Objective Recent significant discoveries of marine clastic gas reservoirs in the Shiqiantan Formation （Shiqiantan Sag， Junggar Basin， NW China） necessitate the refined understanding of depositional systems to guide future hydrocarbon exploration. Methods This study reconstructs paleogeomorphology through isochronous sequence stratigraphic correlation and integrates seismic facies analysis， well-log cross-sections， and core/thin-section observations to delineate depositional facies and establish an evolutionary model. ［Results and Conclusions］ Results reveal a complete third-order sequence in the Shiqiantan Formation， subdivided by paleogeomorphic slope breaks into early transgressive systems tract （TST）， late TST， and highstand systems tract （HST）. During deposition， the sag formed a narrow NW-SE-trending marine seaway featuring rift-controlled troughs， multi-stage slope breaks， fault-bounded steep margins， and erosional valleys. Depositional systems identified include nearshore subaqueous fans， fan deltas， slope fans， and basin-floor fans. Paleogeomorphology exerted primary control on facies distribution： fan deltas and nearshore subaqueous fans developed along southern fault-controlled steep slopes； slope fans dominated the northern multi-stage gentle slopes； basin-floor fans occupied the central trough. Deposition occurred under rapid transgression， forming multi-stage imbricate retrogradational basin-floor/slope fan complexes during TST， overlain by extensive braided delta systems in HST. The TST hosts high-quality mature source rocks. Critically， basin-floor fans and slope fans on the northern gentle slope exhibit "finger-like interdigitation" with marine source rocks—creating optimal "source-on-top" and "source-adjacent" reservoir configurations. These zones represent prime targets for concealed hydrocarbon exploration due to superior charge conditions.

Significance Climate plays an important role in controlling marine organic carbon enrichment， but relevant studies have primarily focused on the mechanism of organic carbon enrichment under high insolation and a warm humid climate， summarized the relationship between organic-rich basin sedimentation and astronomical cycle， and have rarely involved the development mechanism of marine source rocks during deglaciation. Several sets of marine source rocks were deposited during the deglaciation of Proterozoic and Paleozoic in China. Studying and summarizing the mechanism of climate on organic carbon enrichment in basins during deglaciation is helpful for establishing a coupling relationship model between different climatic conditions and organic matter abundance from the perspective of Earth system. Progress Research has found that the main source rocks of Mesozoic and Cenozoic corresponding to large oil and gas fields in China were developed in the greenhouse climate period of middle and high latitudes， whereas the main source rocks of Proterozoic and Paleozoic were mainly developed in deglaciation of middle and low latitudes. Abundant dating data indicate that the source rocks of the deglaciation period were distributed in the Neoproterozoic， Ediacaran-Cambrian transition， Ordovician-Silurian transition， and Permian. The source rock of the second member of the Doushantuo Formation in the Neoproterozoic was formed after the Nantuo glaciation. The Yurtus Formation and Qiongzhusi Formation of the early Cambrian were formed in the cold stage of the early Cambrian， after the North China Luoquan and Ediacaran glaciations. The Longmaxi Formation of the Early Silurian was formed after the Hernant glaciation. The Chihsian Formation of the Middle Permian was formed after the Late Paleozoic global ice age. Through the study of the paleoenvironment， paleoclimate， and productivity of source rocks in each periods， it is found that the unique climatic conditions in the deglaciation period played an important role in the organic carbon enrichment of marine source rocks. High temperatures and humidity in the middle and low latitudes， and high insolation corresponding to high obliquity during the deglaciation favor biological growth， reproduction， and organic carbon enrichment. Glacial meltwater input during deglaciation can cause transgression and import nutrients to the ocean. The chemical index of alteration （CIA） values， which rise sharply and fluctuate significantly between 50 and 90 in each period， reflect the warming and unstable climate characteristics during the deglaciation， corresponding to enhanced chemical and freeze-thaw weathering， leading to increased terrestrial nutrient input and productivity enhancement. During the initial stages of ice melting， strong equator-polar temperature differences and sea surface wind lead to intense upwelling， increasing surface water primary productivity， whereas surface biological blooms and oxygen consumption create a reducing environment conducive to organic matter preservation. Volcanism is more intense during interglacial deglaciation， the input of nutrients from volcanic eruptions increases the productivity of the marine surface， and the input of reducing substances improves the preservation conditions of organic matter. Conclusions and Prospects Under the control of Milankovitch cycles， high insolation， warming， and fluctuating climate lead to enhanced rock weathering， extensive marine transgression， intense upwelling， and frequent volcanism， collectively resulting in increased water productivity and anoxic reduction during deglaciation， thereby promoting organic carbon enrichment and source rock formation. This relationship model between climate and source rock formation during deglaciation reflects the control of the ocean-land-atmosphere earth system on organic carbon enrichment under the control of climate cycles. In the future， combining climate simulations and quantitative calculations， certain predictions can be made regarding the distribution of large shale oil and gas fields.

