The characteristics and palaeogeography of the fine-grained sediments are significant for the exploration and development,assessment and prediction,and the engineering technical design of the oil-gas-bearing fine-grained sediments. The rapid development of fine-grained sediments,especially the oil-gas-bearing fine-grained sediments in recent years has brought a series of new research hotspots. In this paper,the definition,classification and characteristics of fine-grained sediments are discussed. The structure of fine-grained sediments and shale,research hotspots and development direction are clarified based on the detailed literature review. The international research in the fine-grained sediments is mainly focused on the micro-structure and the formation mechanism of the fine-grained sediments,especially the shale that has a special significance of oil and gas. Except the shale characterization,the occurrence and migration of shale gas are also widely studied. There are many methods of palaeogeographic restoration of fine-grained sediments,but the applicability is different. The quantification,synthesis of multiple factors,data integration and data deep mining,artificial intelligence will be the research direction of palaeogeographic restoration of fine-grained sediments. The optimum evaluation on the oil-gas bearing fine-grained sediments requires the delicate study of the sediment characteristics and the controlling factors,which leads to the two main development directions of “more macroscopic” and “more microscopic”.

Many new advances and understandings have been made in the study of Chinese ichnology in recent ten years,and key results are outlined below: (1)84 ichnogenera and 211 ichnospecies have been identified and 28 types of ichnoassemblages and 26 types of ichnofabrics have been established in the Phanerozoic marine and transitional deposits;44 ichnogenera and 107 ichnospecies have been identified and 22 types of ichnoassemblages and 10 kinds of ichnofabrics have been established in the Mesozoic and Cenozoic terrestrial(alluvial fan,fluvial and lacustrine)deposits;(2)The impact of the Cambrian Substrate Revolution on the evolution of early benthic organisms and ecological effects was discussed;(3)The response relationship of 3 kinds of palaeo-oxygenation(aerobic,dysaerobic and anaerobic)facies to ichnofossils and their deposition backgrounds was proposed;(4)Many carbonate micro-shapes(single mineral and aggregates)related to microbes were found in limestones of the Lower Permian Taiyuan Formation of the northern China,and six kinds of carbonate micro-shapes were distinguished by observing Zoophycos burrow fillings,which are spheroid-like,rhabditiform,areatus,net-like,vase-like and cerebrum-like. Meanwhile,it was described that many complex and delicate biogenetic structures formed by a series of life activities of trace-makers with symbiotic micro-organisms;(5)The sedimentary setting evolution of a complex ichnofossil Zoophycos was revealed,i.e.,the bathymetrical shift of Zoophycos from the littoral-neritic to bathyal-abyssal environments synchronized with the tiering shift from shallow to deep;(6)It was suggested that the composition and distribution characteristics of ichnofossils in a sedimentary sequence of storm and turbidity currents in the littoral-neritic and lacustrine environments and nearby P-Tr boundary,and the ecological system evolution law of ichnofossils and their trac-makers before and after the P-Tr event was demonstrated,which provides a reliable ichnologic information for the ecosystem recovery process after the P/Tr great extinction;(7)A large amount of theropod,sauropod and ornithopod dinosaur tracks were found in the Jurassic and Cretaceous of many places in the eastern,northern and western China;(8)Many types of microbially induced sedimentary structures were distinguished in the clastic deposits of the Mesoproterozoic and Upper Permian-Lower Triassic in the North China Platform;(9)The bioturbation and ichnofabrics in clastic and carbonate rock reservoirs and their impact on reservoir physical properties were analyzed in detail,a three-dimensional geological model of bioturbation changes was established,and the manifestation and research methods of bioturbation type reservoirs were put forward. These results have greatly promoted the development and progress of ichnological field including ichnotaxon,ichnofacies,ichnofabrics,geomicrobiology and application in the event deposits and reservoir geology in China.

Microbialites have been defined as the organosedimentary deposits that have accreted as a result of a benthic microbial community trapping and binding sediment and/or forming the locus of mineral precipitation or teemed as the benthic microbial deposits. Microbial carbonates are understood as the calcified microbialite as well as the geological record of calcified bacterial-algal mats and biofilms or the most spectacular example of the MISSs(microbially-induced sedimentary structures). On the basis of the meso-structures,microbialites can be subdivided into four types,inlcuding stromatolites,thrombolites,leiolites and dendrolites. Leiolites were named relatively late and characterized by relatively structureless,aphanitic,mesostructures,lacking lamination or clotted structures. No examples of modern leiolite have been published and only few examples have been described or discerned in stratigraphic records since it was firstly named,which led to difficulties in the study of leiolites. Leiolites from the Miaolingian Gushan Formation at the Shengshuiyu section in Sishui,Shandong Province,are predominated by massive micrite and expressed as a biostromal microbial reef. They,together with the ribbon-like marls and micrite,make up the subtidal carbonate cycle belonging to the relatively deep-water deposits. Thus,a sedimentary succession of this type of subitidal carbonate cycles constitutes a forced regressive system tract of a third-order sequence of the Miaolingian Formation at the Shengshuiyu section,which represents a typical record of the falling stage of a third-order relative sea-level change. Within dense and massive micrite of the leiolites,a common and uneven high-density preservation of Girvanella is marked by densely tangled masses of fairly thin-walled,unbranched filaments or tubes,which indicates that these leiolites possibly resulted from the sophisticated calcification of cyanobacterial mats. In addition,a particular micro-fabric is marked by the microspar tubes of filaments within dense and dark micrites and makes up an irregular-shaped and mm-to cm-long clump. This micro-fabric is similar to the Lithocodium,described as the Lithocodium-like fabric,which can be regarded as colonies of calcified cyanobacteria. Importantly and interestingly,this micro-fabric was interpreted as the sponges including spicula sponges or non-spicula sponges in studies of microbial carbonates in recent years,which resulted in a highly different cognition. According to its fundamental feature,i.e. the microspar filaments or tubes within the dense and dark micrite,I deduced that it could be the residue of calcified Nostoc colonies with common sheathes. The microspar filaments or tubes might represent the calcified residues of filaments or tubes;the dark and dense micrite might have been formed by calcification of sheathes. This interpretation is further supported by its intergrowth with Girvanella. Leiolites predominating the biostromal microbial reefs of the Miaolingian Gushan Formation at the Shengshuiyu section not only provide an example of the Cambrian leiolites but also provide insights into the cyanobacterial calcification without the help of CCM mechanism and the sophisticated calcification of multiple cyanobacterial mats predominated by both the Girvanella and the residue of the Nostoc colonies. This study broadens further understanding of a particular calcification of cyanobacteria and a cyanobacterial blooms that are paralleled with the metazoan exploration in typical calcite sea during the Cambrian under a high atmosphere CO₂ and O₂.