Following the discovery of the Anyue extra-large gas field in central Sichuan,significant exploration breakthroughs have been achieved in Member 2 of Dengying Formation in the Penglai area of northern Sichuan Basin in recent years. However,the exploration and development of this region remain constrained due to the complex microfacies types,considerable burial depth and strong heterogeneity of microbial mounds and shoals within the Dengying Formation. Based on core data from Well Wang-1C—a fully cored scientific exploration well located in northern Sichuan Basin—this study systematically investigates the microfacies types,sedimentary evolution,and reservoir characteristics of microbial mounds-bank complex in the Member 2 of Dengying Formation. The main findings are as follows: (1)Four primary dolomite lithofacies are identified in the Member 2 of Dengying Formation at Well Wang-1C: microbial dolomite,granular dolomite,crystalline dolomite,and dolomite breccia. Among these,laminated stromatolite dolomite,thrombolite dolomite,botryoidal dolomite,and mud-crystalline dolomite are the most developed. (2)Member 2 is predominantly composed of restricted platform deposits,with key microfacies including basal mound,core mound,mound crest,grain bank,and subtidal flat. (3)Three distinct microfacies assemblages can be recognized within the microbial mound-bank system: (Ⅰ)subtidal flat-basal mound-core mound,(Ⅱ)subtidal flat-grain dolomite bank,and(Ⅲ)subtidal flat-basal mound-core mound-grain dolomite bank. Each assemblage exhibits a substantial thickness,typically ranging from 25 to 50 m. (4)Reservoir spaces in the microbial mound-bank complex include primary pores and secondary dissolution pores. Reservoir development is primarily controlled by microfacies distribution,high-frequency sea-level fluctuations,and diagenetic processes. Notably,the microbial mound-core and mound-crest facies exhibit the most favorable reservoir properties,followed by granular shoal facies,whereas the subtidal flat facies displays the poorest physical characteristics. The above results can provide geological theoretical basis for the prediction of the distribution of ultra-deep microbial mound and flat reservoirs,and the exploration and development of natural gas in the northern Sichuan Basin.

During the Ordovician-Silurian transition,the southern Sichuan Basin experienced the superposition of global and regional geological events,resulting in the complexity of its underwater topography. This complexity led to the variation of the environment during the deposition period of the Longmaxi Formation shale in this area,which in turn controlled the development of shale lithofacies. This study takes the Longmaxi Formation shale in Changning area of southern Sichuan Basin as the research object. Based on drilling wells,seismic data and various analytical test data,the underwater topography and lithofacies development characteristics during the depositional period of the Longmaxi Formation in this area were studied. The results show that: (1)The underwater topography during the depositional period of the Longmaxi Formation in Changning area has a pattern of alternating high land and depression,which can be further divided into underwater high land,slope area and depression;(2)The lithofacies types of the Longmaxi Formation shale in Changning area are diverse and the reservoir differences are obvious. The shale lithofacies have obvious “storage and gas content control” characteristics. Among them,the mixed siliceous shale lithofacies(S-2)and the calcareous siliceous shale lithofacies(S-3)are the dominant lithofacies of the Longmaxi Formation shale in Changning area,which have the “three highs”(high TOC,high porosity and high total gas content)characteristics;(3)The underwater topography pattern has an obvious differential control effect on the development of shale lithofacies. The underwater high land is mainly occupied by mixed shale lithofacies association(M)and siliceous shale lithofacies association(S),the slope area is dominated by siliceous shale lithofacies association(S),and the depression is mainly composed of mixed shale lithofacies association(M)and siliceous shale lithofacies association(S). The mixed siliceous shale lithofacies(S-2),the best lithofacies with high TOC,high gas content and high porosity,is mainly distributed in the slope area and the area close to the underwater high land. This study is of great significance for revealing the distribution law of shale lithofacies and predicting the planar distribution of favorable shale lithofacies.

