A Genetic Mechanism for Chemical Compaction in Mudstones: Case Study from the Xihu Depression, East China Sea Shelf Basin

Mudstone compaction is widely used in the estimation of subsidence caused by sediment load, basin modeling, and overpressure genesis. The boundary for the depth at which chemical compaction in mudstones begins in the Xihu Depression is across two wells; in Well A, the depth is 2200 m, while in Well D, it is 1750 m. The porosity shows a gradual decreasing trend, followed by a sharp decrease and then an increase. Compared with Well A, the pores in Well D show a faster reduction (resulting in higher compaction) with increasing burial depth. The compaction process is controlled by clay mineral transformation and temperature increase. At a temperature range of 65–105 °C, local dissolution of quartz and pyrite, as well as precipitation of plagioclase, occur in Well D. The quartz, pyrite, and plagioclase contents in Well A do not significantly change, while K-feldspar corrosion and illitization are dominant. At a temperature range of 125–135 °C, diagenesis is reversed. The mixed layer increases across a low range, while chlorite and kaolinite contents increase; the dissolution of dolomite and the intermittent dissolution of calcite cause a local increase in pore size at 4460 m in Well A and 3300 m in Well D. The report values of geothermal gradient raise the depth limit of chemical compaction in Well D compared to that in Well A, thus accelerating the process of illitization and the cementation rate of quartz, and becoming the leading cause of the steeply decreasing trend of porosity in Well D. A compaction model for the Xihu Depression has also been established, which involves mechanical compaction, coexistence of chemical compaction and mechanical compaction, and chemical compaction. In the chemical compaction stage, the chemical/diagenetic compaction of mudstones locally increases the pore size. Moreover, abnormally enlarged pores became important reservoirs in the Xihu Depression.