In recent years, the Upper Paleozoic Fengcheng Formation of the Mahu sag has been the new exploration target of shale oils in the Junggar basin, NW China. However, due to its highly alkaline depositional condition, the shales of the Fengcheng Formation have suffered intense diagenetic modification and the primary lithology of its shales remains uncertain. XRD results reveal that the shales of the Fengcheng Formation are characterized by low contents of clay minerals (<10%) and high contents of felsic minerals (60%~80%). Thin sections show that there are three types of felsic shales in the Fengcheng Formation. The first type is composed of detrital silt- sized (4~63 μm) quartz and feldspar grains, typically defined as siltstone. The second type of felsic shales contain abundant cherts, which are siliceous shales. The third type of felsic shales mainly consist of cryptocrystalline felsic matrix. The rare existence of vitric pyroclasts and clays make it difficult to classify the third type of felsic shales of Fengcheng Formation as either mudstone, siltstone or tuffite. Backscattered electron imaging and high- resolution scanning electron microscopy reveal that the matric felsic minerals are mainly composed of authigenic and detrital quartz, albite, and K- feldspar. Detrital K- feldspar and albite have been intensively replaced by authigenic quartz and albite. Authigenic euhedral K- feldspar, albite, and quartz crystals, range from 2 μm to more than 10 μm in size. These phenomena together suggest that parts of quartz and feldspar minerals in the shales of the Fengcheng Formation are authigenic, not detrital in origin, which is different from the felsic shales in other lacustrine basins in China. After reviewing the mineral compositions and diagenetic modifications of sediments in modern and Cenozoic alkaline lakes over the world, this study proposes that the felsic shales of the Fengcheng Formation resulted from multiple diagenetic alteration of original clayey shales. Detrital clays and silt- sized quartz, as well as occasionally ashes, reacted extensively with saline, alkaline solutions and contributed to the formation of SiO2- rich and Al- rich gels. This was the key process responsible for the disappearance of clay minerals and volcanic materials in the shales of the Fengcheng Formation. Metastable silicate and silica minerals precipitated from the gels and gradually transformed to more stable K- feldspar to albite. The modified felsic shales in the Fengcheng Formation, with abundant quartz and feldspars and little clay minerals, largely increased intercrystalline porosity and the brittleness of shales and contributed to the formation of high- quality shale reservoirs.