Ediacaran Dengying Formation Research Articles

Hydrocarbon accumulation and reconstruction in the Ediacaran Dengying carbonate reservoirs of the southeastern and central Sichuan Basin, SW China

The carbonate reservoirs of the Ediacaran Dengying Formation (Z2dn) in the southeastern Sichuan Basin (SESB) and central Sichuan Basin (CSB) experienced different tectonic movements during the Caledonian, Yanshanian, and Himalayan orogenies, resulting in distinct hydrocarbon accumulation processes in these two regions. Based on a detailed analysis of diagenetic sequences, pore-fluid pressure evolution, hydrocarbon charge history, and regional tectonic structural analysis, the hydrocarbon accumulation processes in the Dingshan and Ziyang areas can be divided into three stages, namely Triassic to Jurassic, Jurassic to Cretaceous, and from Late Cretaceous to the present. The first stage was characterized by the formation of paleo-oil pools, marked by an episode of oil charge in the Z2dn reservoirs, as evidenced by solid bitumen and bitumen-bearing inclusions. During the second stage, pore fluid pressure increased from normal to overpressure due to oil cracking in the favorable preservation conditions, leading to the formation of paleo-gas pools in the SESB and CSB. The timing of gas charge in the Dingshan and Ziyang area was at 173 Ma and 165 Ma, respectively, according to the trapping temperature of gas inclusions. The third stage was marked by a pressure drop from overpressure to normal pressure, possibly due to gas dissipation during the Yanshanian-Himalayan uplift. The destruction of paleo-gas pools in the Dingshan area, accompanied by gas leakage through faults and fractures, was caused by intense tectonic movement in the SESB. In contrast, the low gas productivity in the Ziyang area of the CSB was attributed to the migration of natural gas into the Weiyuan area. The fluid evolution of carbonate reservoirs and various tectonic characteristics reveal similar hydrocarbon accumulation but different reconstruction processes in the SESB and CSB, providing greater insight into the mechanism of hydrocarbon enrichment in the Sichuan Basin and other multi-cyclonic composite basins.

Late Ediacaran to Early Cambrian stratigraphic correlation and its geological implications in the northwestern Sichuan Basin: insights from phosphorus, isotopes, and small shelly fossils

The characteristics of elements, isotopes, and small shelly fossils were investigated for Late Ediacaran to Early Cambrian stratigraphy division and to discuss their geological implications in the northwestern Sichuan Basin. The results reveal that small shelly fossils can be detected in the high-phosphorous section, with the concentration of phosphorus mainly ranging from 2% to 8%, suggesting that this interval belongs to the Early Cambrian, which is also consistent with the carbon isotopic composition results. In addition, the Early Cambrian is denudated in the Sichuan Basin due to tectonic movement, and the characteristics of some isotopes and small shell fossils are different from those in other basins. It can be proposed that P content can support the recognition of lithological boundaries, and the high phosphorus content can be used as a reference to identify the top and bottom boundaries of the Maidiping Formation in the study area. According to the elemental compositions in the Ediacaran Dengying Formation, the variations in Si, Al, Fe, and K contents are similar in the platform area and rift area, suggesting that the third and fourth member of the Dengying Formation are also developed in the Deyang–Anyue Rift. The results suggest that both the Deng-4 member and Maidiping Formation feature contemporaneous deposition of different facies in the northwestern Sichuan Basin. The strata consist of shale intercalated with thin carbonate rock deposits in the Deyang–Anyue Rift, while carbonate rock deposits in the platform. The Deyang–Anyue Rift expanded gradually in the Late Ediacaran and eventually filled in the Early Cambrian. The data in this study illustrate that elemental compositions, isotopes, and small shelly fossils can be combined to correlate the Late Ediacaran to Early Cambrian strata and provide new evidence for Deyang–Anyue Rift evolution. The results offer some new insights for deep oil and gas exploration in the Sichuan Basin and for the tectonic–depositional–environmental–biological synergistic evolution in the Late Ediacaran to Early Cambrian transition.

Logging Identification Method for Reservoir Facies in Fractured-Vuggy Dolomite Reservoirs Based on AI: A Case Study of Ediacaran Dengying Formation, Sichuan Basin, China

