The Chang 8 Member in the Longdong area of the Ordos Basin hosts significant petroleum resources, demonstrating substantial potential for tight oil exploration and development. Astronomical forcing exerts a discernible influence on the evolution of its petroleum system. To elucidate the impact of Milankovitch orbital cycles on organic enrichment and the development of source rocks, reservoirs and cap rocks, we conducted a high-resolution cyclostratigraphic analysis of the Chang 8 Member stratigraphy. This study utilized gamma-ray (GR) well log series as the primary dataset. This lacustrine succession preserves distinct Milankovitch cycles, including similar to 405 ka long eccentricity, similar to 125 ka short eccentricity, obliquity, and precession periods, with eccentricity cycles showing particularly strong expression. These diagnostic eccentricity signals provided the framework for delineating high-frequency sequences. Subsequent astronomical tuning and base-level reconstruction constrain the depositional age of the Chang 8 Member to 242.22-241.23 +/- 1.4 Ma. During this interval, the lacustrine system exhibited a pronounced trend of base-level fall followed by rise, punctuated by higher-frequency fluctuations. Milankovitch cycles govern the development of high-quality reservoirs and cap rocks and organic enrichment by modulating climate and lake-level fluctuations. These orbital forcings drive weathering processes, control fluvial sediment supply and lacustrine accommodation space, and influence biological productivity. Our results demonstrate a pronounced association between the long eccentricity cycle (similar to 405 ka) and enhanced reservoir quality development, while the short eccentricity cycle (similar to 125 ka) exhibits a stronger correlation with organic matter enrichment, cap rocks, and source rock formation. Ultimately, the interplay of eccentricity cycles jointly governs the formation of the hydrocarbon system within the continental Chang 8 Member.

The temporal framework of the Ediacaran Period remains fragmentary, thereby hindering progress in understanding the coevolution of ocean oxygenation and multicellular eukaryotes. This study presents Milanković cycles and carbon isotopic profile of the Doushantuo Formation of South China, establishing a continuous astrochronology from 635.1 ± 0.6 Ma to 568.3 ± 6.8 Ma. The most negative carbon isotope excursion, Shuram excursion of South China (EN3/DOUNCE), started no earlier than 584.2 ± 5.2 Ma and persisted for over 12.2 ± 1.4 Myr. Compared to the Shuram excursion recorded in open-ocean settings, the Shuram excursion of South China exhibits temporal heterogeneity in terms of potentially earlier onset, prolonged duration, and slower triggering, suggesting a heterogeneous pattern of global oceanic oxygenation. In addition, the prominent carbon cycle perturbations during the Shuram excursion in South China display sea-level paced oscillations. The newly established ∼67-Myr high-resolution astrochronology refines the current understanding of the evolutionary tempo and dynamics of Ediacaran macroscopic organisms and oceanic environment.

We conducted in-depth research on the hydrocarbon generation and expulsion characteristics of the Permian humic source rocks and the evolution of the reservoir gas content in the eastern margin of the Ordos Basin. Using low-mature shale from the Palougou section, thermal simulation experiments were conducted in a closed system. Rock-Eval pyrolysis, total organic carbon (TOC) analysis, vitrinite reflectance (EasyR(o)) analysis, and gas chromatography (GC) were performed on sample residues and products. Based on isothermal adsorption experiments combined with the hydrocarbon expulsion conceptual model, we established evolution models for thermogenic gas (G), gas-in-place (GIP), adsorbed gas (V-ad), and free gas (V-free). The results are as follows: (1) The hydrocarbon generation and expulsion stages can be divided into the immature (EasyR(o)<0.77%), oil generation (EasyR(o) = 0.77-1.54%), oil cracking (EasyR(o) = 1.54-2.51%), and secondary heavy hydrocarbon gas cracking stages (EasyR(o) = 2.51-3.61%). Based on aromatic and phenolic features, the humic source rocks have a significant late gas potential at EasyR(o)>1.99%; (2) The volume of G is 5.89-9.25 cm(3)/g. The V-ad is positively correlated with the pressure and TOC, and negatively with the temperature, ranging from 0.67 to 13.34 cm(3)/g. The hydrocarbon expulsion efficiency (eta(pg)) is 65.28-86.62%, and the GIP is 5.02-7.38 cm(3)/g. Furthermore, the hydrocarbon generation intensity (I-St) and expulsion intensity (I-Pt) are (0.42-8.62) & times; 10(8)/km(2) and (1.89-21.75) & times; 10(8) t/km(2), respectively. In the study area of Daning-Jixian, the amount of hydrocarbon generation (Q(St)) is 25.94 & times; 10(8) t and expulsion (Q(Pt)) is 9.17 & times; 10(11)t; (3) A key well Daji3-4 in the study area contains the strata from the Benxi Formation to the P(1)s(2)(2) member, and the main hydrocarbon generation center is located in the lower Shan(2)(3) layers, with the current free gas volume of 0.41 cm(3)/g. Rapid subsidence began at around 144 Ma during the Late Cretaceous, which increased the geothermal gradient, leading to increased maturity and a significant increase in gas content.

