Hydrocarbon Migration-accumulation Mechanism And Accumulation Model Of Paleogene In The Northern Sub-sags,Raoyang Sag

With the relationship between reservoir-forming dynamic and resistance and effective prediction of hydrocarbon reservoirs as the core,hydrocarbon migration-accumulation mechanism is discussed deeply and this paper makes the systematic research on the hydrocarbon accumulation process and accumulation model of Paleogene in the northern sub-sags of Raoyang sag,applying the theory and method of petroleum geology,organic geochemistry and structural geology.Through the study of geologic and geochemical characteristics and thermal evolution of source rocks,the hydrocarbon generation characteristics of source rocks in Sha-3 member and Sha-1 member of sub-sags in north of Raoyang sag are compared and analyzed.Although the hydrocarbon generation potential of source rock in Sha-1 member was better than that in Sha-3 member,the thermal evolution degree was low and the hydrocarbon generation capacity was weak in Sha-1 member.The hydrocarbon generation characteristics of the different sub-sags were different.The hydrocarbon generation capacity in Maxi sub-sag was stronger with source rock of Sha-3 member as the main hydrocarbon generation layer,while the main hydrocarbon generation layer was Sha-1 member in Renxi sub-sag.Basin modeling technology and thermometry of fluid inclusions are applied to reconstruct the paleopressure in Sha-3 member and Sha-1 member of sub-sags with the study of the current pressure field in the study area.The evolution of the paleopressure can be divided into three stages: pressure primitive accumulation stage,pressure release stage and pressure reaccumulation stage,and the differences in paleopressure development characteristics of different sub-sags at different stages are significant.During the pressure primitive accumulation stage(31 Ma ~27.2 Ma before present),the overpressure of Sha-3 member was strong in the middle and south of Maxi sub-sag,and the paleopresure was relatively low in Renxi sub-sag.During the pressure release stage(27.2 Ma ~25 Ma before present),the formation was uplifted and the abnormal pressure was released.During the pressure reaccumulation stage(25 Ma ~0 Ma before present),the abnormal pressure reaccumulated with the secondary burial and the overpressure strength increases significantly since 12 Ma.According to the reservoir properties and the lower limit of reservoir property,the evolution of the lower limit pressure of the effective reservoirs in the study area is restored,and the development characteristics of the reservoir-forming resistance in different sub-sags during the oil and gas filling period are clarified.In the late depositional stage of Dongying Formation,the reservoir-forming resistance of Sha-3 member was weak with the value less than 1MPa.During the deposition period of the Minghuazhen Formation,the reservoir physical properties in sub-sags deteriorate with the burial and the reservoir-forming resistance became stronger.In the present day,the lower limit of the effective pressure of the reservoirs was 6.25 MPa and 3.59 MPa in Sha-3 member and Sha-1 member of Maxi sub-sag,and 4.81 MPa and 3.86 MPa in Renxi sub-sag.The relationship between reservoir-forming dynamic and resistance is defined with the simulation of the excessive paleopressure evolution and quantitative characterization of lower limit of reservoir displacement pressure,and the drive types can be divided into three kinds of overpressure driven,buoyancy driven and mixed driven.There are differences in the type and range of oil and gas driving in different sub-sags at different periods.In the late depositional stage of Dongying Formation,the oil and gas in Sha-3 member of Maxi sub-sag were mainly driven by overpressure,and the drive type in Sha-1 member was the mixed driven.The drive type of Sha-3 member and Sha-1 member in Renxi sub-sag was changed from buoyancy driven to overpressure driven.During the deposition period of the Minghuazhen Formation,the overpressure develops in sub-sags and the drive type of Sha-3 member and Sha-1 member was overpressure driven,which provided sufficient power for hydrocarbon accumulation.Based on the characteristics and the effectiveness evaluation of the pathway system,the types of the pathway system in sub-sags can be divided into three types: “T” type vertical pathway system,stepped lateral pathway system and subtle type pathway system according to the control fuction in the process of hydrocarbon migration and accumulation and the relationship between the elements.The composition,distribution and reservoir forming control of different pathway system types are different.The “T” type vertical pathway system controled the hydrocarbon enrichment with multiple series of strata,which was mainly distributed in the adjacent fault zone of Maxi sub-sag.The stepped lateral pathway system provided an effective migration channel for vertical or lateral migration of oil and gas,which was distributed in all sub-sags.The subtle type pathway system controled the intrasource hydrocarbon accumulation of Maxi sub-sag,which was distributed in the deep of Maxi sub-sag.The hydrocarbon migration and accumulation processes in different periods are reconstructed with the simulation of the three-dimensional streamline migration,and the hydrocarbon migration-accumulation mechanism is discussed through the study of the relationship between reservoir-forming dynamic and resistance and the pathway system in sub-sags.There were two hydrocarbon accumulation periods in the study area.In the late depositional stage of Dongying Formation,only the Sha-3 member in Maxi sub-sag got the hydrocarbon generation threshold and reservoirs had small accumulation resistance.The oil and gas mainly accumulated in the sandstone of the Sha-3 member in Maxi sub-sag.In the late depositional stage of Minghuazhen Formation,the source rock has entered the primary generation and expulsion period,and sub-sags entered the main hydrocarbon generating period.In this period,the reservoir-forming resistance became stronger,while the reservoir-forming dynamic was sufficient with the overpressure development in sub-sags.The intrasource oil and gas driven by the overpressure formed the lithologic reservoir and structral-lithologic reservoir.The extrasource oil and gas migrated vertically along the fault to the shallow trap during the oil source faults active stage,while the oil and gas driven by the overpressure and buoyancy migrated and accumulated along the sand body and faults during the oil source faults inactive stage.With the comprehensive analysis,two types of the hydrocarbon accumulation mode of Paleogene in the northern sub-sags of Raoyang sag are established,which is “intrasource-overpressure driven-subtle passage” and “extrosource-mixed driven-fault passage”.The former developed in the deep source rock of the sub-sags with a single source,and there are two terms of reservoir-forming with the late hydrocarbon accumulation as the main accumulation period.During the late hydrocarbon accumulation,oil and gas accumulated inside intrasource by overpreesure.The latter developed in the fault zone near sub-sags with the Sha-3 member and Sha-1 member source rock.Driven by the overpressure and buoyancy,oil and gas migrated in vertical direction along the oil source fault.