Abstract
The dolostone reservoir of the Middle Permian Maokou Formation in Eastern Sichuan has good prospects for oil and gas exploration. Study of dolomitizing genesis of the Maokou Formation is essential for predicting the distribution of the dolostone reservoir. Petrography, in situ geochemistry, Sr-Mg isotopes, and fluid inclusions were carried out on samples from the Maokou Formation in Eastern Sichuan in order to discuss the dolomitizing process. Based on mineral and textural characteristics, dolomites were divided into four components: partially clouded dolomite (PCD), mosaic-like dolomite (MLD), cloudy-centered and clear-rimmed dolomite (CACD), and saddle dolomite (SDD). Results indicate that the Maokou Formation in Eastern Sichuan mainly experienced two stages of dolomitization. PCD, MLD, and cloudy-centered dolomite (CCD) were formed during the early dolomitization. They all show turbid crystal planes and bright orange-red CL and have similar trace element contents, 87Sr/86Sr ratios, and rare-earth patterns, indicating that they might be formed in the same fluid. This is a period when dolomitizing fluids mainly migrated along pores or microcracks and replaced protogenetic calcites, which occurred in the shallow burial stage of the Maokou Formation before the Late Permian. Clear-rimmed dolomite (CRD) and SDD were formed in the late stage of dolomitization. They all have clean crystal planes and darkly red CL. CRD of the ERY profile has trace element contents, 87Sr/86Sr ratios, and rare-earth patterns similar to SDD of the HLCH profile and Well TL6, inferring that both may be formed in the same fluid. Combined with high SrO contents and homogenous temperatures of fluid inclusions of CRD and SDD and Mg-isotopic compositions, they were generated by hydrothermal dolomitization. The hydrothermal fluid stage is related to the movement of the Emeishan Large Igneous Province, which was made up of basaltic magmatic fluids mixing with the surface water. The hydrothermal fluid mainly migrated upwards along structural fractures or faults and filled in structural fractures, occurring in the Late Permian to Middle-Late Triassic.
Highlights
The genesis of dolomite has been a perplexing problem for geologists [1]
In order to discuss the lateral changes of dolomitization in the Middle Permian Maokou Formation, samples were collected from the ERY profile, the HLCH profile, and Well TL6 (Figure 2)
Dolomites were subdivided into four components: partially clouded dolomite (PCD), mosaic-like dolomite (MLD), cloudy-centered and clear-rimmed dolomite (CACD), and saddle dolomite (SDD)
Summary
The genesis of dolomite has been a perplexing problem for geologists [1]. In the past two decades, more and more geologists paid attention to the hydrothermal dolomitization model due to lots of reports of deeply buried dolostones [8,9,10]. Hydrothermal dolostones are products of the multistage dolomitization and may have undergone complicated diagenetic processes [12,13,14]. Geochemistry and stable isotope composition of carbonate minerals or rocks are closely associated to the diagenetic environment and provide important information regarding paleosalinity, paleotemperature, and paleoclimate. The composition of stable isotopes is of great significance to the diagenetic environment of carbonate rocks, and is important for distinguishing the genesis of dolomite [15,16,17,18]. Distribution of major and trace elements in strata reflects changes of the diagenetic environment to a certain extent [19,20,21,22]
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