The Tonian Period (1000–720 Ma) bore witness to the transition from a prokaryote-dominated marine ecosystem to one characterized by the proliferation of eukaryotes. This fundamental shift has generally been attributed to evolving marine redox states. Here, we present sedimentological and geochemical analyses of the early Tonian Huainan, Feishui, and Huaibei groups in the Xuhuai basin of the North China craton. Multiple redox proxies show consistent, water depth-dependent variations across the Xuhuai basin. Excess barium contents and Ba/Al ratios further highlight spatial variations in primary productivity which ultimately regulate basinal redox structures. We propose that a shallow-water oxygen minimum zone sandwiched between the oxic/suboxic mid-depth and surface layer water masses occur in the oligotrophic Xuhuai basin, which is analogous to, but much shallower than modern oxygen minimum zones. Such marine redox architectures may benefit the maintenance of a bioavailable nitrate reservoir in the ocean, foreboding the subsequent expansion of eukaryotes. A shallow-water anoxic zone sandwiched between the oxic surface layer and mid-depth water masses occurs in the oligotrophic early Tonian Xuhuai basin of North China craton. It is analogous to, but much shallower than, modern oxygen minimum zones.

The organic-rich shales from the Lower Cretaceous Dalazi Formation in northeastern China are well known for their rich terrestrial fossils and significant hydrocarbon potential. However, the exact depositional environments of these shales remain unclear. Here, integrated geochemical proxies, including iron speciation (Fe-HR/Fe-T, Fe-py/Fe-HR), pyrite sulfur isotopes (delta S-34(py)), and major- and trace-element enrichments were carried out on these organic-rich shales from the Zhixin section of the Yanji Basin and the Beigou section of the Luozigou Basin in Northeast China. This study aims to provide insights into the contemporaneous continental chemical weathering intensities accompanied with the lake redox conditions during the deposition of the Dalazi Formation shales. Low chemical index of alteration (CIA) values were detected in the shales, indicating low weathered continental environments during the depositional period of the shales. The reduced water availability under dry climate evidenced by plant fossils likely contributes to limiting the intensity of chemical weathering with low CIA values. All samples exhibit Fe-HR/Fe-T ratios >0.38 and Fe-py/Fe-HR ratios far below 0.7, displaying the water column characterized by anoxic ferruginous conditions with limited HS- availability during the shale deposition. The reconstruction of redox condition is consistent with the observations of the low pyrite contents, minimal well-formed framboidal pyrite crystals and covariation patterns of Mo-EF and U-EF in the shales. With low CIA values, the less continental input to the lake probably gives rise to the anoxic ferruginous deep-water conditions via limiting sulfate availability, which is consistently supported by high delta S-34(py) values (12.2 +/- 9.6 parts per thousand), low pyrite contents, and the remarkable negative correlation between the CIA and delta S-34(py) values. Considering the high content of organic matter up to 6.38 wt% and abundant fossil collections in the shales, this finding highlights the beneficial role of a sustained and stable anoxic ferruginous environment for preserving organic matter and carbonaceous fossils in the Dalazi Formation.

