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Abstract A significant deposition of black shales occurred during the Mesoproterozoic Oxygenation Event (MOE). In order to investigate the hydrocarbon generation potential and organic matter enrichment mechanism of these shale deposits, we studied the Xiamaling Formation shale in the North China region as a representative sample of the Mesoproterozoic shale. The research involved organic petrology, organic geochemistry, mineralogy, and elemental geochemistry. The following observations were made: (1) The depositional environment of the Xiamaling Formation shale can be categorized as either oxic or anoxic, with the former having shallow depositional waters and high deposition rates, while the latter has deeper depositional waters and slower deposition rates. (2) Anoxic shales exhibited significantly better hydrocarbon generation potential compared to shales deposited in oxic environments, although the latter still demonstrated high hydrocarbon generation potential. (3) Shales deposited in anoxic environments displayed higher paleoproductivity compared to those deposited in oxic environments. The high deposition rate in oxic environments slowed the decomposition and mineralization of organic matter, leading to the formation of high-quality shales. In contrast, the strong paleoproductivity, along with favorable preservation conditions, accounted for the high hydrocarbon potential of anoxic shales.

The mid‐Proterozoic (∼1.8–0.8 Ga) ocean‐atmosphere system is hypothesized to have experienced fluctuations in redox conditions with transient oxygenation events. One of these happened at ∼1.4 Ga, and it is speculated that this event may link to the emplacement of large igneous province (LIP) at this time. However, direct evidence for this relationship remains to be proved. Here, we report Hg/TOC, P, and trace element concentrations across the ∼1.4 Ga oxygenation event in the Xiamaling Formation of North China. A prominent increase in Hg/TOC is slightly earlier than that of nutrient contents (especially P), pyrite and TOC abundances, suggesting that this distinct oxygenation event was likely the result of LIP activity at ∼1.4 Ga, which increased nutrient and sulfate supply from continental weathering to the ocean, sustaining elevated primary productivity, organic carbon and pyrite burial. This study indicates that LIP weathering could trigger transient oxygenation events during the mid‐Proterozoic. Plain Language Summary: The mid‐Proterozoic (∼1.8–0.8 Ga) retains persistent low oxygen concentrations in atmosphere and shallow seawater but is occasionally punctuated by transient oxygenation events. Multiple lines of evidence from sedimentary geochemistry have argued a global oxygenation event at ∼1.4 Ga, and speculated that this event was closely related to large igneous province (LIP) activity at this time. Direct link between LIP activity and the oxygenation event, however, has rarely been addressed yet. As mercury in sedimentary rocks has been widely used to trace volcanism, we therefore report Hg/TOC, P, and trace element concentrations across the ∼1.4 Ga oxygenation event. The result shows that a prominent increase in Hg/TOC is clearly recognized in the shales that stratigraphically just underly the layers recording the oxygenation event, suggesting a close causal link between LIP emplacement and the oxygenation event. Accompanied with this Hg/TOC ratio peak, increased nutrient contents (especially P), pyrite and TOC abundances are also observed. Based on these observations, we proposed a model that the Xiamaling oxygenation event was the result of the LIP's emplacement at ∼1.4–1.35 Ga, which increased nutrient and sulfate supply from continental weathering to the ocean, sustaining elevated primary productivity, organic carbon and pyrite burial. Key Points: High Hg/TOC ratios associated with large igneous province (LIP) emplacement were detected in the ∼1.4 Ga Xiamaling Formation shales of North ChinaHigh P content accompanied with this interval indicates enhanced phosphorous input from LIP weathering to the oceanA causal link between the LIP weathering and oxygenation event at ∼1.4 Ga was suggested [ABSTRACT FROM AUTHOR]

