This paper studies the characteristics

diagenesis and forming mechanism of the carbonate reservoirs in the Lower Triassic Feixianguan and Jialingjiang formations in the middle part of the western Sichuan Depression based on the outcrop observation and microscopic analysis of core samples

and in combination with the test results of physical property

trace element

and carbon and oxygen isotope analysis of some samples. The results show that grain bank reservoirs are developed in some intervals of the Lower Triassic and they are dominated by mid-lowporosity and lowpermeability reservoirs. The reservoir lithologies are dominated by oolitic dolomite

oolitic limestone

and micritic dolomite with residual oolitic psammitic texture. Diagenesis of the grainy carbonates in the Feixianguan Formation varies a lot from northeast to southwest

changing from cementation in submarine diagenetic environment to dissolution and cementation in burial diagenetic environment. The Jialingjiang Formation is dominated by diagenesis in burial environment. Comprehensive study shows that dissolution and dolomitization are the key factors that control the development of grainbank reservoirs. Petrological and geochemical characteristics indicate that some oolitic dolomite and dolorenite of the Feixianguan and Jialingjiang formations are the products of burial dolomitization

and that burial dissolution might be caused by the mixing and cooling of basin brine.