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Lacustrine beach ridges are often used to reconstruct past lake levels. However, the sedimentary architecture involved in their sedimentary evolution remains unclear, particularly for embayed beaches. To better understand the sedimentary processes, the sedimentary architecture and geomorphology of an embayed beach on the southern of Qinghai Lake were investigated using ground penetrating radar (GPR), trenches, and modern beach observation. The embayed beach's deposits are composed of three major sedimentary facies: washover deposition, fair-weather swash deposition, and lagoonal deposition. According to the variations in sedimentary facies, the formation of the embayed beach can be divided into two stages: lake-level highstand and lake-level lowstand. During lake-level highstand stage, geomorphic changes primarily occur in the cross-shore direction. A single beach ridge with washover lobes and sheets developed along the entire embayed beach. During lake-level lowstand stage, the sediments are mainly transported alongshore within the embayed beach. The northern end shows progradational complex deposits including beach ridge, lagoon and beach system, while only a beach ridge is present at the southern end. Hydrodynamic changes in two stages lead to the heterogeneity of sediment distribution, reflected in the morphology and the sedimentary architecture. The interaction of topography and hydrodynamics controls the formation and evolution of the embayed beach. This study improves the understanding of the sedimentary processes involved in embayed beaches. [ABSTRACT FROM AUTHOR]

Ancient deep-lacustrine fans serve as prolific hydrocarbon reservoirs and provide records of climatic and tectonic activity. The deep-lacustrine fans of the Lower Jurassic Sangonghe Formation in Turpan Basin have become the most important exploration targets as stratigraphic traps. An integrated analysis of field profile investigation, core examination, well logging and seismic reflection profile interpretation, and grain size distribution patterns has enabled the identification of a series of deep-lacustrine fans. The deep-lacustrine fans of the Lower Jurassic Sangonghe Formation, deposited in the Turpan Basin consist of sandy debris flow, bottom current reworking and turbidite lithofacies. The deep-lacustrine fans are depicted by the following geological and geophysical features: (1) abundant soft-sediment deformation structures, and dish structures, normal graded beddings and inverse graded bedding structures developed in the strata, (2) the grain size distribution on the C–M diagram is parallel to the C = M baseline and cumulative probability curves display a bi-segment pattern with the suspension components to be the dominant parts, (3) gamma-ray logging curve are inverse-cycle features and seismic reflection profiles are characterized of poor continuity, high amplitudes, scattered and lenticular shapes. The deep-lacustrine fans are favorable target reservoirs, and have become the most important exploration targets in the Turpan Basin. [ABSTRACT FROM AUTHOR]

The shale of Cambrian Niutitang Formation is one of the most important shale gas reservoirs in the Middle Yangtze region, South China. Nanopores are important reservoir spaces for shale gas. The study of pore characteristics and its formation controlling factors is of great significance for the exploration and development of shale gas in the Niutitang Formation. In this study, two drilling core samples and two profile samples were investigated. The laser Raman, X-ray diffraction analysis, FE-SEM, and low-pressure N2 and CO2 adsorption methods are used to study the geochemical and mineral compositions of the shales, and the nanopore characteristics and its formation controlling factors of the shales are discussed. Results show that the shales have the following features: (1) rich in organic matter (OM) and the total organic carbon (TOC) contents ranging from 2.65 to 11.33%, with an average value of 6.81%. (2) The main mineral components of these shales are quartz, feldspar, clay minerals, calcite, and a small amount of pyrite. (3) The samples from the center of the Middle Yangtze region (EY1 well and BGP profile) have a higher maturity value ( Rmc R 0 % = 4.0 %) relatively to the samples from the south of the Middle Yangtze region (XD1 well and JF profile), where the Rmc R 0 % is 2.8%. (4) The maximum N2 adsorption capacity of the four groups of shale samples ranges from 5.090 to 27.333 cm3/g, with an average value of 15.571 cm3/g, while the maximum CO2 adsorption capacity ranges from 2.686 to 5.567 cm3/g, with an average value of 3.776 cm3/g. The higher maturity leads to the EY1 well having the lowest average maximum N2 adsorption capacity, while most of the OM pores in the EY1 well and BGP profile samples are smaller than 10 nm in diameter. The tectonic uplift leads to the profile samples having more meso-macropores than the core samples. The dissolution pores and granular mineral support are the key factors for the better pore structure of the high-evolution stage Niutitang Formation shale. [ABSTRACT FROM AUTHOR]

