Keywords: 
• mixed reservoirs /
• controlling factors /
• QHD29-2E structure /
• Bohai Sea

Abstract: The development of the high-quality reservoir in medium-deep sections often decides hydrocarbon enrichment. The depth of medium-deep sections in the Bohai sea is approximately greater than 3 000 m with poor reservoir physical properties. We found a mixed reservoir, including terrigenous detrital and Carbonate oolitics and bioclasts, with excellent physical properties in the 1^st and 2^nd member of the Shahejie Formation (E₃s₁₊₂) in the QHD29-2E structure, and the research of the controlling factors, which have significant implications for similar areas. The features of lithology and reservoir space were revealed by observations of core, rock thin section, scanning electronic microscope (SEM), examining the physical properties, and X ray diffraction (XRD). The isotope fluid inclusion method and reservoir forming mechanism were further addressed. We determined that the rocks in E₃s₁₊₂ are composed mainly of terrigenous detrital bearing-carbonate mixed rocks, terrigenous detrital-carbonate mixed rock, and carbonate bearing- terrigenous detrital mixed rock with high porosity and low-very low permeability; for the reservoir space, primary porosity, including intergranular and body cavity pores, as well as secondary pores like moldic pores, were generally found. According to research, snail-rich mixed shoal formed due to strong lake wave action in a saline lacustrine basin. Additionally, dolomitization in the earlier stage and the latter secondary erosion are dominant factors affecting the formation of the excellent reservoirs in the QHD29-2E structure.