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).