火山岩储层斜长石选择性溶蚀的岩石学特征和热力学条件

Characteristics of petrology and thermodynamics of selective dissolution of plagioclase in volcanic reservoir rocks

长石溶孔是火山岩中重要的孔隙类型之一,火山岩的主要孔隙类型中长石溶孔所占的比例在1.24%～60.61%之间,在储层中占有重要地位。从岩相学、岩石化学CIPW标准矿物成分、热力学等方面论证了斜长石的选择性溶蚀。岩相学特征表现出斜长石选择性溶蚀的特征,即钙长石溶蚀时钠长石没有溶蚀,同时出现斜长石选择性溶蚀时钠长石和钾长石生长的现象。斜长石选择性溶蚀得到了CIPW标准矿物成分的支持,在CIPW标准矿物成分中长石的端元组分主要为钠长石和钾长石,而钙长石绝大多数为0或接近0。热力学研究表明,在同样的温度、压力和地层水成分的条件下,在斜长石中优先溶解钙长石组分。当地层水中Na+和K+浓度达到饱和时发生钠长石和钾长石沉淀,出现斜长石选择性溶蚀的同时发生钠长石和钾长石生长的现象。

Dissolved hole of the feldspar is one of the most important pore types in volcanic rocks which accounts for 1.24% to 60.61% of all and plays an important role in reservoir. Authors analysed the selective dissolution of plagioclase in aspects of petrography, petrochemical CIPW standardized mineral composition and thermodynamics. Petrographic features have clearly indicated the existence of selective dissolution, anorthite dissolves while albite doesn＇t, at the same time when plagioclase dissolves, albite and orthoclase grows. Selective dissolution ofplagioclase has been supported by the evidence of CIPW standardized mineral composition where the most endmember composition ofplagioclase is albite and orthoclase, anorthite content is close to 0. It is further validated by thermodynamics study that under the same temperature, pressure and fluid conditions, anorthite is firstly dissolved within plagioclase. Deposition ofalbite and orthoclase happens when Na＋ and K~ saturates in fluid. At this time, when plagioclase dissolves, albite and orthoclase grows.