The Mesoproterozoic Xiamaling Formation (∼1.4 Ga) is considered to be a set of promising strata for shale oil and gas exploration and production in the Yanliao Basin, North China Craton. This study aimed to investigate the organic matter accumulation mechanism, and hydrocarbon generation and expulsion characteristics of the Xiamaling Formation, using various methods, including scanning electron microscopy, rock-eval pyrolysis, and elemental geochemistry. Multiple geochemical proxies indicate that Unit 3—Unit 1 of the Xiamaling Formation was deposited in a climate that alternated between highly humid and semiarid conditions, with the strong-to-weak terrigenous influx. The redox conditions of bottom water were undulatory oxic in Unit 3 and the upper part of Unit 1, but anoxic in Unit 2 and the lower part of Unit 1. Sulfurization of organic matter was the main reason for the high TOC content in Unit 3. However, the high TOC content in Unit 2 and the lower part of Unit 1 was caused by an oxygen-deficient water column. Unit 2 and the lower part of Unit 1 exhibit a significant shale oil potential owing to low maturity, high hydrogen index (HI) values, and prevalence of type Ⅰ, II1, and II2 kerogens. [ABSTRACT FROM AUTHOR]

Heat carried by deep fluid might greatly affect hydrocarbon generation and pore space in shale. Dyke intrusion carrying high levels of heat may be a means by which to explore the influence of deep fluid on shale reservoirs. This study evaluates hydrocarbon generation and analyzed the evolution of shale storage space in the third member of the Xiamaling Formation in the Zhaojiashan section, Hebei Province, based on experimental data such as TOC, SEM, VRo, low‐temperature N2 adsorption and high‐pressure mercury injection. The results show that the dyke intrusion reduced the shale TOC content drastically—by up to 77%—and also induced instantaneous hydrocarbon generation over a range about 1.4 times the thickness of the intrusion. Furthermore, the dyke intrusion might transform organic pores in surrounding shales into inorganic pores. There were two shale porosity peaks: one appeared when VRo = 2.0%, caused by the increase of organic pores as thermal maturity increased, the other occurred when the VRo value was between 3% and 4%, caused by the increase of inorganic mineral pores. It can be concluded that dyke intrusion can be an effective tool with which to study how deep fluid affects instantaneous hydrocarbon generation and pore space in shale. [ABSTRACT FROM AUTHOR]

There is increasingly valued attention on whether the matter and energy carried in the deep fluids can significantly change the hydrocarbon (HC) generation of low-mature source rocks. Previous studies suggest that the upward movement of deep fluids to sedimentary basins will change the HC generation evolution mode of low-mature source rocks, and the matter and energy carried by the fluid will transform the evolution process as transient events. However, there is a lack of quantitative evaluation of the specific changes of gaseous HC generation in the process of modification. In this study, the effect of deep fluids on HC generation and evolution of low maturity source rocks were quantitatively studied through simulation experiments of the gold tube closed system. We quantitatively selected hydrogen and catalysts (ZnCl2 and MoS2) to conduct catalytic hydrogenation of kerogen and explore the quantitative effects of deep fluids on HC generation in low-mature source rocks. Through the experimental results, it is found that catalytic hydrogenation has significant changes in a HC generation transformation of organic matter (OM). With the increase of catalytic hydrogenation reaction intensity, the maximum gaseous HC generation yield is 3.16–3.24 times that of the control groups without hydrogenation. In the relatively low-temperature stage (

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The rate of net primary production in the Proterozoic ocean was suggested to be no more than 10% of its modern value (Laakso and Schrag, 2019), however, in the Mesoproterozoic Xiamaling Formation, the export production values could reach 20%–150% of the present-day Equatorial Atlantic average values (Zhang et al., 2016). Here, we report Zn and Cu isotope data for black shales from the Xiamaling Formation to illustrate the biogeochemical cycling of Zn and Cu in the Mesoproterozoic ocean. The 65Cu-enriched signature in the authigenic fraction is similar to that in bioauthigenic Cu of the modern marine sediments. The Zn isotope ratios of sediments deposited in euxinic conditions are commonly higher than those of clastic sediments, indicating light Zn sinks in the coeval ocean. Combined with previously reported Mo isotope data, the proportion of organic carbon to total carbon burial in the Mesoproterozoic was about as half as that at present, which is larger than the previous estimation—a quarter of today's value (e.g., Ozaki et al., 2019) and is evidenced by a wide distribution of black shales. The organic burial may be ascribed to the increasing phosphorus inputs from large igneous provinces and consequently high primary productivity, which has spurred the hypothesized atmosphere-ocean oxygenation at ∼1.4 Ga. [ABSTRACT FROM AUTHOR]