Objective The Qigebrak Formation developed abundant microbial carbonates， which are the favorable target for deep to ultra-deep oil and gas exploration. However， the existing stratigraphic division scheme of the Qigebrak Formation remains controversial， hindering the analysis of reservoir depositional evolution and distribution prediction. Further in-depth research and clarification are urgently needed. Methods This study focused on the Shiairik section in the northwest Aksu area of the Tarim Basin to define the depositional environment and sea-level change， and conduct a sequence stratigraphic division of the Qigebrak Formation based on the analysis of high-precision depositional facies and the Fischer plot. Results Ten main facies are recognized from the Qigebrak Formation and grouped into a carbonate ramp platform. Two subfacies； i.e.， inner and middle ramp， are developed under this setting. The inner ramp consists of the tidal flat， lagoon， tidal channel， and grain shoals. Eight subtypes of peritidal cycles， two subtypes of shallow subtidal cycles and two subtypes of middle-ramp cycles are identified. Based on the stacking patterns reflected in the Fischer diagrams， analysis of orders of depositional facies and proportion of subtidal facies， the Qigebrak is divided into four third-order T-R sequences （SQ1-SQ4）. Among these sequences， SQ1 only records the regressive system tract in the Qigebrak Formation， whereas SQ4 only preserves the transgressive system tract. Conclusions This study suggests that （1） it is reasonable to divide the Qigebrak Formation into four sequences； （2） the top of the Qigebrak Formation could have experienced the million-year-scale exposure and erosion， which would favor the formation of scaled reservoirs.

Objective Limestone-marlstone alterations （LMAs） are important archives of climatic， hydrological， and geological events. However， as a typical endmember of the LMAs， knowledge regarding the genesis of the LMAs with eyelid-eyeball structure from First member of the Maokou Formation in the Sichuan Basin is not widely accepted. This not only seriously affects environmental research of the LMAs with eyelid-eyeball structure， but also affects the efficiency of its exploration. Dolomitization is developed in the LMAs with eyelid-eyeball structure （mainly in the eyelids） of First member of the Maokou Formation of well DB1 during the penecontemporaneous and early diagenetic phase can be used to constrain the depositional environments of carbonate rocks. Methods Petrological and geochemical analyses have been conducted in detail， the process and the fluid properties of dolomitization has been discussed to constrain the process of LMAs with eyelid-eyeball structure. Results Three types of dolomitization developed in First member of the Maokou Formation， which are muddy dolomitic （EDM-I）； limy dolomitic （EDM-II）； and dolomitic marlstone （EDL）. EDM-I has the highest degree of dolomitization， distinct bands of cathodoluminescence， a flat rare earth partitioning pattern， higher δ¹³C and δ¹⁸O values， but lower Y/Ho values relative to seawater； it was dolomitized by penecontemporaneous seawater mixed with a small amount of meteoric freshwater during the penecontemporaneous phase. EDM-II has a middle degree of dolomitization， a left leaning rare earth partitioning pattern， and values of δ¹³C， δ¹⁸O， ⁸⁷Sr/⁸⁶Sr， and Y/Ho similar to seawater； it was dolomitized by seawater with high Mg/Ca， which was induced by frequent sea-level rise and fall changes during the syn-sedimentary and penecontemporaneous phase. EDL has the lowest degree of dolomitization， a left leaning rare earth partitioning pattern， higher δ¹³C and δ¹⁸O values relative to EDM-II， and seawater-like Y/Ho values. EDL was dolomitized by the sealed seawater of the syn-sedimentary phase that has been alterated with the wall rocks in the early diagenetic stage. Conclusions The dolomitization of the LMAs with eyelid-eyeball structure by weakly evaporating seawater and seawater mixed with freshwater during the penecontemporaneous and early diagenetic phase requires that the LMAs deposited in a shallower water. The high mud content， high strontium isotope and rare earth element values， low carbon and oxygen isotope values，and Y/Ho values of the eyelid-like marlstone relative to the eyeball-like limestone indicate that the depth of water for the eyelid-like marlstone is shallower than that of the eyeball-like limestone during deposition. Therefore， the LMAs in First member of the Maokou Formation were produced under cyclic oscillation in the depositional environment， and their genesis may have been mainly influenced by sedimentary process.