As an important target for deep to ultra-deep carbonate oil and gas exploration, Fractured-Vuggy dolomite reservoirs have strong heterogeneity. Accurate characterization of reservoir facies is crucial for their exploration and exploitation. Three methods, including the unsupervised intelligent clustering method of improved Fuzzy C-means clustering Algorithm Based on Density Sensitive Distance and Fuzzy Partrition (FCM-DSDFP), the fusion method of Principal Components Analysis (PCA) dimensionality reduction and noise reduction, and the principle of clustering feature analysis are applied to identify reservoir facies based on logging data. Based on the PCA method, the logging response characteristics of the reservoir facies are excavated, and the fusion characterization data of dimensionality reduction and noise reduction are extracted. The FCM-DSDFP unsupervised intelligent clustering method, a model that approximates the subsurface conditions is established, and the reliability of the model is tested according to the elbow rule and silhouette coefficient. Combining drilling core observation, Fractured-Vuggy type, partially cemented Fractured-Vuggy type, Pore-Vuggy type, Pore Type I, Pore Type II, Tight Type I, and Tight Type II are divided in the Dengying Formation 4th Member. Fractured-Vuggy type, partially cemented Fractured-Vuggy type, Pore-Vuggy Type I, Pore-Vuggy Type II, Pore Type I, Pore Type II, and Tight Type are divided in the Dengying Formation 2nd Member, respectively. Two methods were applied to verify the reservoir facies identification results based on intelligent algorithms. The first method is to compare the identification results with the reservoir facies types identified by core observations (Well PT103 and PS13). The second method is to verify the recognition results of intelligent algorithms by utilizing the relationship between reservoir facies types and bitumen. The test results show that the accuracy of the reservoir level identification is higher than 0.9, and the applicability is better than the commonly used algorithms such as FCM and K-means.

Formation mechanism of deep to ultra-deep carbonate reservoir: a case study of the Ediacaran Dengying Formation, Penglai area, Sichuan Basin

The formation of high-quality deep to ultra-deep carbonate reservoirs presents a topic worthy of thorough exploration. Focusing on the Ediacaran (Sinian) Dengying Formation in the Penglai area of the Sichuan Basin, we analysed genesis mechanisms of deep to ultra-deep marine carbonate reservoirs through drilling/logging, seismic data and analytical tests, which showed the dolomite reservoir primarily consists of secondary solution pores and cavities, exhibiting characteristics of vertical overlap and horizontal continuous distribution. The microbial mound beach, which represents the main dominant facies type within the reservoir, can be categorised into three types: high-quality reservoir, medium-quality reservoir and poor reservoir. The reservoir has undergone five stages: syngenetic to quasi-syngenetic, shallow burial, supergene, burial and deep burial adjustment stages. It is postulated that the development of deep to ultra-deep carbonate reservoirs is governed by sedimentary facies, with the type and structure of carbonate influencing the development and evolution of reservoir pores. Multiphase diagenetic fluids further modify and enhanced the properties of the reservoirs. Deep and ultra-deep carbonate reservoirs exhibit strong heterogeneity, and high-quality reservoirs are characterised by multi-factor joint control and multi-stage composite genesis. The lithological and structural–lithological traps associated with them commonly cluster in large areas so effective identification of favourable sedimentary facies and reservoirs is crucial for predicting deep to ultra-deep carbonate reservoirs, characterising traps and improving the success rate of drilling.

Retrogradation of carbonate platforms on a rifted margin: the late Ediacaran record of the northwestern Yangtze Craton (SW China)

The Late Ediacaran Dengying Formation, located in the Sichuan Basin of the northwestern Yangtze Craton, is of significant interest in oil and gas exploration due to its abundant pores and vugs within microbial mound-shoal complexes. However, there is still uncertainty regarding the spatiotemporal distribution and controlling factors of the platform margin. This study comprehensively analyzes the retrogradation pattern of the Dengying Formation platform margin using seismic data, well logs, field outcrops, and petrological characteristics. Our findings reveal that the Dengying Formation strata surrounding the rift basin at the northwestern of the Yangtze Craton can be divided into three main depositional facies: basin facies, slope facies, and platform margin facies. Additionally, based on the integration of lithological, log, and seismic characteristics, the Dengying Formation is subdivided into four third-order sequences, with five sequence boundaries and three seismic facies identified. Supported by sequence stratigraphy and geophysical data, we have reconstructed the tectono-sedimentary evolution of the multiple platform margins on the eastern side of the Deyang-Anyue rift in the Sichuan Basin during the late Ediacaran. Our findings indicate that the platform underwent two phases of retrogradation. The second-stage platform margin underwent retrogradation towards the interior, spanning a distance between 10 and 80 km, based on the initial configuration established by the first-stage platform margin. The main controls for progradation and retrogradation of carbonate platforms are eustatic sea-level changes and tectonic activity. Eustatic sea-level changes can be divided into constructive and destructive phases. Constructive phases are commonly observed in highstand systems tracts, while destructive phases are often associated with transgressive systems tracts and are related to platform retrogradation processes. However, sea-level changes alone cannot fully control the process of platform retrogradation. The thermal subsidence following mantle plume events likely played a significant role in the retrogradation of the platform in the study area. During this period, tectonic processes controlled the geometry of the platform and the deposition of carbonates in the platform margin-slope-basin environment. Additionally, karst-related mound-shoal complexes developed extensively along the platform margin of the Dengying Formation in the northwestern Yangtze Craton. The Lower Cambrian dark shales represent high-quality hydrocarbon source rocks, while the Dengying Formation exhibits an optimal source-reservoir configuration.