Sedimentary records suggest that the mid-Proterozoic (ca. 1.8–0.8 Ga) was persistently characterized by a greenhouse climate despite significantly lower solar luminosity compared to modern levels. To maintain greenhouse conditions, the partial pressure of carbon dioxide (pCO2) must have remained elevated, possibly indicative of key differences in the complexities of the carbon cycle compared to the modern. Modeling has suggested that high pCO2 was likely maintained by elevated rates of "reverse weathering:" marine authigenic clay formation, a process that consumes alkalinity and generates CO2. This process is kinetically slow in modern marine environments, yet is hypothesized to have been enhanced during the mid-Proterozoic due to the greater availability of important species for clay authigenesis such as silica and ferrous iron. This hypothesis is testable using the geological record, because enhanced reverse weathering would lead to the formation of abundant marine authigenic clays. However, the distribution of marine authigenic clays in the Proterozoic sedimentary record has not been paid sufficient attention. In this study, we report the presence of authigenic clays (glauconite and berthierine) from the Xiamaling Formation (ca. 1.4 Ga), North China. The glauconite-berthierine horizons occur as millimeter- to centimeter-thick laminae interbedded with muddy siltstone and feature detrital grains supported by the clay matrix. In places, these layers were partially reworked to form soft and cohesive intraclastic sands, suggesting a syndepositional origin. We hypothesize that marine iron cycling in the iron- and silica-rich mid-Proterozoic oceans may have facilitated the formation of authigenic iron-rich clay during the deposition of the Xiamaling Formation. The formation of iron-hydroxides on the seafloor—and the local increase in pH caused by subsequent dissimilatory iron reduction—could have resulted in the absorption of SiO2, Al(OH)3, and Fe(OH)2 to form soft, cohesive and noncrystalline Fe(OH)3-SiO2-Al(OH)3-Fe(OH)2 gels. These gels would have subsequently converted to glauconite/berthierine through aging. The transformation from glauconite-rich layers to berthierine-rich laminae was likely facilitated by a greater availability of Fe(II), and therefore higher Fe(II)/TFe and Fe/ Si ratios. We suggest that the relatively rapid formation of syndepositional, seafloor berthierine and glauconite layers in the basal Xiamaling Formation is the result of enhanced reverse weathering during this time. This study provides an important geological support for carbon cycle models that invokes enhanced reverse weathering rates in the mid-Proterozoic ocean that may have helped to maintain a high-baseline pCO2 during this time. [ABSTRACT FROM AUTHOR]

The accumulation of oxygen is one of the most important characteristics that distinguish Earth from other planets in the solar system, which is also considered to be the key factor influencing the birth and evolution of complex life forms. The oxygenation process of the Earth surface has long been viewed to be episodic with two critical intervals occurring in the early Paleoproterozoic (2.45–2.10 Ga) and the late Neoproterozoic (0.80–0.54 Ga), with a 1.3-billion-year-long low oxygen period in between. Recently, increasing independent works carried out by different scientific teams in the Yanliao Basin, North China are demonstrating that the atmospheric oxygen concentrations had reached >4% PAL (present atmospheric levels) at least during 1.59–1.56, 1.44–1.43, and 1.40–1.36 Ga. These estimated values are higher than the previously recommended values of <0.1–1% PAL. Such a scenario discovered in the Yanliao Basin is consistent with the synchronously deposited strata in Australia and Siberia, pointing to a Mesoproterozoic oxygenation event (1.59–1.36 Ga) between the two major oxygenation intervals during the Proterozoic. This Mesoproterozoic oxygenation event is coupled with the break-up of the Columbia (Nuna) supercontinent, the formation of organic-rich shales and Fe-Mn deposits, and the early innovation of eukaryotic algae, indicating that the geological and biological co-evolutionary processes control the Earth surface system. [ABSTRACT FROM AUTHOR]

