Objective Organisms are extremely responsive to the environments in which they live， and the traces they create are the result of interactions between organisms and their environment. To a certain extent， the traces reflect the properties of the depositional environment. Biological sedimentary structures are a powerful tool in understanding the ecology and depositional processes of modern coastal sedimentary environments. Methods The composition and distribution of modern biological traces in the tidal flat environments of the Pearl River Delta were studied by field observations and 3D reconstruction. Results The key understanding obtained by the study was that the distribution of organisms in the supratidal， intertidal and subtidal varies greatly， and the biological traces are mainly distributed in the intertidal zone. In the supratidal zone， mainly Ligia oceanica and Uca arcuata are present. The main traces are Uca arcuata level feeding traces， excretion traces， trackways， and intra-layer I-shaped dwelling burrows. Biological traces in the intertidal mud flat environment mainly include feeding， excretion， trackways and I-shaped dwelling burrows of Uca arcuata， crawling traces and Y-， U- and I-shaped dwelling burrows of Periophthalmus cantonensis， and trackways and I- and Y-shaped dwelling burrows of Orisarma dehaani. Biological traces in the mixed intertidal flat environment mainly include excretion traces and intra-layer I-， U-， and Y-shaped dwelling burrows of Perinereis aibuhitensis， Y-， U- and I-shaped dwelling burrows of Ligia oceanica， and crawling traces of Vittina coromandeliana and Neritina pulligera. Biological traces in the sandy intertidal environment include feeding and excretion traces of Ocypode cordimand and intra-layer I-shaped dwelling traces. The subtidal environment was not accessible for observation and no biological traces were found. Conclusions The study of modern intertidal biological traces in the Pearl River Delta supplements modern sedimentological data on tidal flat environments in the Pearl River Delta， and provides modern empirical evidence for palaeoichnology and palaeoenvironmental reconstruction.

Objective The widespread siderites in the marine strata of the Toarcian Oceanic Anoxic Event （T-OAE） are believed to have resulted from a low sulfate concentration and increased terrestrial iron supply. Similar large amounts of siderite have also been reported in paleolakes in the same period. However， the mechanism of lacustrine siderite development and its connection with the T-OAE remain unexplored， but is crucial to a better comprehension of lacustrine biogeochemical cycling in the early Toarcian. Methods Mineralogical and carbon isotope geochemical analyses were performed on siderite samples from the lacustrine Anya section of the Ordos paleolake. The sources of iron and carbon， as well as the sedimentary environment， were investigated by integrating major and trace element data， which allowed exploration of the formation mechanism of siderites during the T-OAE period of climatic change. Results The coexistence of pyrites and siderites was observed by scanning electron microscopy （SEM）. The value of inorganic carbon isotopes （δ¹³C_carb） varies from -11.25‰ to +16.32‰， and the organic carbon isotope （δ¹³C_org） value ranges between -32.03‰ and +14.33‰. Light rare earth elements （REEs） are enriched and heavy REEs are depleted， with Eu anomalies ranging from 0.69 to 0.85. Conclusions The iron in the Anya siderites is derived from terrestrial inputs； the carbon included HCO₃^– produced by methanogenesis， as well as carbon released by organic matter decomposition and aerobic methane oxidation， either bacterially in the suboxic zone or archaeally below the oxic-anoxic interface immediately after methane production. The pore fluids during siderite formation were a suboxic to anoxic environment. During the T-OAE， intensified continental weathering and increased surface runoff transported abundant Fe²⁺ ions into basin lakes whose widespread anoxia， abundant organic matter and low sulfate concentrations created favorable conditions for extensive siderite deposition.

Objective Tidal forces influence the form and scale of marine shelf deltas and the development and evolution of branch channels. The quantitative control mechanism of different amplitude tidal ranges on the dam body and branch channels of continental shelf deltas has garnered significant scientific interest. Methods In this study， a single factor control method was used to analyze the evolution of the key geologic bodies in the continental shelf delta under different tidal ranges using Delft3D hydrodynamic simulation software to establish four control groups： no， small， medium， and spring difference. Results The experimental results show that the down-cut depth and width of the branch channel decrease with the increase of the flow distance when there is no tidal effect. When tidal action is added， the number of distributary channels decreases with the increase of tidal range， but the down-cut degree and width of the distributary channels increase. The width-to-depth ratio of the branch channel ranges from 2-10 when there is no tidal action， and decreases to zero when there is a low tidal range. Under the influence of middle and spring tidal ranges， the width-to-depth ratio of the branch channel corresponds to 0-3 and 2-4， respectively. Conclusions The results show that tidal action is beneficial for increasing the broad-to-thickness ratio of sand bodies， and the delta morphology changes become more stable with the increase of tidal range. The number of branch channels in the delta under the influence of tides decreases significantly owing to the periodic influx and retreat of tides in the lateral direction. This promotes the formation of lateral channels and tidal channels. Under the influence of middle-spring difference， the lateral migration distance of the branch channel is shorter， the branch channel is more stable， and the dam body is more isolated， providing an important reference for reservoir configuration analysis of similar underground sediments.

