Abstract
Isotopic rollover refers to that δ13C value of a gas component decreases with maturity. Its occurrence is closely related to high productivity of shale gas. Isothermal confined pyrolysis experiments (gold capsules) were performed to simulate this phenomenon on whole rock Lucaogou and kerogens Saergan, Wuerhe and Fengcheng in the absence (anhydrous) and presence of added water (hydrous) at 50 MPa, 372 °C and heating duration 0–672 h, corresponding to 0.96–1.85 EASY%Ro. For kerogen Saergan isolated from source rock with hydrogen index (HI) 159 mg/g TOC and 1.10–1.30% Ro equivalent, none of δ13C1, δ13C2 and δ13C3 showed any rollover in both anhydrous and hydrous experiments. For Lucaogou whole rock with HI 856 mg/g TOC and 0.50–0.60%Ro, both δ13C2 and δ13C3 showed rollover in anhydrous experiments while all δ13C1, δ13C2 and δ13C3 showed rollover with greater magnitude in hydrous experiments starting at 1.49–1.64 EASY%Ro. For kerogens Wuerhe and Fengcheng isolated from source rocks with HI of 550 and 741 mg/g TOC, and 1.18 and 0.96%Ro respectively, both δ13C2 and δ13C3 demonstrated rollover in anhydrous experiments while only δ13C2 showed rollover with minor magnitude in hydrous experiments starting at 1.47–1.53 EASY%Ro. The different effects of water on isotopic rollover among samples Lucaogou, Wuerhe and Fengcheng can be ascribed to rate related isotopic fractionation. Higher generation rate leads to minor isotopic fractionation and rollover magnitude. It was suggested that isotopic rollover likely occurs in a source rock having higher amount of initial retained oil prior to bulk oil cracking and currently within the major stage of oil-cracking to gas (1.50–2.00%Ro).
Highlights
Isotopic rollover refers to that δ13C value of a gas component decreases with maturity
We aimed at the influence of the amount of the retained oil on the occurrence of isotopic rollover of gas components in source rocks, and performed confined pyrolysis experiments on four samples to simulate the isotopic rollover in laboratory
We suppose that kerogen Saergan had suffered a heating process prior to present pyrolysis experiment, e.g., from 250 °C to 386 °C or 404 °C at 2 °C/h, achieving a thermal stress of 1.10 or 1.30 EASY%Ro
Summary
Isotopic rollover refers to that δ13C value of a gas component decreases with maturity. For the experiments on whole rock Lucaogou and kerogens Wuerhe and Fengcheng, the yields of wet gases (C2 to C5) do not show any clear decreasing trend, and have not reached the maximum values
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