Geological characteristics and shale oil potential of the lacustrine immature to low mature shale oil system: a case study of the second member of Kongdian Formation in Cangdong Sag, Huanghua Depression, Bohai Bay Basin, China

Abstract

ABSTRACT The Second Member of Kongdian Formation (Ek2) shale oil system is a set of thick organic-rich lacustrine shales with great shale oil resources potential. The shales are oil-prone with high organic matter abundance (samples with TOC > 2% accounting for 72.6%) and low maturity (the Ro of samples between 0.42% and 0.84%). The Ek2 shale system is dominated by shales, sandstones, and carbonate rocks, which can be subdivided into various-mixed lithologies. The shales are primarily composed of clay minerals, quartz, feldspar, calcite, dolomite, pyrite, siderite, and zeolite, with no obvious predominant minerals. The brittle mineral content is high with an average content of 73.86%, which is conducive to reservoir stimulation. Physical properties and oil saturations vary significantly among different lithologies. Tight sandstones (average porosity 8.66%, average permeability 2.207 × 10−3 μm2) and dolomites (average porosity 5.26%, average permeability 1.910 × 10−3 μm2) have better exploration potential than shales (average porosity 2.46%, average permeability 0.937 × 10−3 μm2). Considering Ro distribution, the slope zones are more favorable than Kongdian Structure Belt laterally and the lower intervals of the Ek2 shale are more potential for shale oil accumulation than the upper interval vertically.