Characteristics and origin of dolomitic tuffin the Permian Fengcheng Formation in Wu-Xia area of Junggar Basin
Wang Junhuai1, Liu Yinghui2, Wan Ce1, Zhu Xiaomin2,3, Wang Xiaojun1, Ge Jiawang2, Zhang Xin2
1 Research Institute of Petroleum Exploration and Development,Xinjiang Oilfield Company,PetroChina,Karamay 834000,Xinjiang; 2 College of Geosciences,China University of Petroleum(Beijing),Beijing 102249; 3 State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum(Beijing),Beijing 102249
Abstract：The dolomitic tuff of Fengcheng Formation in Wu-Xia area of Junggar Basin is considered to be a set of hydrocarbon-abundant carbonate reservoir with special geologic origins. Based on the geological background study including tectonic,sedimentation and volcanic activities in the Permian Fengcheng Formation,and combined with the macroscopic occurrence of the dolomitic tuff cores,the dolomitic rocks could be divided into four types:Rippled,massive,flocculent and diffuse dolomitic tuff. The characteristcs of the four types with the dolomitic tuff examined under microscope are:The ripple one has fine-grained and poor crystal shape,but the other three ones have silt-grained and fine-grained,and there is fogged center and clear margin structure and residual annulus alternation structure widely formed. The geochemical analyses of C-O isotopes and trace elements suggest that the fluid which promotes dolomitization may probably come from the sedimentary formation water in Fengcheng Formation, which is from semi-closed shore-shallow lakes and Mg-rich marine seawater upwelling from the Permian Jiamuhe Formation and Carboniferous. The inclusion homogenization temperature indicates that the uniform temperature for calcite is among 60～70℃ while 85～105℃ for the dolomite. The rippled dolomitic rocks are penecontemporaneous,but the massive,flocculent and diffuse dolomitic rocks are metasomatized lately and the original rocks of the metasomatism are volcanic material(tuff). Volcanic glass in the volcanic tuff material is easily to take hydrolysis alteration,the products of which are montmorillonite,feldspar and quartz. Both of the alteration of montmorillonite to illite and feldspar hydrolysis can separate out plenty of Ca2+. Then,the combination of Ca2+ and CO2 or CO32- form the alteration products calcite,taking a strong dolomization reaction under the metasomatism of rich magnesium fluid environment later.
Wang Junhuai,Liu Yinghui,Wan Ce et al. Characteristics and origin of dolomitic tuffin the Permian Fengcheng Formation in Wu-Xia area of Junggar Basin[J]. JOPC, 2014, 16(2): 157-168.
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