The Mesozoic Khorat Group in NE Thailand consists of a series of continental red-beds, divided here into five formations, which unconformably overlie the lithologically-similar Nam Phong Formation. The reservoir quality of the Khorat Group decreases progressively with increasing age, owing to burial compaction and diagenesis. Compactional fabrics suggests a maxim um burial depth of more than seven km, although the Sao Khua Formation (in the middle of the Khorat Group) shows early calcite cements which stabilised the sediment fabric at burial depths of about one km. Porosities vary from 11% in the uppermost part of the Khorat Group (Khok Kruat Formation) to 4.9% in the Nam Phong Formation. The ratio of secondary grain-dissolution porosity to primary porosity increases with age (and depth). Authigenic minerals consist of quartz (1.3 -9.3%), calcite (0-26.5%) and kaolinite (0-4.0%). The occurrence of detrital clays (0.8-9.6%) in particular causes downgrading of the porosity of many of the sandstone samples examined.