Editorial for the Special Issue “Clay Minerals: Roles in Oil and Gas Generation, Drilling, and Enhanced Recovery”

Clay minerals are fundamental components of sedimentary basins and play a dual role in the petroleum industry according to Zhuang et al. (2024) [1]. On one hand, they act as critical constituents of source rocks, cap rocks, and reservoirs, influencing hydrocarbon generation, migration, and trapping mechanisms [2,3]. On the other hand, their unique physicochemical properties—such as swelling capacity, cation exchange, and large specific surface area—make them indispensable functional materials in drilling fluids and cementing operations [4,5] (contribution 1). However, these same properties can also present significant engineering challenges, including wellbore instability and formation damage [6].

This Special Issue, “Clay Minerals: Roles in Oil and Gas Generation, Drilling, and Enhanced Recovery,” was curated to explore the critical interactions between clay minerals and hydrocarbon systems. The collection of nine papers published herein represents a blend of comprehensive reviews and cutting-edge original research. These contributions span the spectrum from fundamental mineralogical characterization of reservoirs to the development of novel clay-based composites for high-performance drilling and cementing applications.

A significant portion of this Special Issue addresses the perennial challenges of drilling in complex formations, particularly regarding rheology control and shale inhibition. As exploration moves toward deeper and harsher environments, the limitations of traditional bentonite-based fluids become apparent. In a comprehensive review, Zhuang et al. (2024) [5] explore the application of nanofibrous clay minerals, specifically palygorskite and sepiolite, in water-based drilling fluids (WBDFs). They highlight how the unique nanostructure of these minerals offers superior salt resistance and thermal stability compared to conventional smectites, proposing them as a new generation of rheological additives. Building on the theme of high-performance additives, Zhang et al. (2025) (contribution 2) present a novel hydrothermal carbon/bentonite composite (HCBC). Their work demonstrates that these composites not only stabilize rheology but also significantly reduce filtration loss and improve lubrication in high-temperature environments by scavenging free radicals, thereby protecting polymers from degradation.

Two contributions focus specifically on maintaining wellbore stability in reactive shale formations. Wang et al. (2024) (contribution 3) synthesized a small-molecule quaternary ammonium salt (TEE-2) via N-alkylation. Their experimental evaluation proves that this inhibitor effectively suppresses the hydration swelling and dispersion of clay minerals through intercalation and adsorption, offering a promising solution for maintaining wellbore integrity. Complementing this, Ai et al. (2024) (contribution 4) provide a review of mechanical inhibitors. Moving beyond chemical inhibition, they discuss the use of physical plugging agents—including carbon-based and mineral-based nanomaterials—to seal microfractures in shale, thereby preventing pressure transmission and fluid invasion.

Understanding the mechanisms of failure is as crucial as preventing them. Gu et al. (2025) (contribution 5) analyze drilling losses in the Mesozoic reservoirs of the Zhengning oilfield. Through a combination of mineralogical analysis and rock mechanics, they identify that the hydration–dispersion of illite/smectite mixed layers, combined with the propagation of natural fractures, are the primary drivers of wellbore instability in this region. Similarly, Fang et al. (2025) (contribution 6) investigate the impact of hydration on the Gulong shale oil reservoir. Using micro-CT and SEM, they reveal a complex trade-off: while hydration initially increases porosity and permeability through clay dispersion, prolonged exposure leads to crack propagation and a reduction in core strength. Their findings regarding the impact of fracturing fluid concentration offer vital theoretical support for balancing reservoir stimulation with long-term stability.

The presence of clay minerals significantly complicates the petrophysical evaluation of reservoirs. Two articles in this issue address the difficulties in interpreting logging data in clay-rich formations. Yu et al. (2025) (contribution 7) investigate the phenomenon of high resistivity in the ultra-deep shale of the Mahu Sag. By characterizing the mineral composition, they reveal that the reservoir is dominated by brittle minerals (quartz/feldspar) with low clay content. Their findings clarify that high resistivity is driven by the mineral framework and porosity characteristics, providing a theoretical basis for better drillability prediction.

In a detailed review, Navarro-Perez et al. (2025) (contribution 8) examine the petrophysical controls of greensand reservoirs, which are rich in the iron-bearing clay mineral glauconite. They highlight how the paramagnetic and conductive nature of glauconite distorts conventional well logs (such as NMR and resistivity), leading to biased porosity and saturation estimates. The review suggests integrated workflows involving core analysis and spectral gamma-ray logging to overcome these interpretation challenges.

Finally, the utility of clay minerals extends beyond drilling fluids into well construction. Ventura et al. (2025) (contribution 9) evaluate the use of palygorskite as an extender agent in lightweight cement pastes. Their performance analysis indicates that palygorskite-based pastes exhibit lower viscosity and yield stress compared to bentonite-based pastes while maintaining satisfactory stability. This suggests that fibrous clays can be effective alternatives for cementing operations in formations with low fracture gradients.

The research presented in this Special Issue underscores the versatility and complexity of clay minerals in the energy sector. From acting as the root cause of wellbore instability to serving as the solution in the form of advanced fluid additives, clay minerals remain at the heart of petroleum engineering challenges.

We hope this collection provides valuable insights for researchers and engineers working on drilling fluid formulation, reservoir characterization, and well construction. We sincerely thank the authors for their high-quality contributions and the reviewers for their constructive feedback, which ensured the scientific rigor of this Special Issue.