--> Hydrocarbon Fluid Charge History and Pore-Pressure Evolution in the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin

AAPG ACE 2018

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Hydrocarbon Fluid Charge History and Pore-Pressure Evolution in the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin

Abstract

The hydrocarbon fluid evolution including charge episode, timing and pressure features in sandstone reservoirs in the Pinghu Formation in the Pinghu slope belt of Xihu depression were investigated using a combination of methods such as fluid inclusion, basin modeling, thermal simulation experiment, K/Ar dating of authigenic illite and well-logging. The hydrocarbon generation history established by basin modeling and thermal simulation experiment indicate that the coal-measure source rock in the lower Pinghu Formation entered into oil window in the late Oligocene and reached oil-generating peak (around 0.75%Ro) in the middle Miocene, and then oil generation decreased. It entered into gas window (Ro>0.7%) in the middle Miocene and generated gas continually until present day. Fluid inclusion analysis suggests that there are three stages of oil-gas charges. The first orange-red fluorescing oil charge occurred in the early Miocene (circa 20.7~15.8Ma). The second yellow-green fluorescing oil charge occurred in the middle-late Miocene (circa 12.6 to 5.5Ma). The third blue-white, blue fluorescing oil and gas charges occurred in the Pliocene and Quaternary(circa 3.8 to 0 Ma). The charge timings determined by fluid inclusions are well coincided with K-Ar dating and hydrocarbon generation history. During this period from the Miocene to the present day, the pore pressure evolution history reconstructed by fluid inclusion PVTx analysis and previous numerical modeling indicate that the Pinghu Formation experienced pressure accumulation-release-accumulation possesses. Currently, moderate overpressures are present. In the middle-late Miocene (circa 12~7.2 Ma), oil generation and tectonic compression may result in the first-phase weak overpressure generation. In the late Miocene, the sudden uplift of formations leaded to pressure drop. In the Pliocene and Quaternary (circa 5.0~0 Ma), kerogen to gas by maturation occurred intensively and is mainly responsible for the second-phase moderate overpressure

generation. Besides, long-distance vertical or lateral transfer of pressure may also make minor contribution.