Petrology and Stable Isotopes of Patchy Dolostone of Ediacaran Dengying Formation in South Qinling, Central China: Implication for the Diagenetic History

Petrology and Stable Isotopes of Patchy Dolostone of Ediacaran Dengying Formation in South Qinling, Central China: Implication for the Diagenetic History

Microbially induced fabrics in the Upper Ediacaran coated grain dolostone, Northeast Sichuan Basin, SW China: Implications for reconstruction of the paleoceanic environment

In the Upper Ediacaran Dengying Formation of the Sichuan Basin, the presence of abundant, well-preserved microbial carbonates provides a unique opportunity to study Precambrian paleoceanography and microbial carbonate origins. Particularly, the underexplored coated grain dolostones of this formation, characterized by distinct microbially induced fabrics and intragranular dissolution, offer crucial insights for understanding late Ediacaran microbial mineralization and diagenetic sequences. Utilizing selected typical samples, we conducted detailed petrographic and geochemical analyses, revealing a dynamic interplay between microbial processes and sedimentary dynamics. We observe the coexistence of constructive micrite envelopes with microbially induced fabrics such as clots and laminations, highlighting microbial biomineralization's key role in fabric formation. The sedimentary dynamics critically determines the formation processes of the coated grains: low-energy settings foster grain agglomeration and consolidation through clot precipitation between grains, while high-energy settings favor smaller grains binding to microbial mats. Geochemically, micrite envelopes play an essential role in preserving distinct rare earth element (REE) signatures. The weak negative Ce anomalies and positive Eu anomalies within these envelopes point to a suboxic to anoxic depositional environment, directly indicative of the microenvironmental conditions conducive to microbial mineralization processes. Furthermore, our study sheds light on the structural evolution of coated grains with hollow nuclei, proposing that their internal pore formations are influenced by both mineral instability and selective dissolution by meteoric freshwater. These findings not only provide fresh insights into complex diagenetic processes in the Dengying Formation but also substantially advance our understanding of early microbial life and environmental adaptations during the Precambrian.

Formational stages of natural fractures revealed by U-Pb dating and C-O-Sr-Nd isotopes of dolomites in the Ediacaran Dengying Formation, Sichuan Basin, southwest China

Deep core (>4.9 km) from Ediacaran Deng IV Member algal dolomites in the Gaoshiti-Moxi block in the Sichuan Basin, southwest China, reveals multiple generations of dolomite-lined and dolomite-filled opening-mode fractures. Three progressive stages of fracture formation are marked by crosscutting relations visible in the core, by acoustic emission experiments revealing evidence of past stress directions, and by fluid inclusions, U-Pb ages, C-O-Sr-Nd isotope patterns, and rare earth element data for dolomite cements in fractures, which document ages and differing thermal conditions and fluid compositions during fracture. In calcite-filled fractures, U-Pb ages and carbon and oxygen isotope signatures vary greatly, indicating that fractures developed with intensified tectonic activity marked by regional structures and with enhanced diagenetic alteration. In stage I, WNW-striking opening-mode fractures formed that contain dolomite deposits precipitated from basinal fluids between ca. 549 Ma and ca. 532 Ma. At this time, the Sichuan Basin experienced Xingkai taphrogenesis (rifting) from the late Neoproterozoic to early Cambrian. The central Sichuan paleo-uplift was undergoing ENE extension, and preexisting ESE- and nearly E-W−striking faults were oblique to the ENE principal stress orientation. This led to a local stress field favoring dextral shear near fault zones accommodated by the fractures. In stage II, ENE-striking fractures that are younger based on crosscutting relations contain dolomite deposits from basinal fluids with ages from ca. 423 Ma to ca. 411 Ma. Contemporaneous with Xuefeng thrusting, the central Sichuan paleo-uplift was in a NNE-striking transpressional stress field, which likely further generated ENE-striking fractures. In stage III, nearly N-S−striking fractures formed in the Gaoshiti-Moxi block. High-temperature fluids related to the Permian Emeishan large igneous province invaded these fractures from ca. 260 Ma to ca. 256 Ma. At this time, the Sichuan Basin was uplifted under the influence of the Emei taphrogenesis in the late Permian, and the central Sichuan paleo-uplift was subjected to E-W−striking extension. In fractures in these carbonate rocks, micro-computed tomography imaging reveals that macropores (>10 μm, 12.1%−21.8%) and small pores (2−10 μm, 76.6%−86.1%) dominate the dolomite mineral deposits, and that there are few (1.6%−1.8%) micropores or nanopores (<2 μm). Medium-sized throats (1−3 μm) are the main connecting channels. We infer that fractures served as conduits for fluid migration, leading to the dissolution of matrix pores adjacent to the fractures. This secondary porosity not only enhances reservoir storage capacity but also augments reservoir connectivity. Our study shows that in situ U-Pb dating and full-diameter rock acoustic emission data can effectively constrain the timing of fractures. By integrating this information with regional tectonic sequences and fracture diagenetic sequences from combined relative timing, geochemistry, and rock mechanics evidence, we clarify the factors controlling fracture formation.

