[en] A suite of lacustrine mixed marlstone is developed in the lower third member of the Shahejie Formation (Es3^l) in the Shulu Sag, Bohai Bay Basin, and it is a tight oil reservoir which acts as both source bed and reservoir bed. According to the rock fabric characteristics, the marlstone in Es3^l can be divided into two types: lamellar marlstone (stratigraphically mixed) and massive marl (structurally mixed). Both of them contain minerals dominated by calcite, followed by dolomite, and minor quartz, feldspar and clay minerals. The marlstone in Es3l is a typical ultra-low porosity and ultra-low permeability tight reservoir. Its pore space is classified into two types and eight sub-types, and the matrix pores are mainly nano-scale pores. The lamellar marlstone reservoir shows better physical properties and pore structure than the massive marlstone reservoir. Both the chloroform asphalt “A” content and rock pyrolysis S0+S1 are high, and oil-bearing is universally observed. The development of “sweet spot” is jointly controlled by sedimentary facies, rock fabric, organic matter abundance and maturity, structural fractures. (paper)