Characteristics of paleopressure evolution and its influencing factors in sub-sags of hydrocarbon-bearing sag: A case study of Raoyang sag in Jizhong depression

WANG Xin; JIANG Youlu; CAO Lanzhu; LIU Hua; ZHANG Wenjie

Abstract：To determine the paleopressure evolution characteristics of sub-sags in hydrocarbonbearing sags,taking Raoyang sag of Jizhong depression as the research object,based on the study of the formation pressure,basin modeling technology and thermometry of fluid inclusions were applied to reconstruct the process of paleopressure evolution in different sub-sags and investigate the influencing factors with the analysis of the detection logging and oil testing datas.The paleopressure evolution in sub-sags experienced three stages:primitive accumulation-releasing-reaccumulation and the significant difference of the paleopressure evolution characteristics in different sub-sags was observed.During the large-scale hydrocarbon generation and expulsion period(12-0 Ma),the overpressure caused by the hydrocarbon generation and expulsion of the source rock is the main reason for the high growth rate of paleopressure in sub-sags.The results show that the double overpressure structure exists in Dongying formation-Sha-1 member and Sha-3 member,including the lower pressure stronger than upper in Hejian,Maxi and Liuxi sub-sag while the upper pressure stronger than lower in Renxi sub-sag.The strongest overpressure develops in Hejian sub-sag with the pressure coefficient of 1.58.The result of the paleopressure restoration shows that the overpressure in sub-sags of Raoyang sag develops with the highest pressure coefficient of 1.51 during the accumulation period,while the low overpressure in the adjacent structural belts develops with the pressure coefficient of 1.29.The lithology,thickness and deposition rate determined by the sedimentary character of different sub-sags influence the contribution of disquilibrium compaction pressurization mechanisms.The abnormal pressure accumulates rapidly during the large-scale hydrocarbon generation and expulsion period and the paleopressure variation rate which can reach 0.030 Ma-1 in Maxi sub-sag increases along with the growth of the difference between hydrocarbon generation and expulsion.