Molecular geochemical evaluation of shale oil mobility: a case study of shale oil in Jiyang Depression

Shale hydrocarbon potential, shale oil mobility and shale compressibility are three important factors of geological evaluation for shale oil exploration and development. Among them, shale oil mobility is constrained by the pressure system, shale hydrocarbon potential, shale oil composition and occurrence space. Since there were no standardized methods or criteria for shale oil movability evaluation, a combination of multiple parameters (pyrolysis parameter S₁, S₁/w(TOC) and median pore-throat radius) together with exploration and development practice have been used for comprehensive characterization. Based on the experimental analysis of a large number of core samples for shale oil exploration in the Jiyang Depression, a coupling relationship among shale oil molecular composition, shale hydrocarbon potential and shale oil occurrence space was found, and a molecular geochemical evaluation model for shale oil mobility was established. The study revealed that when the ratio of ∑nC_20^-/∑nC_21⁺ was<1, shale pore throat radius was generally>20 nm. Under this condition, pyrolysis S₁ was usually>3 mg/g and S₁/w(TOC) was>100. The ratio of ∑nC_20^-/∑nC_21⁺ basically remained constant with the increase of S₁, the ratio S₁/w(TOC) and shale pore throat radius. This indicated that when shale pore throat radius is large, hydrocarbon molecular diffusion is not affected by diffusion energy barriers, and shale oil molecule occurrence in pore throats is relatively homogeneous, so shale oil has a good fluid mobility. When the ratio of ∑nC_20^-/∑nC_21⁺ was>1, shale pore throat radius was usually<20 nm, pyrolysis S₁ was commonly<3 mg/g, and the ratio of S₁/w(TOC) was<100. The ratio of ∑nC_20^-/∑nC_21⁺ was increasing quickly with the decrease of S₁, S₁/w(TOC) and shale pore throat radius. It indicated that when the shale pore throat radius becomes small, diffusion energy barriers limited the diffusion of macromolecular hydrocarbon components in pore throats, thus shale oil has a poor fluid mobility and low molecular weight hydrocarbon is the main constituent to flow easily. In such a condition, shale has a poor content of free oil. The lowest limit of pore-throat radius for shale oil flow in the Jiyang Depression is approximately 20 nm.