Source rocks in Dongying depression are characterized by rapid and continuous deposition and large thickness

as well as high heat flow

high geotherm and high content of organic matters. Large thickness of mudstones are developed in the main source rock intervals(3

rd

and 4th members of Shahejie Formation). Source rocks are mainly deposited in the forms mudstone inlaid with sandstone

sandstone enclosed within mudstone and pure mudstones. As a result

overpressured systems are well developed in mature source rock intervals

and episodic expulsion becomes the fundamental mode of hydrocarbon expulsion in Dongying depression. The episodic expulsion of hydrocarbons in the overpressured systems in the depression can be classified into "tectonic episode" and "pressure episode".The mechanism of "tectonic episode" is destruction caused by tectonic activities

and hydrocarbons migrate along fault planes and tectoclase

while the mechanism of "pressure episode" is the accumulation and release of the "residual energy" in the overpressured systems

and hydrocarbons migrate along microfissures induced from fracturing. Hydrocarbon expulsion efficiency and hydrocarbon index in the overpressured systems with episodic expulsion are remarkably higher than that in the upper source rock interval

and they are remarkably lower than that in the lower source rock interval. Episodic expulsion of hydrocarbon has important significance in petroleum exploration

due to its characteristics of high energy and rapid migration.