Formation and preservation processes of Ediacaran microbial dolomite pore system, evidence from the Qigebrak Formation, Tarim Basin, China

Ancient microbial dolomite reservoirs exhibit distinct advantages in both their formation and preservation processes, making them significant hosts for hydrocarbon resources. However, the interaction of multiple geological factors complicates the assessment of ancient microbial dolomite pore systems and the reconstruction of their differential evolutionary processes. This study investigates the morphological characteristics and evolution of pore systems within Ediacaran microbial dolomites. Through quantitative analysis of pore structures using nuclear magnetic resonance (NMR) T₂ curves, supplemented by petrographic and scanning electron microscopy observations, rock fabric analysis, a robust method for pore system identification based on four NMR T₂ curves was established. Comparative analysis of differential characteristics and evolutionary processes of pore systems in diverse microbial dolomite types. Spongiomicrobialite and stromatolite exhibit diverse primary porosity, including microbial mold and framework pores, as well as dissolution pores, resulting in a wide range of porosity. In contrast, residual microbial rocks are dominated by secondary pores, while thrombolite displays limited porosity. The effective preservation of microbial dolomite pores is influenced by the interaction between microbes and the Precambrian marine environment. Early microbially mediated dolomitization conferred significant resistance to compaction and pressure dissolution in microbial carbonates. In elevated terrains above the wave base or within the upper sections of highstand system tracts (HST) quasi-sequence groups, microbial dolomites rich in primary pores are more prone to dissolution by diagenetic fluids, which can modify original sedimentary structures. Conversely, environments characterized by poorly developed primary pores and restricted fluid flow tend to preserve the original structures.