Objective The redox states of the Early Ediacaran ocean have long been a research hotspot， particularly its material composition and distribution. The formation process of sedimentary barite nodules in the lower part of the Doushantuo Formation was studied to explore the redox variation of the Ediacaran ocean immediately after the Snowball Earth event using sedimentology and petrology. Additionally， a detailed analysis of the formation process of barite nodules in the lower part of the Doushantuo Formation in the northern part of the Yangtze Block， specifically at the Jinguadun profile， was conducted to infer its depositional environment. Methods Analysis on sedimentary environments of barite nodules from calcareous mudstone of the lower Doushantuo Formation in Jinguadun section were developed via sedimentologic and petromineralogical methods. Results Based on field observations， the surrounding rock layers were found to exhibit a distribution pattern around the nodules. Examining the contact relationships between the nodules and their internal bedding， in accordance with the summary by Sellés-Martínez in 1996， indicates that the barite nodules in this profile formed during the early burial stage. The barite， quartz， and pyrite are the three top authigenic sedimentary minerals in the nodules. The mineral distribution is stratified in the barite nodules. Specially， the inner layer contains dense pyrite aggregates dominated by euhedral-to-subhedral pyrite grains and quartz minerals that grow interwoven together. From the core to the outer edge， radial aggregates were developed in barite and quartz. This study preliminarily demonstrates that the formation of authigenic sedimentary minerals occurring in different parts of the barite nodules precipitate in different redox zones. First， in the sulfate reduction zone within the sediment， an area enriched in organic matter forms a loose outer shell primarily composed of pyrite aggregates. Subsequently， as burial depth increases towards the sulfate-methane transition zone， barite begins to deposit around the pyrite outer shell， causing the high-content pyrite aggregates to gradually become denser. The isolating effect of the pyrite aggregates results in a relatively closed internal space， impacting the deposition of stable barium ions and sulfate radicals by barite. In a slightly acidic environment， the process of pyrite generation promotes the stable deposition of quartz， leading to an internal mineral composition predominantly consisting of quartz. Simultaneously， the external environment evolves into a radial mixture of interwoven barite and quartz. This process highlights the interaction between pyrite and barite under different geological conditions and their influence on the deposition of barium ions and sulfate radicals. The Ba²⁺ sources in the barite likely originate from the methanogenic zone， where dissolution of labile and biogenic barite associated with organic matter occurs. The upward spreading Ba²⁺ ions react with downward diffusing seawater sulfate radicals and deposit barium sulfate in the upper front of sulfate-methane transition zone. Conclusions In summary， the barite nodules form in the early diagenetic soft sediments， exhibiting a phased development. In the initial stages， they are primarily composed of pyrite and quartz； in the later stages， barite and quartz deposition becomes dominant. With the increase in sulfate concentration， it is suggested that in the Early Ediacaran oceans， the sulfate reduction interface may have descended into the sediments near the water-rock interface. As a result， methane in the sulfate-methane transition zone underwent oxidation， thereby reducing their emissions into the atmosphere.

