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Understanding microbial carbonate is one of the most challenging issues in the field of carbonate sedimentology and reservoir. The biotic and abiotic processes that control microbial carbonate precipitation are becoming more established, but the influence of subsequent diagenesis on microbial carbonate reservoirs has not been adequately studied. Here, we describe microbial carbonate from the lower Cambrian Qingxudong Formation in southeastern Sichuan Basin to assess their formation, textures and subsequent diagenesis. Six stages of calcite cementation (Calcite-1 to Calcite-6) and four stages of dissolution (Dis-1 to Dis-4) were identified in microbialites of the Qingxudong Formation. Stromatolites and thrombolites have a different porosity evolution. Sedimentary processes are the fundamental controlling factors of creating pre-existing pores in stromatolites. For thrombolites, the formation of pre-existing pores is attributed to island dissolution and dolomitization in near-surface processes. Microbial metabolic activities contribute more to the dissolution and dolomitization than mixing water. Meteoric water dissolution (Dis-1) in synsedimentary processes and microbial dissolution (Dis-2) in near-surface processes are the fundamental control factors of porosity creation in microbial carbonate of Qingxudong Formation. Thermochemical sulfate reduction (TSR) is a double-edged sword for the porosity development. The destructive effect of TSR on microbial carbonate reservoirs is greater than the constructive effect. The microbial carbonate reservoirs, especially the fabric-destructive dolomite (Dol-2) evolved by thrombolites, have great potential for gas exploration in the Qingxudong Formation. This study is especially useful for further understanding deeply buried microbialite reservoir formation and development, and deep hydrocarbon exploration in this basin and elsewhere worldwide. [ABSTRACT FROM AUTHOR]

In this paper, the genesis of dolomite in Longwangmiao Formation is analysed through field investigation and geochemical methods, and dolomization model is established, which is very important for the next reservoir prediction in Longwangmiao Formation. According to the crystal size, the Longwangmiao Formation dolomite can be divided into two types: microcrystalline dolomite (MID) and macrocrystalline dolomite (MAD). Both types of dolomite have higher Al content than lime mudstone, indicating that during the deposition process of the Longwangmiao Formation, terrigenous materials are continuously mixed into the sediment, which significantly affects the geochemical characteristics of the Longwangmiao Formation. The rare earth element (REE) composition of the two types of dolomite is consistent with that of lime mudstone (LM), with an obvious δCe positive anomaly and no δEu anomaly, which is a typical distribution pattern of marine carbonate sediments. The microcrystalline dolomite has a higher Na element content, lower δ13C value, and higher δ18O value, suggesting that it is formed in a highly saline evaporation environment. Most of the macrocrystalline dolomite has a residual grain texture. The elements, as well as the isotopic geochemical characteristics of the dolomite, are consistent with those of lime mudstone, indicating that macrocrystalline dolomite is formed early and inherits the geochemical characteristics of the precursor limestone. We believe that the dolomitization model of the Longwangmiao Formation is an evaporative concentration + reflux seepage model. In the late sedimentary period of the Longwangmiao Formation, the sea level declined and a large area of lagoon appeared in the restricted and semi-restricted platform. As the evaporation and concentration continued, precursor minerals such as high-magnesium calcite and aragonite mud experienced chemical precipitation. In this process, a large amount of Ca2+ was consumed, resulting in an increase in the Mg2+/Ca2+ ratio. Then, the precursor minerals underwent dolomitization, and microcrystalline dolomite began to form. At the same time, owing to the difference in density between the high-salinity concentrated seawater and the pore water existing in the early limestone, the concentrated seawater flowed back and penetrated downward, causing the dolomitization of the underlying strata, and then the formation of macrocrystalline dolomite. The degree of dolomitization was mainly affected by the development degree of the granule beach and the thickness of the strata. • The highlights of this article are as follows: • This paper provides a case study of terrigenous clastic dolomite. • This paper analyses the genesis of the dolomites of Longwangmiao Formation and establishes a dolomitization model. • We analysed a large number of samples and provided basic geochemical data for Longwangmiao Formation. [ABSTRACT FROM AUTHOR]