Thermal maturity evaluation by sonic log and seismic velocity analysis in parts of Upper Assam Basin, India

Vitrinite reflectance is an important index used in assessment of thermal maturity of petroleum source rocks. Vitrinite reflectance typically increases with depth of burial as a function of time and temperature and is accompanied by a decrease in log-derived interval transit time. The relationship between these parameters has been investigated in the Upper Assam (Tertiary) Basin, India. An equation that relates sonic log transit time to vitrinite reflectance has been derived. Thus, interpretation of the sonic log enables a quantitative estimation of thermal maturity that is not affected by the problems frequently encountered in measurement and interpretation of vitrinite reflectance data. Further, the approach has been extended to the determination of thermal maturity from seismic velocity analysis. The results suggest that seismic data can be used as an additional maturity evaluation tool which can provide preliminary estimates of maturity in undrilled areas of the basin.