The distribution of open-fracture in Eogene abnormal overpressure mudstone and shaleof Changdong-Nanpi Sag in Huanghua Depression suggests that the mudstone and shale withopen fractures centralized in certain depth interval in vertical

but laterally the overpressurezone is regionally distributed. The porosity

the content of ascanite

the residual total hydrocarbons/total organic carbon (TH/TOC) and ααα20S/20S+20R C

29

of the mudstone and shale decrease more than that of the upper and lower beds. It thus proved that so long as the openfractures form

a large scale of continuous fluid expulsion would start and the fractures wouldnot close until the expulsion potential exhaust. According to the distribution of the open fractures

the main zone of oil and gas migration could be determined.