Mud shale can serve as source or cap rock but also as a reservoir rock, and so the development of pores or cracks in shale has become of great interest in recent years. However, prior work using non-identical samples, varying fields of view and non-continuous heating processes has produced varying data. The unique hydrocarbon generation and expulsion characteristics of shale as a source rock and the relationship with the evolution of pores or cracks in the reservoir are thus not well understood. The present work attempted to monitor detailed structural changes during the continuous heating of shale and to establish possible relationships with hydrocarbon generation and expulsion by heating immature shale samples while performing in situ scanning electron microscopy (SEM) imaging and monitoring the chamber vacuum. Samples were heated at 20°C/min from ambient to 700°C with 30 min holds at 100°C intervals during which SEM images were acquired. The SEM chamber vacuum was found to change during sample heating as a consequence of hydrocarbon generation and expulsion. Two episodic hydrocarbon expulsion stages were observed, at 300 and 500°C. As the temperature was increased from ambient to 700°C, samples exhibited consecutive shrinkage, expansion and shrinkage, and the amount of structural change in the vertical bedding direction was greater than that in the bedding direction. At the same time, the opening, closing and subsequent reopening of microcracks was observed. Hydrocarbon generation and expulsion led to the expansion of existing fractures and the opening of new cracks to produce an effective fracture network allowing fluid migration. The combination of high-resolution SEM and a high-temperature heating stage allowed correlation between the evolution of pores or cracks and hydrocarbon generation and expulsion to be examined.

Marine red beds (MRBs) are common in sedimentary records, but their genesis and environmental implications remain controversial. Genetic models proposed for MRBs variably invoke diagenetic or primary enrichments of iron, with vastly different implications for the redox state of the contemporaneous water column. The Xiamaling Formation (ca. 1.4 Ga) in the North China Platform hosts MRBs that offer insights into the iron cycling and redox conditions during the Mesoproterozoic Era. In the Xiamaling MRBs, well-preserved, nanometer-sized flaky hematite particles are randomly dispersed in the clay (illite) matrix, within the pressure shadow of rigid detrital grains. The presence of hematite flake aggregates with multiple face-to-edge ("cardhouse") contacts indicates that the hematite particles were deposited as loosely bound, primary iron oxyhydroxide flocs. No greenalite or other ferrous iron precursor minerals have been identified in the MRBs. Early diagenetic ankerite concretions hosted in the MRBs show non-zero I/(Ca+Mg) values and positive Ce anomalies (>1.3), suggesting active redox cycling of iodine and manganese and therefore the presence of molecular oxygen in the porewater and likely in the water column during their formation. These observations support the hypothesis that iron oxyhydroxide precipitation occurred in moderately oxygenated marine waters above storm wave base (likely <100 m). Continentally sourced iron reactivated through microbial dissimilatory iron reduction, and distal hydrothermal fluids may have supplied Fe(II) for the iron oxyhydroxide precipitation. The accumulation of the Xiamaling MRBs may imply a slight increase of seawater oxygenation and the existence of long-lasting adjacent ferruginous water mass. [ABSTRACT FROM AUTHOR]

The Mesoproterozoic Era (1.6–1.0 Ga), long regarded as an interval of sluggish biotic evolution and persistently low atmospheric-oceanic oxygen levels, has become the subject of recent controversy regarding putative large-scale oxygenation events. In this study, we conducted a comprehensive investigation of redox, productivity, seawater sulfate concentrations, and hydrographic conditions for the ∼1.4–1.32-Ga Xiamaling Formation in the shallow Hougou and mid-depth Huangtugui sections in the Yanshan Basin (North China). Integrated Fe-trace metal data suggest that bottom-water redox conditions mainly underwent a three-stage secular evolution: (1) suboxic (-oxic), (2) predominantly anoxic, and (3) oxic. Integrated productivity (TOC, P/Al, and Cu-Zn-Ni enrichment factors) and depositional system (major and trace elemental enrichment patterns) data suggest varying productivity levels during deposition of the Xiamaling Formation, with high productivity fueled mainly by intense open-marine upwelling. In combination with published data from the deep Xiahuayuan and Chicheng sections, a refined stratigraphic correlation framework was developed for the Xiamaling Formation based on the basinal upwelling event and existing lithostratigraphic framework. Within this new framework, sulfur concentration and pyrite δ34S (δ34S py) data suggest a rise in marine sulfate concentrations at ∼1.36 Ga. Furthermore, strong spatiotemporal redox heterogeneity was observed within the ∼1.4–1.32-Ga Yanshan Basin, i.e., strong bottom-water anoxia (or euxinia) in deep sections and less anoxic (even oxic) conditions in shallow sections, although all sections experienced high productivity owing to oceanic upwelling. We propose a productivity-driven shelf oxygenation model to explain this spatiotemporal redox heterogeneity within the context of eustatic changes, an upwelling event, and the rise of oceanic sulfate concentrations at ∼1.36 Ga. In this model, from initially low atmospheric oxygen levels, increasing surface productivity in shelf areas due to elevated nutrient supplies from upwelling not only intensified bottom-water anoxia through organic matter export but also stimulated expansion of the ocean-surface oxic layer through photosynthetic O 2 release, contributing to the development of contrasting bottom-water redox conditions between shallow and deep sections during the upwelling event identified in this study. Our model helps reconcile conflicting observations concerning Mesoproterozoic marine redox conditions, and provides new insights into putative Mesoproterozoic oceanic oxygenation events and their relationship to the evolution of early eukaryotes. [ABSTRACT FROM AUTHOR]