The Chang 7 member of the Upper Triassic Yanchang Formation in the Ordos Basin is considered to hold the main source rocks for conventional and unconventional oil and gas. The lamination or lithology alteration in vertical and lateral directions, even over a short distance, is a common feature in lacustrine source rocks. The differences in the geochemical characteristics of black shales, dark mudstones and interbedded sandstones have been scarcely reported, and their influences on the petroleum generation potential and shale oil potential are not clear. To this end, 22 core samples were collected from the Lower and Middle Chang 7 (C7-3 and C7-2) members of the Triassic from well CYX in the Qingcheng area. By conducting a series of geochemical analyses including TOC, Rock-Eval pyrolysis yields, bitumen extraction and quantification, and the separation and quantification of saturates, aromatics, resins and asphaltenes, along with biomarker analyses, several results were found. Firstly, the C7-3 and C7-2 source rocks are thermally mature and have entered into the stage of hydrocarbon generation. The C7-3 and C7-2 source rocks have a good to very good hydrocarbon generation potential especially the C7-3 black shales. Secondly, terrigenous source input is more abundant in C7-2, whereas the source input of phytoplankton, algae or microbial lipids is more abundant in C7-3. Moreover, a high TOC content basically corresponds to low wax indexes, terrigenous/aquatic ratios (TARs), and Pr/nC17 and Ph/nC18 ratios and high C27/C29 regular sterane ratios, which suggests that the source input of phytoplankton, algae or microbial lipids is favorable for OM accumulation. Third, analyses of the molecular composition of saturated fractions in shales and interbedded sandstones and the production index demonstrate the migration of petroleum from organic-rich source rocks to their organic-lean counterparts. The C7-2 dark mudstones could be considered as a potential "sweet spot" since their oil saturation index (OSI) was the highest among all the studied samples and they are more enriched in light aliphatic fractions. [ABSTRACT FROM AUTHOR]

In recent years, significant progress in shale gas exploration has been achieved in the Upper Ordovician (Wufeng Formation)-Lower Silurian (Longmaxi Formation) shales in the Upper Yangtze area, South China. Although many studies have been carried out on the Upper Ordovician-Lower Silurian shales, the controlling factors causing organic matter accumulation of these shales remain controversial. This study uses trace-element geochemistry and sedimentological methods to evaluate terrigenous input, redox conditions and primary productivity to explore the mechanisms of organic matter accumulation. The variation of terrigenous fraction elements (Al, Th and Sc) concentrations reflect a mixed influence of sea-level change and weathering. The sea-level of the Upper Yangtze Sea went through two cycles of transgression to regression during the Ordovician-Silurian transition. The Linxiang Formation, Kuanyinchiao Bed and the upper part of Longmaxi Formation developed during the periods of regression, whereas the Wufeng Formation and the lower part of the Longmaxi Formation developed during the periods of transgression. The paleo-productivity indexes of TOC content, ratios of Ba/Al and P/Al, and redox conditions proxies of Mo concentration, ratios of U/Th and V/Cr generally display similar variation patterns with respect to the sea-level changes. High TOC contents and Ba/Al and P/Al ratios indicate the paleo-productivity was high on the sea surface, as shown by relatively good positive correlations between Th vs. TOC, and Sc vs. TOC. This indicates that the paleo-productivity was controlled by the nutrients input through weathering. The good positive correlations between redox conditions indexes (U/Th and V/Cr ratios) with TOC content reflects reductive preservation conditions (anoxic to euxinic), thus implying they were an important controlling factor for organic matter accumulation. Nevertheless, redox conditions were closely associated with sea level change and organic matter decomposition. Therefore, the sea-level change and weathering were the primary controlling factors for organic matter enrichment across the Ordovician to Silurian transition. [ABSTRACT FROM AUTHOR]