Objective Reconstruction of atmospheric carbon dioxide concentrations （pCO₂） during the Cenomanian-Turonian （C-T） has been previously limited to stage-scale temporal resolutions， which have greatly constrained its effectiveness in unveiling the extreme greenhouse climate of the C-T period and perturbations in the global carbon cycle. Methods To enhance our understanding of pCO₂ fluctuations and the paleoclimatic response during the C-T "greenhouse climate，" this study reconstructed high-resolution atmospheric pCO₂ and mean annual precipitation （MAP） during the C-T period by analyzing the stable carbon and oxygen isotopes of paleosol carbonates from the Upper Cretaceous Nanxin Formation in the Qamdo Basin， southeastern Xizang. Results Our results reveal a gradual decline in pCO₂ during the Early Cenomanian period， followed by an increase in the Middle and Late Cenomanian stages. Moreover， we observed significant fluctuations in pCO₂ during the Early and Middle Turonian stage. These findings align with the variations in pCO₂ throughout the C-T period that have been estimated using the stomata ratio method of plant fossils and the geochemical models. In addition to the long-term pCO₂ fluctuations， the paleosols of the Nanxin Formation also documented two crucial carbon cycle perturbations during the Cenomanian period： Oceanic Anoxia Event 2 （OAE2） and Mid-Cenomanian Event （MCE）. The atmospheric pCO₂ rose by 309 μmol/mol and followed by a rapid decline of 520 μmol/mol across the initiation of MCE， accompanied by a positive shift of 1.25‰ in organic carbon isotopes （¹³C_org）. During OAE2， there were two distinct CO₂ pulses， with the maximum pCO₂ concentration coeval with the lowest ¹³C_org. The pCO₂ increased up to approximately 1 300 μmol/mol at the OAE2 boundary， followed by a reduction of approximately 17% owing to substantial organic carbon burial during OAE2. The increased concentration of CO₂ throughout the Mid-Late Cenomanian intensified the greenhouse effect and elevated atmospheric humidity， resulting in significant negative shifts in ¹³C_org that were decoupled with inorganic carbon isotopes. Our conclusion further suggests that there would be an increase of 511 mm/yr in MAP as the atmospheric pCO₂ elevated from 500 μmol/mol to 1 000 μmol/mol during the C-T period. Conclusions We assumed that the elevated atmospheric pCO₂ and climatic sensitivity during the mid-Cretaceous considerably intensified the hydrological cycle， contributing to an extreme greenhouse climate period.

Objective Upward shallowing sequences are widely developed in shallow-water carbonate environments， and the identification of upward shallowing sequences in deep-water carbonate rocks has a unique advantage in analyzing the evolutionary process of shallow-water carbonate sedimentary environments. Analyzing the high-precision upward shallowing sequences of the late Cambrian Xixiangchi Formation in the Upper Yangtze region can provide important references for understanding the late Cambrian global environmental changes. Methods Detailed field sedimentological dissection and indoor microfacies analysis were conducted on the upper Cambrian Xixiangchi Formation in Chengkou town along the northern margin of the Upper Yangtze. Carbonate rock decimeter- and meter-scale upward shallowing sequences were identified， and their compositional characteristics and vertical superposition relationships were analyzed to explore the sedimentary environment and its evolution in the late Cambrian of the Upper Yangtze region. Results Based on detailed field observations and thin section identification， and referring to the Flügel sedimentary microfacies classification scheme， 14 microfacies types were identified in the Xixiangchi Formation at the Chengkou section， including subtidal sedimentary microfacies MF0-MF10； intertidal sedimentary microfacies MF11-MF12； and supratidal sedimentary microfacies MF13. These microfacies include： MF0 argillaceous shale， MF1 micrite， MF2 bright crystal gravel limestone， MF3 bright crystal sand limestone， MF4 muddy siltstone， MF5 dolomitic microcrystalline limestone， MF6 gravel dolomite， MF7 bright crystal sand dolomite， MF8 sandy gravel dolomite， MF9 sandy dolarenite， MF10 sandy dolomite， MF11 laminated dolomite， MF12 bamboo leaf-like gravel dolomite， and MF13 grain dolomite with dissolution pores. Based on microfacies interpretation and vertical stacking relationships， C1-1 to C1-6 decimeter-scale upward shallowing subtidal， C2-1 to C2-6 decimeter-scale upward shallowing subtidal to intertidal， and C3-1 to C3-2 decimeter-scale upward shallowing intertidal to supratidal sedimentary sequences were identified. These decimeter-scale upward shallowing cycles constitute three secondary transgressive-regressive sedimentary cycles， which from bottom to top， constitute the rapid transgression-slow regression upward shallowing sedimentary cycle of the Xixiangchi Formation. Secondary cycle I was developed by subtidal sedimentary microfacies， secondary cycle II was developed by subtidal to intertidal sedimentary microfacies， and secondary cycle III was developed by intertidal to supratidal sedimentary microfacies. From bottom to top， the thickness of subtidal sedimentary microfacies in the lower part of each secondary cycle decreases successively， whereas the thickness and exposure degree of intertidal to supratidal sedimentary microfacies in the upper part of each secondary cycle gradually increase. Conclusions Based on the identification of upward shallowing sedimentary sequences and the analysis of their compositional characteristics and vertical stacking relationships， the Xixiangchi Formation in the late Cambrian was significantly influenced by high-frequency sea level changes， strongly controlled by tides， waves， and storms. It experienced early rapid transgression and late slow regression， and underwent multiple secondary transgressive-regressive cycles， forming a limited carbonate platform sedimentation model characterized by mixed tidal flat subfacies， micritic lagoon subfacies， and intraplatform grain shoal subfacies. Furthermore， the development of paleo-weathering crust at the top indicates significant regression and erosion events in the late Cambrian in this region. By comparing with chronostratigraphic and evolutionary characteristics of platforms in the Yangtze Platform， North China Platform， Tarim Platform， United States， and Australia， it further confirms the global sea level fall events in the late Cambrian.

