Study on Reservoir Characteristics and Distribution Law of Water-Flooded Remaining Oil in the Dongying Formation of Block A, Chengbei Low Uplift, Bohai Bay Basin

Block A of Chengbei Low Uplift is a key development block in the Bohai Bay Basin. Since it was put into development, a certain scale of oil and gas production has been achieved. However, there are also problems such as unclear in reservoir characteristics, rapidly increase in water cut during water-flooding development, and unclear distribution of remaining oil, which seriously affect the efficient potential tapping of oil and gas in this area. In this paper, by comprehensively using data such as seismic, logging, mud logging, core experiments and development performance, the study on reservoir characteristics and water-flooding remaining oil distribution law in Block A is carried out. The research results show that the reservoirs of the 9th sand group (Ed₉) and the 8th sand group (Ed₈) of the Dongying Formation, the main oil-bearing intervals in the study area, are turbidite channel sand bodies formed in the deep lake-semi-deep lake facies. The reservoir lithology is mainly medium-fine-grained lithic arkose, and the reservoirs are mainly medium-porosity and medium-low permeability reservoirs, but the heterogeneity is extremely prominent. The Ed₈ reservoir is mainly developed in the 6th sublayer (Ed₈⁶), and the Ed₉ reservoir is mainly developed in the 5th sublayer (Ed₉⁵). The sand bodies in these intervals have good continuity and large distribution area, showing the characteristics of thinning from southwest to northeast. The development stage of the study area can be divided into four stages: natural energy development stage, enhanced water injection stage, sparse well pattern water-flooding stage and comprehensive adjustment stage. In the initial stage, natural energy development basically produced no water; subsequent enhanced water injection supplemented the formation energy, but the water cut rose rapidly, and the water flooding in the eastern wells was serious. Horizontally, the remaining oil in Ed₉-Ed₈ is mainly distributed in the southwest of the study area; vertically, the remaining oil is mainly distributed in the 1st single sand body of Ed₈ (Ed₈^6-1), the 1st single sand body of Ed₉⁵ (Ed₉^5-1) and the 2nd single sand body of Ed₉⁵ (Ed₉^5-2) . Sand body thickness is the main controlling factor for the distribution of remaining oil, and areas with large sand body thickness tend to have high remaining oil abundance. The structural shape also has a good controlling effect on the distribution of remaining oil, and the southern part of the block often has high remaining oil abundance because it is located in the high structural position. The research results can provide a basis for adjusting the development plan and improving the recovery rate in this area, and more importantly, it can provide a reference for the development of similar oil and gas reservoirs in the Bohai Bay Basin.