The formation of the “Conchostracan shale” layer at the top of the lower section of the Middle Jurassic Shaximiao Formation in the Sichuan Basin marks a major shift in the paleolake depositional environment and palaeogeography. This transition corresponds to the faunal succession of the Shulong-Emeilong fauna to the Mamenchisaurus fauna. Reconstructing the sedimentary and palaeogeographic transformation during this period is crucial for understanding the evolution of dinosaur faunas in the Sichuan Basin. Through the buried characteristics of Conchostracan fossil assemblages and whole-rock geochemical analyses,samples from the lower “Conchostracan Shale” layer,as well as the mudstone and siltstone samples from both the upper and lower sections of the Shaximiao Formation,were examined. The results indicate that Conchostracans thrived in warm,humid shallow lacustrine environments. A transition in sedimentary environments—from deltaic systems to shallow lacustrine,fluvial,and deltaic systems—occurred between the lower and upper sections of the Shaximiao Formation. Analysis of paleoclimate indices(CIA,CIW),redox indicators(δCe,V/Cr,U/Th,V/(V+Ni),V/Sc),and paleosalinity proxies(B/Ga,Sr/Ba)reveals a dynamic evolution of the paleoenvironment within the Shaximiao Formation. Palaeoclimatic conditions shifted from semi-humid and semi-arid to warm and humid,and later reverted back to semi-humid and semi-arid. Concurrently,the paleo-water environment experienced a transition from oxidizing and weakly reducing conditions to more reducing conditions,followed by a return to oxidizing and weakly reducing conditions. Paleosalinity remained predominantly freshwater throughout,with only localized,minor salinization observed in the upper sections. These findings suggest that during the late Middle Jurassic,the Sichuan Basin experienced significant precipitation,leading to the establishment of a warm and humid palaeoclimate and a reducing lake environment conducive to the extensive proliferation of Conchostracans. Subsequently,a shift toward drier climatic conditions likely drove the adaptive evolution of herbivorous dinosaur lineages,transitioning from the Shulong-Emeilong fauna to the Mamenchisaurus fauna.

Tectonic-sedimentary co-evolution in continental faulted lacustrine basins controls the development of source-reservoir systems and the processes of hydrocarbon accumulation and enrichment. Through meticulous analysis of core samples,drilling and logging data,and interpretation of high-resolution 3D seismic data,this study clarifies the sedimentary system characteristics and facies distribution patterns in the Chagan sag during the rift stage,revealing the co-evolution mechanism between tectonic activities and sediment filling. The research identifies two third-order sequences within the rift stage: (1)During the initial rifting phase(SQ1),the NE-trending Maoxi Fault and NW-trending Tula’ge Fault evolved from segmented activation to radial linkage,creating a highly compartmentalized rift framework. This structural pattern controlled the development of proximal,short-axis alluvial fan-fan delta systems,while intense fault displacement combined with warm-humid climates promoted the formation of semi-deep to deep lacustrine facies.(2)In the intensive rifting phase(SQ2),the NE-trending basin-controlling fault system dominated the sedimentary framework: volcanic activity during the first sub-member of the Suhongtu Formation occupied accommodation space,leading to widespread shallow lacustrine facies,while intra-rift volcanic topography fragmented braided river delta sand bodies. During the second sub-member,rapid volcanic accumulation outpaced subsidence,causing source depletion along the northern Maoxi Fault and southern Hule Fault segments,and redirecting sediment supply from the west to the northwest along the northern Hule Fault,thereby altering sandbody filling patterns. Coupling between high-displacement faults and humid climates during SQ1 controlled the development of major source rocks. Volcano-tectonic interactions during SQ2 modified sedimentary paleotopography,regulating reservoir sandbody distribution(e.g.,volcanic barriers fragmenting sand bodies;rapid accumulation triggering source system migration). Volcanic interlayers with stratified vesicles form effective reservoirs when connected to deep hydrocarbon sources via fault systems. The tectono-sedimentary synergistic evolution model established in this paper can provide guidance for reservoir prediction and exploration deployment in the study area,and also offer an analog model for the analysis of fault-depression basins with similar geological settings.