Determination of multistage oil charge processes in the Ediacaran Dengying gas reservoirs of the southwestern Sichuan basin, SW China

The Sichuan Basin has experienced multi periods of tectonic uplift event, accompanied with multiple hydrocarbon accumulation and phase transitions processes in the deep and ultra-deep reservoirs. The formation of paleo-oil reservoir plays an important role in the distribution of present-day gas fields, yet many records of early oil reservoirs have been destroyed. A series of methods including, petrography, cathodoluminescence, fluid inclusion, in-situ U–Pb geochronology, and basin modeling, were used to determine the multistage oil charge process in the Ediacaran Dengying Formation of the Weiyuan area, SW Sichuan Basin. The lace-like dolomite lamination and three stages of dolomite cement were petrographically, and geochronologically distinguished in the Dengying Formation, dated at c. 542 Ma, 486–482 Ma, 410 Ma, and 270 Ma, respectively. Three stages of bitumen in the Dengying reservoir indicated three episodes of oil charge. The first and second oil-charging events occurred at 500–486 Ma and 410–400 Ma, by combining the modeled timing of oil generation with mineral ages of the two generations before and after the solid bitumen. The modification of the Weiyuan paleo-oil pool occurred during the Caledonian tectonic uplift, after the second oil-charging event. The timing of the third oil charge was at 270–230 Ma, according to the Th value of aqueous inclusions coeval with secondary bitumen inclusions in the stage 3 dolomite cement (CD-3), close to the timing of main oil generation for the Lower Cambrian source rocks. The first oil charge may be a contribution from the Doushantuo source rock, and the Lower Cambrian source rocks provide the major contribution for the third oil charge. This research reveals the timing of multistage oil charging events in the southwest Sichuan basin, and provides a further method for determining the timing of oil charge in multi-cyclonic composite basins.

New Ediacaran tubular fossils from southern Shaanxi, China

Much palaeontology research on biomineralization has been directed to studying the Cambrian radiation of biomineralizing animals. The evolutionary radiation of biomineralization in the Cambrian Period was preceded by weakly biomineralized animals in the late Ediacaran period around 550–539 Ma. Here we report many new tubular fossils from the upper part of the Ediacaran Dengying Formation in the Lijiagou section, Ningqiang area, southern Shaanxi Province, China, consisting of two new genera and species, namely Shufangtubulus inornatus n. gen. n. sp., and Palaeorhopalon spiniferum n. gen. n. sp. The newly discovered materials expand the list of the Ediacaran tubular fossils. The borings first found on the outer surface of Sinotubulites baimatuoensis are similar to those of Cloudina, which may reveal the earliest predator-prey dynamics. The small and delicate drill holes detected on the inner walls of Sinotubulites baimatuoensis are inferred to be infested traces by parasitism.

A new silicified microfossil assemblage from the Ediacaran Dengying Formation in South Shaanxi, China

A silicified fossil assemblage is newly recovered from the acid-macerated residuals of the siliceous dolomite in the late Ediacaran upper Dengying Formation, Tudimiao Section, Ningqiang, southern Shaanxi Province, China. It consists of a large number of tubular and spheroidal fossils. Of them, Cloudina and Sinotubulites can be well identified by the remaining “cone-in-cone” structure or transverse corrugations and longitudinal ridge and multi-layered outer wall, and several other tubular fossils of unknown affinities can also be recognized. However, most of the tubular fossils are generally preserved as steinkerns lacking detailed surface ornamentation and are difficult to distinguish. Morphologically, spheroidal fossils are highly diversified, comprising one-cell to various aggregated ones including two-cell, three-cell, four-cell, five-cell, or other irregular morphotypes. Therefore, three different preservational styles in the weakly biomineralized organisms may be presented in this area, referring to three tubular fossil assemblages in the Gaojiashan biota, i.e. from the lower to upper, a pyritized Conotubus-Gaojiashania in the siltstone-mudstone and calcified Cloudina in the limestone at the mid-upper Gaojiashan Member, Dengying Formation in the Gaojiashan section and at upper Gaojiashan and lower Beiwan members, Dengying Formation in the Shengchangba section, a phosphatized Cloudina-Sinotubulites association in the lower Beiwan Member, Dengying Formation in the Lijiagou section, and a silicified tubular and spheroidal fossil association at the mid-upper Beiwan Member, Dengying Formation in the Tudimiao section.This finding thus may have some broad implications, it not only opens a new taphonomic window for the late Ediacaran weakly biominerlizing organisms, encourages us to explore more extensively the once neglected cherty dolomite, but also considerably expands the geographical range and species richness of the terminal Ediacaran biosphere. More importantly, the finding also indicates that Cloudina having a worldwide distribution in different paleoecologies and biofacies has the greatest advantage of being the stratigraphic marker for correlation with precision to different paleocontinents.

