自生伊利石^(40)Ar/^(39)Ar法定年技术及气藏成藏期的确定

^(40)Ar/^(39)Ar Dating of Diagenetic Illites and Its Application in Timing Gas Emplacement in Gas Reservoirs

为了确定天然气藏的成藏期 ,进行了自生伊利石的40 Ar/3 9Ar法同位素定年实验 .实验重点解决了自生伊利石的40Ar/3 9Ar法定年的几个技术难题 :第一 ,粘土矿物的提纯 ,避免伊利石以外的含K矿物混入 ;第二 ,自生伊利石与碎屑伊利石的分离 ;第三 ,克服核反冲造成的Ar原子丢失 .利用冷冻 -加热循环碎样技术获得高纯度的粘土矿物 ;通过阶段加热得到的年龄谱可以区分自生伊利石与碎屑伊利石 ;利用“显微包裹”技术有效克服了核照射反冲问题 .对鄂尔多斯盆地北部苏里格气田研究发现 ,二叠系储层中的伊利石有 2种年龄图谱 :一种只有自生伊利石的坪年龄 ;另一种图谱既有自生伊利石的坪年龄 ,也有碎屑伊利石的年龄 ,形成二阶式的图谱 .通过自生伊利石的形成时间推断 ,天然气的最早充注时间晚于 16 9～ 189Ma .实验的结果表明 ,冷冻 -加热循环碎样技术可以有效地避免伊利石以外的含K矿物混入 ,是获得高纯度粘土矿物的关键技术 ;自生伊利石40 Ar/3

40Ar/39 Ar dating of diagenetic illites is performed in order to determine the beginning time of gas emplacement in gas reservoirs. A series of technical problems in the 40Ar/39Ar dating of diagenetic illites have been solved in this experiment: firstly, the purification of clay minerals in order to avoid detrital contaminants; secondly, the separation of diagenetic illites from detrital illites; thirdly, Ar recoil problem, avoiding Ar loss from samples. The high refinement of clay minerals was obtained using a gentle disaggregation technique with repetitive freezing-heating cycles. Diagenetic illites were separated from detrital illites through the 40Ar/39Ar age spectra of step-heating. Ar recoil problem was solved by microampoule encapsulation technique. Two kinds of age spectra were recognized during the dating of illites in the Permian reservoir sandstone in the Sulige gas field in the north of the Ordos basin, China. The first kind showed a plateau age of diagenetic illites only. The second kind showed an age of detrital illites as well as a plateau age of diagenetic illites, with a notable two-staircase age spectra. The 40Ar/39Ar dating of illites suggests that the earliest gas accumulation occurred later than 169-189 Ma. This result shows that the freezing-heating cycle technique of sample disaggregation is a key technique for obtaining the high refinement of clay minerals, which effectively avoids contamination from K-containing minerals. The technique of 40Ar/39Ar dating has been improved by a series of techniques as freezing-heating disaggregation, diagenetic illite recognition, and microampoule encapsulation, and hence can be used to determine the beginning time of gas emplacement.