Objective Abundant ichnofossils present in the Eocene Pinghu Formation and Baoshi Formation in the Pingbei area， Xihu Sag. It is of practical significance for the division of sedimentary systems in the Xihu Sag to study the characteristics and distribution of its ichnofabrics and their indication of sedimentary environment. Methods Using core observations， the study identified ichnofossils and ichnofabrics in the Pingbei area. The IDIPA technique was employed to objectively quantify ichnofossil abundance and accurately determine both the bioturbation index and the ichnofabric index. By integrating these findings with ichnological indices， this study summarizes the characteristics of the planar distribution and vertical evolution of ichnofabric in the study area. Results （1） The study identified six ichnogenera of ichnofossils and divided them into four types of ichnofabrics： Planolites ichnofabric， Thalassinoides ichnofabric， Teichichnus ichnofabric and Beaconites ichnofabric. （2） The study area features a high landform in the northwest and a low landform in the southeast. Seawater invades from south to north， and the sedimentary environment transitions from a subtidal zone to an intertidal zone， and finally to a supratidal zone in the northwest where the highest terrain is located. The evolution pattern of the corresponding ichnofabrics follows the sequence Thalassinoides → Teichichnus → Planolites → Beaconites， consistent with regional sedimentary zoning and paleoclimate analysis. （3） From the Pinghu Formation to the Baoshi Formation in the study area， the study found an increase in the size， diversity and abundance of ichnofossils. The tiering transitioned from simple and shallow to complex and deep. The variety of organisms and the degree of bioturbation increased， and the biological habits evolved continuously， indicating that the sedimentary environment changed from anoxic to oxic. Conclusions Based on the characteristics of ichnofossils， ichnofabrics and sedimentary structures， the response relationship between the ichnofabrics and the sedimentary environment was established in the Pinghu Formation⁃Baoshi Formation in the Pingbei area. These findings provide new evidence for the changes in sedimentary environment from the perspective of ichnology in the Xihu Sag during the Eocene.

Objective To clarify whether the western edge of the Kaijiang-Liangping trough is connected to the eastern platform margin of the Pengxi-Wushengtai Depression in the north of the Late Permian Changxing-Early Triassic Feixianguan period in northwestern Sichuan Basin， and to determine the sedimentary facies distribution pattern of the Jiangyou-Jiange Formation in northwestern Sichuan Basin and the Changxing Formation-Feixianguan Formation in the periphery area， and to analyse the evolution process of sedimentary filling to indicate further oil and gas exploration work. Methods Based on the latest drilling， field outcrop， and seismic data， this study systematically analyzed the distribution of lithofacies and paleogeography during the deposition of the Changxing Formation-Feixianguan Formation in the Guangyuan-Jiangyou area， based on the stratigraphic and sedimentary facies division and comparison. Results The intersection area of the Changxing Formation-Feixianguan Formation can identify six sedimentary facies： shelf basin， slope， platform edge， open platform， restricted platform， and evaporative platform. Based on the sedimentary records of the front slope of the platform， the western edge of the Kaijiang-Liangping trough and eastern edge of the Pengxi-Wusheng intra-platform depression were clearly merged to form a unified platform edge belt in the Cilinbao-Shangsi area in northwestern Sichuan Basin. The platform edge belt and front slope show a northwest finger shaped interdistribution， rather than a simple smooth belt distribution. The intersection area of the two troughs in the Changxing Formation presents a sedimentary pattern of two troughs interlocking from southwest to northeast. The connecting area of the platform margin zone developed a northwestern fingered reef beach， developing bioclastic shoals in the 1^st member and biological reefs and shoals in the 2^nd member of the Changxing Formation at the platform edge zone. During the sedimentary period of the 1^st and 2^nd members of the Feixianguan Formation， the sedimentary pattern of Changxing depositional period was inherited； however， in the 1^st member of Feixianguan Formation， only local platform edge oolitic beaches were formed owing to sea flooding. In the stage of the late 1^st to 2^nd member Feixianguan Formation， platform edge oolitic beaches gradually developed and expanded in scope with an overall eastward migration. The slope in front of the platform was prone to sliding and sedimentation. During the sedimentation period of the 3^rd member of the Feixianguan Formation， the sea trough gradually filled， and the finger shaped platform edge zone disappeared， forming a unified restricted platform. Only the local development of oolitic beach. The research area was filled and flattened during the depositional period of the 4^th member of the Feixianguan Formation， forming an evaporative platform facies. The platform margin zone in this region did not migrate northwestward along the finger shaped platform margin zone， but rather exhibited eastward migration characteristics. Conclusions The determination of the Changxing-Feixianguan connectivity and the definition of five sedimentary filling evolution stages in the Kaijiang-Liangping trough and Pengxi-Wusheng intra-platform depression supplements a new understanding of the sedimentary pattern and filling evolution process in the intersection area of the Changxing-Feixianguan trough. It also provides ideas and references for oil and gas exploration in this area.