Hydrocarbon Generation and Residue Features of Ediacaran High-Maturity Source Rocks and Their Significance in Gas Exploration in Sichuan Basin

Evaluating residual hydrocarbons is crucial for assessing shale oil and gas reservoirs, significantly impacting resource evaluation and exploration prospects. Previously regarded as lacking hydrocarbon generation potential, the Ediacaran Dengying Formation (Fm) microbial dolomite in the Sichuan Basin has been re-evaluated for its hydrocarbon generation capabilities. While understanding source rock characteristics is vital for petroleum resource assessment, a comprehensive analysis of the dolomite’s source rocks, encompassing hydrocarbon generation and residual features, remains undocumented. In this study, we thoroughly analyze the total organic carbon and vitrinite reflectance and extensively utilize pyrolysis analysis, gas chromatography and isotopic analysis of the organic kerogen from the Ediacaran Dengying Fm dolomite samples. The findings affirm that the Ediacaran Dengying Fm dolomite indeed serves as a source rock with moderate hydrocarbon generation and residue capabilities. This microbial dolomite was formed in a reducing marine environment with high salinity. Characterized by an averaging TOC of 0.82%, the kerogen is primarily identified as type I, with a minor presence of type II, and underwent thermal maturation up to the post-maturity stage. Throughout its geological history, the maximum intensities for hydrocarbon generation and residues were 4.5 × 107 t/km2 and 3.2 × 107 t/km2, respectively. Additionally, cumulatively generated and residual hydrocarbon quantities amounted to 2.7 × 1012 t and 1.67 × 1012 t, respectively. This study indicates significant exploration potential for the Ediacaran Dengying Fm microbial dolomite. Consequently, the central region in the Sichuan Basin has been identified as a promising area for future exploration endeavors. Our study provides valuable insights for the understanding of shale gas exploration in high-maturity source rock areas.

Geochemical properties, mechanism of formation, and source of solid bitumen in the Ediacaran Dengying Formation from the central to northern Sichuan Basin, China

Several large gas reservoirs, including the Weiyuan and Anyue gas fields, have been discovered in the Ediacaran Dengying Formation in the Sichuan Basin, indicating vast opportunities for deep-petroleum exploration. Understanding the origin and evolution process of these gas reservoirs is crucial for further petroleum exploration and development. Although the burial history of the Sichuan Basin and its surrounding areas is basically dominated by Paleozoic burial and preservation, and Meso-Cenozoic uplift and erosion. However, the source of the Dengying Formation natural gas in the Sichuan Basin is still a subject of controversy between the Ediacaran and Cambrian source rocks due to high thermal maturity, secondary alteration, and multistage tectonic activities. This study aims to examine the investigation on the properties and genetics of Dengying solid bitumens that existed in the central to northern Sichuan Basin, in order to provide insights into the source and evolution history of the Dengying paleo-oil reservoirs. For this purpose, petrologic, elemental, and organic geochemical analyses were used to investigate the properties, origin, and formation mechanism of solid bitumens. The results indicate that these solid bitumens have suffered intense thermal alteration (EqVRo 2.1%–5.3%), and are classified as pyrobitumens. Solid bitumens in the Gaoshiti bulge and the north slope area were subjected to tectonic thermal events (e.g., Emeishan igneous province) and relatively strong thermochemical sulfate reduction (TSR) alteration. These solid bitumens were assumed to be pyrolysis products of light (condensate) oil. In contrast, solid bitumens in the Weiyuan bulge and the Micangshan uplift area were not influenced by tectonic thermal events but experienced weak TSR alteration, and originated from either normal or heavy crude oil. Moreover, a systematic bitumen-source correlation was carried out using δ13Corg values, bound steranes, and trace element distributions. Results reveal that the solid bitumens in the central Sichuan paleo-uplift were mainly derived from the lower Cambrian Maidiping Formation, those in the north slope area originated from the lower Cambrian Qiongzhusi Formation, while those in the Micangshan area were sourced from the Ediacaran Doushantuo Formation. Structural and burial history analysis implies that the differences in the source of the Dengying paleo-oil reservoir (i.e., solid bitumen) between these study areas are attributed to the distribution of source rocks and their contact relationship with reservoirs. This research successfully obtained insights into the source and genetic mechanism of paleo-oil reservoirs by analyzing the properties of high-maturity solid bitumens, which can guide further exploration and development of deep petroleum resources.

