Abstract: Based on observations of available cores,thin-sections and SEM and analysis of rock geochemistry,we studied low-temperature-burial metamorphism(alteration)of volcanics in the Lower Permian at northwestern margin of Junggar Basin.These volcanics include volcanic glass,volcanic clasts,feldspar,and mafic minerals such as pyroxene and hornblende,etc.,which originate from lavas,pyroclastic rocks,and volcaniclastic sandstones or conglomerates.In oil/gas reservoirs containing volcanics,the authigenic minerals that are common but important to pore spaces include chlorite,zeolites(like analcime,heulandite,and laumontite),calcite,albite,and authigenic quartz.Based on the paragenetic relationship of minerals and the composition and migration of elements,we analyzed the microscopic characteristics and origins of authigenic minerals,and established diagenetic evolution models during volcanic alteration.The alteration can be summarized as follows:①volcanic glasses hydrolysis-montmorillization-zeolitization-albitization;②chloritization related to mafic minerals and smectites;③transformation of feldspar to chlorite,calcite,and finally to albite.The alteration of volcanics and the formation of authigenic minerals in the study area were controlled by various factors such as the precipitation of authigenic minerals,composition of neutral-basic host rocks,temperature and pressure,and the chemical behaviors of pore water.The study on alteration process,alteration products,and control factors is helpful to understand the mechanisms that cause the differences of volcanics-bearing reservoir qualities,so as to provide a scientific geological foundation for the predication of high-quality reservoirs.

Key words: authigenic mineral, alteration, diagenetic evolution, volcanics, hydrocarbon reservoir, Junggar Basin