Maturity limit of sweet spot area for continental matrix type shale oil: a case study of lower Es₃ and upper Es₄ sub-members in Dongying Sag, Bohai Bay Basin

The matrix type shale oil will become the main contributer for increasing shale oil reserves and production in China. Maturity is an important factor for the controlling to the distribution of sweet spot area for matrix type shale oil. The vitrinite reflectance suppression ranges of continental organic rich shale with different organic matter type were analyzed, and the maturity limit of sweet spot area for continental matrix type shale oil was discussed taking the lower Es₃ (third member of Shahejie Formation) and upper Es₄ (fourth member of Shahejie Formation) sub-members of the Dongying Sag as a case study. The vitrinite reflectance suppression ranges of continental organic rich shale are positively correlated to organic matter types. The actual maturity (EqVR_o) of organic matter rich shale of the lower Es₃ in subsags of Dongying Sag was mainly at the range of 0.69%-1.05%, and that of the upper Es₄ is mainly at the range of 0.74%-1.20%. Combining with the analysis of geological and engineering sweet spot conditions, the EqVR_o of organic matter rich shale of both two sub-members range 0.74%-1.20%. The deeper areas of four sub-sags in the Dongying Sag all have favorable prospecting of commercial development for matrix type shale oil in the lower Es₃ and upper Es₄ sub-members.