U–Pb Dating of Fibrous Dolomite in the Hydrothermal Dolostone of the Dengying Formation, Central Sichuan Basin, and Its Response to Supercontinent Breakup

Super-deep drilling in the central Sichuan Basin encountered volcanic rocks of the Suxiong Formation, which are overlain by multiple hydrothermal alterations within the upper section of the Ediacaran Dengying Formation. This provides an excellent research opportunity to understand the pre-Cambrian hydrothermal activity and geological evolution of the western margin of the Yangtze Craton. Observations revealed the development of a series of hydrothermal dolomite aggregates characterized by the presence of brown sphalerite within fractures and pores of the carbonate rock. Microscopically, the dolomite exhibited fibrous columnar crystallization, forming radial bands with a gradual decrease in crystallization intensity from the center to the periphery. Cathodoluminescence analysis revealed the presence of approximately eight dolomite bands within the aggregate. U–Pb dating from the inner to outer bands yielded isotopic ages of 781 ± 12 Ma for the second band, 683 ± 12 Ma for the fifth, 562 ± 12 Ma for the sixth, and 545.4 ± 6.9 Ma for the seventh. The in situ rare earth element (REE) distribution patterns of the 781 and 683 Ma dolomite bands exhibited similarities. They both showed low total REE content (∑REE), with significant fractionation between light and heavy REEs. Additionally, they exhibited negative anomalies in cerium (Ce) and europium (Eu), while heavy REEs were relatively enriched. The dolomite bands at 562 and 545 Ma also exhibited similar REE characteristics, with low ∑REE and weak fractionation between light and heavy REEs. They also displayed distinct negative anomalies in Ce and Eu, indicating similar distribution patterns. These findings suggested that formation of the banded hydrothermal dolostone occurred during different tectonic events, and the presence of heavy REE-enriched hydrothermal fluids suggested a deep-seated origin. This study has provided preliminary evidence that the Dengying Formation, previously considered to be of the Ediacaran age, has undergone multiple episodes of deep-seated fluid infiltration and alteration since the Mesoproterozoic Era. Importantly, these events coincide with the rifting of the Rodinia and Pannotia supercontinents, aligning with their respective timeframes. This finding raises questions regarding the stratigraphic division and correlation of the formations in the deeply buried core area of the basin.

Diagenetic evolution and effects on reservoir development of the Dengying and Longwangmiao formations, Central Sichuan Basin, Southwestern China

The deeply buried Lower Cambrian Longwangmiao Formation and Upper Ediacaran Dengying Formation from the Sichuan Basin, China, have a total natural gas reserve up to 3 × 1012 m3. The complex diagenetic evolution and their impacts on the present-day reservoir quality have not been systematically elucidated, hampering the current exploration. Crucially, the integration and comparation diagenetic study on these two formations, which may be able to shed new lights on reservoir formation mechanism, are yet to be systemically evaluated. By compiling geochemistry data, including carbonate U-Pb ages and petrophysics data, coupled with new petrology, trace elements, and strontium isotope data, of various types of diagenetic carbonates, this study aims to decipher the potential links between diagenesis and reservoir development of both formations. Intriguingly, similar diagenetic sequence, which contains five distinctive dolomite phases, is established in both formations. The matrix dolomite (D1) and early dolomite cement (D2) were likely formed by reflux dolomitization, as inferred by their nearly syn-depositional U-Pb ages and elevated δ18O caused by seawater evaporation. The subsequent moderate burial dolomite cement (D3) was most plausibly the product of burial compaction as indicated by its lighter δ18O and slightly younger U-Pb ages compared with D1 and D2. Whereas deep burial dolomite cements (D4 and D5) yield markedly depleted δ18O, elevated 87Sr/86Sr, along with much younger U-Pb ages and higher precipitation temperatures, suggesting that they were likely linked to hydrothermal fluids. Despite the wide occurrence of meteoric and organic acids leaching and thermochemical sulfate reduction, they may have only played a subsidiary role on these reservoirs development. Instead, superior reservoir quality is tightly linked to tectonics as inferred by higher reservoir quality closely related to the well-developed fractures and faults filled with abundant hydrothermal minerals. Notably, good reservoirs in both formations are mainly attributed to high permeability caused by tectonics. Hence, this new contribution emphasizes the crucial role of tectonics on spatially explicit reservoir prediction of deep to ultra-deep (up to > 8000 m) carbonates in the Sichuan Basin, as well as other sedimentary basin analogues in China.

