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

⁴⁰Ar/³⁹Ar 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 the⁴⁰Ar/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.