This paper investigates storm deposit characteristics in tidal flat environments of the Early Ordovician Yeli Formation in southern North China to elucidate their implications for palaeolatitude and palaeoclimate. Through comprehensive field sedimentological observations and carbonate microfacies analysis,five distinct microfacies were identified: intraclastic dolomite,quartz-bearing sandy dolomite,sandy dolomite,medium-crystalline dolomite,and fine-crystalline dolomite. These microfacies,combined with macroscopic sedimentary features,indicate an environmental transition from subtidal through intertidal to supratidal zones. Storm-generated sedimentary structures include scoured surfaces,storm gravel layers,graded bedding,massive bedding,hummocky cross-stratification,rip-up clasts,load and flame structures. Through systematic analysis of structure-environment relationships,six distinct storm deposition sequences were classified based on their formative hydrodynamics: subtidal vortex-dominated,subtidal combined vortex-wave,intertidal hybrid gravity flow-wave,intertidal debris flow,intertidal vortex-controlled,and supratidal tidal current sequences. The established depositional model demonstrates that storm flow dynamics primarily controlled the sequence architecture. The recorded increase in both storm frequency and intensity during the Early Ordovician corresponds to a climatic warming phase within the broader cooling trend. Regional synthesis indicates the eastern North China Block margin faced an open ocean at paleolatitudes of 15°~24°S. The research results not only deepen the understanding of the storm deposition sequence,but also provide key sedimentary data for the reconstruction of the Early Ordovician palaeoclimate and palaeogeography in southern North China.

The depositional environment in the Member 2 of Xujiahe Formation on eastern slope of Western Sichuan Depression has long been controversial. Using newly identified storm deposits as key evidence,this study aims to clarify the depositional environment of the in the Member 2 of Xujiahe Formation and the development characteristics of storm deposits in this area. Based on core observation,thin-section identification,grain-size analysis,and geochemical testing,the following conclusions are obtained: (1)Sedimentary structures such as bidirectional cross-bedding,wave-generated cross-bedding,and tidal bundles are widely developed in the in the Member 2 of Xujiahe Formation on the eastern slope of the western Sichuan Depression. Grain-size and geochemical data indicate that the study area was a high-energy environment characterized by periodic energy fluctuations,and that the water body was brackish to freshwater in nature. These features suggest a marine shoreline depositional environment,which can be further subdivided into four sedimentary subfacies: foreshore,upper shoreface,middle shoreface,and lower shoreface.(2)Typical storm-related sedimentary structures,including mud-clast layers,gutter casts,and hummocky cross-stratification,are developed in the foreshore to middle shoreface subfacies. At least nine episodes of storm deposition are vertically identified,and the sequences are dominated by incomplete A-B-C types,reflecting high-energy,high-frequency,and proximal storm depositional characteristics.(3)Against a regressive background,the storm-deposit development belt gradually migrated from shoreface to foreshore. The thickness of storm layers was relatively greater in the southwestern part during the early stage,but became thicker in the eastern part during the late stage,indicating a northeastward migration of the depositional center. The results demonstrate that the Xu 2 Member on the eastern slope of the western Sichuan Depression represents a storm-influenced marine shoreline depositional environment. Together with the paleogeographic framework of “deep in the west and shallow in the east,”these findings reveal a hydrocarbon accumulation element assemblage characterized by “provenance supply from the western depression,sand enrichment on the eastern slope,and sealing in the east,”thereby providing a geological basis for the selection of favorable exploration targets for tight sandstone in the area.