Pyrite sulfur isotopic composition (delta S-34(py)) is a crucial proxy for reconstructing ancient Ediacaran marine environments. However, recent in situ isotopic analyses of sedimentary pyrite have revealed distinct delta S-34(py) signatures among different pyrite morphologies, indicating that secular changes in bulk delta S-34(py) may reflect variations in proportions of different pyrite morphologies rather than environmental signals. Up to now, intragrain isotopic patterns within individual pyrite grains have not yet been extensively investigated for Ediacaran samples. The absence of this specific data set has hindered our ability to understand current complexities of bulk delta S-34(py) in reconstruction paleoenvironment. This study aims to elucidate delta S-34(py) patterns by conducting in situ isotopic analysis on pyrite grains from Ediacaran drill-core samples. We employed scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), Raman spectroscopy, and nanoscale secondary ion mass spectrometry (NanoSIMS) to examine the crystal texture, element atomic ratios (S/Fe), mineral composition, and in situ isotopic composition within single pyrite grains. SEM-EDS observations reveal that the euhedral/subhedral pyrite crystals generally exhibit uniform mineral texture, although some grains show varying degrees of dissolution edges and surface cavities. Raman spectroscopy demonstrates the possible presence of pyrrhotite mineral within pyrite grains at some specific sites and points. In situ isotopic analysis reveals significant intragrain delta S-34 heterogeneity, with difference reaching up to 69.3 parts per thousand on a micrometer scale. Distinct deposition depths and burial rates may contribute to the varying value ranges and degrees of isotopic heterogeneity amongst pyrite grains. A model suggesting rapid precipitation from numerous nucleation sites simultaneously could account for the observed intragrain heterogeneity of in situ delta S-34 values. Pyrite grains exhibit general atomic S/Fe ratios of > 2, potentially due to the presence of trace elements incorporated into the pyrite structure. The occurrence of pyrrhotite leads to a slightly positive correlation between in situ delta S-34(py) values and S/Fe ratios, yet the S-32-enriched pyrrhotite plays less substantial in generating intragrain delta S-34 heterogeneity of pyrite grains. Our findings reveal clear isotopic heterogeneities within individual pyrite grains, in addition to the notable delta S-34(py) differences among pyrite of various morphologies. These results highlight significant microenvironmental heterogeneity and dynamic sulfur pool mixing on rapid short-term timescale during pyrite growth.

Early to middle Ediacaran organic-rich black shales host several famous fossil biotas and provide key evidence for understanding the coevolution of multicellular organisms and palaeoceanic environment. The water column redox states play critical roles in organic-rich shale deposition. Amongst multiple geochemical redox indexes, iron-speciation chemistry (FeHR/FeT, Fepy/FeHR) in shales constitutes a reliable proxy to reconstruct the first-order redox framework of the ocean basin. However, iron-speciation is a local marine redox proxy, and only compiled data collected from multiple different sedimentary facies record statistically significant changes of oxidation states within a depositional basin. Here we compiled the spatiotemporal distribution of iron-speciation data in early to middle Ediacaran black shales of the Yangtze block in South China. New high-resolution analysis on two outcrops and one drill-core reflects overall ferruginous condition and dominant euxinic condition locally interrupted by oxic states, respectively, generally indicating pervasive anoxic depositional environment for the Member II shales of the Ediacaran Doushantuo Formation in the Lower Yangtze block. Compilation of fourteen Ediacaran sections from shallow-, slope- and deep-water facies demonstrates frequent spatiotemporal oscillation of the redox conditions throughout the Yangtze block. Widespread ferruginous states, accompanied with euxinic zones mainly focusing in lower slope facies, are detected during the early Ediacaran period, in contrast with more widespread euxinic settings in the middle Ediacaran period. Statistical data including new high-resolution results from the Lower Yangtze block show generally low enrichment for redox sensitive elements (RSEs, e.g., Mo, V and U) in the Member II shales probably due to their contemporary low seawater concentrations. An increase of RSEs concentrations occurs in the Member IV shales, which probably reflects more oxidized Earth surface environment during middle Ediacaran period. The increasing Earth oxygenation documents the occurrence of widespread euxinic states during deposition of the Member IV shales. Compilation of spatial distribution of TOC contents in black shales demonstrates their apparent relevance to spatial distribution of the euxinic rather than ferruginous states, which is likely due to the favorable role of sulfurization process on burial and preservation of organic matters. The euxinic states may also facilitate the exceptional preservation of the soft-bodied Ediacaran fossils.