Objective Current domestic and international research on bauxite oil and gas reservoirs is limited in scope. The bauxite reservoir with high-yield industrial gas flow discovered in the Taiyuan Formation， Longdong area， southwestern Ordos Basin， represents a significant advance in the exploration for bauxite gas reservoirs， and the recent identification and production tests of bauxite rock series in the Benxi Formation in the Linxing area at the northeastern margin of the basin suggests promising exploration potential.Further natural gas exploration and development in the bauxite series in the Linxing area requires a comprehensive analysis of the fundamental geology of the sedimentary environment， including the mineral composition， reservoir pore properties and the structure of the bauxite series. In addition， it is essential to identify the factors that control the development of bauxite and which areas are most conducive to such activity. Methods A comprehensive analysis of the sedimentary environment， reservoir characteristics and sedimentary model of the Benxi Formation bauxite rock series in the Linxing area was conducted using logging， seismic data， X-ray diffraction， casting thin sections， scanning electron microscopy， major and trace elements， high-pressure mercury injection， N₂ and CO₂ adsorption and conventional physical properties， and other experimental methods. Results The bauxite rock series in the study area is characterized by a high concentration of diaspore and clay minerals， and has a distinctive three-stage vertical structure. The lower portion is rich in iron， the middle zone contains a higher aluminum content， and the upper section has a lower iron concentration. The pore types include predominantly dissolution pores typically ranging between 2 nm and 50 nm， along with intercrystalline pores， organic pores and microfractures. The sorting and connectivity of the pores are relatively good， with average porosity 3.64% and average permeability 7.24×10^-3 μm². Conclusions The thickness of a bauxite rock series depends upon the sedimentary environment and karst paleogeomorphology. A shallow-water sedimentary environment with high temperature and high humidity， low oxygen levels and alternating marine and terrestrial conditions is optimal for its formation and preservation. A slope， or buried pit， is a crucial area for exploration and development. This understanding provides a geological foundation for the exploration and development of natural gas in bauxite with comparable geological background in this region.

Objective The beach facies dolomite reservoir in the Middle Permian Qixia Formation， central Sichuan Basin， has significant potential for oil and gas exploration， but relatively little systematic study has been reported on its diagenetic sequence and pore evolution. Methods The petrology and pore genesis of the Qixia Formation were investigated by core observation， thin section analysis， cathodoluminescence examination and image recognition techniques， to delineate diagenetic variations and pore evolution in the different lithologies. Results In the Gaoshiti area， four types of rock are identified in the Qixia Formation： micrite limestone， micrite grain limestone， sparite grain limestone， and residual grain dolostone. Four types of reservoir space are observed： intergranular dissolved pores， intragranular dissolved pores， fractures and vugs. Each rock type has undergone distinctive diagenesis： the type and intensity of dissolution vary in the different lithologies. The two kinds of granular limestone are mainly dissolved by meteoric water， and the residual granular dolomite is more strongly dissolved by meteoric water. On this basis， dolomitization and burial period dissolution occur. Recrystallization is more pronounced in the residual grain dolomite， whereas compaction and pressure dissolution is more evident in micrite grain limestone. The key diagenesis and stages influencing pore development vary between the different rock types： the pores in the two types of granular limestone are mainly formed by quasi-syngenetic dissolution， then partially destroyed by compaction and cementation during the burial stage. The dissolution and dolomitization in the parasyngenetic stage result in the formation and preservation of the pores in the residual granular dolomites； dissolution and tectonic fracture at the burial stage are beneficial supplements， but cementation during the shallow burial stage destroys the pores. Conclusions The varied diagenesis has caused different pore evolution， forming four lithologies with a decreasing extent of pore development： residual grain dolostone （plane porosity： 2.10%）， sparite grain limestone （plane porosity： 1.24%）， micrite grain limestone （plane porosity： 0.41%） and micrite limestone （compact）.