The shale gas reservoirs in Longmaxi formation in Pengshui and its adjacent areas were overpressured in geological history

but nowadays they transformed into normal pressure. Through the simulation of formation pressure evolution in the uplifting process

it is revealed that Longmaxi shale was fractured under excessive pressure in this process

resulting in shale gas loss and therefore overpressure release. By overburden pressure permeability test

it is found that when the normal stress on the mudstone fracture surface of Longmaxi formation is more than 15 MPa

that is to say

when the buried depth is more than 1 000 m

the cracks will be closed

but the degree of fracture closure is affected by overconsolidation ratio（OCR） of shale. For the mudstone or shale under the brittle zones

OCR is relatively small

the closure degree of fracture is relatively high

the overpressure may not be completely released

and a certain degree of overpressure is still maintain. But for those above the brittle zones

the lager the OCR

the worse the fracture closure degree

and it is apt to cause the overpressure released completely

finally transit to normal pressure state. There is a significant correlation between OCR and the pressure coefficient of the formation fluid

that is

the higher the OCR ratio

the more normal the pressure will tend to be.