jrye@cug.edu.cn;
Corresponding author (Second). Yang Tian; Tel. 86-27- 69111832; Email
address: 519974963@qq.com.
Abstract: Semi-open hydrous pyrolysis experiments on
coal-measure source rocks in the Xihu Sag were conducted to investigate
the carbon isotope evolution of kerogen, bitumen, generated expelled
oil, and gases with increasing thermal maturity. Seven experiments
corresponding were conducted at 335 °C, 360 °C, 400 °C, 455 °C, 480 °C,
525 °C, and 575 °C, while other experimental factors, such as the
heating time and rate, lithostatic and hydrodynamic pressures, and
columnar original samples were kept the same. The results showed that
the simulated temperatures were positive for the measured vitrinite
reflectance (Ro ), with a correlation coefficient
(R 2) of 0.9861. With increasing Ro ,
lower maturity, maturity, higher maturity, and post-maturity stages
occurred at simulated temperatures (Ts ) of 335–360 °C, 360–400
°C, 400–480 °C, and 480–575 °C, respectively. The increasing gas
hydrocarbons with increasing Ro reflected the higher gas
potential. Moreover, the carbon isotopes of kerogen, bitumen, expelled
oil, and gases were associated with an increased Ro ; among gases,
methane was the most sensitive to Ro . Ignoring the intermediate
reaction process, the thermal evolution process can be summarized askerogen0 (original) + bitumen0(original) → kerogenr (residual kerogen) + expelled oil
(generated) + bitumenn+r (generated + residual) +
C2+ (generated + residual) + CH4(generated) . Among these, bitumen, expelled oil, and
C2-5 acted as reactants and products, whereas kerogen
and methane were the reactants and products, respectively. Furthermore,
the order of the carbon isotopes during the thermal evolution process
was identified as: δ13C 1 <13C 2-5 <
δ13C expelled oil <
δ13C bitumen <
δ13C kerogen. Thus, the reaction and
production mechanisms of carbon isotopes can be obtained based on their
changing degree and yields in kerogen, bitumen, expelled oil, and gases.
Furthermore, combining the analysis of geochemical characteristics of
the Pinghu Formation coal-oil-type gas in actual strata with these
pyrolysis experiments, it was identified that this area also had
substantial development potential. Therefore, this study provides
theoretical support and guidance for the formation mechanism and
exploration of oil and gas in the Xihu Sag.
Keywords:coal-measure
source rock; Xihu Sag; semi-open hydrous pyrolysis; carbon isotope;
maturity; reaction mechanism