Thin section and argon-ion polishing-scanning electron microscope observations were used to analyze the sedimentary and diagenetic environments and main diagenesis of the Permian Fengcheng Formation shales in different depositional zones of Mahu Sag in the Junggar Basin,and to reconstruct their differential diagenetic evolutional processes.The diagenetic environment of shales in the central region of alkaline lake kept alkaline,which mainly underwent the early stage(Ro＜0.5％)dominated by authigenic Na-containing carbonates and K-feldspar and the late stage(Ro＞0.5％)dominated by replacement of Na-containing carbonates by reedmergnerite.The shales from the gentle slope zone of lake margin underwent a transition from weak alkaline to acidic diagenetic environments,with the early stage dominated by authigenic minerals of Mg-containing clay and silicoide and the later stage dominated by the dissolution of feldspar and carbonate minerals.The shales from the transitional zone also underwent a transition from an early alkaline diagenetic environment,evidenced by authigenic dolomite and zeolite,to a later acidic diagenetic environment,represented by reedmergnerite replacement and silicification of feldspar and carbonate mineral.The differences in forming authigenic minerals during early stage determine the fracability of shales.The differences in dissolution of minerals during late stage control the content of free shale oil.Dolomitic shales and silty shales from the transitional zone-marginal zone of alkaline lake have relatively high content of free shale oil and strong fracability,and are favorable"sweet spots"for shale oil exploitation.
维普
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