The characteristics and controlling factors of facies-controlled coastal eogenetic karst: insights from the Fourth Member of Neoproterozoic Dengying Formation, Central Sichuan Basin, China

Karst porosity system is widely developed in the Fourth Member of Neoproterozoic Dengying Formation in Central Sichuan Basin of China. The result of the previous exploration in this area based on the theory of telogenetic karst, which mainly focused on the fracture/fault belt especially in the karst slope, leaves much to be desired. Though a systematically geological research in this study, it is found that the dissolution holes are mainly concentrated on the mud mound deposits and granular rocks. Both of them have a greater primitive porosity and permeability than the micrite that originally formed in low-energy environments, which is facies controlled. On the basis of the above and the fact that a tectonic movement has caused a massive subaerial exposure of the formation that only experienced a shallow burial, it is proposed that the paleokarst porosity system is the product of eogenetic karst. Two-stage eogenetic karsts have been interpreted: the syngenetic exposure caused by the short-term relative sea-level fluctuation and the exposure caused by the uplift of the second phase of Tongwan tectonic movement. The meteoric alteration in syngenetic period is commonly marked by the development of intergranular pores but on a small scale. It is believed to be important for providing a good pore-permeability foundation for the later eogenetic karst. The later exposure occurs immediately at the end of the deposition of the Fourth Member of Dengying Formation that only experienced a shallow burial. Since the effect of compaction and cementation in this shallow burial before uplift is relatively weak, only the micrite has been densified and retaining the majority of original pores in the carbonate mounds and grain shoals as paths for karst flow. Moreover, combined with the geological background and seismic profile, it can be ascertained that the karst occurs in a coastal environment. Twelve meteoric lenses have been interpreted after recovering the karst palaeogeomorphology. The scale of the meteoric lens and the reservoir performance have shown a decreasing tendency from the coastal to the inland area, which also confirms this view. The results of this research have put emphasis on the lithologic differentiation in eogenetic karst and the geographical location of karst development thus might provide a reference for further reservoir prediction in this area and other similar places.