Pore Evolution of Ediacaran Spongiomicrobialites: A Case Study from the Qigebrak Formation, Northwestern Tarim Basin

The Ediacaran spongiomicrobialites in China are a typical and distinctive type of microbialite, characterized by complex pore systems and relatively high reservoir porosity, making them potential high-quality reservoirs for deep oil and gas exploration. A thorough investigation into the pore evolution mechanisms of spongiomicrobialites is of great significance for better understanding the genesis of high-quality microbialite reservoirs. The Ediacaran Qigebrak Formation in the northwestern Tarim Basin contains abundant outcrop and core data of microbialites, with well-developed spongiomicrobialites that are thick and exhibit good porosity, providing an excellent opportunity to study their pore evolution mechanisms. In outcrops, the spongiomicrobialites appear as light gray, thick-bedded units, often with gentle mound-like structures, displaying homogeneous textures and locally developed botryoidal cements. Sedimentologically, this type of microbialite mainly developed in tidal-flat environments within a ramp-type carbonate platform, typically at the tops of high-frequency sedimentary cycles. After deposition, they were prone to high-frequency subaerial exposure and leaching, resulting in significantly better development of primary pores compared to other microbialite types in the middle and lower parts of the cycles. Diagenetically, four generations of typical dolomite cements are recognized in these microbialites: early fibrous dolomite, early bladed dolomite, late fine-to-medium crystalline planar dolomite, and late coarse-grained saddle dolomite. Integrated with U-Pb dating data of the matrix and multiple generations of dolomite cements, the pore evolution history has been reconstructed. The present-day reservoir spaces in the spongiomicrobialites of the Qigebrak Formation were mainly formed during the penecontemporaneous sedimentary stage and the end of the Ediacaran Period, with the formation age of abundant effective pores constrained to before (542.7 ± 8.0) Ma. During the burial stage, the filling of fine-to-medium crystalline dolomite cements in the Ordovician represents the most significant porosity-reducing event, while other diagenetic events show no obvious modification of the pore system.