The sandstone from the Oligocene Xiaganchaigou Formation (E3) in the Lengdong area, Qaidam Basin, China, is extensively distributed and acts as an effective regional reservoir for hydrocarbon accumulation. Based on thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD) image analysis and cathodoluminescence (CL), a comprehensive analysis of the sedimentology, petrography, diagenesis and reservoir quality of the Oligocene sandstone reservoirs of the Xiaganchaigou (E3) Formation in the Lengdong area, Qaidam Basin, are studied. The sandstones in the Lengdong area are classified as argillaceous sandstone, silty sandstone, fine sandstone, medium sandstone, pebbly sandstone, conglomeratic sandstone and conglomerate based on their grain sizes, degrees of sorting, and matrix contents. Five sedimentary facies were identified in the Xiaganchaigou Formation (E3) of the Lengdong area, Qaidam Basin: distributary channel, interdistributary bay, mouth bar, central bar and distal sand sheet facies. Their major diagenetic processes include compaction, carbonate cementation and feldspar and calcite cement dissolution. The porosity and permeability values of the fine sandstone, conglomerate, and conglomeratic sandstone are the highest, followed by those of the medium sandstone and silty sandstone, and the argillaceous sandstone and pebbly sandstone have the lowest porosity and permeability values. The diagenetic sequence in the study area ranges from the eodiagenesis B stage to the mesodiagenetic A stage. Compaction has significantly reduced the primary porosity. Carbonate cements, mainly calcite cements, occlude pores by precipitating in intergranular and intragranular spaces. Dissolution largely contributes to increasing the secondary porosity. The well-sorted and relatively coarser-grained sandstones that formed in higher-energy sedimentary facies (distributary channel, mouth bar and central bar) have a better reservoir quality than the poorly sorted, clay-rich sandstones that formed in low-energy environments (interdistributary bay and distal sand sheet). Based on the comprehensive analysis of petrographic data, diagenesis and sedimentary facies, the reservoir in the Lengdong area is classified into three types: type A, type B and type C. Type A, located in the center of the study area, is the most favorable reservoir, while type C sandstone is the last to consider when looking for a good reservoir. This study can guide hydrocarbon exploration in the Lengdong area, as well as in northwestern China and similar areas throughout the world.
