Abstract
The Guanwushan Formation (GWSF) of Devonian dolomite are extensively developed in the northwest of Sichuan Basin in the Upper Yangtze region, but the properties of dolomitization fluid and the geneses are still unclear. Three types of dolomites can be divided by petrological characteristics: the fine-microcrystalline dolomites (FMD), the fine crystalline dolomites (FCD) and the medium crystalline dolomites (MCD). The order degree of these three types of dolomites increased in turn, and they all showed dark cathodoluminescence (CL) luminescence. The total amount of Rare Earth Elements (∑REE) of the dolomite was low, while the dolomite enriched with light REE and lacking heavy REE presented a distribution pattern consistent with that of limestone. The weak negative anomalies of the Ce and Eu indicated that the dolomites were formed in a weak redox environment with relatively low temperature. The dolomitization fluids were inherited from the original seawater. The respective δ13CPDB values of the three types of dolomites varied a little, indicating that they were not affected by the biological effects. Specifically, the δ18OPDB values of the FMD and FCD dolomites were higher than that of the limestone, indicating that the dolomitization fluid was influenced by evaporation at the penecontemporaneous stage. The interpretations were also supported by the 87Sr/86Sr ratios, as the 87Sr/86Sr ratios of FMD comparable to the Middle Devonian seawater. The δ18OPDB value of the MCD dolomite was lower than that of the limestone. It also showed poor automorphic extent, which clarified that the dolomite experienced more intense dolomitization in greater burial depth and at higher temperatures.
Highlights
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Exploration and Development Research Institute, PetroChina Southwest Oil and Gas Field Company, Sichuan Key Laboratory of Shale Gas Evaluation and Exploitation, Chengdu 610091, China; Abstract: The Guanwushan Formation (GWSF) of Devonian dolomite are extensively developed in the northwest of Sichuan Basin in the Upper Yangtze region, but the properties of dolomitization fluid and the geneses are still unclear
Taking the Devonian Guanwushan Formation (D2 g) dolomites in the northwestern part of the Upper Yangtze Platform as the research object, combined with field outcrop, thin sections, CL, X-ray diffraction analysis, whole-rock elemental analysis, and carbon, oxygen and strontium isotope compositions, this study aims to systematically discuss the properties of dolomitization fluid and the geneses of dolomites, to provide insights into the dolomitization models and to guide the subsequent hydrocarbon explorations in the study area
Based on the microscope observation, they are mainly divided into three types: fine crystalline-microcrystalline one (FMD), fine crystalline one (FCD), and medium crystalline one (MCD)
Summary
The dolomite was first described in 1791, and different theories and models were proposed to explain its origin; there remain many controversies at present [1–3]. Despite the discovery of microbial dolomite in modern sedimentary environments, it does neither show volume significance nor address the origin of thick-layer dolomite in ancient strata. Taking the D2 g dolomites in the northwestern part of the Upper Yangtze Platform as the research object, combined with field outcrop, thin sections, CL, X-ray diffraction analysis, whole-rock elemental analysis, and carbon, oxygen and strontium isotope compositions, this study aims to systematically discuss the properties of dolomitization fluid and the geneses of dolomites, to provide insights into the dolomitization models and to guide the subsequent hydrocarbon explorations in the study area
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