Recently,mixed sedimentation involving clastic,carbonate,and evaporite components has been identified at the base of the Majiagou Formation in the Ordos Basin,offering significant insights into the tectonic-palaeogeographic framework during its early depositional phase. This study systematically examines macroscopic and microscopic petrological features,along with representative depositional sequences,to reconstruct the depositional environment. Integrated geochemical analyses were conducted to investigate the genetic mechanisms,geological significance,and elemental behavior associated with this mixed sedimentary system. Key findings include: (1)The mixed sedimentary assemblage comprises three primary lithological components—clastic rocks,carbonate rocks,and anhydrite.(2)Sedimentary structures indicative of tidal flat environments,including scour surfaces,normal grading,transitional bedding,and brecciated anhydrite,were observed and used to define four subfacies: tidal-dominated channels(F1),muddy/arenaceous dolomitic flat(F2),gypsum-bearing dolomitic flat(F3),and supratidal sabkha(F4).(3)Geochemical data reveal distinct elemental partitioning during sedimentation,with vertical variations reflecting fluctuating terrestrial input driven by sea-level changes. Laterally,rare earth elements exhibit greater mobility compared to terrigenous elements,suggesting longer transport distances. The results indicate that the mixed sedimentation originated from reworking of sandstones derived from the exposed Changcheng System within the central Wushen Banner-Jingbian palaeouplift,triggered by marine transgression during the early stage of Majiagou deposition. This unit thus serves as critical evidence for the existence and persistence of the Wushen Banner-Jingbian palaeouplift during the O₁m₁ to O₁m₂ intervals,which played a critical role in facilitating regional uplift and ensuring sustained supply of terrigenous clastics throughout this period.

The Taiyuan Formation of Ordos Basin is a mixed depositional system of coal-bearing paralic facies developed under an epicontinental sea setting. Current inconsistencies in the understanding of its mixed depositional system,as well as unclear mixed characteristics and controlling factors,have constrained the exploration and evaluation of high-quality reservoirs such as tight limestones and tight sandstones. Based on systematic observations of cores,thin sections,and outcrops of the Taiyuan Formation in the eastern basin,combined with well logging and mud logging data,this study conducted an in-depth analysis of the cyclicity,lithofacies,and lithological characteristics of the Taiyuan Formation. Through a systematic examination of multiscale mixed depositional features and patterns,the main controlling factors and formation processes were clarified,and a mixed depositional model was established. The Taiyuan Formation in the study area constitutes a relatively complete third-order sequence,which can be subdivided into five fourth-order sequences,recording a relatively full transgressive-regressive cycle. The fourth-order sequences exhibit a binary structure of transgressive and highstand systems tracts. The transgressive systems tract is dominated by carbonate platform deposits,with local development of deltaic and barrier sandbar sand bodies,while the highstand systems tract is primarily composed of tidal flat and lagoon deposits. The Taiyuan Formation exhibits mixed deposition at three scales: Cyclic mixing at the macro-scale,primarily controlled by high-frequency sea-level fluctuations and their amplitudes,reflects the influence of temporal periodic changes on mixed stacking patterns;Contemporaneous heterotopic facies mixing at the meso-scale,governed by terrigenous clastic supply,carbonate productivity,and depositional topography,highlights the constraints of spatial environmental variations on the distribution of endogenic and exogenic sediments;and Component mixing at the micro-scale,mainly regulated by terrigenous clastic input,hydrodynamic conditions,and physicochemical factors,reflects the control on the distribution of mixed sedimentary components. The mixed deposition in the Taiyuan Formation is characterized by multiscale development,multi-factor synergy,and dynamic evolution of depositional environments.

Lacustrine deep-water gravity flow sedimentary sand bodies are the crucial targets for oil and gas development. Investigating the depositional characteristics of gravity flow sand bodies at the development scale can complement gravity flow depositional models and provide a basis for efficient exploitation of gravity flow reservoir. Based on cores,well logging and other data,this study examines the depositional characteristics,microfacies classification,and distribution of the Chang 7₁ gravity flow in Heshuinan area of the Ordos Basin at the development scale. We compare the reservoir characteristics between the different microfacies and summarizes the gravity flow depositional model. The results show the presence of six lithofacies types and six lithofacies combinations of the Chang 7₁,including Massive-bedded fine sandstone(Sm),Massive-bedded fine sandstone with mud pebbles and mudstone rip-up clasts(Smm),Normally graded-bedding fine sandstone(Sn),Parallel-bedded muddy siltstone(MSp),Lenticular-bedded silty mudstone(MSl),Massive-like mudstone(Mm). This study subdivides the unconfined channel area into four sedimentary microfacies: main channel,channel margin,overflow bank,mud faice of semi-deep to deep lake mud. The main channel is predominantly formed by sandy debris flow deposits,exhibiting good reservoir properties,mainly consisting of Sm-Mm(MSl)-Sm and Sm-Smm lithofacies combinations. The channel margin is mostly formed by the hybrid flow deposits,exhibiting moderate reservoir properties,primarily developing MSp-Sm and Sn-Sm(Smm)lithofacies combinations. The overflow bank is mainly formed by the turbidity current deposits,with the poorest reservoir properties,dominated by Sn-MSp and Sn-Mm(MSl)lithofacies combinations. The Chang 7₁ gravity flow in the Heshuinan area underwent three evolutionary stages: sandy debris flow,hybrid flow,turbidity current. The gentle topography and complex fluid evolution result in significant lateral facies variations perpendicular to the sediment source direction. During the same period,three main sedimentary facies developed: main channel,channel margin,and overflow bank. Gravity flow depositional characteristics,and sedimentary patterns are investigated in small-scale in this study,providing the theoretical guidance for effective exploitation of gravity flow hydrocarbon reservoir.

