Formation and preservation mechanisms of ultra-deep microbialite-dominated reservoirs in the ediacaran dengying formation from the central to Northern Sichuan Basin, SW China

The microbialites in the Ediacaran Dengying Formation of the Sichuan Basin exhibit significant hydrocarbon potential. Investigating the formation and preservation mechanisms of ultra-deep microbialite-dominated reservoirs is critical for expanding exploration domains. Based on observations of 374 samples from outcrops and cores, thin-section quantification, porosity and permeability testing, and diagenetic sequence reconstruction, this study systematically analyzes reservoir characteristics under different burial conditions and establishes an evolution model for microbialite-dominated reservoirs. The reservoir rocks are dominated by stromatolites and thrombolites, with an average porosity of 4.52% and a geometric mean permeability of 0.127 mD. The deposition of microbialite provided high initial porosity. Penecontemporaneous dissolution expanded along primary pores, forming fabric-selective vugs. Regional exposure karst of the Tongwan period further created non-fabric-selective dissolution cavities and fractures, preferentially affecting high-porosity zones. Cementation and hydrothermal activity during the middle to deep burial realm caused limited porosity reduction, and hydrocarbon charging may have helped to preserve pore space. Comparative analysis of Well PS6 (~ 8 km burial depth), Well MX9 (~ 5 km), and the Hujiaba section (outcrop exposure) reveals that the second member of the Dengying Formation reservoirs are primarily influenced by meteoric fluid dissolution during the Tongwan Movement. In contrast, the key diagenetic processes affecting the fourth member reservoirs predominantly occurred during the near-surface to shallow burial stages. This indicates that microbialite-dominated reservoirs in ultra-deep settings retain moderate porosity despite strong heterogeneity. Therefore, reservoir quality is essentially determined during the near-surface to shallow burial stage, where early dolomitization and hydrocarbon charging are critical for porosity preservation in deep to ultra-deep settings. The results provide a theoretical reference for predicting ultra-deep microbialite reservoirs and guiding global exploration of analogous systems.