Impact of diagenesis on the microbial reservoirs of the terminal Ediacaran Dengying Formation from the Central to Northern Sichuan Basin, SW China

Abstract

The central part of the Sichuan Basin shows an excellent exploration potential in the dolomitized microbial reservoirs of the Dengying Formation. However, the influence of diagenesis on the formation of high-quality reservoirs is unclear. This study aims to establish the parasequence of the diagenesis in microbial reservoirs and decipher its control on the quality of microbialite-dominated reservoirs. The microbialite consists of stromatolites and thrombolites in the Dengying Formation, which firstly underwent meteoric alteration in the early diagenetic stage. Subsequently, the microbialites experienced fibrous and bladed dolomite cementation and dolomitization. Laminated and clotted sedimentary fabrics are well-preserved in dolomite with less than 100 μm in crystal-sizes. The carbon (δ13C), oxygen (δ18O), and strontium (87Sr/86Sr) isotopes in the microbial dolomite are similar to the Ediacaran seawater, indicating the marine origin of the dolomitizing fluid. The carbon, oxygen and strontium isotopes of the fibrous and bladed dolomite cements are closed to the microbial matrix, which means that the diagenetic fluid is originated from the coeval seawater. The medium-coarse crystalline granular dolomite cement filled later than the fibrous cement, and the oxygen isotope of this cement was affected by the recrystallization caused by the temperature rise after burial. Finally, the saddle dolomite cement filled in fractures and veins. The δ13C (1.1 ± 1.3‰) value of saddle dolomite is similar to the matrix and early cement, while the δ18O value is significantly negative (−10.9 ± 1.0‰). The high Mn (751.4 ± 570.0 ppm), Fe (3872.5 ± 3347.9 ppm) concentrations, homogenization temperature (194 ± 26 °C) derived from the fluid inclusion and concomitant silicification suggest its formation of Fe-rich high-temperature burial fluid. For the microbialite-dominated reservoir, its initial porosity is high. The porosity and permeability characteristics show dissolution-specific behavior, suggesting a key factor of meteoric diagenesis in enhancing the reservoir. The early dolomitization enhance the resistance to the compaction and pressure dissolution. For the geophysical properties, the cementation of marine-derived fluids occupies the most pore space (∼28.1%), followed by compaction and stylolitization that reduce porosity by 10–13%. The hydrothermal alteration, organic acid dissolution, and thermochemical sulfate reduction slightly influence the reservoir quality. This study provides further understanding on the mechanism of microbial reservoirs and enlightens the further exploration of Precambrian successions.