Unique dolomite and dolomitic quartzite reservoirs occur in the Middle Permian Maokou Formation in eastern Sichuan Basin.They are commonly believed to be of "fault-controlled hydrothermal"origin.However

most of the explorary wells targeting at these reservoirs are unsuccessful

thus further study needs to be performed.The characteristics and development mechanism of these reservoirs are studied by means of comparative analysis of the outcrops on the periphery of the basin and cores inside the basin.The results indicate the main reservoir rocks in the Maokou Formation are crystalline dolomite

siliceous dolomite and dolomitic quartzite

whose distributions are mainly related to the karst systems

thus are uneven both laterally and vertically.Most of the crystalline dolomite occurs on the periphery of the karst systems

and their reservoir properties are poor.On the contrary

the siliceous dolomite and dolomitic quartzite are mostly distributed within the karst systems and have favorable reservoir properties.The development of reservoirs obeys the law of "facies-controlled karst".Specifically

grain-beach facies provided the material basis for the reservoirs

and facies-controlled eogenetic karst was the key for the formation of high-quality reservoirs.The following hydrothermal dolomitzation made it possible for the preservation of reservoir pores.The siliceous hydrothermal activities and later calcite cementation resulted in the filling of some pores.The unique siliceous dolomite and dolomitic quartzite finally came into being.According to these results

it is proposed to further strength study on "facies-controlled karst" reservoirs in addition to "fault-controlled hydrotherm"reservoirs

so as to improve the accuracy of reservoior prediction.