In this paper, we attempt to clarify the relationships between oilfield water and hydrocarbon distribution of the Triassic Baikouquan Formation, in the slope zone of Mahu depression, Junggar Basin, NW China, based on oilfield water mineralization degrees, diagenesis analysis, formation pressure coefficients, burial history, and thermal evolution processes. The results show that the study area can be divided into four zones according to the mineralization degrees, that is, (1) zone I: NaHCO3 + Na2SO4 type water with residual intergranular pores, porosity usually higher than 10%. The strata of zone I are generally buried shallow and influenced by leaching from atmospheric precipitation. This area contains normal edge and bottom water and has normal formation pressure coefficients (average 1.12); (2) zone II: CaCl2 type water with mainly compaction diagenesis, porosity usually less than 7% and high formation pressure coefficients (average 1.41); (3) zone III: NaHCO3 + CaCl2 type oilfield water with mainly feldspar dissolution diagenesis, porosity usually greater than 9% and high formation pressure coefficients (average 1.53); and (4) zone IV: CaCl2 type water, mainly compacted diagenesis with a few intergranular and feldspar dissolved pores, and high formation pressure coefficients (average 1.58). The mineralization degrees of the oilfield water in zone II, III, and IV are relatively high. The appearance of NaHCO3 type water in the deeply buried areas is usually related to oil and gas charging. Therefore, within the strata, deeply-buried areas (e.g., zone III), where NaHCO3 type water has been developed, are favorable targets for oil and gas exploration. [ABSTRACT FROM AUTHOR]

Multiple indicators (trace fossils, trace metals, and δ13Corg) are used to assess the depositional environments and controls on organic carbon accumulation in the Lower Carboniferous Nanmingshui Formation, Junggar Basin, NW China. The majority of trace fossils belong to the Nereites ichnofacies, representing low energy, oxygen‐poor, and relatively quiet bottom water environments. However, the shallow water trace fossil Planolites also appears in the strata, possibly reflecting variations in the bottom water oxygen contents of the turbidite succession. Redox proxies Th/U, V/Sc, and V/(V + Ni) reveal variable redox conditions; most of the samples are in the range from suboxic to anoxic depositional conditions. Ni and Cu show a positive correlation with TOC, but U and V have no correlation with TOC, also suggesting the redox conditions were mainly suboxic. The P/Al and Cu/Al ratios of all samples are higher than those of Post‐Archean Australian Shales (PAAS), and about two‐thirds of the samples' Ba/Al and Ni/Al ratios are close to or higher than PAAS, suggesting the Early Carboniferous Junggar Basin had relatively high palaeo‐productivity levels. δ13Corg values average − 23.75‰, reflecting a mixture of terrestrial organic and marine organic matter. The indexes of Ba/Al, Cu/Al, Ni/Al, and δ13Corg show good positive correlations with TOC, indicating the palaeo‐productivity played an important role for the organic matter accumulation. On the other hand, the TOC correlates inversely with redox proxies (Th/U, V/Sc, and V/(V + Ni)), suggesting the preservation conditions (anoxia) are not the control factor of organic matters accumulation. Using these multiple approaches, our results indicate that productivity was the primary factor controlling organic matter accumulation. [ABSTRACT FROM AUTHOR]

