New Insights Into the Natural Gas Potential of Paleogene Micro-Fractured Tight Conglomeratic Sandstone, Offshore South China

Abstract

The X field is a recently discovered tight gas field in the near-source deltaic system of the South China Sea. The conglomeratic sandstone exhibits rapid grain size variations and high density, and petrophysical interpretation typically identifies them as dry layers. However, gas breakthroughs occur quickly during formation testing, and multiple seal failures are observed. Simultaneously, drilling processes frequently encounter various problems such as wellbore collapse, drill string obstruction, and mud loss. These contradictions hinder the efficiency of exploration and evaluation decisions, urgently needing the clarification of the genesis and development patterns of micro-fractures, and optimization of drilling design. To investigate the genesis of these micro-fractures, sidewall core samples were collected, and borehole imaging logging was performed, and corresponding geomechanical analyses were conducted. Digital core CT scanning revealed numerous micro-fractures, primarily high angle or near-vertical, partially filled, forming a network. Two competing interpretations exist regarding the origin of these micro-fractures: one suggests they were generated during core retrieval as a result of stress release and mechanical disturbance, while the other interprets these micro-fractures as being associated with strike-slip tectonic deformation. Imaging logging shows both induced fractures and breakout, indicating a significant horizontal stress difference. Geomechanical modeling was established to calculate triaxial stresses, and digital core rock strength was used for calibration. The results showed that the maximum horizontal principal stress SHmax is greater than the vertical stress Sv and greater than the minimum horizontal principal stress Shmin; which is consistent with a strike-slip fault system. Based on core observation and CT scanning, a new micro-fracture identification standard based on borehole imaging logging was established, classifying micro-fractures into four types: induced micro-fractures, high angle micro-fractures, low angle micro-fractures, and partially filling micro-fractures. Seismic interpretation suggests localized magmatic diapiric uplift that may have enhanced fracture intensity around the fold axis. However, statistical analysis from borehole imaging shows that micro-fractures are more likely to develop in massive sandstone (ZH620, ZH621, ZH622, EP242 and EP412 reservoir intervals), while they are less developed in sandstones with cross-bedding (ZH630, EP21, EP23 and EP241 reservoir intervals). Drill stem test was conducted on the bottom two reservoir intervals EP242 and EP412, with 16.9m effective thickness and 32300 m3gas/day, demonstrating the productivity potential of micro-fractured tight sandstone reservoirs and supporting reserve growth in the X field.