THERMODYNAMIC MODEL FOR EXPERIMENTAL SIMULATION OF DISSOLUTION OF THE CARBONATE ROCKS IN THE BURIAL ENVIRONMENTS:AN EXAMPLE FROM THE FIFTH MEMBER OF THE LOWER DEVONIAN MAJIAGOU FORMATION IN THE SHAANXI-GANSU-NINGXIA BASIN

The fifth member of the Lower Ordovician Majiagou Formation in the eastern and central areas of the Shaanxi-Gansu-Ningxia Basin as the carbonate reservoirs in-tercalated with the evaporite sediments in the tidal-flat facies consisting mainly of limestones,dolomite and gypsum(anhydrite)has undergone complex muhiphase dia-genesis,of which dissolution has an important effect on the study area.The present paper deals primarily with dissolution of the carbonate rocks in burial environments. The experiments of dissolution for dolomite were made at the temperatures of 75-130℃ and pressures of 20-30 MPa of burial diagenesis.The results have shown that the dissolution rates of dolomite inerease rapidly with the increasing temperatures and pressures.The temperature of 100℃ and pressure of 25 MPa are believed to be the most favourable conditions for dissolution of dolomite.It is estimated that the sec-ondary porosity created through dolomite dissolution in the deeper burial diagenetic environments should be more common than that formed in the epidiagenesis and shal-lower burial environments.Therefore the dolomite reservoirs in the deep-seated strata should be more ubiquitous than those in the shallow-seated strata.The analysis and calculation for the experimental results of carbonate dissolution in the burial environ-ments were also carried out for the first time based on the thermodynamic theory.The results of calculation have indicated that the limestones and dolomite can be dissolved in acetic acids at different temperatures and pressures of burial diagenesis,and the dis-solution of dolomite may occur more easily than limestones.The thermodynamic mod-el for experimental simulation of the dissolution of carbonate rocks has a great theo-retical significance and practical value in the prospecting and exploration of the oil and gas fields in the Shaanxi-Gansu-Ningxia Basin.