Total organic carbon content (TOC), carbon isotope and biomarker analysis have been done on the source rocks from Lucaogou and Wutonggou formations in Junggar Basin, NW China. (1) the TOC values of the source rocks are more than 5%, and the kerogen types are type I1-II, and the organic matters are mainly algae and bacteria, and are in the mature stage. (2) the depositional environments of the source rocks are weak reduction-weak oxidation, and are of weakly salty water to brackish water environment. [ABSTRACT FROM AUTHOR]

Beach ridges are landforms that exist on beach shorelines and can supply abundant information regarding palaeoenvironments. To better understand the hydrodynamic processes of beach ridges, a quartz microtexture analysis was carried out. In this study, the microtextural features of a total of 1063 quartz grains from seven beach ridge samples and eight reference samples from rivers, a dune, and a nearby beach were observed and analysed by scanning electron microscopy. The qualitative and quantitative results suggest that the quartz microtextures of the beach ridge samples present more fresh surfaces and percussion marks than the river samples but less angularity. The upper sequence samples from the beach ridge present higher values of fresh surfaces and percussion marks and lower dissolution than the lower sequence samples. A cluster analysis shows that the hydrodynamic conditions of Qinghai Lake underwent pronounced changes from weak to strong and then to weak at the present time. Furthermore, quantitative analyses indicate that the beach ridge and river sedimentary environments can be distinguished through the microtextural features of the 250–125 μm size fraction. However, more grain size fractions should be observed and analysed to interpret the hydrodynamic conditions. The quartz microtextural analysis of beach ridges may be an effective method reflecting the palaeohydrodynamic conditions. • Beach ridge and river sedimentary environments can be distinguished using the microtextural features. • All grain size fractions (2000–63 μm) should be observed and analysed to interpret hydrodynamic conditions. • Quartz microtextural analysis may be an effective method to reflect the palaeohydrodynamic conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Intensive hydrothermal activity occurred during the deposition of the Lucaogou Formation. • The enrichment of both Sr and Li in the shales is consistent with a hydrothermal origin. • Hydrothermal activity supplied nutrients that enhanced paleoproductivity. Organic matter enrichment in black shales has frequently been linked with hydrothermal activity. However, it is poorly understood how hydrothermal activity affected paleo-environments and the enrichment of organic matter in ancient lacustrine sediments. The hydrothermal activity recorded in the Middle Permian Lucaogou Formation (LCGF) in the Jimusar Sag, Junggar Basin, Northwest China, provides an ideal sedimentary archive to explore this issue. The mineralogical composition, major and trace element, total organic carbon, and sulfur content of forty-three shale samples of the LCGF were investigated to better understand how hydrothermal activity influences organic matter enrichment in lacustrine shales. The terrigenous influx, redox conditions, paleoproductivity, hydrothermal effects, and organic matter enrichment mechanisms are discussed. The results show that the terrigenous influx is insufficient overall, and the lower section of the LCGF has less detrital materials than the upper section. The redox conditions of the bottom water of the lake in the Jimusar Sag during deposition of the LCGF were suboxic to anoxic, which, combined with relatively high paleoproductivity, is favorable for organic matter enrichment. Hydrothermal activity occurred more frequently during deposition of the lower section compared with the upper section of LCGF. Hydrothermal fluids provided sufficient nutrients to enhance organic matter production during deposition of the LCGF and resulted in organic matter enrichment. [ABSTRACT FROM AUTHOR]

