Liquid saturation evaluation in organic-rich unconventional reservoirs: A comprehensive review

Unconventional organic-rich reservoirs, including oil and gas shales, are the type of rocks located in the subsurface and bearing considerable hydrocarbon resources. Shale gas revolution that started in 2011 raised the question of gas and oil shales properties research all over the world. However, still one of the most critical questions on the subject is the problem of oil and water saturation evaluation in shale reservoirs. An expanded literature survey improved understanding of the current state of the problem, available solutions, including methods, and method-based workflows. Analysis of the obtained information revealed an absence of a universal technique to solve the problem. Application of multi-frequency nuclear magnetic resonance (NMR) coupled with an NMR rock core analyzer optimized for shales forms workflows providing the complete information on the type and volume of fluids present in the pore space of oil shales. The methods delivering incomplete (partial) information include retort analysis, Dean-Stark extraction, rock evaluation (Rock-Eval) pyrolysis, electrical resistivity, and dielectric permittivity measurements. This review aims to provide with comprehensive information on the currently existing methods and workflows used to characterize oil and water saturation in organic-rich unconventional reservoirs.