Abnormal pressure in the Nanpu Sag is prevalent

but its distribution in varying tectonic zones and its genetic mechanisms are rarely studied.So we depicted the architectural features of the pressure for varying tectonic zones in detail based on the data from 1 354 drill stem tests(DST)

repeated formation pressure tests(RFT)

and the well logging curves of 357 wells.The results show that vertically

the formation pressure system can be classified into three zones in the Nanpu sag

namely

the upper normal pressure zone(with burial depth less than 1 800 m)

the middle transitional zone(with burial depth ranging from 1800m to 2 400 m)

and the lower abnormal high pressure zone(with burial depth more than 2 400 m).Large part of the Shahejie Member 3 is controlled by significant abnormal overpressure with pressure coefficient up to 1.9

and the depth from ground to the top of the overpressure zone is about 2 400 m.The middle and upper layers of the Laoyemiao Formation overpressure is less intense with pressure coefficient about 1.2.However

the Dongying Member 1 and 2 offshore

develops abnormal low pressure in some area.In order to further discuss the abnormal pressure mechanisms in the Nanpu Sag

we resorted to methods such as numerical simulation

vertical effective stress-acoustic time cross-plot

and comprehensive analysis of hydrocarbon generation and expulsion for source rocks.Consequently

we propose four conclusions:(1)the large-scale overpressure in Es

3

mainly originates from the sandstone disequilibrium compaction during the Dongying period

and contribution from hydrocarbon generation is relatively minor;(2)in Minghuazhen period

hydrocarbon generation becomes the main mechanism for generating pressure

while the influence from disequilibrium compaction is rather limited;(3)the lower-degree overpressure of the middle and upper layers is thought to be induced by the vertical fluid migration from the deep overpressured formations through active fault zone;(4)the regional uplifting and denudation during the late Dongying period result in pore rebound of rock matrix and fluid shrinkage

which are considered to be the main cause of the abnormal low pressure in the Nanpu sag.