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract As a self-contained source-reservoir petroleum system, shale gas plays have different generation mechanisms from conventional natural gas plays, especially with respect to the effects of residual hydrocarbons on shale gas generation. In this study, closed-system pyrolysis experiments were conducted on a suite of residual shales obtained by the semi-closed pyrolysis of shales from the Neoproterozoic Xiamaling Formation under various conditions. The effects of oil expulsion on the geochemical characteristics of residual organic matter (both bitumen and kerogen) in shale and on shale gas generation in the high maturity stage were investigated. The results indicated that shales with high oil expulsion efficiencies should be depleted in the aliphatic fractions in residual shale. In contrast, most of the aliphatic fractions should remain in shales with low oil expulsion efficiencies. Additionally, oil expulsion can further affect the chemical and isotopic compositions of the gas generated at later stages of thermal maturation. With increasing oil expulsion efficiency, C 1 , C 2 , C 3 , C 1–5 , and C 2–5 gas yields and gas wetness decrease, while C 1 , C 2 , and C 3 carbon isotopic values become heavier. Highlights • The roles of residual hydrocarbons differ between conventional and shale gas plays. • The effects of oil expulsion on shale chemistry and gas generation were explored. • Increased oil expulsion efficiencies result in decreased gas yield and wetness. • High expulsion efficiencies promote the cracking of long-chains in kerogen. • Mixing of kerogen- and bitumen-derived gas impacts shale gas isotopic composition. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is an essential element for life, and the oceanic P cycle is closely coupled with the global carbon and oxygen cycles via the role of P as a limiting nutrient. Since P has only one stable isotope, tracking the P cycle through geological history is challenging. However, phosphate oxygen isotopes (δ18O p) represent a valuable tool for deciphering various P-related reactions, such as intracellular P turnover and enzymatic P regeneration, and have been successfully applied to modern soils and marine sediments. The marine P cycle prior to the Neoproterozoic Era remains controversial due to the scarcity of phosphorite and large uncertainties in estimating seawater phosphate levels. Here, we present δ18O p of carbonate fluorapatite (CFA) from the Mesoproterozoic Gaoyuzhuang and Xiamaling Formations in North China. Despite considerable uncertainties, the derived authigenic δ18O p values are markedly lower than those from modern sites with similar depth and latitude, which imply a warmer climate during the Mesoproterozoic Era compared to the present day. Based on thermodynamic calculations of saturation state with respect to CFA, we suggest that the elevated temperatures could have hindered P authigenesis, partially accounting for the general absence of phosphorite in contemporaneous successions. • Phosphorus occurs as tiny CFA spheroids in Mesoproterozoic organic-rich sediments. • Phosphate oxygen isotopes reveal warm climates during the Mesoproterozoic Era. • The high temperature of Mesoproterozoic seawater inhibited phosphorus authigenesis. [ABSTRACT FROM AUTHOR]

