The Diagenetic Alteration of the Carbonate Rocks from the Permian Qixia Formation as Response to Two Periods of Hydrothermal Fluids Charging in the Central Uplift of Sichuan Basin, SW China

Abstract

The hydrothermal fluid–carbonate rock reaction is frequently regarded to occur in deep-burial diagenesis, and the hydrothermal dissolution is usually distributed and takes place along the faults. Previous studies have suggested that there was hydrothermal fluid activity locally in the Permian Qixia Formation in Sichuan Basin, likely related to the Emeishan basalt eruption. However, the effect of hydrothermal fluids on the carbonate rocks of the Qixia Formation in the central uplift of Sichuan Basin is still unclear. Based on the characteristics and geochemical parameters of the diagenetic minerals, this study aims to reveal the diagenetic alteration related to the hydrothermal fluid–rock reaction in the Qixia Formation and reestablish the diagenetic evolution by using the timing of diagenetic mineral precipitation. The methods include petrographic observation; trace and rare earth element (REE) analysis; C, O and Sr isotope measurement; fluid inclusion temperature measurement and cathodoluminescence analysis. According to the petrographic characteristics, the dolostones are mainly of crystalline structure, namely fine-medium crystalline dolostone, meso-coarse crystalline dolostone, and coarse crystalline dolostone, with the cathodoluminescence color becoming brighter in that order. The limestones from the Qixia Formation are of the bioclastic limestone type, with no cathodoluminescence color. Compared with dolostones, limestones have higher Sr content, lower Mn content, and heavier oxygen isotopes. With the crystalline size of dolostone becoming coarser, the oxygen isotopes of dolostones tend to become lighter. The meso-coarse crystalline dolostone has the highest Mn content and negative carbon isotope. Both limestones and dolostones have an obvious positive Eu anomaly in the Qixia Formation. However, the REE patterns of fine-medium crystalline dolostones are very different from those of meso-coarse crystalline dolostones. It is credible that there were two periods of hydrothermal fluid charging, with different chemical compositions. The first period of hydrothermal fluids could laterally migrate along the sequence boundary. Fine-medium crystalline dolostones were almost completely distributed below the sequence boundary and were dolomitized during the shallow burial period. As products of the hydrothermal fluid–dolostone reaction, the saddle-shaped dolomites in the meso-coarse crystalline dolostones were the evidence of the second period of hydrothermal fluids. As a result, the dolomitization model was established according to the timing of diagenetic mineral precipitation, which can improve that the geological understanding of the effect of hydrothermal fluid activities on the carbonate rocks in the Qixia Formation.

Quantitative diagenesis: Methods for studying the evolution of the physical properties of tight carbonate reservoir rocks

“…For such a technique to be applicable to any particular formation, it requires information about the type, degree and timings of different episodes of diagenesis. This information is potentially available from the study of the mineralogy and microstructure of the formation (e.g., Hollis and Al Hajri, 2022;Mohammed-Sajed and Glover, 2022), from studies of the evolution of geochemical signatures, from the use of cathodolumiescence and also fluid inclusion studies (e. g., Chen et al, 2021;Burley et al, 1989), as well as a number of other combined approaches (Elias Bahnan et al, 2021). For a given formation, such studies may be demanding of time and research resources.…”

Fluid Properties and Genesis of Dolomites in the Devonian Guanwushan Formation of Upper Yangtze Platform, SW China

“…Therefore, most scholars believed that the large number of dolomites developed in geological history were the products of the dolomitization of limestones [4]. Based on that, various dolomitization models have been established, including the hypersaline models by concentrated seawater such as reflux of dolomitization [5], capillary-concentrated dolomitization [6], evaporation pump dolomitization [7]; the normal salinity models such as mixing zone dolomitization [8], normal seawater dolomitization [9,10], the temperatureincreased models by buried stratum or thermal fluids such as buried-compacted dolomi-tization [11], adjusted-compacted dolomitization [12,13]; seawater thermal convection dolomitization [14] and hydrothermal dolomitization [15][16][17]. As the most persuasive and representative of the primary dolomite, the microbial one has attracted close attention from geologists in recent years [18][19][20].…”