While gravity flow deposits have been identified in the Chang 7 member of the Yanchang Formation in the Wuqi area,northern Shaanxi Province,Ordos Basin,no such deposits have been reported in the Chang 8 and Chang 9 members. This study aims to comprehensively reveal the presence,types,and controlling effects of gravity flow deposits in the Chang 7-Chang 9 members on reservoir properties and oil-bearing potential.Through centimeter-scale core observations,grain-size analysis,and well-log interpretation of the Chang 7-Chang 9 members in the Wuqi area,an integrated analysis was conducted to determine the genetic types and characteristics of gravity flow sandstones. Gravity flow deposits are widely distributed in the Chang 7-Chang 9 members of the Wuqi area,dominated by turbidity current deposits,followed by sandy debris flow deposits,with additional gravity-driven slide-slip deposits. Multi-parameter identification criteria for gravity flow deposits in the Chang 7-Chang 9 members have been established: (1)Core structures: slide-slump deposits exhibit deformed and convolute bedding;sandy debris flows show thick massive bedding and mudstone clasts;turbidites display stacked,incomplete Bouma sequences. (2)Grain-size characteristics: sandy debris flows typically show “short-tailed” bimodal distributions(>95% saltation sub-population,steep slope;<5% suspension sub-population,gentle slope). Turbidites generally exhibit unimodal distributions. (3)C-M diagrams: C-M values parallel the C=M baseline,consistent with gravity flow deposition. Comparative analysis of reservoir properties,oil saturation,and well testing results demonstrates that sandy debris flow deposits significantly outper form turbidites. The widespread distribution of gravity flow deposits in the Chang 7-Chang 9 members holds substantial exploration significance. Sandy debris flows should be prioritized as prospective targets for tight oil exploration in the lower petroleum assemblage of the Yanchang Formation,followed by turbidite deposits.

Due to intense tectonic compression,the massive sandstone of the Bashijiqike Formation in the Cretaceous within the piedmont belt of the Kuqa Depression exhibit severe deformation,complex seismic imaging,and high ambiguity,resulting in limited methods and even fewer achievements in the detailed study of provenance transport pathway in the Keshen natural gas accumulation area. In response to this issue,a comprehensive study was conducted on the heavy mineral assemblages of over 400 sandstone samples from more than 40 wells in the Bashijiqike Formation of the Keshen area,as well as experimental analyses including sandstone clastic components and cathodoluminescence on over 600 samples from 26 wells. The results show that: (1)Zircon,tourmaline,garnet,and magnetite are consistently present as heavy minerals in the wells of this area. The presence and variations in the content of stable heavy minerals such as rutile,apatite,and titanite,along with unstable heavy minerals like epidote,chlorite,and mica,lead to differences in heavy mineral assemblages across well zones. Combined with the ZTR index,these variations can help determine the differences in sedimentary provenance pathways among the well zones. (2)The sedimentary source rocks of the thick sandstone mainly come from pre Carboniferous high-grade metamorphic rocks,low-grade metamorphic rocks,intermediate acidic igneous rocks,basic igneous rocks,and sedimentary rocks,which can be subdivided into nine heavy mineral combination zones from east to west. (3)Due to the control of the content of sandstone debris components by primitive sedimentation,the planar variation characteristics of the average content of potassium feldspar,plagioclase,and metamorphic rock debris in sandstone debris components indicate the existence of 5-7 source transport pathways. (4)Based on a comprehensive analysis of the characteristics of source pathways indicated by heavy mineral assemblages and sandstone debris components,it is believed that there were at least seven source transport pathways from east to west during the sedimentation period of the Bashijiqike Formation,among which Zone Ⅱ and Zone Ⅴ may have been channels with longer river extensions. The above understanding provides a basis for the analysis of large-scale sedimentary microfacies and favorable sand body distribution in the Keshen area,supporting the fine research and evaluation of gas reservoirs in this area.

