The faulted lacustrine Bohai Basin in eastern China contains abundant hydrocarbon resources. In these reservoirs, understanding the sandstone diagenesis and the resulting formation water provides a means to unravel the evolution processes in the basin. In most cases, the lack of isotopic and trace element analysis tests in this type of basin limits the research on the origin and evolution of formation water in this area. We have used multivariate statistical methods to classify the geochemical characteristics of the formation water for the Cenozoic Formation of Bonan Sag in the Bohai Bay Basin. Analysis of correlations among the evolution processes of different ions in different types of formation water provides an understanding of the primary factors influencing the ion content. We also evaluate the water-rock interactions of different types of formation water to evaluate their geologic significance, and we find three types. Type I formation water includes a mixture of river water, lake water, and atmospheric precipitation and exhibits weak water-rock interactions. Type II formation water contains primitive freshwater and brackish lake water that has undergone an evolution process similar to that of type I formation water, but that was followed by evaporation and concentration, the dissolution and precipitation of calcite and iron calcite, and feldspar dissolution. Type III formation water, which is a product of rock reconstruction, originates from saline lake sediment water. After undergoing evolution processes similar to those of types I and II, type III formation water is also affected by dissolution of evaporite, albite, dolomite, and iron dolomite. Thus, type III formation water is the product of water-rock interactions such as precipitation, SO₄²⁻ reduction, and pyrite precipitation in which the water-rock reaction controls the development mechanism and characteristics of the reservoir space.

As the dynamic condition of oil and gas migration, the change rate of overpressure has a certain relationship with the mode of oil and gas migration. In order to quantitatively identify the mode of oil and gas migration in overpressure basins, the influence of different transmission modes on pressure attenuation gradient is simulated, and the application and discussion are carried out in Bonan sag of Bohai Bay Basin. The influence of different transmission modes on the pressure attenuation gradient is discussed by single factor simulation method. Taking Bonan sag as an example, the process of overpressure fluid migration and accumulation is discussed combined with the evolution process of pressure attenuation gradient. The results show that fault activity has the greatest influence on pressure attenuation gradient, followed by fault dip angle and fault distance. the influence degree of physical properties of skeleton sand body, sand body dip angle and sand body thickness on pressure attenuation gradient decreases in turn; For the fault-sand consistent transport, it mainly depends on the combined effect of the dip angle and resistance of the transport system. The critical value of the fault or sand body transport in Bonan sag is 58.8MPa/km. On this basis, combined with the evolution law of pressure attenuation gradient, it is considered that the overpressure fluid in the deep sag migrates along the oil source fault to the fault terrace zone and the overpressure center distribution of the oil and gas ring in the deep sag. The overpressure oil and gas mainly migrate laterally along the sand body, and the oil and gas are distributed contiguously. In gentle slope zone, oil and gas distribute sporadically at structural height.