Genesis And Reservoir Characteristics Of Lacustrine Dolomite In The Lower Part Of The Sha-1 Formation In Qikou Depression

The origin of dolomite has been one of major research issues in sedimentology field. Based on full investigation of previous associated research results, with basic theories such as carbonate rocks petrology and reservoir geology as a guide, the dissertation carries out a dedicated study into the formation environment, spatial and temporal distribution and formation mechanism of dolomite in the lower part of the Sha-1 formation of Qikou depression from mineral rocks, fossil specimens and geochemical characteristics, etc, by using core, drilling, mud logging and logging data, combined with microscopic identification and geochemical characteristics analysis. Through the study of reservoir macro features and microstructure, the dissertation explores the reservoir space categories, physical properties, fractures distribution rule and controlling factors, and undertakes favorable reservoir areas distribution prediction.Through the microscopic identification of dolomite samples in the study area, and in combination with X diffraction and scanning electron microscopy analysis, it is concluded that the study area comprises mainly 5 types of dolomites, including microcrystalline dolomite, micritic dolomite, argillaceous dolomite, sandy dolomite, and calcite dolomite or dolomite limestone. Then by virtue of the depth homing of coring internal, the author carries out log interpretation analysis and statistics corresponding to different types of dolomites, and establishes suitable GR and AC curve cross-plots for dolomites type identification in the study area. The distribution of the dolomite in the objective intervals of the study area is determined: microcrystalline dolomite and micritic dolomite are mainly in Qi Jiawu, Zhou Qingzhuang-Liu Jianfang-Wang Xuzhuang , Zhao Jiabao and Huang Hua in Bin-1 and Ban-4 oil formations of the lower part of the Sha-1 formation; argillaceous dolomite is mainly distributed in Cangzhou and Jiucheng area of Ban-4 oil formation and Wang Xuzhuang of Ban-3 oil formation; calcite dolomite and dolomite limestone are mainly in Liu Jianfang- Lianmen area of Ban-3 oil formation, and Qi Jiawu and Zhou Qingzhuang of Ban-2 oil formation; and sandy dolomite is mainly in part area of Zhao Jiabao of Bin-1 and Ban-3 oil formations.The salinizing origin of eastern continental facies lake basin in Paleogene has been unsolved issue, with disputes over ancient salt rock weathering, lake basin water evaporation concentration, volcanics and transgression effect. A variety of analysis and tests of rock samples of Sha-1 lower sub member in Qikou depression show that Sr/Ba ratio, V/Ni ratio and Th/U ratios all indicating marine environment, with Z values distribution for carbon and oxygen stable isotope and strontium isotope values indicating aqueous medium features forming under the co-activation of sea water and lake water. The diagnostic glauconite and phosphorite discovered during thin section observation and calcareous nannofossil and other marine facies fossils found through scanning electron microscopy suggest that Sha-1 lower sub-member in Qikou depression was formed due to the transgression, which not only provides part of Mg²⁺ for the formation of offshore lake basin dolomite, but more importantly, changes the nature of the lake basin water so as to promote the dolomitization.In light of the previous studies on the formation conditions and mechanism of lacustrine facies dolomite, using rock thin section identification and scanning electron microscopy analysis, cathodoluminescence, and X-diffraction analysis, trace elements geochemistry and other research methods, the dissertation concludes that penecontemporaneous dolomite and burial diagenetic dolomite dominates the study area, with penecontemporaneous dolomite mainly locating in Qi Jiawu, Huanghua area and its south part, and Liu Janfang-Zhou Qingzhuang-Wang Xuzhuang and its north mainly consisting mainly of burial diagenetic dolomite.Finally, main reservoir spaces of the study area are structural joints and dissolution pores, with fractures distribution formed by the interaction of lithology and structures, through cores and cast thin section observation. The dolomite reservoirs are mainly microcrystalline dolomite and micritic dolomite. Comprehensive analysis shows that the dolomite reservoirs in the study area are mainly distributed in Bin-1 and Ban-4 oil formation of Sha-1 lower sub-member, and Liu Jianfang-Lianmeng-Zhou Qingzhuang and Zhao Jiapuzi area is most favorable where burial dolomite prevails, and Qi Jiawu area with penecontemporaneous dolomite distribution is also fairly favorable exploration area.