Objective The 1st member of Jurassic Badaowan Formation （J₁b₁） in the slope area of Mahu Sag is a set of sandy （gravel） rock deposits of coal-bearing rock series close to the Jurassic / Triassic （J/T） unconformity. The shale rich conglomerate belt near the bottom of the J/T unconformity has clear oil and gas thickened， and light oil and gas are concentrated in the thin sandstone belt above the coal seam. There is still no research on the origin of oil and gas thickening zone in the relatively deep buried area， the percolation mechanism of atmospheric fresh water to overlying strata on the unconformity surface， and the origin of light oil and gas enrichment above coal seam of coal-bearing rock series. Methods Based on the properties of crude oil and carbon isotope analysis of natural gas， combined with the data of burial history， thermal history， pore evolution history， and laser Raman spectrum analysis of hydrocarbon salt water inclusions， the genetic types of J₁b₁ natural gas and the origin and distribution of hydrocarbon thickening zone and light oil and gas zone are discussed. Results （1） The J₁b₁ coal-bearing rock series can be divided into oil and gas thickening zone and light oil and gas zone based on the characteristics of oil and gas density viscosity. The former is mainly distributed in the upper wall of the fault zone， the fault zone， and the bottom of the slope zone adjacent to the J/T unconformity. The latter is distributed in the thin sandstone zone above the coal seam in the slope zone， and the coal-derived gas content is high. （2） The origin of the shale rich conglomerate oil and gas thickening zone at the bottom of the slope zone adjacent to the J/T unconformity is closely related to the atmospheric freshwater compacting-centrifugal flow oxidation degradation. The “Yanshan normal fault - J/T unconformity - shale rich conglomerate assemblage” and the associated capillary imbibition of wetting water phase led to the formation of atmospheric freshwater compaction-centrifugal flow oil-gas thickening zone closely related to the unconformity in the slope area of relatively deep burial. （3） The thin sandstone reservoir above the coal seam has the characteristics of “self-generated and self-stored （deinspiratory charging in the J₁b₁ coal seam）” and “bottom-generated and upper-stored （hydrocarbon supply from P₁ f source rock）” and has the advantage of light oil and gas enrichment. The coal seam desorption diffused upward through the positive fracture into the thin sandstone at the top， and the methane carbon isotope value was relatively light， which was different from the retained adsorbed gas in the coal seam （δ¹³C₁=-48.2‰ in the former and δ¹³C₁=-43.6‰ in the latter）. The geological stratification effect caused by long distance migration of P₁f source rock oil and gas further promotes the accumulation of light oil and gas in thin sandstone above the coal seam. Conclusions The upper thin sandstone belt of J₁b₁ is the first choice for exploration of light oil and gas enrichment belt. The quantitative lithological logging identification map of coal-bearing rock series with the combined restraint of CNL-IMP was established， and the effective reservoir distribution area of the upper thin sandstone of J₁b₁ with a thickness≥10m was identified in the Yanbei1 three-dimensional （3D） seismic zone.

