DISSOLUTION AND INFILLING OF THE ORDOVICIAN CARBONATE ROCK RESERVOIRS IN THE GAS FIELDS IN THE CENTRAL PART OF THE SHAANXI-GANSU-NINGXIA BASIN

The gas fields in the central part of the Shaanxi-Gansu-Ningxia Basin occur in the car-bonate rock reservoirs in the weathered crust on the top of the Ordovician strata.The stor-age spaces of the carbonate rocks resulted from the interaction of multistage dissolution and infilling,which may be divided into four stages on the basis of the development and evolution.(1) The near-surface early diagenetic dissolution and infilling (O₁);(2) the disso-lution and infilling of the weathered crust (O₂-C₁);(3)the shallow burial dissolution and infilling (C₂),and (4)deep burial dissolution and infilling (P-Q).The formation orders and stages of the chemically precipitated authigenic minerals such as calcite,dolomite,clay min-erals,quartz and gypsium have been distinguished in terms of the study of their infilling or-ders and geochemical properties.Finally,the present paper has dealt with the constructive and destructive effects of dissolution and infilling in different stages on the formation of the storage spaces.