Tight sandstone reservoirs occur in the Palaeozoic Unayzah and Sarah Formations in Rub Al Khali Basin in the Middle East.Thin section analysis reveals that quartz sandstone is the main rock type

with some lithic sandstone

lithic arkose

lithic quartz sandstone and feldspathic quartz sandstone.The main pore type is residual intergranular pore.The cement types and their development degree are closely related with the depositional environment.Measurement of reservoir pore structure through mercury intrusion and SEM indicates that pore throat size distribution is large and the proportion of samples with small pores and narrow throats is large.The capillary pressure cures show that most samples have the poroperm characteristics of reservoir rocks.The testing data indicates that the Unayzah and Sarah formations have good re-servoir quality.On this basis

we determined the lower limits of the reservoir parameters and proposed the evaluation criteria for reservoir classification.The comprehensive evaluation indicates that the Unayzah Formation is moderate-good reservoir

with typeⅠand typeⅡ reservoirs widely developed vertically

whereas the Sarah Formation is moderate-poor reservoir

with typeⅡ and type Ⅲ reservoirs being predominate.