Based on panel data from 2009 to 2021, covering 30 provinces in China, we have been constructed the Rural Financial Risk Index using the objective entropy weighting method to study rural financial ...risk in China systematically from the perspective of spatial distribution. Specifically, we discuss the spatial distribution, regional differences and dynamic evolution of rural financial risk across Chinese four different regions divided into the Northeast, East, Central and West. It's found that Local government debt and Land transfer income are the two primary determinants influencing the level of rural financial risk in China. Furthermore, we conclude the ranking value of rural financial risk across four regions that the central exhibits the highest level, followed by the West, the East, and finally the Northeast, where the reasons for such ranking results as follows. Firstly, although the highest level of risk among provinces in the West is equivalent to that in the Central, there exists a smaller minimum rural financial risk in the former compared to the latter. Then, it should be noted that there's a low-low agglomeration of rural financial risk in the Northeast, while it demonstrates a high-high agglomeration in the Central according to the Moran Index test analysis. Again, there's a declining trend in rural financial risk disparity within the region and an upward trend is observed when comparing different regions (except the East vs West), especially increase largely between the Northeast and Central in past two years after analyzing the decomposition of Dagum Gini coefficient. Moreover, we study the absolute differences and dynamic evolution in different four regions through three-dimensional diagram of kernel density estimation, and it's found that the change of rural financial risk in four regions moved to the right as a whole, while the tail distribution remains inconspicuous. The absolute difference is diminishing in the Northeast, and the two-level differentiation characteristics tend to weaken as a whole in the Central, with a disordered wave peak height observed in both the East and West. Finally, the article presents pertinent policy implications but limitations according to the research findings.
Studies of the nonlinear optical phenomena that describe the light‐matter interactions in 2D crystalline materials have promoted a diverse range of photonic applications. MXene, as a recently ...developed new 2D material, has attracted considerable attention because of its graphene‐like but highly tunable and tailorable electronic/optical properties. In this study, we systematically characterize the nonlinear optical response of MXene Ti3C2Tx nanosheets over the spectral range of 800 nm to 1800 nm. A large effective nonlinear absorption coefficient (βeff∼‐10−21 m2/V2) due to saturable absorption is observed for all of the testing wavelengths. The contribution of saturable absorption is two orders of magnitude higher than other lossy nonlinear absorption processes, and the amplitude of βeff strongly depends on the light bleaching level. A negative nonlinear refractive index (n2∼‐10−20 m2/W) with value comparable to that of the intensively studied graphene was demonstrated for the first time. These results demonstrate the efficient broadband light signal manipulating capabilities of Ti3C2Tx, which is only one member of the large MXene family. The capability of an efficient broadband optical switch is strongly confirmed using Ti3C2Tx as saturable absorbers for mode‐locking operation at 1066 nm and 1555 nm, respectively. A highly stable femtosecond laser with pulse duration as short as 159 fs in the telecommunication window is readily obtained. Considering the diversity of the MXene family, this study may open a new avenue to advanced photonic
devices.
Broadband nonlinear optical responses in few‐layer MXene Ti3C2Tx with values comparable to graphene were systematically investigated. Considering the chemical composition richness of MXene, highly tailorable and efficient optical manipulating is suggested for diverse photonics applications. Compact femtosecond fiber laser in the telecommunication window is demonstrated using Ti3C2Tx as a fast saturable absorber.
To assist in the progression of shale gas exploration in southern China, we aimed to clarify the burial depth of the “sweet window” of shale gas reservoirs. In this study, we statistically explored ...the relationship between burial depths and shale gas production of the Wufeng-Longmaxi Formations in the Sichuan Basin, Southwest China, analyzed the development morphology of fractures under different stress fields and fracturing effects of high-yield strata, conducted a series of triaxial mechanical experiments on shales under the formation conditions, and summarized the characteristics of the shale gas “sweet window” as well as its main affecting factors. The results show some interesting conclusions. First, a “sweet window” of shale gas reservoir burial depths does exist. Second, it is found that the bedding cracks and artificial fracture network formations are affected by burial depth. Third, shale mechanical properties in the “sweet windows” generally have the characteristics of transitioning from brittleness to ductility. Based on the above results, a method for determining the top and bottom depth of the shale gas “sweet window” is proposed. The top boundary of the “sweet window”, i.e., the depth at which shale suffers from brittle fractures, can be determined using the shale's over consolidation ratio (OCR). The bottom boundary of the “sweet window”, i.e., the depth at which shale transitions to a ductile state, can be estimated using triaxial stress experiments. The depth range of the “sweet window” top boundary is 2000–2700 m for the Longmaxi shale in the Sichuan Basin, and the depth range of the bottom boundary is 4200–4400 m. In areas with tectonic stability and low geo-stress differences, the top boundary of the “sweet window” can be shallower than the brittle boundary depth as determined by OCR, and the shale gas buried shallower than 2000 m may be preserved effectively and allow for higher production capacity. In areas with higher brittle mineral content and greater geo-stress differences, the bottom boundary of the “sweet window” may reach 4500–5000 m. The area of the Wufeng-Longmaxi shale burial depth between 3500 m and 4500 m is up to 5.5 × 104 km2 in the Sichuan Basin, which is a potential area for shale gas exploration and development in the future.
