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.