Mechanism of dedolomitization and its rebuilding to reservoir
Zhang Jie1,2, Shou Jianfeng1,2, Wen Yingchu2,3, Wang Yigang2,3, Hu Anping1,2
1 PetroChina Hangzhou Research Institute of Geology，Hangzhou 310023， Zhejiang 2 Key Laboratory of Carbonate Reservoir，CNPC，Hangzhou 310023， Zhejiang 3 PetroChina Southwest Oil and Gas Field Company Exploration and Development Research Institute，Chengdu 610000， Sichuan
Abstract：Dedolomitization is very common in the strata, but it is still in dispute about its formation environment，influencing factors，mechanisms，and the role in reservoir and so on.The recognition marks was analyzed，and detailed classification of dedolomitization from 3 aspects was given: Retaining the original crystal form or not，occurrence mode and forming environment，the manifestation in diagenesis.Dedolomitization can occur in various environments，from the near-surface settings to shallow burial and deep burial environments，even in hypergene deposit and lacustrine environments.The factors influencing dedolomitization mainly include the dolostones chemical constituents and physical properties，degree of order，fluid characters（concentration of SO2-4，Ca/Mg ratio，temperature，pH and partial pressure of CO2）and Fe content of the dolostone. Cathodeluminescence and geochemical analysis can be used to study the dedolomitization mechanism in near-surface，mixed seawater and freshwater，burial and other environments.Dedolomitization process can be divided into two steps, including dissolution of dolomite and the precipitation of calcite. Only the first step occurs to form the rhomb-moldic pore; the two steps take place at the same time; only one step happens or the two the two steps take place in different order.Dedolomitization mostly begins from the core of the dolomite or the interface between the core and the rim，because there are deficits during the growth process and the Ca content is high.Dedolomitization can be considered as a kind of process that the Mg2+ is gradually released.The studies of calcite-dolomite-solution system can help to investigate the transforming condition between calcite and dolomite and the influencing factors.The near-surface dedolomitization is generally relevant to the dissolution of gypsum or anhydrite and the meteoric water of vadose zone and phreatic zone.It is confirmed that the laterally continuous distribution of dedolomitization can indicate unconformity.The study of dedolomitization products can reflect the diagenetic stages and the source of the fluid.Dedolomitization can enlarge the modic pore and inter-crystal pore and improve the reservoir quality，but there is close relationship between the increasing porosity of dedolomitization and latter eluviation.Without the latter leaching，maybe the late burial dedolomitization relevant to calcite precipitation will destroy the primary pores.
