Clay mineral transformations of mesozoic mudstones in the central Junggar Basin, northwestern China: Implications for compaction properties and pore pressure responses

The clay mineral transformation of deeply buried mudstones is closely related to the mudstone microstructure, petrophysical properties, and pore fluid pressure (pore pressure). The influence of the burial history on the microstructure and compaction behavior of mudstones has been the focus of recent research. In this study, the mineral composition, mudstone microfabric and petrophysical properties of Mesozoic overpressured mudstones in the central Junggar Basin were analyzed to explore the influence of clay mineral diagenesis on compaction processes and overpressure responses in diagenetically altered mudstones. The clay minerals of these Mesozoic mudstones are mainly composed of mixed smectite/illite layers (I/S) and illite with small amounts of kaolinite and chlorite, but they do not contain discrete smectite. The Mesozoic mudstone underwent clay mineral diagenesis, the smectite in I/S transformed to illite at depths of 4000–4800 m (temperatures of 90–105 °C), and the I/S changed from the R0 type to the R3 type. The recrystallization and growth of clay minerals continued with increasing stress with depth, leading to the substantial realignment of clay particles observed at the microscale. The combination of the preferential alignment of clay particles and precipitation of authigenic cements caused significant decreases in the porosity of the mudstone. The results of this study support the viewpoint that the effective stress continues to work in diagenetically altered mudstones, although porosity loss is enhanced by clay fabric destabilization caused by clay mineral transformation. The top depth of smectite-to-illite transformations is consistent with the top of the overpressure, indicating that overpressure occurs simultaneously with clay diagenesis in diagenetically altered mudstones. The decrease in porosity and increase in density of mudstone caused by clay diagenesis changes the overpressure logging response to a certain extent and affects the accuracy of the pressure prediction results. The sonic transit time-density cross plot can be used to effectively distinguish the compaction stage and the cause of overpressure in diagenetically altered mudstones. Clay mineral diagenesis cannot be ignored in pore pressure analysis and pressure prediction in deeply buried mudstones. This research is of great importance for deepening the theory of mudstone compaction and has practical significance for pore pressure prediction in deeply buried layers.