• Productivity of marine plankton in Late Ordovician was largely reduced by the deposition of volcanic ashes in the ocean. • Weathering of volcanic ashes reduced the concentrations of atmospheric carbon dioxide. • The Late Ordovician glacial event was triggered by intense volcanic eruptions in a short time. Late Ordovician – Early Silurian bentonites and shales are widely distributed in the western Yangtze plate, South China. This study is to use these rocks to examine the mechanism of global cooling. We present total organic carbon content (TOC) and organic carbon isotope data. The TOC of the shales for the Qiliao (QL) and Shuanghe (SH) sections range from 0.21% to 5.88% with an average of 3.0%, and from 1.48% to 6.34% with an average of 2.91%, respectively. The organic carbon isotopic compositions of the shales from the QL and SH sections are 31.1‰–29.4‰ (average 30.3‰) and −30.8‰–29.1‰ (average 30.0‰), respectively. In combination with previous studies of the Katian to Hirnantian glaciation, two conclusions can be summarized: (1) shales in close proximity to volcanic ash (bentonite) layers exhibit obvious TOC reduction, which indicates that the deposition of volcanic ash inhibited the productivity of marine plankton; (2) the organic carbon isotope compositions of shales close to volcanic ash layers present positive excursions, reflecting the decline of the atmospheric carbon dioxide after the volcanic ruption. The main reason is probably that the intense volcanic eruptions created abundant volcanic ash for weathering. It is probably 840 × 1015 kg (1.9 × 1019 mol) carbon dioxide was consumed in the process of volcanic ash weathering, this would make the carbon dioxide content of the atmosphere drop to the threshold level, which triggered the global glacial event during the Early Hirnantian stage, and then the Earth entered into an ice age in the Late Ordovician. Therefore, this study provides a new point of view to understand the climate variation in the Earth's geological evolution history. [ABSTRACT FROM AUTHOR]

• Reviews on tectonics and paleoenvironment of the Junggar Basin are present. • An updated chronostratigraphic framework is provided for Permian Junggar Basin. • Dry climate and evaporative environment prevailed during deposition of source rocks. • Stable salinity stratification and limited dilution promoted organic matter burial. • Organic carbon burial in lakes may act as a significant carbon sink in Permian. The Junggar Basin in northwestern China is a typical "walled" sedimentary basin that has a prolonged basin evolution history from the Paleozoic to Cenozoic and is rich in petroleum resources. Permian strata reach thousands of meters in thickness and have been intensively studied for their record of regional tectonics and paleoenvironments. They have also been exploited for oil and gas for the past several decades. In this paper, we summarize perspectives on tectonic evolution, stratigraphic framework, sedimentary environments, and the paleoclimate of the Permian Junggar Basin and the adjacent Turpan Basin. Further, we discuss the mechanism and consequences of massive organic carbon burial. Based on a compilation of published radiometric dating datasets, an updated chronostratigraphic framework was proposed for the Junggar Basin. This framework constrains the ages of two main petroleum source intervals, the Fengcheng Formation and the Lucaogou and equivalent formations, as Early Permian and Middle Permian, respectively. Although a global warming and drying trend was present throughout the Permian Period, the regional climate over the Junggar Basin was drier in Early Permian and more humid in the Middle-Late Permian with short-term climatic fluctuations. During the deposition of the two petroleum source intervals, dry and temperate climate prevailed and evaporative lacustrine environment dominated. Organic matter enrichment was promoted by stable salinity stratification and limited dilution by inorganic sediments, leading to one of the richest lacustrine source rock deposits in the world. The average organic carbon burial rates were 11.2 gC/m2/yr for the Lucaogou Formation and 3.5 gC/m2/yr for the Fengcheng Formation, lower than the average organic carbon burial rate in modern lakes. However, considering the large areas of global tectonic lakes in the Permian Period, organic carbon burial in lacustrine environments could act as a significant carbon sink and impact the global carbon cycle and climate evolution. [ABSTRACT FROM AUTHOR]