Nitrogen is an essential nutrient for all life on Earth. It is, and likely has been, a critical limiting nutrient controlling primary production rates in the oceans. The marine N budget is regulated by the balance between nitrogen loss as N 2 (and N 2 O) in low-oxygen settings in the oceans including marine sediments and coastal oxygen-minimum zones and nitrogen inputs, mainly from nitrogen fixation. How this balance has changed through Earth history, particularly considering changing degrees of ocean oxygenation, is a frontier research area. Here, we explore ancient ocean nitrogen dynamics as revealed through stable nitrogen isotopes in the ca. 1400 million year old (Ma) Xiamaling Formation of North China. We compare these dynamics to other Mesoproterozoic-aged settings and to modern environments. Through these comparisons, we see that Mesoproterozoic (1600 to 1000 Ma) oceans experienced both nitrogen replete- and nitrogen-deplete conditions with isotope systematics very similar to modern analogues. We provide models explaining the ancient ocean nitrogen dynamics, and we conclude that Mesoproterozoic oceans generally maintained extensive regions of nitrate-replete conditions. These nitrate-deplete conditions could have corresponded with extensive areas of deoxygenated seawater or vast nitrate-replete marine environments similar to modern oxygen-minimum zone environments. We also evaluate, generally, what nitrogen isotope systematics can and cannot tell us about nitrogen limitation in ancient oceans. [ABSTRACT FROM AUTHOR]

Berthierine and chamosite are iron-rich clay minerals that share similar chemical compositions. Berthierine forms at low temperature (25-45 °C) during early diagenesis and may transfer to chamosite at temperatures of ≥70 °C. Because the formation of berthierine and chamosite requires significant amount of Fe2+ supply, their presence in marine sediments is often used as a mineral proxy for ferruginous conditions in porewater. Recent studies reveal that the Precambrian oceans were characterized by pervasive ferruginous water-column conditions that may favor the formation of iron-rich clay minerals like berthierine and chamosite. To evaluate if ferruginous water-column conditions in the Precambrian ocean played a role on iron-rich clay mineral formation, we conducted an integrated petrographic, mineralogical, and geochemical study on the chamosite -and glauconite-bearing strata of the Mesoproterozoic Xiamaling Formation (~1.40-1.35 Ga) in North China. Petrographic, XRD, SEM, and EDS analyses show that the chamosites of the Xiamaling Formation was transferred from glauconite, with berthierine as an intermediate mineral phase during early diagenesis. Geochemical analyses indicate that a complete transformation from glauconite-dominated to chamosite-dominated end-members (samples) requires an addition of a large amount of Fe (16.9 wt%), Mg (2.4 wt%), and a small amount of Al (1.4 wt%), but a simultaneous release of Si (11.8 wt%) and K (6.0 wt%). Considering that the glauconite- and chamosite-bearing strata are devoid of iron-rich detrital minerals (e.g., biotite and iron oxides) and lack evidence of hydrothermal alteration, the required Fe2+ for glauconite-berthierine-chamosite transformation was most likely from Fe2+-rich (ferruginous) seawater, which may have promoted glauconite-berthierine transformation at the very early diagenetic stage when Fe2+ exchange between porewater and seawater was still available. This interpretation is consistent with the high FeHR/FeT (but low Fepy/FeHR), Fe/Al, and V/Al ratios from the hosting strata that support ferruginous depositional environments. Because most Precambrian strata have passed the oil window temperature (>50-150 °C), the preservation of berthierine would be rare and chamosite should be the representative iron-rich clay mineral. Thus, the abundance of chamosite in fine-grained, marine siliciclastic sediments may be used as a mineral indicator of ferruginous water-column conditions. [ABSTRACT FROM AUTHOR]