Organic-rich shales from the Triassic Yanchang Formation in the Ordos Basin in China are mainly derived from aquatic organisms with type II kerogen. A reverse maturity trend, derived from the commonly used biomarker maturity parameter Ts/(Ts + Tm) occurs in the depth profiles of the Chang 73 submember and the Chang 8 member. In contrast, maturity proxies derived from aromatic compounds show a normal sequence for more deeply buried Chang 8 samples exhibiting higher maturity levels, as expected. To explain the abnormal phenomenon, multiple controlling factors-including the paleoredox condition, water salinity, the clay mineral content and composition, biodegradation, the primary migration fractionation effect, and organofacies-are considered. It was found that organofacies BC (HIo - 400-250) appear in the Chang 8 samples, while organofacies B (HIo - 400-650) appear in the Chang 73 submember. The results suggest that variation in organofacies has a significant impact on Ts/(Ts + Tm) values, even for samples with the same kerogen type and similar source input, and is primarily responsible for the reverse maturity trend in the depth profiles. (c) 2024 Sichuan Petroleum Administration. Publishing services by Elsevier B.V. on behalf of KeAi Communication Co. Ltd.

The Chuaria is the most common fossils in the Ediacaran Lantian biota (approximately 600 Ma) of South China which represents one of the earliest-known macroscopic eukaryotic fossil assemblages with high morphological diversity. Although great majority of the Lantian Chuaria fossils are preserved as carbonaceous compressions, Chuaria has been reported to be commonly preserved as pyritization enveloped by aluminosilicate minerals in the upper Member II of the Lantian Formation. The outer minerals that envelope the pyrite layer have been assigned to quartz or aluminosilicate clay minerals. However, their exact mineral compositions as well as whether these minerals have contributed to the exceptional preservation of Ediacaran Lantian Chuaria, remain unclear. We employed light microscopy, field emission scanning electron microscopy (FESEM), energy-dispersive spectroscopy (EDS), and Raman spectroscopy techniques to revisit the preservation of pyritized Chuaria from the Lantian biota. Our taphonomic experiments show that pyritized Chuaria fossils are enveloped by platy minerals that are a complex mixture of quartz and magnesium-rich aluminosilicates. Integrated mineral and structural pattern analyses show that these quartz and clay minerals are both secondary overgrowth on Chuaria internal mold, which probably formed as a diagenetic product concurrent with carbonate dissolution in the Lantian black shales. Their formation is probably regulated by local micro-environment near the Chuaria bodies. These enveloped minerals may facilitate preservation and identification of Chuaria fossils, but they are probably not involved in the initial fossilization process as the early diagenetic pyritization has done.

The Middle Triassic Tongchuan Formation in the Ordos Basin of northwestern China is a typical lacustrine deposit. A major component of this formation is thick layers of organic‐rich shales that are probably a potential hydrocarbon source and preserve the earliest known Mesozoic‐type lacustrine ecosystem. The exact depositional environment of the shales in the Tongchuan Formation, however, remains unknown. To address this question, we carried out high‐resolution carbon (δ13Corg) and sulphur (δ34Spy) isotope analysis as well as undertook total organic carbon (TOC)/pyrite contents and pyrite morphology investigation, and framboidal pyrite size measurements in shales of the Bawangzhuang section of the southern Ordos Basin. Remarkably high TOC (23 ± 9%) and pyrite (7 ± 3%) contents were obtained from the shales, which indicate a large amount of organic carbon and pyrite burial during shale deposition. Framboids are the dominant pyrite morphology in the pyrite crystals and show large and variable mean diameters (7.0 ± 1.7 μm to 14.3 ± 6.8 μm) across the section, indicating oxic–dysoxic bottom water during shale deposition. δ13Corg and δ34Spy revealed narrow and less variable values, ranging from −31.8‰ to −28.1‰, and −4.1‰ to 4.9‰, respectively. The δ13Corg values suggest balanced and consistent carbon cycles. Integrated with pyrite content and morphological patterns, consistent δ34Spy values probably demonstrate a relatively open environment for the formation of sedimentary pyrite, and thus a shallow chemocline that was quite close to the water‐to‐sediment interface during shale deposition. Overall, the organic‐rich shales of the Tongchuan Formation were probably deposited under oxic–dysoxic bottom‐water conditions. Shallow chemocline depth combined with moderately high sedimentation rate and high primary productivity may have played crucial roles in the deposition and formation of the organic‐rich shales in the Tongchuan Formation. The shallow chemocline also facilitates the fossil preservation in a lacustrine environment.

