致密砂岩气藏地层水地球化学特征——以天环坳陷北段上古生界为例

Geochemical Characteristics of Formation Water in Tight Sandstone Gas Reservoir:Taking Upper Paleozoic in Northern Part of Tianhuan Depression as an Example

采用砂岩岩石铸体薄片、扫描电镜等方法对地层水赋存介质特征进行研究,通过常规离子浓度测试结果分析地层水的地球化学性质,探讨储层岩石学特征、成岩作用与地层水地球化学性质之间的成因联系,揭示天环北段上古生界山1—盒8段致密砂岩气藏地层水地球化学特征、成因及其地质意义。结果表明:①地层水为CaCl_(2)型,Cl^(-)、Na^(+)和Ca^(2+)是主要的离子类型,其浓度控制了地层水总矿化度,具有Ca^(2+)富集、Mg^(2+)和Na^(+)亏损特征;②地层水地球化学性质与成岩作用密切相关,浓缩作用是影响地层水矿化度的主要因素,火山岩岩屑溶蚀是Ca^(2+)富集的主要原因,方解石白云石化和高岭石绿泥石化是Mg^(2+)亏损的主要原因,Na^(+)来源不足和吸附作用是Na^(+)亏损的主要原因;③地层水具有低钠氯系数、高变质系数和低镁钙系数特征,变质程度高,表明水岩作用强烈,为较封闭的水体环境,有利于天然气藏的保存;④地球化特征的成因决定地层水地球化学参数与储层质量的关系,高渗透率砂岩的地层水中以Ca^(2+)含量低、Mg^(2+)含量高为主要特征,具有较低的变质系数和较高的镁钙系数。

The characteristics of occurrence medium of formation water were studied by means of casting thin section of sandstone rock and scanning electron microscope,The geochemical properties of formation water were analyzed by conventional ion concentration testing,the genetic relationship between the petrological characteristics and diagenesis of reservoir and the geochemical properties of formation waterwere discussed,and the geochemical properties and genesis of formation water and their geological significance in the tight sandstone gas reservoir of the upper Paleozoic Shan1-He 8 member inTianhuannorthern section are defined.The results show that:(1)The formation water is CaCl_(2) type,and Cl^(-),Na^(+) and Ca^(2+) are main ions.Their concentration determines the total salinity of formation water,which characterizesby enrichment of Ca^(2+) and lack of Mg^(2+) and Na^(+).(2)The geochemical properties of the formation water are closely related to the characteristics of its occurrence medium,concentrating is the main cause of affecting formation water salinity,debris dissolution of volcanic rock is the main cause of Ca^(2+) enrichment,the dolomitization of calcite and the chlorite of kaolinite are the main causes of lack of Mg^(2+),and the insufficient sources and the adsorption of Na^(+) are the main causes of Na^(+) lack.(3)The formation water has low sodium-chloride coefficient,high metamorphic coefficient and low magnesium-calcium coefficient.The high metamorphic degree indicates that the hydrolithogenesis is strong,and the water environment is relatively closed,which is conducive to the preservation of natural gas reservoir.(4)The genesis of the geochemical characteristics of the formation water determines the relationship between the geochemical parameters of formation water and reservoir quality.The formation water in high permeability reservoirs is mainly characterized by low content of Ca^(2+) and high content of Mg^(2+),and is of a low metamorphism coefficient and a high magnesium-calcium coefficient.