Gas-bearing shales have become a major source of future natural gas production worldwide. It has become increasingly urgent to develop a reliable prediction model and corresponding workflow for identifying shale gas sweet spots. The formation of gas-bearing shales is closely linked to relative sea-level changes, providing an important approach to predicting sweet spots in the Wufeng-Longmaxi shale in the southern Sichuan Basin, China. Three types of marine shale gas sweet spots are identified in the shale based on their formation stages combined with relative sea-level changes: early, middle, and late transgression types. This study develops a prediction model and workflow for identifying shale gas sweet spots by analyzing relative sea-level changes and facies sequences. Predicting shale gas sweet spots in an explored block using this model and workflow can provide a valuable guide for well design and hydraulic fracturing, significantly enhancing the efficiency of shale gas exploration and development. Notably, the new prediction model and workflow can be utilized for the rapid evaluation of the potential for shale gas development in new shale gas blocks or those with low exploratory maturity.

Efficient exploration and exploitation of shale oil and gas depend heavily on the accurate prediction of hydrocarbon sweet spot distribution. Meanwhile, the spatiotemporal differentiation and coupling of endogenetic components in fine-grained sediments produce a key effect on sweet spot distribution. Through an in-depth analysis of typical oil and gas-bearing shales both at home and abroad, we systematically reveal the types and distribution patterns of endogenetic components in fine-grained sediment, as well as the mechanisms by which these components control shale oil and gas sweet spots. The results indicate that the major endogenetic components include biogenic silica, biogenic carbonate minerals, biogenic phosphate minerals, authigenic organic matter, and intraclasts. These components are primarily concentrated in zones that terrigenous clastics pass by, zones that are difficult to reach for terrigenous clastics, areas influenced by upwelling ocean currents, and nearshore areas experiencing rapid transgression. The dissolution-reprecipitation process of biogenic silica, among others, leads to the formation of authigenic quartz, which inhibits organic matter degradation through hydrogen bonding. While the biogenic carbonate and phosphate mineral cementation maintains reducing environments by decreasing pore water mobility. All these are conducive to organic matter enrichment. The biogenic silica in endogenetic component-rich shales, through its synergy with organic matter, contributes to the development of pores and fractures, which in turn leads to the formation of a distinctive organic pore-microfracture coupling structure. Furthermore, the rigid support and pore networks provided by the biogenic silica frameworks provide, along with swelling stress and dissolution induced by the thermal evolution of organic matter, collectively drive the formation of a three-dimensional microfracture system. Endogenetic components play a determinant role in the distribution of shale oil and gas sweet spots by influencing organic matter enrichment, pore structure, gas adsorption, seepage characteristics, and rock brittleness. Accordingly, shale oil and gas sweet spots generally feature high endogenetic component content, elevated nano-scale porosity, abundant microfractures, and high brittle mineral content. Presently, there is a need for further investigation into the genesis and enrichment mechanisms of endogenetic components in fine-grained sediments, as well as the quantitative relationships of these components with the physical properties and brittleness index of reservoirs. Moreover, the mechanisms underlying the dynamic controlling effects of the endogenetic components on organic matter preservation and pore evolution during diagenesis warrant further exploration. In the future, it is necessary to further identify and analyze the endogenetic components using methods such as multi-scale characterization, artificial intelligence (AI), and interdisciplinary integration, with a view to supporting the efficient exploitation of shale oil and gas.