Geological background, analytical methods and results.

埃迪卡拉纪普遍缺乏有效的同位素年龄和生物地层资料,碳同位素(δ13Ccarb)常用来作为其内部地层划分与对比的标准.目前,关于埃迪卡拉纪扬子地台δ13Ccarb地层的研究多集中在中上扬子区,扬子地台东缘下扬子区的研究则较少.本文详细分析了扬子地台下扬子区邵家山剖面地层序列和δ13Ccarb特征,发现邵家山剖面以碳酸盐岩为主,为该时期典型的浅水相沉积;δ13Ccarb值分布范围为-2.6‰ ～ 5.7‰,均值为0.9‰±0.3‰.剖面的下部、中下部、中上部以及上部的δ13Ccarb负漂移幅度分别为1.6‰、4.1‰、7.6‰和3.2‰,可分别对应峡东地区的EN 1/CANCE、EN2/BAINCE、EN3/DOUNCE和EN4/BACE.对比分析扬子地台典型的8个浅水相剖面发现:盖帽白云岩广泛发育EN1/CANCE,δ13Ccarb分布在-6‰～0‰之间;EN3/DOUNCE在各剖面均有发育,漂移幅度均大于4‰,最大可达15.6‰;EN2/BAINCE仅在峡东地区的剖面发育,漂移幅度可达8.2‰;EN4/BACE在大多数剖面发育,漂移幅度为1.2‰ ～ 8.7‰.其中,EN4/BACE可作为广泛采用的埃迪卡拉系—寒武系界线的划分依据,而EN1/CANCE和EN3/DOUNCE可作为扬子地台埃迪卡拉系内部地层划分对比的重要标志.

Major radiations of microscopic and macroscopic eukaryotes occurred respectively in the early and middle Ediacaran Period. Various hypotheses have been proposed to attribute these evolutionary events to changes in ocean redox conditions. To date, published models of the Ediacaran ocean in South China have largely been focused on the Upper and Middle Yangtze blocks, as opposed to the slope and basin sections of the Lower Yangtze Block, where the oldest complex macrofossils of the Ediacaran (i.e. the similar to 600 Ma Lantian Biota) are preserved. To achieve a holistic understanding of the record of ocean chemistry in the entire Yangtze Block, we carried out carbon (delta C-13(carb) and delta C-13(org)) and sulfur (delta S-34(CAS) and delta S-34(pyr)) isotopic investigations of four new sections (i.e., Wujialing, Meishuxia, Wangfu and Zhushuwu) in the Lower Yangtze Block, in addition to one previously reported drill core at Lantian. The Doushantuo and Lantian formations at these sections represent shelf margin, upper slope, lower slope, and deep basin environments. In conjunction with previous work on sections of the Upper and Middle Yangtze blocks, our study reconstructs the redox structure of the entire Ediacaran ocean in South China. The new results show profound delta C-13(carb) negative excursions in the slope and deep basinal sections, which can be correlated to the EN3 and Shuram excursions at a global scale. Notably, the delta C-13(carb) and delta S-34(pyr) profiles from shelf to basinal sections show distinct spatial and temporal patterns, which can be explained by model of a redox stratified ocean during the early and middle Ediacaran Period. The finding of isotopically light (S-34-depleted) pyrite from this time suggests that the oceanic chemocline extended to intermediate depth. Integrating this study with existing ecological and taphonomic data, it is likely that marine redox stratification in the early Ediacaran limited the spatial distribution of early metazoans in the Lower Yangtze Block.