The co-development of alluvial fans and aeolian deposits in arid regions is a common phenomenon. While wind reworking and redeposition on fan surfaces are widespread,research on their interactive sedimentation and detailed internal architecture characterization is relatively scarce. Based on modern remote sensing images and field outcrops,this study conducted detailed observations and dissection of the lithofacies,lithofacies associations,and planar distribution within the middle part of the modern Dunhuang alluvial fan. The study area primarily develops 9 lithofacies and 5 lithofacies associations,identifying two main sedimentary types: braided channel deposits and aeolian deposits. Among these,the large-scale cross-bedded sandstone lithofacies(St)and parallel-bedded sandstone lithofacies(Sp)represent typical aeolian deposits. The braided channels mainly develop 5 lithofacies associations: cross-bedded conglomerate facies-small-scale cross-bedded sandstone facies(Gt-Scs),clast-supported conglomerate facies-small-scale cross-bedded sandstone facies(Gcm-Scs),clast-supported conglomerate facies-massive sandstone facies(Gcm-Sm),lateral accretionary thin-bedded conglomerate facies-parallel-bedded sandstone facies(Gla-Sp),and clast-supported conglomerate facies-sandy conglomerate facies/pebbly sandstone facies(Gcm-Gs/Sg). Horizontally,the braided channels of the alluvial fan develop adjacent to the aeolian dunes. Vertically,aeolian sandstones and braided channel sandy conglomerates are interbedded,reflecting the alternating dominance of these two geological agents during different periods. Based on this,a depositional model controlled by cycles of dominant and subordinate agents on the lateral margins of the middle fan under an arid to semi-arid climatic background was established,characterized by “aeolian processes advance when fluvial activity retreats” and “fluvial processes advance when aeolian activity retreats”. During arid periods with prevailing monsoons,wind was the dominant geological agent,depositing thick large-scale cross-bedded and parallel-bedded sandstones. During wet periods with abundant rainfall and glacial meltwater,fluvial processes dominated the fan surface,resulting in braided channel features. This study enriches the depositional architectural models of alluvial fans and provides theoretical support for the genetic identification and hydrocarbon exploration of sandy conglomerate reservoirs within alluvial fans.

The Longmen Mountains region is one of the key areas for Devonian geological research in China. However,controversies persist concerning stratigraphic correlation and paleoenvironmental studies in this area. This study focuses on the Middle Devonian Guanwushan Formation and the Lower Carboniferous Zongchanggou Formation in the northwestern Sichuan Basin. Systematic field surveys of outcrop sections were conducted,complemented by thin-section petrographic analyses. A total of 210 carbonate rock samples were analyzed for carbon and oxygen isotope compositions. The results indicate that the middle part of the Guanwushan Formation in the northwestern Sichuan Basin was deposited under relatively higher paleosalinity and shallow-water conditions,representing a favorable stage for dolomite formation. The Guanwushan Formation exhibits conformable contacts with the overlying Shawozi Formation and the underlying Jinbaoshi Formation. The Devonian-Carboniferous boundary is marked by a regional unconformity,resulting in the absence of the upper Guanwushan Formation and the lower Zongchanggou Formation. Within the Guanwushan Formation,δ¹³C values range from -3.72‰ to 3.19‰(average: 0.61‰),while δ¹⁸O values vary between -7.30‰ to-1.11‰(average: -4.49‰). In the Zongchanggou Formation,δ¹³C values range from-4.47‰ to 2.08‰(average: -0.92‰),and δ¹⁸O values range from-5.83‰ to-0.64‰(average: -3.52‰). Paleoenvironmental reconstruction indicates that both the Guanwushan and Zongchanggou formations were deposited in typical marine settings. During deposition of the Guanwushan Formation,seawater temperatures ranged from 15 ℃ to 40 ℃,with an average of 25.32 ℃,suggesting a subtropical to tropical climate. In contrast,during deposition of the Zongchanggou Formation,seawater temperatures varied between 15 ℃ and 34 ℃,averaging 23.71 ℃,reflecting a temperate-tropical climate transition and revealing a cooling event across the Devonian-Carboniferous boundary. Furthermore,the multi-stage positive and negative excursions in δ¹³C were controlled by high-frequency sea-level fluctuations and periodic exposure of continental shelves,reflecting perturbations in the paleo-oceanic carbon cycle. The overall trends in carbon isotope variations across three measured outcrops in the study area show consistency and are broadly comparable to those documented in sections from Guangxi(China),France,and Germany,demonstrating their utility for global Devonian-Carboniferous stratigraphic correlation and subdivision. This study reveals the carbon isotope geochemical characteristics of the Devonian-Carboniferous strata in the northwestern Sichuan Basin,providing key evidence for regional isochronous stratigraphic correlation. It reconstructs the evolution of paleo-seawater temperatures and the mechanisms of carbon cycle fluctuations,identifies the favorable period for dolomite development,and holds significant implications for understanding paleoenvironmental evolution along the eastern Paleo-Tethyan margin and for hydrocarbon reservoir prediction.