Increasing evidence shows that the recovery of marine ecosystems after the Permian/Triassic mass extinction (PTME) was coupled with environmental amelioration. Recently, abundant insects and complete trophic structures were unraveled from the Ladinian (late Middle Triassic) in the Ordos basin, central China, deciphering recovery of lacustrine ecosystem from the PTME. Coeval environmental settings, however, remain unclear. In the Ordos basin, the Ladinian succession is dominated by black organic-matter-rich shales of the Chang 7 Member of the Yanchang Formation, and their depositional environments and palaeo-productivity are reconstructed based on major and trace element concentrations, pyrite framboid size and morphology, and total organic carbon (TOC) contents. High values of TOC and phosphorous (P) concentrations, as well as more enrichment of trace-metal elements such as Copper (Cu) and zinc (Zn) all indicate a rather high primary productivity. Such a high productivity during the deposition of the Chang 7 3 sub-member is also reinforced by and the presence of abundant organic-matter-enrich laminae in black shales and the diverse chrysophytes, as well as intensive occurrence of tuff layers and possible hydrothermal fluids. Analyses of framboidal pyrite size and morphology as well as C org /P ratio and trace-metal elements such as molybdenum (Mo) and uranium (U), indicate the dysoxic to oxic conditions in water columns of the Ordos lake during the late Middle Triassic. The Ladinian-Carnian cooling likely promoted the occurrence of dissolved oxygens in the lake. As a result, although the massive black shales of the Chang 7 3 sub-member were deposited and suggest a rather high primary productivity, they also indicate slightly oxygen-poor to oxygenated conditions of water columns, which are inductive for lacustrine biota to inhabit, and also offer hospitable habitats for freshwater lake ecosystems to recover from the PTME. • A rather high primary productivity occurred in a Ladinian freshwater lake. • Slightly oxygen-poor to oxygenated water conditions developed in a Ladinian lake. • Ordos lake offered hospitable habitats for freshwater ecosystems to recover. [ABSTRACT FROM AUTHOR]

Geological records of Milankovitch cycles provide a temporal framework for reconstructing Earth climate evolution and mapping out the history of the ancient Solar System. Understanding the fundamental celestial mechanics driving astronomical rhythms during the Permian Period from stratigraphic record is challenging because of the inherent limitations of astronomical solutions, the chaotic nature of Solar System motion, and the paucity of cyclostratigraphic studies of geological records. The Junggar and adjacent basins of northwestern China constituted a large tectonic lake in the Middle Permian, which deposited one of the thickest and richest petroleum source rock intervals in the world. Constructing an accurate timescale for the Permian Junggar Basin is critical to understand the processes that controlled organic carbon burial as well as paleoclimatic and paleoenvironmental background conditions. In this study, detailed spectral analyses of natural gamma ray (NGR) data were performed on the lacustrine muddy-dominated Lucaogou Formation from five exploitation wells. The results reveal significant meter-scale sedimentary cycles of 37.8 m, 10.7–10 m, 3.85–3.2 m, and 2.0–1.58 m, reflecting oscillations in lithologies (variations in changing clay content within the mudstone, sandstone and dolomite). These cycle wavelength ratios match well with those of the Milankovitch cycles predicted for Middle Permian Period. The depositional duration of the Lucaogou Formation was estimated at ~3 Myr. Correlation of the ~1.2 Myr obliquity modulation cycles among the NGR logs, sedimentation rates, and lake levels of the Junggar Basin, as well as with the Middle Permian global sea level, suggests that long-term astronomical "grand cycles" tightly forced climatic and sedimentary processes throughout the Lucaogou Formation. According to two independent approaches, we reconstruct Middle Permian astronomical parameters (Earth precession constant: k and fundamental frequencies terms: g i ; i = 1, 2, 3, 4, 5) and calculate astronomical periods (eccentricity, obliquity and precession). These results document astronomical forcing of Paleozoic lake systems, constrain Earth-Moon orbital evolution, and refine the fundamental astronomical frequencies of the Solar System during the Middle Permian. • First report of Milankovitch cycles in Middle Permian terrestrial strata • The Lucaogou Formation, in Junggar Basin was deposited at ~270–267 Ma. • The "grand cycle" influenced climate and depositional processes in middle Permian. • Two approaches are used to estimate the Middle Permian astronomical parameters. • Cyclostratigraphic analyzing refined the Earth-Moon history in the Paleozoic. [ABSTRACT FROM AUTHOR]