地质历史时期新元古代大气氧含量普遍较低.在硫酸盐还原细菌作用下,作为海洋重要的氧化性离子,陆源硫酸根离子有效促进了深层海水的氧化进程.在此过程中,硫元素在硫酸根和黄铁矿之间发生显著同位素分馏,其分馏程度可反推当时古海洋的氧化还原状态.沉积地层中的黄铁矿普遍具有多种形态,不同形态黄铁矿的形成环境多有不同.如草莓状黄铁矿多形成于底层缺氧水体或沉积物的浅表面,而大颗粒单晶黄铁矿或脉状黄铁矿则多沉积于成岩早期的沉积物孔隙或形成于成岩后期的热液改造.与草莓状黄铁矿不同,大颗粒单晶或脉状黄铁矿的硫同位素组成并不能反映沉积时期的古海洋氧化还原条件.判定沉积地层中不同形态的黄铁矿及形成过程,是获得有效反映海洋沉积环境硫同位素组成特征的基本前提.简要总结了地质历史时期沉积地层中的黄铁矿类型及矿物形成过程,并以华南埃迪卡拉纪蓝田组岩芯样品为例,识别出各个样品中的黄铁矿形态组成特征,对比分析了全岩黄铁矿与样品中大颗粒黄铁矿硫同位素组成差异.研究结果表明:不同岩性样品中黄铁矿的形态种类及含量均存在差异.页岩样品保存有更好形态的自形晶以及草莓状黄铁矿;碳酸盐岩样品中具有较多自形晶以及他形晶黄铁矿,并且其中的少量草莓状黄铁矿遭受后期成岩作用而发生不同程度的晶体蚀变.样品中大颗粒黄铁矿的硫同位素值(δ34SL-pyr)通常显著高于全岩黄铁矿的硫同位素值(δ34ST-pyr),最大差值可达48.5‰.在利用黄铁矿的硫同位素组成来反推当时古海洋环境时,需要区分不同形态黄铁矿,仔细剔除大颗粒黄铁矿,降低成岩期黄铁矿对样品中硫同位素组成的影响.更细致的微区黄铁矿硫同位素分析工作将依赖于SIMS分析测试手段进行.

The Ediacaran Doushantuo Formation in South China is a prime target for geobiological investigation because it offers opportunities to integrate chemostratigraphic and paleobiological data. Previous studies were mostly focused on successions in shallow-water shelf facies, but data from deep-water successions are needed to fully understand basinal redox structures. Here, we report δ13 Ccarb , δ13 Corg , δ34 Spyr , δ34 SCAS , and δ15 Nsed data from a drill core of the fossiliferous Lantian Formation, which is a deep-water equivalent of the Doushantuo Formation. Our data confirm a large (>10‰) spatial gradient in δ13 Ccarb in the lower Doushantuo/Lantian formations, but this gradient is probably due to the greater sensitivity of carbonate-poor deep-water sediments to isotopic mixing with 13 C-depleted carbonate cements. A pronounced negative δ13 Ccarb excursion (EN3) in the upper Doushantuo/Lantian formations, however, is spatially consistent and may be an equivalent of the Shuram excursion. δ34 Spyr is more negative in deeper-water facies than in shallow-water facies, particularly in the lower Doushantuo/Lantian formations, and this spatial pattern is interpreted as evidence for ocean redox stratification: Pyrite precipitated in euxinic deep waters has lower δ34 Spyr than that formed within shallow-water sediments. The Lantian Formation was probably deposited in oscillating oxic and euxinic conditions. Euxinic black shales have higher TOC and TN contents, but lower δ34 Spyr and δ15 Nsed values. In euxinic environments, pyrite was predominantly formed in the water column and organic nitrogen was predominantly derived from nitrogen fixation or NH4+ assimilation because of quantitative denitrification, resulting in lower δ34 Spyr and δ15 Nsed values. Benthic macroalgae and putative animals occur exclusively in euxinic black shales. If preserved in situ, these organisms must have lived in brief oxic episodes punctuating largely euxinic intervals, only to be decimated and preserved when the local environment switched back to euxinia again. Thus, taphonomy and ecology were the primary factors controlling the stratigraphic distribution of macrofossils in the Lantian Formation.