestanes, and triaromatic steroids, the Group I and II oils were clearly identified and shown to be derived from the lower Es3 and upper Es4 source rocks, respectively. Significantly, it was discovered that three oil samples collected from the Eocene Kongdian Formation and the Ordovician reservoirs exhibit completely different steroid and isotopic compositions from those of the Group I and II oils. These oils, as a distinct oil group (Group III), have the characteristics of abnormally high concentrations of C 29 steranes, 24-nordiacholestane, 27-nordiacholestane, C 28 triaromatic steroids, C 27 4α-methyltriaromatic steroids, and C 29 4α-methyl-24-ethyltriaromatic steroids, and low concentrations of 24-norcholestane, 27-norcholestane and triaromatic dinosteroids, and 13C depletion. Although the related source rock of Group III oils needs to be further clarified in the study area, the 13C depletion, abundant C 29 steranes, and low 24-norcholestane content may suggest greater bacteria/algae inputs (e.g., green algae rather than dinoflagellates) than both the lower Es3 and upper Es4 source rocks. Identification of the Group III oils indicates that the norcholestanes and triaromatic steroids may be more effective molecular fossils for the classification of crude oils in complicated petroleum exploration fields. [ABSTRACT FROM AUTHOR]

Research on noble gases is important which can provide more information on gas origins and sources due to their special chemical inertness and genesis related to nuclear processes. However, few studies have been carried out on noble gases in the largest un-compartmentalized gas field in the Tarim Basin. In this study, molecular and isotopic compositions of noble gases and hydrocarbon gases in the Kela 2 gas field were analyzed. Hydrocarbon gases are the dominant gases, accompanied by minor N 2 , CO 2 and extremely low contents of noble gases in the Kela 2 gas field. He content is generally 1 order of magnitude higher comparing to that of air, while Ne, Ar, Kr, and Xe contents are 1–3 orders of magnitude lower than corresponding atmospheric values. The δ 13 C 2 value indicates natural gases in the Kela 2 gas field are typical coal-derived gases. Noble gases in the Kela 2 gas field are typical crustal derived source, mainly originating from decay of crustal radioactive elements. Natural gases in the Kela 2 gas field are speculated to be derived from the coal measures source rocks of Triassic to Jurassic with major contribution from coals and secondary contribution from mudstones. [ABSTRACT FROM AUTHOR]

• Datangpo Formation Mn carbonates in South China have μm-scale ooidal structures. • Black shale-hosted Mn carbonate could form via microbial or physiochemical pathways. • Formation pathways correspond with post-glacial atmosphere–ocean composition. • Massive storage of inorganic carbon: Mn carbonate phase can influence carbon cycle. Manganese-rich deposits in the lower member of the Datangpo Formation (DTP) (ca. 663–654 Ma) in South China formed in the aftermath of the Cryogenian Sturtian glaciation. The Mn in the DTP occurs dominantly as rhodochrosite and Ca-rhodochrosite. A hydrothermal origin of the Mn2+ is shown by the rare earth element distribution and significantly high Mn/Fe ratios (3–19, average = 10.1). Previous studies suggested a microbially-mediated process for controlling the DTP black-shale hosted Mn carbonate deposits. However, detailed reports on the formation mechanisms of micro-scale (<2–5 μm) ooid-like Mn carbonates in the DTP have rarely been published. Systematic petrography and geochemical analyses in this study demonstrate the coexistence of two types of micro-scale ooidal-like Mn carbonates formed through two distinct mechanisms, either dominated by microbially-mediated or physiochemically-forced pathways. The Type I Mn carbonate has relatively larger grain size of 2–5 μm and exhibits a radial-concentric microfabric that shows signs of growth banding in the form of alternating light and dark laminae, which mainly express variation in Ca and Mn concentrations. The initial precipitation phase of the Type I Mn carbonate is interpreted to be Mn oxide/hydroxide, based on positive Ce anomalies and selective enrichments of particular trace elements. Novel evidence indicates that the capture of Mn as a carbonate phase directly from the water column by primarily precipitated calcite, which is referred to as the Type II Mn carbonate, has also contributed to the DTP Mn-rich deposits. Multiple roles of organic matter in Mn carbonate formation have been established: (1) catalysed Mn-redox cycling; (2) trapping and transportation of initial mineral precipitates to sediments; (3) serving as a carbon source; (4) regulating the morphology of the Mn carbonate. As a key link for understanding Cryogenian carbon and Mn cycling, specific formation pathways for the DTP Mn-carbonates are likely to have been controlled by given atmospheric-oceanic compositions (including oxygen level, p CO 2 , and redox conditions) in response to major geological and biological events during the interglacial period. In turn, massive storage of inorganic carbon and phosphorous in Mn carbonate phases would have had a substantial influence on biogeochemical carbon cycling during the Cryogenian. [ABSTRACT FROM AUTHOR]