Oil Retention in Shales: A Review of the Mechanism, Controls and Assessment

Abstract

Shale oil is a vital alternative energy source for oil and gas and has recently received an extensive attention. Characterization of the shale oil content provides an important guiding significance for resource potential evaluation, sweet spot prediction, and development of shale oil. In this paper, the mechanism, evaluation and influencing factors of oil retention in shales are reviewed. Oil is retained in shales through adsorption and swelling of kerogen, adsorption onto minerals and storage in shale pores. Quite a few methods are developed for oil content evaluation, such as three-dimensional fluorescence quantitation, two-dimensional nuclear magnetic resonance (2D NMR), solvent extraction, pyrolysis, multiple extraction-multiple pyrolysis-multiple chromatography, logging calculation, statistical regression, pyrolysis simulation experiment, and mass balance calculation. However, the limitations of these methods represent a challenge in practical applications. On this basis, the influencing factors of the oil retention are summarized from the microscale to the macroscale. The oil retention capacity is comprehensively controlled by organic matter abundance, type and maturity, mineral composition and diagenesis, oil storage space, shale thickness, and preservation conditions. Finally, oil mobility evaluation methods are introduced, mainly including the multitemperature pyrolysis, 2D NMR, and adsorption-swelling experiment, and the influencing factors of movable shale oil are briefly discussed. The aim of this paper is to deepen the understanding of shale oil evaluation and provide a basis for further research.

Mass Balance-Based Method for Quantifying the Oil Moveable Threshold and Oil Content Evaluation of Lacustrine Shale in the Paleogene Shahejie Formation, Nanpu Sag, Bohai Bay Basin

“…Copyright 2021 Energy). (b) Concept image to show the oil existing states in shale in the initial stage of hydrocarbon generation (reproduced with permission from ref ( 21 ). This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License).…”

“…There are several methods to quantify the HET, among which the hydrocarbon generation potential method is widely applied due to its high efficiency and simplicity. 18 , 21 − 23 …”

“…Therefore, the OSI of the HET can be viewed as the OMT value of shale. There are several methods to quantify the HET, among which the hydrocarbon generation potential method is widely applied due to its high efficiency and simplicity. ,− …”

Depositional Environment Variation and Organic Matter Accumulation Mechanism of Marine–Continental Transitional Shale in the Upper Permian Longtan Formation, Sichuan Basin, SW China

“…Petroleum is an essential driver for human society development, and shale gas is gradually becoming the main focus of unconventional oil and gas exploration. − Successful exploration in marine shale of North America provides a great example for shale gas exploration; in 2020, the shale gas production of North America is approximately 7330 × 10 8 m 3 . The shale gas exploration in China started relatively late, but significant achievements had been made, such as the daily production of marine shales of Wufeng–Longmaxi in the Sichuan Basin can reach 3000 × 10 4 m 3 .…”

Quantification of Imbibed Heptane in Shale Rocks Determined by Edited T1–T2 Nuclear Magnetic Resonance Relaxation Experiments at High Magnetic Field

“…As the determination of fluid imbibition, wettability, and mobility are very complex problems, − many studies use model compounds to obtain information that contributes to the understanding of real formation fluids. Dodecane has been used as a model for light hydrocarbons to investigate, for instance, the effect of TOC on shale wettability and swelling of kerogen using binary mixtures of solvents with different solubilities and electron donor capacities. , Storage and transport properties of water and oil, modeled by n-dodecane, in crushed shale rocks were also determined by vacuum imbibition.…”