Reservoir characteristics and the occurrence mechanism of shale gas outside of the Sichuan Basin are the research hotspots of normally pressured shales in China. Taking shales on the Anchang syncline from the periphery of the Sichuan Basin as an example, X-ray diffraction, organic geochemistry, and rock physical experiments were carried out to analyze the reservoir characteristics and their main geological controls on the normally pressured shales. The mineralogical results show that the studied shales from the Anchang syncline are mainly siliceous shales with a high quartz content (average of 57%). The quartz content of these normally pressured shales is of biological origin, as shown by the positive correlation between the quartz and organic carbon (TOC) contents. The average porosity of the studied shales is about 2.9%, which is lower than shales inside the Sichuan Basin. Organic matter pores are likely the primary storage space of the normally pressured shale gas, as shown by the positive relationship between the TOC content and porosity. However, scanning electron microscopy observations on the studied shales show that the pores in these normally pressured shales are poorly preserved; many pores have been subjected to compression and deformation due to tectonic movements. Compared to shales inside the Sichuan Basin, the effective thickness of shales outside of the Sichuan Basin is thin and the stratum dip is large. Thus, shale gas outside of the Sichuan Basin is apt to escape laterally along the bedding of the strata. After losing a significant amount of shale gas, the gas pressure decreases to normal pressure, which makes it difficult for the pores to resist compaction from the overlying strata. This is probably why most shale gas reservoirs outside of the Sichuan Basin are normally pressured, while the shale strata inside the Sichuan Basin are commonly overpressured. This study provides insights to understand the pore development and hydrocarbon occurrence on normally pressured shales outside of the Sichuan Basin.

The Early Carboniferous Dawuba Formation within the Yadu-Ziyun-Luodian aulacogen (YZLA), southern China, represents a significant shale gas play formed within a dynamic platform-basin transitional sedimentary system influenced by multi-sphere coupling (the dynamic interaction of tectonic, climatic, and oceanic spheres). This setting generated pronounced heterogeneities in lithology and gas content. Integrating field geological surveys and well data, this study comprehensively evaluates the mineralogy, geochemistry, structural features, and gas content of these shales to decipher the key factors controlling gas enrichment, particularly in this tectonically complex area. A crucial finding is the distinction between low-energy, “thick-bedded” deposits and higher-energy, “thin interbedded” sections characterized by frequent alternations of mudstone and argillaceous limestone (0.5–5 m single layers, mudstone:limestone ratio 1:1–1:2). Significantly, these thin interbedded sections demonstrate superior reservoir quality, exhibiting higher porosity (∼2.9%) and notably higher gas content (∼2.5 m³/t). We reveal that in the YZLA’s complex structural setting, shale gas content is primarily governed by preservation conditions dictated by the unique litho-mechanical properties of the interbedded strata, rather than traditional controlling factors like TOC abundance or matrix porosity. The frequent interlayering promotes stable micro-fractures, supported by clay-rich horizons and effectively sealed by compact limestone, thereby enhancing secondary storage capacity and permeability for free gas. Consequently, these thin interbedded sections represent the most favorable intervals for shale gas accumulation, identifying them as the prime targets for achieving Early Carboniferous shale gas breakthroughs in the YZLA.

China has focused on the exploration and development of shale gas resources to reduce its reliance on coal and shift to cleaner energy sources. While significant progress has been made in the Sichuan Basin, unlocking the shale gas potential in other regions of South China has proven challenging due to the complex geology and mountainous terrain. In 2021, Well QSD-1 was deployed in southwestern Guizhou and achieved a daily shale gas flow of 11,011 m³ in the Dawuba Formation, marking the first time an industrial gas flow had been obtained from shale gas drilling in the marine strata of the Upper Paleozoic in China. This breakthrough has deepened the understanding of the southern China Carboniferous marine strata and highlighted key aspects of the formation: (1) Sedimentation occurred in alternating platforms and basins, with most organic-rich shale developed in sloping and basin areas; (2) the formation exhibits favorable static indicators, with a relatively thick section (over 200 m), and an organic carbon content of approximately 1%; (3) the intercalation of argillaceous limestone and shale intervals is conducive to the preservation of shale gas within the formation. These results demonstrate the potential for the Upper Paleozoic in South China to become a significant shale gas producer, which could contribute significantly to China’s energy security. Furthermore, exploring shale gas in the region may have positive economic and environmental impacts, including reducing China’s dependence on coal and decreasing greenhouse gas emissions.