The provenance of black shales is primarily investigated using elemental geochemistry and mineralogy. Specifically, elemental geochemical approaches are employed to analyze the process of sediment transport and deposition, as well as ascertain the nature of parent rocks and tectonic setting through the analysis of major and trace elements, and rare earth elements (REEs). Mineralogical techniques, on the other hand, serve to identify the locations of provenance areas and pathways of sediment transport by examining variations in mineral composition. The main limitation of this methodology is that it can only determine potential provenance areas, leaving their specific locations and extents, as well as sediment transport pathways unclear. This study focuses on the provenance of black shales in the Wufeng-Longmaxi formations in the southern Sichuan Basin. The results indicate that the active elements and compounds in the shales include Si, Al, Ca, Fe, Ba, Ta, Th, and Zr and that the inactive elements and compounds encompass Ti, P, Na, Mn, Yb, Lu, and Tm. The parent rocks exhibit low compositional maturity overall, having undergone low to moderate degrees of chemical weathering and transformation under warm and humid climatic conditions. In contrast, sedimentary sorting and recycling produce minimal impacts on variations in the elemental characteristics of the parent rocks. The parent rocks consist mainly of felsic igneous rocks, with tectonic setting dominated by a collision environment, followed by a rift environment locally. The sediment originates primarily from the Qianzhong-Xuefeng paleo-uplift, succeeded by the Leshan-Longnvsi paleo-uplift. Specifically, the sediment in well block Y203, the Changning and Luzhou areas, and the northeastern part of the southern Sichuan Basin is derived principally from the Qianzhong-Xuefeng paleo-uplift, while that near well W207 in the Weiyuan area originate primarily from the Leshan-Longnvsi paleo-uplift. Provenance analysis reveals multiple factors influencing the contents of major and trace elements, and mineral components in the sediment. Therefore, to accurately determine the provenance areas of shales and their distribution, it is necessary to integrate the stable isotope analysis, high-resolution geochemical techniques, multi-index, multidisciplinary study, and big data analytics.

Shale has horizontal bedding of diverse origins in differential permeability. An integrated analysis of core data, full-diameter images of enlarged thin section and scanning electron microscope (SEM) images of argon-ion-milled samples, shows that the gas-bearing Wufeng-Longmaxi shale in southern Sichuan Basin develops four types of horizontal bedding, that is, grading type composed of claystone, grading type composed of siltstone and claystone, alternating siltstone and claystone type, and page type. The grading type of claystone constitutes the multi-layer superimposed siltstones in parallel, with siltstone and claystone depositing in graded bedding. The grading type composed of siltstone and claystone is composed of parallelly alternating silty and clayey beds, where the silty beds mainly in clast-supported texture, has abrupt boundary at the bottom and gradual contact with the clayey bed on top, featuring normal grading as a whole. The alternating siltstone and claystone type features abrupt contact between both beds and lamina with no grading. The page type constitutes parallel bedding of very thin clayey lamina with weakly normal grading. The four types of horizontal bedding are of different genesis. The grading type of claystone and grading type of siltstone and claystone are of relatively low-energy turbidite origin, with the former derived from even weaker hydrodynamic conditions; the alternating siltstone and claystone is of contourite origin such as shelf facies; and the page type is of pelagic origin from suspended sediment deposition. The horizontal bedding serves to directly affect shale permeability. The page type is characterized by the abundance of organic matter and organic pores, ranking top in permeability; the alternating siltstone and claystone type takes the second place in permeability with better sorting; while the two grading types come at last in permeability with poor sorting and low organic matter content.

This study provides a comprehensive analysis of global advances in the study of the characteristics, origin, and organic matter enrichment model of marine fine-grained sediments. These sediments, mainly composed of clay minerals, quartz, carbonate minerals, and organic matter, are predominantly transported by four geological agents: wind, hypopycnal plumes, hyperpycnal plumes, and bottom currents. They exhibit clayey and silty laminae; single, sequential, and alternating laminasets; and massive, graded, and alternating beds. Covering three major types of sedimentary facies, namely turbidite, bottom current, and pelagic to semi-pelagic facies, as well as their transition types, the sediment demonstrates two organic matter enrichment models under high productivity and enhanced preservation. In the high productivity-derived model, the organic matter enrichment in black shales typically involves upwelling currents, oxygen-minimum zones (OMZ), and nearshore photic zone euxinia (PZE). In the enhanced preservation-related model, the enrichment principally involves restricted basins, improved restricted basins, irregular bed form, expanding puddles, transgressive chemocline, and transgressive nearshore (TN) zones. Current challenges in the research on the genetic mechanism and facies models of marine fine-grained sediments primarily include non-standard terminology, undefined origins of different mineral compositions, and the difficulty in distinguishing among three sedimentary facies: fine-grained turbidite, contourite, and semi-pelagic facies. Further research is required to address these issues.