Differential Tight Reservoir Pore Structures under the Control of Gravity Flow and Delta Front Sedimentary Systems

The microscopic pore structure has an important effect on the oil-bearing properties of tight reservoirs. It’s significant to clarify the characteristics and influencing factors to reveal the enrichment characteristics, the grading and evaluation of tight oil. Taking the lower third member of the Shahejie Formation in Bohai Bay Basin in China as an example, the pore structure characteristics and influencing factors under different sedimentary systems are studied by using HPMI, NMR, SEM, and other methods. The results show that tight sandstone reservoirs have developed gravity-flow turbidity current and delta front sedimentary facies, each corresponds to three different pore structures. Coarse pore-coarse throat, medium pore-medium throat and fine pore-medium throat are developed in turbidity current reservoirs, while delta front reservoirs have developed medium pore-coarse throat, medium pore-fine throat and fine pore-fine throat. According to experimental results, the pore structure of turbidity current reservoirs is much better than that of delta front reservoirs. The microscopic pore structure is mainly controlled by the diagenetic facies, and the six types of pore structure are formed under the combined influence of different degrees of compaction, cementation and dissolution. These conclusions have a certain guiding role in tight oil exploration and classification evaluation of sweet spots.