Pyrolysis of oil at high temperatures: Gas potentials, chemical and carbon isotopic signatures

Although the gas cracked from oil has been believed to be one of the important sources in highly matured marine basins, there are still some debates on its resource potentials and chemical and isotopic compositions. In this study a Cambrian-sourced marine oil sample from the Silurian reservoir of well TZ62 in the central Tarim basin was pyrolyzed using sealed gold tubes with two different pyrolysis schemes: continuous pyrolysis in a closed system and stepwise semi-open pyrolysis. The results show that the maximum weight yield of C_1–5 gases occurs at EasyRo=2.3% and the residual gas potential after this maturity is only 43.4 mL/g, about 12% of the yield of 361 mL/g at EasyRo=2.3%. Combined with the results of kinetic modeling, the main stage of gas generation from oil cracking is believed within the EasyRo=1.6%–2.3%. The increase in the volume yield of C_1–5 gases at EasyRo>2.3% in a closed system is mainly related to the re-cracking of previously formed C_2–5 wet gases, not the direct cracking of oil. The stepwise pyrolysis experiments show that the gas from the cracking of residual oil at EasyRo>2.3% is characterized by very high dryness index (higher than 92%) and heavy methane carbon isotopes ranging from −28.7% to −26.7%, which is quite different from the gases from the continuous pyrolysis in a closed system. The kinetic modeling of methane carbon isotope fractionation shows that the carbon isotopes of methane within the main stage of gas generation (EasyRo<2.3%) are far lighter than the carbon isotopes of the precursor oils under a geological heating rate of 2 °C/Ma. The above observations and results provide some new clues to the accurate recognition and objective resource evaluation of oil cracking gas in highly mature marine basins. Supported by Key Project of the Chinese Academy of Sciences (Grant No. KZCX2-YW-114-2), National Natural Science Foundation for Distinguished Young Scholars (Grant No. 40625011), National Natural Science Foundation of China (Grant No. 40802025) and Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. 5407341801)