Thermodynamic Analysis Of Feldspar Diagenesis In The Upper Paleozoic Reservoir In Ordos Basin

The Upper Paleozoic in Ordos Basin is important horizon of exploration for natural gas,the reservoir is generally a tight reservoir,sandstone had experienced stronger diagenesis,the alteration of feldspar during the diagenetic process change the characteristics of sandstone and physical properties of the reservoir.Therefore,it will be beneficial to establish prediction model of the Upper Paleozoic in Ordos Basin what find out the change of feldspar during the diagenetic process.About the change of feldspar during the diagenetic process by previous research,most of the scholars think it is dissolution of feldspar,the author put the slice analysis as the main means combined with microscopic observation of casting thin sections,cathode luminescence and SEM,and thermodynamics for dissolution of feldspar are analyzed by the theories of thermodynamics from the phenomenon of process of diagenesis of feldspar starting with,and the alteration sequence of the anorthite,the albite and the potash feldspar are analyzed by analyzing the range of temperature for the the alteration of feldspar,and the main conclusions are as follows:1)Diagenesis of feldspar of the Upper Paleozoic in Ordos Basin is mainly appeared as the metasomatism and dissolution of feldspar,the main diagenesis include carbonation of the anorthite,carbonation of the albite,kaolinization of the albite,illitization of the albite,kaolinization of the potash feldspar,illitization of the potash feldspar.2)Based on calculation of Gibbs free energy and analysis of enthalpy change and entropy change what is reaction process of diagenesis of feldspar,the increment of Gibbs free energy of these reaction process of diagenesis of feldspar are all below zero what include carbonation of the anorthite and kaolinization of the albite in stage of biogas,kaolinization of the potash feldspar and kaolinization of the albite in the early stage of cracking gas,illitization of the potash feldspar and illitization of the albite in a closed system of the stage of cracking gas under the pressure and temperature of the process of burial diagenesis,namely these reaction can react spontaneously during diagenetic process,but it beneficial to kaolinization of the albite at lower temperature;it beneficial to illitization of the potash feldspar at higher temperature.3)By thermodynamic analysis of dissolution,results indicate that stability of the anorthite,the albite,the potash feldspar are higher than stability of calcite,so calcite has the properties of easy dissolution compare to those feldspar.It is thought that dissolution pore are mainly caused by the dissolution of calcite that is formed by carbonation of feldspar.4)By thermodynamic analysis,the range of temperature for carbonation of the anorthite is about 52?;the range of temperature for carbonation of the albite is about 57?;the range of temperature for kaolinization of the albite is about 73?;the range of temperature for illitization of the albite is about 163?;the range of temperature for kaolinization of the potash feldspar is about 122?;the range of temperature for illitization of the potash feldspar is about 162?.So the main feldspar's diagenetic reaction sequence is: carbonation of the anorthite ? carbonation of the albite ? kaolinization of the albite ? kaolinization of the potash feldspar ? illitization of the potash feldspa ? illitization of the albite.By research of the Upper Paleozoic in Ordos Basin,the anorthite produced calcite in stage of biogas and accompanied with a small amount of carbonation of the albite;kaolinization of the albite in the middle stage of biogas;kaolinization of the potash feldspar in the early stage of cracking gas;illitization of the potash feldspar in a closed system of the early or the middle stage of cracking gas and accompanied with illitization of the albite.