Abstract
The Sinian Dengying Formation in China hosts one of the oldest gas-bearing reservoirs in the world. With a long geological history, it has developed into the reservoir rocks, whose quality depends on chemical dissolution and cementation. Despite presence of cavities and pores in the algal dolomites of the Dengying Formation, they were mostly filled with various forms of dolomite cements. Therefore, the investigation of these dolomites and their evolution is significant for analyzing the mechanism of pore space preservation. There are five types of cements in the Dengying Formation, which were well sampled for lab measurements such as thin section, cathodeluminescence, minor elements, isotope analysis of carbon, oxygen and strontium, in attempt to explore the fillers’ geochemical characteristics and the diagenetic fluids. They are characterized as follows: firstly, fibrous rim dolomite cement is a typical product formed at sea bottom, and geochemically very similar to the matrix (micrite and microcrystalline dolomite), which could be a reflection of the geochemical characteristics of water at that time; secondly, the diagenetic fluids of the foliated dolomite cements tend to be inheriting, derived from the shallow burial “imprisoned” seawater; thirdly, fine- to medium-grained dolomite cements with fairly high strontium isotopes and low carbon and oxygen isotopes were precipitated chiefly in freshwater during the period of uplifting caused by Tongwan tectonic movement; fourthly, coarse-grained dolomite cements bear a relatively high percentage of Mn element, but low in Fe and Sr, with carbon and oxygen isotopes remarkably shifted to negative values, which all show that they are related to hydrocarbon maturation and migration, and corresponding Thermochemical Sulfate Reduction; lastly, saddle-shaped dolomite cements are hydrothermally associated, and they were formed in high-temperature brine, which had passed through the areas rich in radioactive strontium isotopes.
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