The Middle Jurassic black rock series in the Qiangtang Basin serve as significant hydrocarbon source rocks due to their high organic matter contents. The Biluo Co section was chosen as the research target. Based on paleontological stratigraphy and by analyzing the total organic carbon(TOC)contents as well as variations in carbon and oxygen isotopes,this study discussed the spatio-temporal distribution patterns of carbon and oxygen isotopes from the black rock series of the Bathonian-Callovian stages(Middle Jurassic),and the characteristics of the paleoenvironment and paleoclimate during this period. Results show that the TOC contents range from 0 to 1.5%,δ¹³C_org,δ¹³C_carb and δ¹⁸O values vary between-24‰ and-26‰,-4.819‰ and 2.323‰(mean values of-0.659‰),-13.96‰ and 0.66‰(mean values of-7.37‰),respectively. According to the global carbon isotope correlations,the negative inorganic carbon excursions in the early Bathonian is associated with the enhanced terrigenous input triggered by the Early-Middle Jurassic regression event. The positive shifts of δ¹³C_carb from the Middle-Late Bathonian to the Early Callovian correspond to the event of a substantial surge in marine primary productivity. During the Callovian-Early Oxfordian,a widespread positive carbon isotopes anomalies and high TOC contents of shales occurred in regions such as the Qiangtang Basin,Saudi Arabia,Paris Basin,and Polish Carpathians,which shows that the organic-rich black rock series are closely related to large-scale marine transgressions and primarily driven by the increase in the burial flux of organic matters under the backdrop of global sea-level rise. Moreover,paleontological and petrological characteristics,and redox-sensitive proxies(e.g.,V/(V+Ni),V/Cr,Ni/Co ratios) suggest that the black rock series deposited in a relatively reducing sedimentary environments. This study highlights that the carbon and oxygen isotopes of Middle Jurassic black rock series have the global distribution patterns influenced by multiple factors,including redox conditions,sea-level fluctuations, burial dynamics of organic matters, and so on.

A large-scale marine regression occurred during the depositional period of the Middle Jurassic Xiali Formation in the North Qiangtang Depression,accompanied by the widespread deposition of gypsum-bearing clastic sediments. Based on integrated petrological characteristics and elemental geochemical analyses of clastic rocks from the Xiali Formation in the Shenglihexi section and Well QK-1,we systematically analyzed the intensity of chemical weathering,provenance characteristics,and tectonic setting of these sediments,which is critical for understanding the evolutionary history and evaporite response of the Jurassic basin system. The results indicate that the Chemical Index of Alteration(CIA)values of these clastic rocks are significantly influenced by grain size and sediment sorting,but minimally affected by sedimentary recycling;thus,the CIA values of mudstone samples reflect the intensity of chemical weathering in the source region. Mudstones from the Xiali Formation in Well QK-1 exhibit considerable variation in CIA values(60-77),with negligible influence from potassium metasomatism,indicating that the source area underwent generally moderate to weak chemical weathering. In contrast,mudstones from the Shenglihexi section show significant effects of potassium metasomatism;after correction for this influence(CIAcorr),their values range from 80 to 85,suggesting intense chemical weathering in the corresponding source region. Discriminant diagrams based on La/Th vs. Hf,Co/Th vs. La/Sc,La/Yb vs. 􀰐REE,and A-CN-K plots,along with pronounced negative Eu anomalies,collectively indicate a predominantly felsic rock provenance for these clastic deposits. Clastic sediments from the western and central segments of the North Qiangtang Depression were deposited in a continental collision setting,whereas those from the eastern segment were formed in an arc-related tectonic environment. This interpretation aligns well with the established tectonic framework of the North Qiangtang Depression during the Middle Jurassic. During this period,the central and western regions functioned as a weakly magmatic intracontinental foreland basin,while the eastern region likely developed as a back-arc foreland basin,induced by southward subduction of the Bangonghu-Nujiang Tethys Ocean. Multiple transgressive-regressive cycles,combined with a relatively arid palaeoclimate and restricted paleogeographic conditions in the North Qiangtang Depression during the Early to Middle Jurassic,may have created favorable conditions for the formation of extensive gypsum-rich evaporites within the Xiali Formation.

