Observation of the Middle Permian outcrops and thin sections of cores from typical wells in Sichuan Basin and its periphery reveals that the Middle Permian consists mainly of micro-dolomite

medium-macro-crystalline dolomite and calcitic dolomite.And the petrophysical properties analysis shows that reservoir quality of the medium-macro-crystalline dolomite is the best

so it can act as effective reservoir.

Various geochemical analyses in combination with the geological background indicate the medium-macro-crystalline was formed by the deep thermal fluids.There are five major evidences concerning their genesis.The medium-macro-crystalline shows the characteristics of saddle-dolomites and is associated with some low temperature hydrothermal minerals;Gas-liquid hydrocarbon fluid inclusions with homogenization temperature higher than the surrounding rocks occur in the medium-macro-crystalline

and the salinity of two phase fluid inclusions tested based on the freezing point temperature is three to five times of normal sea water

indicating high salinity digenetic fluids;The stable isotope values are extremely biased

and they all are less than -9‰ and mostly are up to -13‰;Their Sr isotopes have the characteristics of mantle-derived medium-macro-crystalline dolomite

indicating the existence of deep thermal fluids;The lateral and vertical distributions of dolomite vary dramatically

and dolomites are found to distribute along the deep-large faults and in areas with volcanic rocks.Most of the intergranular dissolved pores of medium-macro-crystalline dolomite are filled by asphalt.Comprehensive analysis based on the bury history of typical wells

homogenization temperature of fluid inclusions and hydrocarbon generation history of source rock reveals that the medium-macro-crystalline dolomites were formed early

at least prior to the Middle-Late Triassic

when large amount of hydrocarbons were charged.Based on the above mentioned analysis

a hydrothermal genetic model was established for the medium-macro-crystalline dolomite was established by using the typical wells and seismic profiles.