Study On Reservoirs’ Pressure System And Genetic Mechanism In The Santanghu Basin

The Santanghu Basin is a small-sized composite basin developed on the superimposition of the Permian residual basin and Mesozoic depressional basin. It has mainly experienced the Hercynian, Yanshanian and Himalayan tectonic movements. Multiple oil-bearing series including lithologic, carbonate and volcanic petroleum reservoir have been found from the Carboniferous to the Jurassic. The fluid pressure systems are different in different petroleum reservoirs. This dissertation discusses the mechanisms of different fluid pressure systems, the relationship between pool-forming factors and pressure and the important influence of pressure evolution in studies of pool-forming pattern and finding of the favorable exploration targets.Abnormally low pressure system is mainly developed in the Jurassic and the Triassic , while normal-abnormally high pressure system is developed in the Permian and normal-abnormally low pressure is developed in the Carboniferous in the Santanghu Basin. Tectonic uplifting is regarded as the main reason of generally abnormal low pressure in the Santanghu Basin and diagenesis has significant influence in the formation of abnormally low pressure of the Jurassic due to the consumption of stratigraphic water. Low compaction, hydrocarbon generation and tectonic compression also lead to the abnormally high pressure of the Permian. As though it experienced pressure reliefing in later period, the Permian keeps the normal pressure due to its continuous hydrocarbon-generating ability. As to the Carboniferous, volcanic rock epigenesist is the main factor influencing the pressure. The relationship between reservoir characteristics and stratigraphic pressure indicate that the better porosity-permeability and the weaker heterogeneity, the higher the pressure coefficient is in the Xishanyao Formation in the Malang Sag . It is indicated that the sealing reservoir’s pressure is abnormally low and the opening reservoir’s pressure is normal in the Carboniferous because the opening degree of the fissures result in the connecting quality of the reservoir space and outerior space. The abnormally low pressure system of the Jurassic and the abnormally low-normal pressure system of the Carboniferous develop bottom- generation and top-preservation petroleum reservoirs, while the normal-abnormally high pressure system of the Permian develops self-generation and self-preservation petroleum reservoirs. Both the Carboniferous and the Jurassic petroleum reservoirs experienced the pressure evolution of normal pressure- abnormally high pressure - abnormally low pressure from their main hydrocarbon-formation period(Late Yanshanian Age) to the present. Several accumulation models have been summarized and favorable exploration targets are predicted based on the studies of the pressure and reservoir-formation evolution.