The complexity of shale oil reservoirs within alkaline lacustrine basins complicates the accurate identification of desserts. This study focuses on the Fengcheng Formation in Mahu sag of China' s Junggar basin. We carried out lithofacies typing, evaluated full- aperture reservoir space systems using high- pressure mercury injection and low- temperature nitrogen adsorption experiments, and analyzed sedimentation and diagenesis to understand reservoir formation mechanisms. Our results reveal four distinct lithofacies types in the Fengcheng Formation: stratified dolomitic siltstone, lamellar felsic shale, lamellar lime- dolomite shale, and stratified felsic- lime dolomite. Stratified dolomitic siltstone exhibits the largest full- aperture reservoir space (1. 81 cm3/g) and the highest macropore volume (0. 51 cm3/g). Its porosity is attributed to primary intercrystalline pores within microcrystalline quartz, dissolution pores in feldspar and alkaline minerals, and a significant number of dissolution and structural fractures, resulting in excellent pore- fracture connectivity among these four lithofacies types. Lamellar felsic shale, with a full- aperture reservoir space of 1. 32 cm3/g and a macropore volume of 0. 34 cm3/g, primarily forms spherical dissolution pores in talc and mica, as well as organic pores. Locally dissolved fractures contribute to moderate pore- fracture connectivity. Both Lamellar lime- dolomite shale and stratified felsic- lime dolomite exhibit intragranular pores, residual intergranular pores, and small- diameter dissolution pores. The former has a total pore volume of 0. 49 cm3/g and a macropore volume of 0. 13 cm3/g, while the latter has values of 0. 41 cm3/g and 0. 08 cm3/g, respectively. Both two types of lithofacies display poor pore- fracture connectivity. The average pore diameter in these reservoirs positively correlates with felsic mineral content and negatively correlates with dolomite content. Consequently, stratified dolomitic siltstone is identified as the dominant lithofacies. The formation of shale oil reservoirs in this setting is controlled by sedimentation and diagenesis. Fine- grained gravity flow sedimentation delivers highly porous silty sediments into the deep lake environment. Subsequent dissolution, recrystallization of silicon within the alkaline lake, and fracture formation create the excellent storage space observed in intercrystalline pore and dissolved fracture. These findings provide valuable insights for evaluating shale oil desserts in similar geological settings.