Abstract:

Undercompacting mudstone's roof and floor （UMRF) are the mudstone surface layers that are tighter than the central undercompaction, and are close to the normal compacting mudstone. UMRF develops in upper and lower parts of  thick-layered mudstone with low permeability. This paper, by the Laws of Low-speed Flow, analyzed formation process of the UMRF from the microscopic mechanism, and proposed the resistance force formula of the fluid passing UMRF. The mudstone with the UMRF as a cap rock, can be divided into three types: the continuous sedimentary type, the uplift type and the fracture type. The sealing ability of the fracture type mudstone cap rock is equal to the floor's displacement pressure plus the roof's overpressure when it breaks down. The other two types of mudstone cap rocks  sealing abilities are equal to the floor's displacement pressure plus the roof's displacement pressure. When the mudstone with the UMRF as a source rock, in the continuous type, the expulsion direction of the oil and gas is controlled by the permeability of UMRF; In the fracture type, a large number of oil and gas migrated upward along the cracks. In addition, the expulsion direction of the source rock is also controlled by the fault. The research of UMRF's formation mechanism can help to evaluate the sealing ability of the mudstone cap rock and to understand the muddy source rock's expulsion direction in the primary migration.

Key words: displacement pressure, undercompacting mudstones roof and floor, oil and gas, sealing ability, migration method