This study investigates the shale of the upper sub-member of Member 4 of Shahejie Formation in the Minfeng subsag,Dongying sag,integrating core and thin-section observations with elemental geochemistry to reconstruct paleoenvironmental conditions. An innovative multi-method analytical framework was employed—including scanning electron microscopy coupled with cathodoluminescence(SEM-CL),focused ion beam-scanning electron microscopy(FIB-SEM),laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS),stable isotope(C,O,Sr)analysis,fluid inclusion microthermometry,and U-Pb geochronology—to systematically characterize diagenetic mineral evolution and elucidate the reservoir formation mechanism. Results indicate that deposition of the target interval occurred under an arid to semi-humid climate,within a fine-grained,brackish-deep lacustrine setting characterized by strong redox stratification. High-frequency vertical cyclicity is evident,reflecting a progressive facies transition from carbonate-rich mixed shale → mixed shale → feldspar-rich mixed shale. Total organic carbon(TOC)ranges predominantly between 1.8wt% and 6.0 wt%,with abundant organic matter accumulation attributed primarily to high sedimentation rates and robust terrigenous and/or endogenous organic input—rather than prolonged bottom-water anoxia alone. Granular calcite and fibrous calcite exhibit distinct geochemical signatures: divergent rare earth element(REE)patterns,contrasting δ¹³C and δ¹⁸O values,and significantly different U-Pb ages. Notably,columnar-to-fibrous calcite laminae oriented near-horizontally yield a broad U-Pb age spectrum(31.2±4.3 Ma to 25.1±4.2 Ma),spanning the full early-to-middle diagenetic interval. Illite comprises 80%~100% of the clay mineral assemblage;its formation is dominantly controlled by montmorillonite-to-illite transformation,while feldspar dissolution represents a major silica source for authigenic quartz precipitation. Authigenic quartz is ubiquitously observed within argillaceous laminae and calcite laminae. Under overpressured conditions,co-precipitation of authigenic quartz and recrystallized calcite effectively buffered porosity loss during cementation—thereby preserving critical pore space and facilitating subsequent pore enhancement via acidic fluid dissolution during later diagenesis. This interplay constitutes a key reservoir-quality preservation and improvement mechanism in deeply buried lacustrine shales.

The Qiongzhusi Formation in the Upper Yangtze Region is currently a key target for shale gas exploration. To reconstruct the sedimentary paleoenvironment of the Lower Cambrian Qiongzhusi Formation in southern Sichuan Basin,this study takes the Qiongzhusi Formation shales from Well A1 as the research object,and explores the weathering intensity,paleoclimatic characteristics,tectonic setting,and provenance features during their deposition through geochemical analyses of major elements,trace elements,and rare earth elements. The results reveal enrichment of TFe₂O₃,MgO,and K₂O,and depletion of SiO₂,Al₂O₃,and Na₂O in major elements. Among trace elements,Mo and Cd are strongly enriched,whereas Sr is significantly depleted. The REE distribution patterns are comparable to those of the upper continental crust,showing a right-inclined “V”shape with LREE enrichment,flat HREE trends,distinct LREE-HREE fractionation,and notable negative Eu anomalies. CIA values range from 48.16 to 69.36(average 64.03),and A-CN-K diagrams indicate moderate chemical weathering under a warm and humid climate. The Th/Sc-Zr/Sc plot suggests a first-cycle sedimentary origin for the shale,with no evidence for significant sedimentary recycling. Provenance diagrams(K₂O-Rb,TiO₂-Zr,Co/Th-La/Sc,and Th/Sc-Zr/Sc)consistently imply intermediate to acidic felsic igneous sources. Major element discrimination and REE abundances(ΣREE=136.51~297.68×10^-6,avg. 189.23×10^-6,higher than average continental crust)support a passive continental margin setting. Based on paleoclimatic conditions,provenance characteristics,and tectonic setting,it is inferred that the Lower Cambrian Qiongzhusi Formation shales in southern Sichuan were deposited in a passive continental margin under a warm,humid climate with moderate chemical weathering,predominantly sourced from intermediate-acidic felsic igneous rocks of the Kangdian-Yunnan craton with negligible hydrothermal influence.