Development characteristics and transformation mechanism of clay minerals in the organic-rich shale layers of saline-alkali lake:A case study of Fengcheng Formation in Mahu Sag,Junggar Basin
To systematically study the development characteristics and transformation modes of clay minerals in alkaline lake shale,and explore the development of clay minerals in alkaline lake sedimentary environments,Based on scanning electron microscopy(SEM)observation and X-ray diffraction,this paper investigates the characteristics and dynamic transformation of clay minerals in different sedimentary environments of the Fengcheng Formation shale in Mahu Sag.In the sedimentary center of the alkaline lake,only a small amount of clay minerals(<5%)are developed,mainly characterized by smectite(S)and illite-smectite mixed layers(I/S).The detrital clay minerals were dissolved in a strongly alkaline diagenetic environment,transforming into au-thigenic silicate minerals such as K-feldspar.Moreover,the presence of CO32-/HCO3-can delay the transforma-tion of clay minerals in the sedimentary center.In the edge slope zone,the content of clay minerals(average 11.57%),which is mainly the illite(I),chlorite(C),and I/S is higher than that in the sedimentary center.The diagenetic environment is transformed from strongly alkaline to weakly alkaline-weakly acidic,and the primitive detrital clay minerals have not been dissolved and can be preserved in the early diagenetic stage,and il-litization and chloritization occurr with the increase of stratigraphic temperature and pressure.The transforma-tion of clay minerals can produce many brittle minerals such as authigenic quartz and K-feldspar,which not only increases the brittleness of shale reservoirs but also generates secondary pores and fractures,and is beneficial for the exploration and development of shale oil and gas.
Clay mineralsDiagenetic evolutionAlkaline Lake BasinFengcheng FormationMahu Sag
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