Abstract:

The dolomitic rocks of the Permian Fengcheng Formation are formed under special geologic conditions, and are
important hydrocarbon reservoir rocks in Wu-Xia area in the Junggar Basin. Data of petrologic characteristics, SEM EDS and
stable isotope composition indicate that dolomitizing host rocks are volcaniclassic rocks. Dolomitizing fluids probably came from
brine from nearshore and shallow-water lake deposits in the Fengcheng Formation, which formed under arid climatic conditions,
and from residual Mg-rich seawater from underlying Jiamuhe Formation and Carboniferous strata. Tuff has a significant content
of plagioclase, in which anorthite and labradorite hydrolysis by CO2 can be couple with calcite precipitation. Later, Mg-rich brine
percolate and replace calcite of the first stage to precipitate dolostones. The dolomitizing fluid has been driven by the thermal
convection flow generated by volcanic eruption. At the same time, the overthrust of the Wu-Xia growth fault have speeded up the
flow of deep Mg-rich water upwards, and induced the water to quickly penetrate and horizontally migrate in the strata. Fracture
and fault play important role in controlling the formation of dolostone and the distribution of favorable reservoir.

Key words: diagenetic dolostone, volcaniclassic rocks, Fengcheng Formation, Wu-Xia area, Junggar Basin