Geological heterogeneity of shale sequence and evaluation of shale oil sweet spots in the Qingshankou Formation, Songliao Basin

The lacustrine sediment of the Cretaceous Qingshankou Formation in the Songliao Basin is rich in organic shale, and the well Guye 1 has achieved a breakthrough in oil exploration of Gulong shale of deep lake facies. To further evaluate the geological characteristics of the shale sequence in different facies zones of the lacustrine basin and evaluate the characteristics of shale oil sweet spots in the continental lacustrine basin, we study the geological heterogeneity of shale sequences in different facies zones of the lacustrine basin regarding the environmental difference for lacustrine shale formation in the Qingshankou Formation. It is suggested that the shale sequences deposited in the freshwater lacustrine basin can be divided into seven types according to lithofacies, namely the organic-rich lamellar clayey shale (TOC > 3%), lamellar clayey shale, felsic shale, lamellar shell shale, massive mudstone, limestone and dolomite respectively, which are evaluated in terms of total hydrocarbon generated and retained, hydrocarbon mobility, reservoir property, compressibility and oil production capacity. The geological and engineering sweet spots of shale oil are thereby proposed. Based on the contents of TOC and S₁ (pyrolysis hydrocarbon content), we group the geological sweet spots of the Qingshankou Formation shale into Type Ⅰ, Ⅱ, and Ⅲ. The TypeⅠsweet spot is generally characterized by TOC content greater than 3% and S₁ greater than 4 mg/g; the Type Ⅱ by TOC content ranging between 1.5%and 3%, and S₁ between 1.0 and 4 mg/g; the Type Ⅲ by TOC content less than 1.5% and S₁ less than 1.0 mg/g. The semi-deep lacustrine and deep lacustrine facies are dominated by sweet spots of Type Ⅰ and Ⅱ, while the shales of shallow lacustrine facies by sweet spots of Type Ⅱ and Ⅲ. The Qingshankou Formation shale in Songliao Basin is selected as the major oil pay zone after a comprehensive analysis of the oil-bearing property, percolation coefficient, compressibility, characteristics of source rocks and physical properties of different facies zones.