Objective The reticulated laterite in southern China is a good geological carrier for Quaternary environmental changes， but there is still a weakness in microscopic research on sandstone reticulation. Methods Through micro-area analysis techniques， geostatistical and factor analysis， and other methods， we conducted elemental geochemical analysis on the white vein micro-area of the sandstone and reticular red clay in the Langxi profile. Results （1）The content of iron group elements such as Fe₂O₃ （1.14%-13.29%） and Mn （87×10^-6-3 230×10^-6） in sandstone layers increased from the center of the white veins to the periphery. However， the spatial distribution of other major elements and stable elements such as Ti （1 294×10^-6-2 454×10^-6） and Zr （171 ×10^-6-197×10^-6） showed the opposite trend. The results of the reticulated red soil layer are consistent. （2） Ti/Zr （sandstone layer： 7.23-12.89； reticulated red soil layer： 12.82-21.84） and Ti/Al₂O₃ （sandstone layer： 0.013-0.018； reticulated red soil layer： 0.044-0.062） were divided into two groups through cluster analysis and scatter plots. The results show that they have different provenances. （3） The leaching intensity of elements such as Al₂O₃ （17.3%）， Fe₂O₃ （73.91%）， and MnO （76.68%） in the sandstone white vein micro-area and Al₂O₃ （15.78%） ， Fe₂O₃ （70.39%）， and MnO （74.84%） in the reticulated red soil layer white vein micro-area decreased from the center of the white veins to the periphery. （4） The common factors in the white vein micro-area of the sandstone layer and the reticulated red soil layer are that they reflect the leaching migration of iron and iron group elements in the white vein， as well as the relative enrichment process of constant elements such as Al₂O₃， SiO₂， and K₂O and stable elements such as Ti and Zr. Conclusions During reticulation， the leaching of iron group elements dominates， but other elements also have a certain degree of migration. Additionally， it exhibits spatial characteristics of strong leaching at the center of the network pattern and weak leaching at the periphery. Affected by local environmental differences， the development of reticulation is a spatial process that expands from the center of the reticulation to the periphery. The material composition inside the reticulate pattern is non spatially homogeneous. The sandstone and reticulated red soil layers have different material foundations， and the larger reticulation in the sandstone layer indicates that it experienced a relatively humid and hot climate during its development.

Objective Mixed sediments were widely developed in the lower First member of the Shahejie Formation in the Lixian slope and their influence on the development of hydrocarbon reservoirs is of great significance. The aim of this study was to clarify their properties to determine the target area for exploration and development in the next step. Methods The types， distribution regularity， control factors， sedimentary model and mechanism of high-quality reservoir formation in mixed sediments were analyzed from cores， thin sections， seismic well logging and laboratory analysis data. Results Two types of mixed sedimentation occur in the study area： mixed sedimentary rock and mixed sedimentary strata. Three major categories of mixed sedimentary rock （mixed sandstones， mixed mudstones and mixed carbonate rocks） are further divided into six types. The mixed sedimentary strata are divided into 16 types in four lithofacies assemblages： terrigenous rock-carbonate rock， terrigenous rock-mixosedimentite， carbonate rock-mixosedimentite， and mixosedimentite. The lower First member of Shahejie Formation in the Lixian slope has typical mixed features， which occur in eight microfacies types： underwater distributary channel microfacies， mouth bar sheet microfacies， underwater diversion bay microfacies， mud flat microfacies， sandy beach bar microfacies， carbonate beach bar microfacies， inner beach microfacies and semi-deep lake mud microfacies. The mixed sediments display rapid phase transformation laterally， and multiple rock types are frequently superimposed vertically. The mixed sediments are distributed differently at different intervals， two mixed depositional models are evident， one developed during lowstand periods， and one during highstand periods influenced by climatic and water environments， tectonic background， paleogeomorphology， sediment supply and lake level changes. The mixed sediments play a significant role in reservoir control； the sandy beach bars and carbonate beach bars that were developed in high-energy facies zones contain the most favorable mixed sedimentary microfacies. A closed saline water environment is the foundation of reservoir formation， controlling the formation and distribution of high-quality reservoirs； sedimentary cycles control the degree of development and distribution. Conclusions The paleogeomorphology highland in the southwestern region contains favorable areas for mixed beach bar development and exploration.