Editorial: Distribution and Development of Faults and Fractures in Shales

The study of fractures and faults has long been recognized as a cornerstone of shale reservoir characterization, given their profound influence as primary storage spaces and critical seepage channels [1,2,3,4,5]. These geological features directly govern the enrichment patterns, preservation conditions, and ultimate productivity of shale hydrocarbons, making their understanding indispensable for successful exploration and development strategies [6,7,8,9]. This Special Issue was conceived to synthesize the latest advancements and insights regarding fractures and faults in shale reservoirs, drawing from the interconnected fields of mineralogy, geology, geochemistry, and geophysics.

The response from the scientific community has been strong and insightful. The published contributions collectively advance our knowledge across multiple facets of shale reservoir systems, from pore-scale dynamics to basin-scale tectonic histories (Contributions 1–5). This editorial aims to synthesize the key findings of these studies, highlighting the progress made and pointing towards future research directions.

The articles included in this Special Issue cover a diverse geographical and geological spectrum, from continental to marine–continental transitional shales, and employ a wide array of methodological approaches. They can be broadly categorized into several thematic areas that reflect the current frontiers of research.

A fundamental understanding of how pore networks evolve and interact with hydrocarbon generation is critical for assessing reservoir quality. The study by He et al. (Contribution 1) provides a detailed investigation into the Lower Jurassic continental shale gas reservoirs in the northeastern Sichuan Basin. Their work elegantly bridges the gap between pore structure characterization and the dynamic process of gas accumulation.

The inherent heterogeneity of shale reservoirs is often rooted in their depositional environment and lithofacies. Sun et al. (Contribution 2) address this complexity in their evaluation of the Lower Permian Fengcheng Formation source rocks in the Junggar Basin’s Hashan area. By classifying the source rocks into distinct lithofacies—terrigenous clastic, dolomitic mixed, tephra-bearing mixed, and alkaline mineral-bearing mixed—they demonstrate a direct link between rock type and hydrocarbon potential.

The effectiveness of hydraulic fracturing, a key technology for shale resource development, is heavily dependent on the geomechanical properties of the reservoir, particularly its brittleness. Lai et al. (Contribution 3) present a detailed analysis of the brittleness of shale oil reservoirs in the Liushagang Formation, Beibuwan Basin. They identify a suite of internal and external factors controlling brittleness, with organic matter maturity, horizontal stress difference, and brittle mineral content being the most significant.

The pore system in shales is notoriously complex, and this is especially true for marine–continental transitional facies, which represent an emerging exploration frontier. Zhang et al. (Contribution 4) delve into the pore structure and heterogeneity of coal-bearing shales from the Longtan Formation in the South Sichuan Basin. Using nitrogen adsorption–desorption data and fractal theory, they quantitatively characterize the micro-scale heterogeneity.

Understanding the absolute timing of fracture formation and its relation to hydrocarbon migration is a long-standing challenge in structural diagenesis. Fattah et al. (Contribution 5) make a groundbreaking contribution by directly dating a multi-phase natural fracture system in the Jurassic source rocks of NE Iraq using U-Pb geochronology.

The collective body of work presented in this Special Issue marks significant strides in our comprehension of faults and fractures in shale systems. Several overarching themes emerge:

Integration of Process and Time: The studies move from static characterization to dynamic, process-oriented models that incorporate the temporal evolution of pores, fractures, and fluids. The combination of thermal experiments with direct geochronology is particularly powerful in this regard (Contributions 1, 5).

Embracing Heterogeneity: The contributions consistently highlight the profound influence of depositional lithofacies and diagenetic overprinting on reservoir properties, necessitating predictive, facies-based models and quantitative heterogeneity analysis (Contributions 2, 4).

Bridging Scales and Disciplines: From the nanoscale pore to the basin-scale fracture system, and from geochemistry to geomechanics and structural geology, a multi-scale, interdisciplinary approach is proving essential (Contributions 1, 3–5).

Looking forward, several key areas warrant continued focus. The integration of advanced imaging techniques with fluid-inclusion studies and geochemical analysis will further refine our models of fracture–cement–hydrocarbon interactions [10,11]. The predictive modeling of fracture networks in subsurface reservoirs, calibrated by studies such as those presented herein, remains a critical goal. Furthermore, as exploration moves into more complex and challenging environments, such as deeply buried or tectonically stressed shales, understanding the mechanical behavior and long-term sealing capacity of faults will be paramount.

We hope that the research compiled in this Special Issue will serve as a valuable reference and a source of inspiration, fostering continued innovation in the science of shale fractures and contributing to the efficient and responsible development of shale energy resources.