Isothermal confined pyrolysis on source rock and kerogens in the presence and absence of water: Implication in isotopic rollover in shale gases

Isotopic rollover refers to that δ(13)C 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 δ(13)C(1), δ(13)C(2) and δ(13)C(3) 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 δ(13)C(2) and δ(13)C(3) showed rollover in anhydrous experiments while all δ(13)C(1), δ(13)C(2) and δ(13)C(3) 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 δ(13)C(2) and δ(13)C(3) demonstrated rollover in anhydrous experiments while only δ(13)C(2) 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).