Using rock-eval S4Tpeak as thermal maturity proxy for shales
Bodhisatwa Hazra, Deependra Pratap Singh, Prasenjeet Chakraborty, Pradeep Kumar Singh, Santi Gopal Sahu, A.K. Adak
Abstract
The Rock-Eval pyrolysis-stage derived parameters such as free hydrocarbons (S1), heavier pyrolysis-hydrocarbons (S2), pyrolyzable carbon (PC) and pyrolysis T max (from S2 curve) have received considerable interest for source-rock screening and thermal maturity assessment. On the other hand, the Rock-Eval oxidation-stage S4CO 2 curve, which gives the amount of residual carbon (RC), only recently has received some interest. While the pyrolysis-stage S2 temperature-peak (T max ) is conventionally used as a maturity proxy, in this work we show that the temperature-peak of S4CO 2 curve (S4T max ) can also be used as a thermal maturity proxy for shales. For overmature and low-TOC shale samples, showing asymmetric S2 shape and concomitantly producing doubtful T max , the S4 curves showed symmetric nature and consequently the S4T max was observed to be a reliable thermal maturity estimate. While the S4T max clearly resolved immature and overmature shales, for the early mature and peak mature shales the S4T max showed overlapping values. S4T max of pre-pyrolyzed and pyrolyzed masses showed good positive correlation with differential scanning calorimetry temperature-peak (DSCT peak ), and consequently indicated its applicability as a thermal maturity proxy. When early mature pre-pyrolyzed samples were directly analyzed using the Rock-Eval oxidation stage, the S4 curves showed formation of two sub-peaks, and consequently the T max was observed to decrease. It is recommended that analysts and interpreters should thoroughly cross-check S2 curves before reporting data, and in case of asymmetric or unreliable S2 curves, the S4T max can be used as a maturity proxy.