193 / 2019-06-13 15:10:39

Two episodes of dolomitization in Middle Permian Maokou carbonate rocks, Eastern Sichuan Basin

Sichuan Basin, Middle Permian, Maokou Formation, Early dolomitization, Hydrothermal activities

Dolostones in Middle Permian Maokou Formation in the Sichuan Basin contain abundant natural gas resources and are significant gas-producing unites, but their origin is kept very controversial for years. The paper selects the dolostone samples from drilling wells and a field section in eastern Sichuan Basin, and tries to discuss the dolomitization processes in Maokou middle-ramp carbonate rocks by using lithological and geochemical methods. The dolostones in research area are usually found in middle to upper part of Maokou Formation with the thickness varied from 3.0 to 46.8m, and the lateral distribution extends as a NW-SE belt along a basement fault. Dedolomitization caused by ancient weathering and leaching can be observed on the top of Maokou Formation. Field and core observation shows that the dolostones contain numerous tension fractures, fully-filled or partly-filled by coarse dolomite and calcite as veins, presenting a breccia structure. Four structural constituents are identified under the microscope: (I) Silt-fine sized dispersive euhedral dolomite floating in lime-mud matrix; (II) Fine sized planar-s dolomite (mostly subhedral crystals packed as a mosaic texture); (III) Fine-middle sized planar-e dolomite with “cloudy center” and “clear rim” (sucrose texture with high intercrystalline porosity); (IV) Coarse saddle dolomite in veins. Type I, II and type III “cloudy center” dolomite look muddy on crystal surface. SEM (Scanning Electron Microscope) and BSE (Backscattered Electron) images confirmed that there are lots of micropores and tiny calcite residuals on dolomite crystal surface which may be the reason of dirty crystal face. EMPA (Electron Microprobe Analysis) indicate the dolomite poor in Iron, Manganese and Strontium, rich in Calcium, but Magnesium always lower than that of stoichiometric dolomite. However, type III “clear rim” and type IV veins dolomite are usually clean and tight without micropores on the face. Although the “clear rim” and veins dolomite are poor in Iron, Manganese and Strontium as well, the Calcium and Magnesium are always close or even higher than stoichiometric dolomite. The order degree of type IV veins dolomite ranges between 0.65 and 0.71. And type I, II and type III matrix dolomite have a wide range of order degree from 0.42 to 0.90, but main interval between 0.54 and 0.75, which resemble that of veins dolomite. Lithological and major element analysis indicate there are two main episodes of dolomitizaton in the formation of Maokou dolostones; type I, type II and type III “cloudy center” dolomite were formed during the first episode of dolomitization; and type III “clear rim” and type IV veins with similar elementary compositions are attributed to the second episode of dolomitization. Both dolomitizations happened very early, possibly from synsedimentary to shallow burial period. In situ Strontium of the four types of dolomite suggests that both dolomitizations were probably related to the eruption of E’meishan basalt. The dolomitic fluids were featured by mixing sources from both basaltic hydrothermal fluid and crustal water. Moreover, with the weakness of magmatic hydrothermal activities, the proportions of crustal water increased gradually in the dolomitic fluids. The tension fractures were proved to be effective channels for fluids mixing and migrating to the surrounded sediments.