Overpressure Evolution Recorded in Fluid Inclusions in the Dongpu Depression,Bohai Bay Basin,North China

The Dongpu depression,like other depressions in the Bohai Bay Basin,is characterized by overpressure,however,the published studies have not clearly given the overpressure formation mechanism and paleo-pressure evolution history in the Dongpu depression.In addition,the current researches are mainly focused on the northern Dongpu depression,while rare investigation has been conducted in the southern part.In this context,it is urgent to explore the comprehensive overpressure evolution and formation mechanism in the Dongpu depression in academic and industrial fields.This paper provided a systematic analysis of fluid inclusions,including fluid inclusion petrography,microthermometry,vapor phase filling degree measurement and PVT-x modeling.By using homogenization temperature-burial history projection,the trapping time of the oil inclusions are determined.Four pressure evolution stages are identified in both the northern and the southern Dongpu depression.The results indicate that the pressure accumulation stage is derived by disequilibrium compaction and hydrocarbon generation in the northern part because of the salt rock distribution,while the stage is only derived by hydrocarbon generation in the southern part because of no salt rock distribution.The pressure release stages are mainly caused by the tectonic uplifting movements at the end of Dongying and Minghuazhen Formation.The overpressure in most areas in the northern part has been preserved till now,which is considered as a result of the good plasticity and sealing ability of salt rock that prevent the pressure leakage by smearing on the fault.Areas without salt rock distribution show normal pressure,which means the salt rock is an important factor that contributes to the overpressure preservation.However,there are exceptions that in Baimiao and Qiaokou,the overpressure still exists even without salt rock distribution,which is due to fault closure.

Mixing Characteristics of Oil Inclusions with Different Thermal Maturities in the Wenliu Uplift,Dongpu Depression,Bohai Bay Basin,North China

Multiple types of oil inclusions with different fluorescence colors have been detected in the northern Dongpu depression.Recent evidences show that these inclusions may have been trapped simultaneously or during a very short period.Therefore,whether these oils were mixed before trapping is unknown.In this study,we analyzed the petrography and fluorescence spectral characteristics of oil inclusions in the Wenliu uplift in the northern Dongpu depression,and assessed the data with the oil mixing ratio curve obtained in the previous experiment.The results show that there are three types of oil inclusions(type I,type II and type III)with yellow,green and blue fluorescence colors,corresponding to low-mature,medium-mature and high-mature oil,respectively.The"pure"oil inclusions,do exist in type II and type III group of oil inclusions,showing the medium-mature oil was generated from the source rock rather than being formed by the mixture of high-mature and low-mature end oils.Most of the oil inclusions are mixtures of high-mature and medium-mature end oils from the sub-sags to the Wenliu uplift,the mixing degree increases to close to 50%.The oil between the Qianliyuan sub-sag and the Wenliu uplift was mainly mixed by medium mature oils,whereas the oil between the Liutun sub-sag and the Wenliu uplift was mainly mixed by high mature oil.