will occupy the top of the structure, so it is known as gas cap; oil will gather in the lower wings ind water with greater density will gather under oil. as In reservoir the oil-water contact surface is called ller interface. and it shall be the oil bearing boundary when projected onto the horizontal plane. Strictly speaking, oil-bearing boundary should be classif ied as inner boundary and outer boundary. In lact, average of the two oil boundaries is generally taken as the oil-bearing boundary. The intersecting line of oil-gas interface and reservoir top is called gas boundary. If the outer-ring of reservoir connects to source that supply for reservoir, then the reservoir is called open-reservoir, the projecton ovei protile is called supply boundary( Figure 1-1), If the outer-ring of reservoir is closed and its altitude is the same as oil-water interface. then the reservoir is called closed reservoir. and its overall protile is called closed boundary According to the distribution of oil. gas and water. w outside the oil-bearing boundary is called edge water; if the reservoir is of large thickness or relatively flat structure that makes the water under the oil then the water is called bottom water In actual oil tield. it is rare to scc reservoir with only a single oil laver, most of reservoirs are The litho not always the same from layer to layer. Therefore. the distribution and features of oil, gas and water should be understood first before oil tields development Figure 1-1 Reservoir with recharge area Figure]-2 Oil and gas boundar 1.1.2 Concept of various pressures Reservoir pressure is the character of formation energy, if the pressure field of reservoir is lavers. In the oil field development process, the concept of different pressures are oflen encountered, some will introduced as follows Initial formation pressure p T'he reservoir pressure is the fluid pressure within the pores in the middle of oil formation reservoirs remain a state of relative equilibrium before development generally, and at th int the fluid pressure is called initial formation pressure. When th latively big. the mid-lepth ol each usually not the same, wells lie on the top of structure have a shallower mid-depth while the lie in the wings have a deeper mid-depth, The total