Study On Mechanism Of Dedolomitization Of The Ordovician Ma-5 In Yichuan-Huanglong Area

The effect of dedolomitization is the reaction of dolomite with a high Ca/Mg fluid,which is being replaced by calcite?or other minerals?after dolomite dissolution or dissolution of dolomite.In this paper,the secondary limestones developed in the Ordovician Ma-5 Member of the Yichuan-Huanglong area of the Ordos Basin which is used as the research object.The typical wells of the dedolomitization is performed by microscopic slice identification,cathodoluminescence,scanning electron microscopy,electron probe and other experimental methods.By using major elements and trace elements,rare earth elements,strontium isotope,carbon and oxygen isotopes and other geochemical analysis methods the types of secondary limestones,and the diagenetic environment,diagenesis,diagenetic evolution stage and mechanism of dedolomitization are studied,the two stages of dedolomitization and the secondary calcite formed under the two stages were proposed innovatively,achieving the following results and understanding:?1?Through the analysis of mineralogy and petrological characteristics,it is believed that the dedolomitization plays a role in the formation of two types of secondary calcite,including the preservation of the original dolomite crystal form red secondary calcite;medium-coarse grain,mainly produced in cracks or solution cavities'white secondary calcite.Cathodoluminescence shows that the red secondary calcite is dark orange and has a bright edge structure,indicating that diagenetic environment may be shallow burial environment;while secondary calcite dim-medium bright orange-red light,indicating that the diagenetic environment may be medium-deep Buried environment.?2?Red secondary calcite has low Sr and high Fe and Mn values,which may be formed in the surface or near-surface environment affected by fresh water;while white secondary calcite contains higher Sr element,lower Fe and Mn elements.The diagenetic environment is a relatively closed burial or deep burial environment.?3?Analysis of rare earth elements shows that dedolomitization occurs under both oxidative and anoxic conditions.The transition of diagenetic environment from oxidizing to anoxic environment may mean shallow burial and deep burial conversion from the surface or near-surface;the diagenetic fluid of red secondary calcite may be from the atmospheric fresh water,while white secondary calcite may be related to hydrothermal or deep fluid.?4?The relative negative?¹³C value and slightly negative?¹⁸O value of red secondary calcite indicates that the process may have the participation of atmospheric fresh water and is located in the near surface environment;The white secondary calcite has a slightly negative?¹³C value and significantly negatively?¹⁸O value,indicating the formation environment is formed by a mixture fluid of carbon from the rock and CO₂ from the hydrocarbon in the medium-deep burial period.?5?Red secondary calcite has lower Sr element content and the wilde distribution range of ⁸⁷Sr/⁸⁶Sr value,indicating that the fluid source is atmospheric fresh water;white secondary calcite has the obviously high Sr element content and low ⁸⁷Sr/⁸⁶Sr value,indicate that the formation process is related to sea-sourced fluids.?6?Combining the sedimentary background and paleo-karst characteristics of the Ordovician Ma-5 in the Yichuan-Huanglong area,it is believed that the dedolomitization in this area occurs in the surface and near-surface,medium-deep burial environments.Secondary limestone in these two environments has two aspects:the first stage is leached by atmospheric fresh water in the near-surface environment,which is characterized of the incomplete dedolomitization of red secondary calcite;the second stage is in medium-deep burial environment,first influnenced by acidic fluids mixed with carbon in the rock and CO₂ from hydrocarbons,and then under the influence of sea-borne fluids.secondary limestones form white secondary calcite by thorough dolomitization.