•There is a “sweet window” for burial depths of gas exploration.•Bedding cracks usually behave in a “cracked and unbroken” state in the “sweet window”.•The range of the “sweet window” can be quantitatively determined by experiments.•The boundaries of the “sweet window” are 2000-2700 m and 4200-4400 m, respectively in eastern Sichuan Basin.•The “sweet window” could reach 4500–5000 m due to high content of brittle minerals and large geo-stress difference.
•Origin of steroid acids by biodegradation of steranes observed for the first time.•Aerobic bacteria can oxidize the terminal methyl group of steranes to form steroid acids.•The substitution at C-24 ...affects isomerization of C-5 for steroid acids.•Terminal carboxylation at side chain of steranes generates 25S and 25R steroid acids.
Steroid acids are unique molecular fossils composed of a steroid carbon skeleton linked to a carboxyl group. These biomarkers contain special geological and biological clues in their structures. However, the origin of steroid acids remains a subject of debate, and there is a lack of direct evidence in previous studies. Here, we present experimental evidence for the origin of steroid acids. The origin of steroid acids during biodegradation in crude oil was observed for the first time. Molecular structures of these steroid acids reveal that they arose from steranes via bacterial oxidation of sidechain terminal methyl groups. Our results suggest that microbial activities indeed contribute to the formation of carboxyl groups on the eukaryotic steroid skeletons, and we believe steroid acids are a promising biomarker for both geomicrobial processes and ancient eukaryotic life in the rock record.
In shale gas reservoirs, scientific consensus indicates that the main pore types are organic matter pores, mineral pores and micro-fractures. The organic matter pores are crucial to porosity, and in ...many cases they form the main pore type favorable for shale gas enrichment. Dissolution pores can influence shale gas enrichment and preservation. Although the various types of dissolution pores have been reported, few studies have identified the characteristics of mineral dissolution pores, let alone the development of dissolution pores and their effect on shale gas enrichment. Based on observations from shale gas wells in the Sichuan Basin, this paper studied the types of dissolution pores in the Wufeng Formation and the Longmaxi Formation. The study shows that the types of dissolution pores are carbonate dissolution pores, feldspar dissolution pores, and quartz dissolution pores and that carbonate dissolution pores are the main type. The formation time of the dissolution pores can be identified through analyzing the time of the mineral generation and the time at which the solid bitumen and/or minerals filled in the dissolution pores. We found that the dissolution pores in the secondary enlargement of minerals formed later than those in the original minerals. The dissolution pores without solid bitumen may have formed later than the dissolution pores with solid bitumen filling. Given that the dissolution pores had formed before the peak stage of oil generation, the dissolution pores should be well connected through the dissolved trench, which would largely improve the reservoir volume for hydrocarbon storage and matrix permeability. If there is no solid bitumen filling in the dissolution pores and the formation of the dissolution pores began after the crude oil was charged but before the peak stage of gas generation, these conditions will have some destructive effects on shale gas enrichment. In the worst-case scenario, if the dissolution pores formed after the peak stage of gas generation, the development of the pores will likely destroy the shale gas enrichment and can convert reservoir pressure to a normal pressure. The new findings in this paper reveal that the formation time of the dissolution pores has a controlling effect on shale gas enrichment in the Wufeng Formation and the Longmaxi Formation in the Sichuan Basin; these findings also provide valuable guidance for future research on shale gas reservoirs around the world.
•Three types of dissolution pores are identified and the carbonate dissolution pores are the main type.•The formation time of dissolution pores can be identified by analyzing mineral generation and solid bitumen filling.•The effects of dissolution pore on shale gas enrichment vary with its formation time.•The dissolution pores formed after the peak stage of gas generation will destroy the shale gas enrichment.