To refine the use of graptolite and solid bitumen as thermal proxies at overmature conditions, we evaluated their evolution via Raman and infrared (IR) spectroscopies, reflectance, and geochemical screening using high-temperature pyrolysis experiments in comparison to naturally matured samples. Naturally matured samples included marine shales from the overmature Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations (herein referred to as Wufeng-Longmaxi) of the Sichuan Basin, China. Immature samples for pyrolysis experiments included Mesoproterozoic (Ectasian) Xiamaling marine shale from Hebei, China (graptolite absent) and graptolite-bearing Ordovician (Tremadocian) Alum marine shale from Grönhögen, Sweden. Pyrolysis experiments at 360 °C for 10 days and 27 days (hydrous conditions), and 450 °C 3 days, 550 °C 3 days, and 550 °C 10 days (all anhydrous) created Xiamaling and Alum residues with solid bitumen and graptolite, respectively, of similar and higher maturity compared to the naturally matured Wufeng-Longmaxi samples. Compositional proxies from IR spectroscopy were inconclusive. Raman spectral properties including Raman band separation (RBS) and the full-width at half-maximum of the G-band (G-FWHM) exhibited robust positive and inverse relationships with increasing reflectance in pyrolysis residues, respectively. RBS values were systematically lower in artificially matured samples versus naturally matured samples with equivalent reflectance values, whereas G-FWHM values were systematically higher. This observation indicates that the limited duration or low internal reactor pressure of pyrolysis experiments is insufficient to allow alignment or ordering of aromatic carbon arrays in both organic matter types, relative to natural maturation occurring at geologic time scales. Reflectance of graptolite was typically higher than co-occurring solid bitumen in Wufeng-Longmaxi samples and the untreated Alum sample, suggesting inherently higher aromaticity. However, aromaticity in pyrolysis residues was systematically higher in solid bitumen compared to graptolite with equivalent reflectance values, indicating a higher kinetic barrier to molecular rearrangement in graptolite versus solid bitumen. These data further clarify differences in the molecular evolution of sedimentary organic matter in natural versus analogue laboratory environments and distinguish the properties of individual organic matter types in response to thermal stress. [ABSTRACT FROM AUTHOR]

Abstract: Based on analysis on the coexistent relationship between igneous rock intrusion and bituminous sandstone of the Xiamaling Formation in northern Hebei (Qnx), the accumulation time of the Xiamaling Formation bituminous sandstone reservoir is defined by the methods of organic petrology and isotope chronology. According to field investigation, observation of thin-section and the shallow drilling show, and based on the previous results of oil seepages and bitumen spots in the Qnx base sandstone, thirty-one new oil seepages (bitumen spots), including twenty-one visible black bitumen spots, were discovered in the Qnx. The distribution of paleo-reservoirs in the Qnx bituminous sandstone was identified, and 4 paleo-reservoirs, i.e. Shuangdong paleo-reservoir in Pingquan, Longtangou paleo-reservoir in Lingyuan, Lujiazhuang paleo-reservoir and Tashan paleo-reservoir in Kuancheng, have been named. The reserves of the paleo-reservoirs are 0.7-1.0 billion tons, which are calculated by the volume method with the solid bitumen content. Organic petrology analysis shows that two kinds of solid bitumen with diferent reflectivity characteristics were developed. Class structure of natural coke is discovered in solid bitumen, which is the first direct evidence to prove that severe thermal alteration occurs in the Qnx base sandstone bitumen. The age of Qnx gabbro-diabase baddeleyite by SHRIMP U-Pb dating is 1 327 Ma±2 Ma, which indicates that the accumulation time of the Xiamaling Formation bituminous sandstone is 1 400 Ma to 1 327 Ma. So, it is the oldest oil accumulation in China. [Copyright &y& Elsevier]

Abstract: We report SHRIMP U–Pb zircon ages from K-bentonite beds from different locations in the Xiamaling Formation near Beijing at the northern margin of the North China Craton (NCC). The 1379±12 Ma and 1380±36 Ma ages obtained in our study correlate with similar data (1368±12 Ma) obtained from recent studies and assign a Mid-Mesoproterozoic (Ectasian Period) age for the formation. In addition, a detailed bed-by-bed study of the K-bentonite sequence indicates that the present dated successions northwest of Beijing can be correlated fairly well with that of the well-known Meso- to Neoproterozoic standard section in Jixian to the east of Beijing. The new data call for a major revision of the previous Meso- to Neoproterozoic chronostratigraphic subdivision of the northern NCC, with a revised calibration into the early to middle Mesoproterozoic Era. Our data suggest that the Tawuia-bearing assemblage in the lower part of the Xiamaling Formation in the NCC could be older than ~1380 Ma, and is not limited to a representative feature of the Neoproterozoic Little-Dal-Group-type macroalgal biota. Combined with other similar discoveries, we conclude that the macro-evolution of Proterozoic eukaryotic biotas, represented by macroalgal assemblages, would have probably been a long, protracted, and somewhat monotonous process. On the other hand, reassessment of the paleomagnetic database for these successions, combined with stratigraphic correlations and similarities in the macroalgal assemblages on the NCC and Laurentia, support a configuration in which the NCC was in close proximity to the Laurentia–Siberia–Baltica grouping during much of the Mesoproterozoic to Neoproterozoic time. Furthermore, our study suggests recent assumptions linking δ 34SCAS and δ 13Ccarbonate fluctuations in the Jixian section with the assembly and breakup of Rodinia must now be reconsidered, as these fluctuations may correlate better with the tectonics associated with the Columbia supercontinent cycle. [Copyright &y& Elsevier]

