Paleo-karst zone and its control on reservoirs in the fourth member of the Sinian Dengying formation in the Moxi area, central Sichuan Basin

Paleo-karst controls reservoirs in the fourth member of the Sinian Dengying formation in the Sichuan Basin. It is crucial to characterize the paleo-karst zone architecture to understand better the distribution of reservoirs controlled by the multi-stage ancient water tables. We divide vertical paleo-karst zones using cores and formation micro-scanner images based on the groundwater movement and development of pores and fractures. Then we analyze positions and variations of paleo-karst water tables according to the characteristics of different paleo-karst zones. Besides, we illustrate the distribution of underflow subzones and the development of reservoirs in each subzone. This paper reveals the relationship between paleo-karst landform relative elevation, paleo-karst zone thickness and reservoir thickness. Thus, we build the paleo-karst evolutionary pattern in the study area. Research suggests that vadose, underflow, and slow flow zones develop in the intended interval. We identify four underflow subzones in response to four paleo-karst water tables. Compared with the vadose zone, the underflow zone is thicker and holds more reservoirs, mainly distributed in the upper part of the single underflow subzone. Higher paleo-karst landform can possess a thicker vadose zone and thinner underflow zone. Besides, an excellent positive linear correlation exists between reservoir thickness and paleo-karst zone thickness. The thickness of the subzone constrains the development of reservoirs in it. The second and third underflow subzones are the most favorable intervals, owning more reservoirs.