Deposition of black shale of the Lower Cambrian Niutitang Formation (NTT) of the Yangtze Block, South China, in association with the Ediacaran-Cambrian (E-C) transition was accompanied by widespread ...formation of phosphate nodules. Petrological and geochemical studies of the nodules and host sedimentary rocks were carried out to elucidate hydrographic conditions of the Early Cambrian ocean. Our results reveal that NTT phosphate nodules are composed principally of concentrically banded carbonate fluorapatite (CFA) that likely reflects changing Eh and pH conditions contemporaneous with diagenetic nodule growth. Accumulation of organic-rich sediment and nodule growth may have been induced and sustained by contemporaneous volcanic and hydrothermal activities on the Yangtze Block. The introduction of voluminous nutrients with volcanic ash, including phosphate and aqueous Fe and Si, promoted the bloom of plankton, including sponges and lower algae leading to organic matter enrichment. Productivity may have been further enhanced by upwelling of phosphate-rich bottom water that resulted in widespread phosphogenesis and formation of phosphate nodules. The hyalophane-quartz association documented from phosphate nodules suggests the possible interaction of K-feldspar-rich volcanic ash and Ba-rich hydrothermal fluids at lower pH levels in association with nodule growth. The formation of phosphate nodules of the Niutitang Formation, then, reflects the complex interaction among primary productivity, volcanic and hydrothermal activity, and the accumulation of organic matter at a critical period of Earth history.
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•Phosphate nodules display a concentric structure that may be controlled by changing Eh and pH during the diagenesis.•Discovery of hyalophane-quartz associations reflects possible interaction of volcanic ash and Ba-rich hydrothermal fluids.•Volcanic and hydrothermal activities appear to be critical to the formation of phosphate nodules during the E-C transition.
The upper Ediacaran-lower Cambrian (551–526 Ma) sedimentary succession of the Yangtze Block, South China, hosts abundant chert that formed contemporaneously with the Neoproterozoic Oxidation Event ...and the Cambrian Explosion. The effect of biological evolution, especially the rise of silica-secreting organisms, on marine Si cycling during the Ediacaran-Cambrian (E-C) transition remains controversial. This paper describes results of a petrological and geochemical investigation aimed at elucidating the origin of the chert and silica biogeochemical cycling during the E-C transition. Our results reveal that the studied chert is dominated by quartz and carbonate. The widespread presence of relict carbonate in inferred peritidal chert deposits suggests that carbonate replacement was an especially common silicification pattern of this facies, whereas the lack of carbonate precursors in subtidal strata argues in favor of primary and biogenic silica precipitation in deposits of this facies. Silicon isotope compositions, relative abundances of Al, Fe, and Mn and rare earth element (REE) + Y concentrations suggest that seawater rather than high-temperature hydrothermal fluid was the primary silica source of the studied cherts. Secular variation of chert Si isotopes reveals that marine silica cycling was closely linked with biological activity. Although silicification was abiological during the Precambrian, it may have been facilitated by microbial activities, especially the widespread formation of cyanobacterial mats. The flourishing of siliceous sponges and radiolarians in the early Cambrian ocean may have served as a global silica sink thus enhancing biological control of the silica cycle after Cambrian time. The studied chert deposits not only demonstrate the silica-rich nature of the global ocean during the E-C transition, but also record the transition of silica cycling from abiological to biological processes and the effect of the evolutionary radiation of silica-secreting organisms during the Cambrian Explosion on silica cycling.
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•Carbonate replacement is the principal silicification pattern of peritidal facies.•Primary and biogenic silica precipitation dominates silicification of subtidal facies.•Seawater was the primary silica source of widespread E-C chert deposits.•Post-Cambrian marine silica cycling is closely linked to biological activity.
•Stromatolites and thrombolites constitute major proportion of ancient Ediacaran gas reservoirs.•Buildup of early reservoir spaces was dominated by cavities in microbialite framework and karst ...vugs.•Petroleum charge and pyrobitumen coat were beneficial for porosity preservation in later burial.•Acidic fluid conditions related to presence of H2S and CO2 linked to petroleum crack and TSR prevent cementation.
The ancient Ediacaran Dengying Formation consists of microbialites with unusually high porosity that represent a large proportion of the Weiyuan-Anyue gas reservoirs in southwestern China’s Sichuan Basin. However, a clear model for the development and preservation of these microbialite reservoirs has yet not been determined. We used samples from outcrops and borehole cores to characterize these microbialites and propose a model for early development and subsequent reservation of their porosity. Primary porosity in the microbialite framework was mainly represented by fenestrate vugs and elongated cavities between microbial layers that were formed during initial sedimentation as well as secondary karst vugs and caves formed by meteoric karst resulting from subsequent tectonic uplift; these features represent the early development of the microbialite reservoirs. Subsequent filling of vugs by early-stage radial dolomite (RD) and late-stage saddle dolomite (SD) occurred during the deep burial stage, with δ18O and homogenization temperature patterns suggesting continuous cementation. However, most vugs were not cemented due to combination of (1) a rigid dolomitized framework, (2) a closed burial system restricting the flow of mineralized fluid, (3) the presence of petroleum or pyrobitumen, and (4) acidic fluid conditions related to high concentrations of H2S and CO2. The third and fourth factors were linked to presence of hydrocarbons and had made big contribution to porosity retention during deep burial diagenetic stage.
The Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation marine shale are the major shale gas exploration and development target in the Middle-Upper Yangtze region. Although three ...national shale gas demonstration areas have been built and several high-yield shale gas wells have been discovered in the Wufeng Formation and Longmaxi Formation, shale gas in these areas has not been fully commercialized because the enrichment degree of shale gas varies greatly in different tectonic settings. By analyzing the formation processes and distribution characteristics of the organic-rich shale, reconstructing the burial history and hydrocarbon generation history, and comparing the shale gas enrichment and preservation conditions in different tectonic settings in South China, this paper points out that (1) The craton depression and foreland basin on the cratonic margin are favorable zones for the development of organic-rich shale. Because the Yangtze plate is small, the foreland basin is greatly affected by the input of terrigenous clastic and the thickness and area of organic-rich shale in the craton basin is larger than that in the foreland basin. The organic-rich shale is mainly distributed in south Sichuan deep-water shelf and west Hubei-east Chongqing deep-water shelf, and the enrichment interval is the Dillcellograptus complexus-Cystograptus Vesicolus graptolite zone from the Wufeng Formation to the bottom of Longmaxi Formation are favorable intervals for shale gas enrichment. (2) Since the Late Jurassic, significant differential intracontinental tectonic deformation, uplift and denudation occurred in the Middle-Upper Yangtze region. Moderate compression and uplift were beneficial to the development of micro-fractures and bedding fractures in shale, resulting in good preservation conditions of shale gas reservoirs with a high-pressure coefficient. In contrast, strongly deformed areas, where faults and high-angle large fractures were developed, are not conducive to the preservation of shale gas reservoirs. (3) The craton depression of the Middle-Upper Yangtze region, where organic-rich shale was deposited, is favorable to shale gas enrichment because it provides a good material basis, and also a relatively stable and compression-resistant tectonic environment for later tectonic deformation and uplift-derived destruction. Based on the analysis and evaluation of tectonic construction–transformation conditions under the macro-control of plate tectonics, it can well explain the existing commercial shale gas discoveries and failure cases and can guide the optimization of future exploration targets.
•►Deep water in the craton depression controlled the distribution of organic-rich shale.•Stable tectonic settings are favorable for shale gas enrichments.•Two depressions in Yangtze Craton are the most favorable areas for shale gas accumulation.
The factors controlling silica enrichment of the Lower Cambrian sedimentary succession have been disputed for a long time. This paper describes results of a petrological and geochemical investigation ...of Lower Cambrian shale deposits of the intraplatfrom basin of the Yangtze Block, South China, with the goal of elucidating potential controls on silica enrichment of these deposits. Excess silica (Sixs) concentrations in the studied deposits suggest that silica was generally enriched in Lower Cambrian units, including the Maidiping Formation (MDP) and the overlying Qiongzhusi Formation (QZS). Greater excess silica concentrations in the MDP relative to the QZS may reflect the influence of three factors: continental weathering, aeolian input, and biological activity. Corrected chemical index of alteration (CIAcorr) values and Ti/Al ratios of the MDP suggest that strong chemical weathering increased the delivery of dissolved silica to the global ocean during the episodes of warm, humid paleo-climate conditions. This, in tandem with diminished aeolian input that accompanied the humid climate, favored establishment of a silica-rich ocean during Fortunian and early Stage 2 time. The presence of coeval massive chert-phosphorite deposits may be attributed to blooms of phytoplankton and siliceous organisms (mainly sponge spicules) induced by upwelling of nutrient-rich bottom waters (e.g., Si and P). However, the abundance of low-diversity sponge spicules was likely insufficient to achieve silica supersaturation of seawater by itself so that direct precipitation of aqueous silica from Si-enriched seawater and pore water may have partly contributed to silica enrichment of the MDP. Diminished CIAcorr values and elevated Ti/Al ratios of the QZS suggest the increased terrestrial and aeolian inputs into the global ocean under a relatively cool, arid paleo-climate condition during middle Stage 2 to the Stage 3 time thereby reducing aqueous silica concentrations. The present study suggests that the interaction among paleo-climate fluctuations, increased delivery of nutrients to the ocean, and redox variability was critical to the diversification of sponge spicule and the appearance of radiolarian during the Cambrian Explosion.
•Fortunian to the early Stage 2 age deposits of the Yangtze Block, South China, are relatively enriched in silica.•Major factors controlling the enrichment of silica of the Lower Cambrian deposits have been proposed.•Early Cambrian climate fluctuations exerted considerable impact on silica enrichment and the evolution of siliceous organisms.
