RESERVOIR CHARACTERISTICS AND ITS MAINLY CONTROLLING FACTORS OF DOLOMITE IN THE LATE EDIACARAN DENGYING FORMATION, CENTRAL SICHUAN BASIN,SW CHINA

The late Ediacaran Dengying Formation is one of the oldest carbonate reservoir in the world. Due to the remarkable heterogeneity of carbonate reservoirs, especially karst reservoirs, the research on the development characteristics and genetic mechanism of carbonate reservoirs have always been a focus and difficulty of sedimentary and reservoir geology research globally. In 2011, well GS1 in the Gaoshiti-Moxi tectonic belt in central Sichuan Basin produced more than 1 trillion m³, showing good exploration prospects in this area. The analysis in central Sichuan is essential because of the old stratum, deep burial, and complicated diagenesis.

To fully understand the reservoir genetic types in D4 member (the fourth of Dengying Formation), studies on the core and field profiles as well as reservoir analysis are undertaken. The controlling factors of sedimentary facies, high-frequency sequence boundaries, diagenesis and tectonic movement on the reservoirs are analyzed. Three types of reservoirs are identified, which are early-stage karst mound-shoal reservoirs , late-stage karst superimposed reservoirs and other factors impacted reservoirs (fracture reservoirs and hydrothermal-tectonic reservoirs). The shoal and microbial mounds developed in the platform margin and platform interior have the best physical properties. The shoal reservoirs spaces include acicular-shaped and dissolved pores and holes. For the microbial mounds reservoir, the reservoir spaces are dominated by dissolved pores and caves form enlarged dissolution stripes along the laminae. The fourth-order sequences strictly control the development of the early-stage karst mound-shoal reservoirs, and the high-quality reservoirs generally develop at the top of the high-level system domain. The key factors for the improvement of reservoir quality are meteoric water karstification and organic acid karstification. The meteoric-water karstification controlled the late-stage karst superimposed reservoirs which spaces are honeycomb appearance caves and dissolved vugs. The organic acid karstification enlarges the existing pores and fractures .The fractures formed by the tectonics improve the permeability of the reservoir and play a vital role in the formation of high-quality reservoirs.