A new age, provenance and tectono-sedimentary setting of the middle Dengying Formation of the terminal Ediacaran in the western Yangtze Block, South China

The widespread terminal Ediacaran Dengying Formation in the western Yangtze Block, South China, is dominated by thick marine dolomite successions deposited in shallow-water platform environments. However, a distinctive succession of mixed clastic and carbonate rocks occurs widely in the middle Dengying Formation, representing sedimentary environment variations. The absolute age of the mixed sedimentary succession is poorly constrained, and tectonic and sedimentary settings remain controversial. In this study, we report a new zircon U–Pb age of 547.3 ± 4.8 Ma from a volcanic ash bed intercalated within a mudstone succession (ca. 40–100 cm thick) in the middle Dengying Formation in the western Yangtze Block. This age is consistent with previously published ages of the middle Dengying Formation or its equivalents in the Yangtze Block and in other blocks worldwide. Field investigations show that the mudstone succession overlies above an unconformity resulted from the lower Dengying Formation weathering and shows a deepening upwards depositional succession representing a period of transgression. Mineralogical analyses show that the mudstones are composed of clay minerals (ca. 68.7%, illite and illite/smectite mixed layers), quartz (24.2%) and minor K-feldspar, indicating a high compositional maturity. Redox sensitive element results (e.g., Mo and U) indicate an increasing anoxic setting upwards. Both elemental geochemical and Lu-Hf-O isotopic analyses suggest that the siliciclastic sediments were moderately weathered (average CIA = 71) and were derived from felsic-intermediate rocks, such as granites and granodiorites (or geochemically-equivalent rocks). We suggest that the mixed sedimentary succession represents as a short-lived, regionally-correlated sea level change event, rather than a tectonic uplift event. Our geochemical data reveal that the mudstone succession accumulated most likely in a passive margin setting, supporting the proposition that the Yangtze Block was located on the periphery of the Gondwana supercontinent during the Ediacaran Period.

Hydrothermal silicification in Ediacaran Dengying Formation fourth member deep dolomite reservoir, Central Sichuan Basin, China: Implications for reservoir quality

The Ediacaran Dengying Formation fourth member in Central Sichuan Basin is the deep dolomite gas reservoir with the largest natural gas reserves in China, providing an excellent example for understanding the effect of hydrothermal silicification on deep dolomite reservoir quality. By integrating petrology, geochemistry and physical property measurement, this study aims to reveal the genesis of different quartz phase and their corresponding diagenetic events, and to discuss the temporal relationship of diagenetic events and their effects on the quality of deep dolomite reservoir in Dengying fourth member. Three phases of quartz cement are identified in Ediacaran Dengying Formation deep dolomite reservoirs of Central Sichuan Basin as follows: (1) first‐stage quartz filled in primary reservoir spaces, (2) second‐stage quartz mainly filled in secondary reservoir spaces, and (3) third‐stage quartz filled in secondary reservoir spaces with Mississippi Valley‐Type minerals. By analysing petrographic and temporal relationships between these fabrics, it is suggested that the first‐stage quartz precipitated during the syndiagenetic stage, the second‐stage quartz precipitated during middle diagenetic stage, and the third‐stage quartz precipitated during middle diagenetic stage as well. Silicification during syndiagenetic stage is characterized by replacement of cryptocrystalline quartz and cementation of first‐stage quartz within matrix dolomite (MD). After syndiagenetic silicification, the mineral composition of the MD changed from magnesium calcium carbonate to silica, and the later constructive diagenetic alterations were almost ineffective. The distribution of syndiagenetic silicification during syndiagenetic stage gradually decreases from the platform‐interior to the platform‐margin. The second‐stage and third‐stage quartz cements, derived from two‐stage hydrothermal silicification in the middle stage, are both destructive for reservoir preservation. This study, combined with coeval vapour–liquid inclusions Th data and thermal‐burial models, constrains the time of the second‐stage quartz to ca. 403–408 Ma, suggesting an hydrothermal silicification related to the Caledonian movement. The time of the third‐stage quartz cement is constrained to ca. 259.4 Ma, associated with Late Permian eruption of Emeishan flood basalts.

Terminal Ediacaran microbialite lithofacies associations with paleo-environmental constraints in a high-frequency sequence stratigraphic framework of Sichuan Basin, SW China