Zircon grains of magmatic origin from tuffite layers in the Xiamaling Formation at Zhaojiashan Village, Xuanhua area, Hebei Province, were used for zircon dating with a Sensitive High-Resolution Ion Microprobe (SHRIMP II), which gives a weighted mean 207Pb/206Pb age of 1366±9 Ma. It shows a very similar age (1368±12 Ma) as the zircon dating from the Xiamaling Formation in Western Hill, Beijing. This age proposes that the Xiamaling Formation in the North China plate should be of Mesoproterozoic, instead of Neoproterozoic based on K-Ar, Ar-Ar dating. The new zircon age also indicates the development of macrofossils algae from the Mesoproterozoic age. [ABSTRACT FROM AUTHOR]

A suit of oil shales, predominated by black argillaceous silicalite and finely laminated black-brown shale, has been discovered in a set of carbonaceous-siliceous mudstone formations (350 m in thickness) in the third member of Xiamaling Formation of the Upper Proterozoic Qingbaikou Series (900-873 MaBP), Xiahuayuan, Hebei Province, China. The oil shale, combustible with strong bitumen odour, has super-high TOC contents ranging from 21.4% to 22.9%, bitumen “A” contents from 0.58% to 0.88% and oil length from 5.29% to 10.57%. The ultrathin section observation of the shale and the identification of its kerogen demonstrate that its hydrocarbon-generative parent material is mainly benthonic Rhodophyta whose specific tetrasporangia are legible and abundant. It is rarely reported in the literature that such a hydrocarbon-generative parent material, composed mainly of Rhodophyta and with extraordinarily high contents of TOC and bitumen “A”, developed into a set of high-quality source rocks. The extracts of the oil shale are characteristic of richness in 17α(H)-diahopanes and n-alkyl tricyclic terpenoids but low in steranes. Such a biomarker feature is obviously different from that of the extracts from other Proterozoic marine carbonate source rocks of the studied area. Since the biological constitution of this oil shale is rather simple, it is clear that these biomarkers most likely represent to certain extent the specific molecular constitutions of the benthonic Rhodophyta identified in the ultrathin sections of the samples. Studies on its lithologic association and depositional sequences suggest that this suit of the carbonaceous-siliceous mudstone formation, which contains oil shales, was probably developed in an undercompensation deep-bay environment when a maximum transgression occurred during the formation of the third member of Xiamaling Formation. The high concentration of SiO2 in this organic-rich rock and the positive correlation between TOC and some trace elements such as P, Cu, Ni, W and Mo indicate that this suit of rocks was affected by activities of bottom thermal currents as deposited. [ABSTRACT FROM AUTHOR]

Iron formations (IFs) are traditionally considered to be limited during 1.8−0.8 Ga. However, there are recent reports of siderite-dominated IFs within this time interval, such as the 1.40 Ga Xiamaling IF in North China and the 1.33 Ga Jingtieshan IF in Qilian. To further explore the crystallization and formation mechanisms of siderite, an authigenic siderite crystal from the Xiamaling IF was fully scanned using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Multi-element imaging with a spatial resolution of 5 μm revealed an obvious rim structure of the siderite crystal, which might record the crystallization and growth processes. The Al- and Fe-enriched zone in the core of siderite crystal might be an iron-bearing nucleus, and the formation of rim structure was related to the transition from a closed crystallization environment to a semi-closed growth environment. These results, combined with carbon isotope evidence from the siderites and surrounding shales, suggest that vigorous dissimilatory iron reduction that can provide Fe2+ and HCO3− to the pore water is a key factor to form the siderite-dominated Xiamaling IF. [ABSTRACT FROM AUTHOR]