Accumulation contribution differences between lacustrine organic-rich shales and mudstones and their significance in shale oil evaluation

The differences in organic matter abundance and rock composition between shale and mudstone determine the discrepancy of their contributions to the formation of conventional and shale oil/gas reservoirs. The evaluation criteria of source rocks are different in the future exploration in self-sourced petroleum systems. Shales are deposited in deep/semi-deep lacustrine, with low sedimentation rate and chemical depositions of various degrees, while mudstones are mostly formed in shallow lacustrine/lakeside, with high deposition rate and density flow characteristics. Three factors contribute to the enrichment of organic matter in shales, including the “fertility effect” caused by volcanic ash deposition and hydrothermal injection, excessive and over-speed growth of organisms promoted by radioactive materials, and deep-water anaerobic environment and low sedimentation rate to protect the accumulation of organic matter from dilution. Lamellations in shales are easy to be stripped into storage space, and acid water produced during hydrocarbon generation can dissolve some particles to generate new pores. The massive mudstones with high clay content are of poor matrix porosity. Shales with high total organic carbon, developed laminations, relatively good reservoir property, and high brittle mineral content, are the most favorable lithofacies for shale oil exploration and development. It is necessary to conduct investigation on the differences between shale and mudstone reservoirs, to identify resources distribution in shale and mudstone formations, determine the type and standard of “sweet-spot” evaluation parameters, optimize “sweet-spot areas/ sections”, and adopt effective development technologies, which is of great significance to objectively evaluate the total amount and economy of shale oil resources, as well as the scale of effective exploitation.