Oil physical status in lacustrine shale reservoirs - A case study on Eocene Shahejie Formation shales, Dongying Depression, East China

A suite of 24 lacustrine shale core samples from the Eocene Shahejie Formation of the Dongying Depression, East China have been geochemically and petrologically characterized to investigate shale oil production potential and its constraints. Rock-Eval analysis indicates that majority samples are organic-rich with average TOC content of 3.69 wt%, dominated by Type II kerogen at early to peak oil generation stage. Shale oil physical status was defined as free, adsorbed and residual oils based on sequential extraction from different particle sizes under cold and Soxhlet extraction conditions. Quantitative data illustrate that free oil dominates the total extractable organic matter (EOM) yields (with an average of 66%). EOM bulk compositions are dominated by the saturated hydrocarbons, which decreases sequentially from free to residual status, corresponding to the increase of polar fractions. Several major factors, including TOC content, brittleness index, clay mineral content and thermal maturity, which may control relative proportions of different status oils in shale reservoirs are discussed herein. TOC and clay mineral content exert critical impact on adsorption ability and retention proportion, while brittleness index shows a positive correlation to EOM amounts. Thermal maturity plays the principle role in controlling hydrocarbon generation and then the expulsion behavior which in turn have major impacts on the total extract yields. Relatively low maturity coupled with high clay mineral contents were considered as main restrictions on shale oil producibility in the study area. Our sequential extraction results also demonstrated marked compositional fractionation during primary migration in shale reservoirs.