The terminal Ediacaran shallow marine microbialites are significant for paleo-environment reconstruction and petroleum exploration. Their depositional and paleo-environmental characteristics during high-frequency sea-level fluctuations were paid less attention, which limited the understanding of their evolution and related reservoir characterization. Microbialites from the fourth member of the upper Ediacaran Dengying Formation, Gaoshiti-Moxi area, Sichuan Basin, SW China are important records for studying their lithofacies associations and paleo-environmental constraints in high-frequency depositional sequences. Petrographic and geochemical analyses (in-situ major and trace elements and carbon and oxygen isotopes) indicate microbialites depositional and paleo-environmental evolution on a rimmed platform within a fourth-order depositional sequence. From late TST (transgressive systems tract) to early HST (highstand systems tract), thick-bedded dark-grey dolomudstone-thin-bedded spotted thrombolite dominates the platform margin; while thick-bedded dark-grey dolomudstone-thin bedded dark-grey wavy stromatolite dominates the platform interior. From late HST to early TST of the next fourth-order sequence, the platform margin is dominated by thick-bedded spotted thrombolite—dolograinstone, while the carbonate interior is dominated by thin-medium-bedded layered thrombolite—light-grey wavy stromatolite. Salinity correlates well with microbialite lithofacies associations: the platform interior layered thrombolite and wavy stromatolite enriched lithofacies associations are characterized by higher salinity than that of the platform margin spotted thrombolite enriched lithofacies associations. The microbialite lithofacies associations are less constrained by redox: platform interior layered thrombolite and wavy stromatolite enriched lithofacies associations are slightly more reduced than the platform interior spotted thrombolite enriched lithofacies associations. The redox analysis of the microbialites also indicates low oxygen levels of shallow marine in the study area during terminal Ediacaran, with possible oxygenation at the Ediacaran-Cambrian boundary. The study suggests that the terminal Ediacaran microbialite development was controlled by both high-frequency sea-level fluctuations and paleo-environmental factors such as salinity and redox. Platform margin spotted thrombolitic reefs from late fourth-order HST to next early fourth-order TST serve as favorable microbialite reservoirs. Platform interior layered thrombolites and wavy stromatolites are regarded as potential reservoirs, which deserve more detailed depositional-paleo-environmental research.

A comparison of hydrothermal events and petroleum migration between Ediacaran and lower Cambrian carbonates, Central Sichuan Basin

Hydrothermal minerals and associated organic matter have been frequently found in the Ediacaran and lower Cambrian carbonate reservoirs, separated by internal shale source rocks, in the Central Sichuan Basin. This work provides a thorough comparison of petrography, geochemical signatures, fluid inclusions and ages between the hydrothermal minerals in the Ediacaran Dengying Formation (Fm) and the lower Cambrian Longwangmiao Formation. This comparison leads to a better understanding of the behaviour and migration history of hydrothermal fluids and petroleum. During the Ediacaran–Cambrian transition, the flux of silicon-rich hydrothermal fluid triggered by seafloor-related volcanic events, resulted in the exclusive occurrence of silicification in the Dengying Fm. During P3–T3, two different hydrothermal systems were found in the Dengying and Longwangmiao formations. The hydrothermal fluids in the Dengying reservoirs were dominated by a mixture of basement-derived Mississippi Valley Type [MVT] ore fluids and shale-derived oil-bearing brines, while those in the Longwangmiao Fm were mainly the oil-bearing brines. The upwards migration of the MVT ore fluids, characterized by very hot, saline, radioactive and Eu-rich conditions, was promoted by the activity of the Emeishan mantle plume 259.4–248 Ma and then blocked by the lower Cambrian shales and trapped only within the underlying Dengying reservoirs. The expulsion of the warm, less saline and moderately radioactive brines and oil was induced by thermal maturation of the organic-rich shales, which might have occurred over a longer duration (261–218.7 Ma). During 170.7–125.8 Ma (J2–K2), the combined effects of the overmature source rocks and Yanshanian tectonic compression facilitated the migration of much hotter shale-derived brines, as well as gas hydrocarbons, across the Ediacaran to lower Cambrian carbonate reservoirs. This comparative study provides some insights into the behaviour of hydrothermal fluids and petroleum in carbonates with internal organic-rich shales.

A knowledge graph for standard carbonate microfacies and its application in the automatical reconstruction of the relative sea-level curve

The reconstruction of high-resolution sea-level variation curves in deep time based on the standard carbonate microfacies knowledge graph (SMFKG) is of great scientific significance for exploring the Earth system evolution and predicting future sea-level and climate changes. In this study, the concepts, attributes, and relationships among standard carbonate microfacies (SMF) are comprehensively analyzed; an ontology layer is established and its data layer is constructed using thin-section descriptions; and finally, the SMFKG is established. Additionally, based on the knowledge graph, an application for automatically identifying SMF using identification markers and reconstructing the high-resolution relative sea-level variation curve using the SMF and facies zones is compiled. Then, all thin sections of the late Ediacaran Dengying Formation in the western margin of the Yangtze Platform are observed and described in detail, the SMF and facies zones are identified automatically, and the relative sea-level curve is reconstructed automatically using the SMFKG. The reconstruction results show that the Yangtze Platform experienced four sea-level rise and fall cycles in the late Ediacaran, of which two intense regressions led to subaerial-exposed unconformities in the interior and top of the Dengying Formation, which is highly consistent with previous research results. This shows that the high-resolution relative sea-level variation curve in deep time can be reconstructed efficiently and intelligently using the SMFKG. Additionally, in the near future, the combination of an automatic digital slide-scanning system, machine-learning techniques, and the SMFKG can achieve one-stop fully automatic SMF recognition and reconstruction of high-resolution relative sea-level variation curves in deep time, which has a high application value.