Overpressure is common in the Liutun salt-lake sag in the Dongpu depression and obviously control the formation of mudstone fractures and affects the hydrocarbon accumulation in these fractured mudstone reservoirs.Therefore

it is important to look into the genesis of overpressure in the salt-lake sag.We analyzed the relationship between the gypsum and overpressure based on the geophysical and geochemical data and combined with the analysis of overpressure genesis mechanisms.The results show that the undercompaction caused by sealing process of the gypsum cap rocks was the major mechanism for the formation of overpressure in salt-lake sags of the Dongpu depression where the deposition rate was low.Further quantitative evaluation of the undercompaction indicates its contribution to the overpressure is 53.068%-71.416%(62.758% in average).This leads to a conclusion that the extensive undercompaction caused by the strong sealing capacity of gypsum cap rock in the salt-lake sag is the determinative factor of the overpressure

especially the ultra-strong overpressure.Hydrocarbon generation and clay-mineral dehydration of inter-salt mudstone also can contribute to a certain limited extent to the formation of overpressure.Based on the above analysis

we propose that the properties of the gypsum cap rocks are the pivotal factors affecting and controlling the formation of overpressure in salt-lake sags.