During the Early Cambrian,organic-rich shales of the Liuchapo Formation and Niutitang Formation developed in northwestern Hunan,exhibiting good shale gas exploration potential. To address the issues of insufficient precision in Early Cambrian stratigraphic correlation and unclear mechanisms of carbon isotope evolution,this study systematically collected 133 carbonate rock samples from the drilling core of the Well Xiangzhangdi-1 (with a well depth of 2018.25 m and a core recovery rate of 98%). Analyses were conducted on the evolution trends of carbon and oxygen isotopes( {{\delta }^1}^3C, {{\delta }^1}^8O),as well as studies on shift events and stratigraphic correlation. Five negative {{\delta }^1}^3C anomalies(with amplitudes up to 12.1‰)and three positive anomaly events(with amplitudes approaching 9‰)were identified in the Early Cambrian slope facies area of northwestern Hunan,revealing their synchrony with global carbon cycle events such as BACE and ZHUCE. The results show that the Ediacaran-Cambrian boundary(BACE event)corresponds to the peak of negative {{\delta }^1}^3C shift(-9.8‰),consistent with records from Siberia,Morocco,and the South China platform facies. Positive shift events(ZHUCE,CARE)are coupled with intervals of high organic matter abundance(with TOC peak values reaching 10.5%),indicating that enhanced carbon burial driven by biological prosperity is the main controlling factor for positive {{\delta }^1}^3C shifts. This study establishes a carbon isotope stratigraphic framework for Early Cambrian boreholes in the South China slope facies,providing a chemostratigraphic basis for regional shale gas sweet spot prediction.

A massive Tertiary salt deposit containing 10 billion tons has been identified in the Huanggang buried sag of southwestern Shandong Province. This deposit is a rare super-large rock salt deposit in eastern China. To investigate the material sources and depositional environment of this salt formation,we conducted a comprehensive analysis of salt mineral assemblages,stratigraphic distributions,and 87Sr/86Sr isotopic compositions in halite and anhydrite from a 520-meter core(Borehole YZK-3)situated at the basin margin. Among the salt minerals,carbonate minerals mainly contained calcite,dolomite,ankerite and minor aragonite. Sulfate minerals contained anhydrite,with minor gypsum and celestite. Chloride minerals were composed of only halite. Based on their distribution and genesis,the paleolake evolution during the Middle-Late Eocene to Oligocene progressed through five distinct stages: freshwater→brackish→saline→brackish→freshwater lake systems. The⁸⁷Sr/⁸⁶Sr ratios measured in halite and anhydrite(0.711227-0.71177)significantly exceed contemporaneous seawater values. This isotopic disparity,combined with existing hydrothermal research data,demonstrates the absence of marine influences in the brine system.Substantial contributions from deep hydrothermal fluids mixed with meteoric waters was an important source of salt material.

The braided river delta sandbodies containing coal developed in the Lower Jurassic in the piedmont southwestern Tarim Basin,the overall exploration level is relatively low,with significant differences in the properties of source rocks and reservoir characteristics in different regions,which in turn restricts the oil and gas exploration process. This article takes the sandy reservoirs of the Lower Jurassic in the Fusha and Aketao regions as examples,and fully utilizes various rock and mineral testing and analysis methods to analyze the differences in the properties of the parent rock and reservoirs. The results show that the source rocks in the Fusha area are mainly Early Proterozoic metamorphic rocks. The reservoir exhibits characteristics of “poor feldspar and rich metamorphic rock debris”(Q_57.3F_4.8R_37.9),and the high content of plastic debris leads to strong burial compaction in the later stage,with a compaction reduction of up to 20.11%. Late cementation and dissolution were weak. The reservoir space is a small amount of residual primary pores,and the current porosity is only 8.98%. Aketao area is the Late Paleozoic-Triassic extrusive rock,and the reservoir exhibits the characteristics of “rich feldspar and extrusive rock debris”(Q_33.2F_17.5R_49.3). The Compaction is weak,and the early cementation is strong,The high content of soluble components and the influence of organic acids lead to strong dissolution,with a dissolution pore increase of 4.03%. The development of intergranular and intragranular dissolution pores improves the reservoir quality,and the current porosity is 12.95%. Comprehensive research suggests that the properties of the parent rock lead to differences in the petrological characteristics,diagenetic intensity,diagenetic evolution sequence,reservoir space types,and pore evolution processes of the Lower Jurassic sandstone reservoirs in the piedmont southwestern Tarim Basin. This study provides theoretical support for the prediction of high-quality sandstone reservoirs in the Jurassic period in the piedmont southwestern Tarim Basin.