Types and characteristics of sweet spots of marine black shale and significance for shale gas exploration: A case study of Wufeng–Longmaxi in southern Sichuan Basin

The types, identification standards, and distribution rules of sweet spots of black shale directly affect the optimization of exploration intervals and the design of drilling targets. Therefore, taking the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation (Wufeng–Longmaxi) in southern Sichuan Basin as an example, according to the theories of petrology and sequence stratigraphy, the relationships between relative sea level changes or fracture types and shale reservoir quality were analyzed. The results are obtained in five aspects. First, the sweet spots of Wufeng–Longmaxi black shales can be divided into two categories: sedimentary and fractured. The sedimentary sweet spots are subdivided into four types: early transgression type, rapid transgression type, late transgression type, and nearshore transgression type. The fractured sweet spots are divided into two types: network microscopic fracture type and network macroscopic fracture type. Second, the sweet spots of early transgression and nearshore transgression types are dominated by hemipelagic sediments, mixed sediments of siltstone and clay, and TOC is lower. The sweet spots of rapid transgression type are mainly characterized by pelagic sediments, with well-developed microcrystal quartz, high TOC, and well-developed organic pores. The sweet spots of late transgressive type are dominated by contourites, with lower TOC and relatively developed inorganic pores. Third, the reservoirs of fractured sweet spots have well-developed macroscopic and microscopic fractures, but the matrix porosity and permeability may be lower. Fourth, the sweet spots of early transgressive type are distributed in the graptolite zones WF2–WF3, those of rapid transgressive type are distributed in the graptolite zone LM1, those of late transgressive type are distributed in the graptolite zones LM2–LM5, those of nearshore type are distributed in the graptolite zones LM1–LM4, and those of network microscopic fracture type are mainly distributed in the graptolite zone LM1. The distribution of network macroscopic fractured sweet spots is controlled by the scale of faults and the deformation degree of structural transition zones. Fifth, in the graptolite zones LM6–LM8, where clay mineral contents are high and diagenetic contraction joints are well developed, sweet spots of network microscopic fracture type can be formed. The structural transition zones and adjustment zones have the characteristics of weak deformation and weak transformation, with well-developed network macroscopic fractures, which can form sweet spots of network macroscopic fracture type. The conclusion is that significant breakthroughs have been made in the exploration of sweet spots of rapid transgression type and nearshore transgression type in southern Sichuan Basin, and those of early and late transgression types are important targets for future exploration. The sweet spot classification based on sedimentology and fracture characteristics provides technical support for the in-depth exploration and benefit development of shale gas. © 2023 Natural Gas Industry Journal Agency. All rights reserved.