6. Conclusions
This study focused on the carbon isotopic evolution of kerogen, bitumen, expelled oil, and gases during the pyrolysis of coal-measure source rocks in the Xihu Sag under semi-open conditions. In addition, we investigated the relationships between their carbon isotopes and maturity. As a result, the following conclusions were drawn:
Four thermal evolution stages occurred in the pyrolysis samples. 1) Lower maturity stage at Ro = 0.82–1.09% (Ts = 335–360 ℃), characterized by rapid oil and lower gas generation. 2) Maturity stage at Ro = 1.09–1.65% (Ts = 360–400 ℃), characterized by the generation of lighter liquid hydrocarbons and wet gaseous hydrocarbons. 3) Higher maturity stage at Ro = 1.65–2.3% (Ts = 400–480 ℃), characterized by the thermal cracking of oil to wet gases. 4) Post-maturity stage at Ro = 2.3–3.24% (Ts = 480–575 ℃), characterized by the second cracking of oil-gas to methane.
Combining the carbon isotopes of kerogen, bitumen, expelled oil, and gases during thermal evolution, it was found that there was an obvious relationship between them. 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) . The process contained both reactants and products, and only methane remained the product in each case. Therefore, the carbon isotope of methane was the lightest, and the general order was δ13C 1 <13C 2-5 < δ13C expelled oil < δ13C bitumen < δ13C kerogen. During the thermal evolution of organic matter into hydrocarbons, 12C and13C were primarily enriched in the former and latter generated products, respectively. Among the products, the correlation between the methane carbon isotope and Ro was the most obvious, indicating that the methane carbon isotope can better characterize the corresponding maturity. Thus, based on the pyrolysis products and the carbon isotopes of oil and gases, we can clearly define the different formation mechanisms at different thermal evolution stages.
Finally, by combining the analysis of geochemical characteristics of the Pinghu Formation coal-oil-type gas in actual strata with these pyrolysis experiments, it was concluded that this area also had substantial development potential.
Acknowledgments:
We thank CNOOC Shanghai Branch for providing valuable opportunity for sampling and geological data. We thank Sinopec Wuxi Institute of Petroleum Geology for providing valuable opportunity for experimental conditions and technical support. In addition, this study was supported by National Science and Technology Major Project of China (No.2016ZX05027001-005),National Natural Science Foundation of China (No. 40172051).