CHARACTERISTICS OF FORMATION AND DISTRIBUTION OF PROLIFIC SAGS IN THE NORTHERN JIANGSU BASIN

The oil-and-gas fields discovered so far in the Northern Jiangsu Basin mainly distribute in the Gaoyou,Jinhu and Qintong prolific sags locating in the Dongtai Depression which is in the south of the basin.The formation conditions are as follows: 1) During the migration of subsidence center from west to east in the basin,the sustainable and rapid subsidence type of sags such as Jinhu,Gaoyou and Qintong are created with thick source rocks,premium quality organics and high degree of thermal evolution;2) In the west part of Jinhu and Gaoyou sags which locate in the entrance area of big rivers to lakes in the west of the basin,the favorable Funing Formation reservoir facies belt corresponding to Fu₂ Formation premium source rocks is generated.While in the east part of Qintong and Gaoyou sags which locate in the center of the basin,in addition to many favorable combinations of source-reservoir-seal rocks formed with primary oil and gas in the lower part,the vertical migration of oil and gas is fairly dynamic,and the upper secondary oil-and-gas pools have conducive reservoir-forming conditions;3) In the resi-dual depression area of the Wubao movement,namely the Gaoyou,Jinhu and Qintong sags,source rocks are buried with great depth,resulting in high degree of thermal evolution,rich abundance of oil and gas,and good development of secondary oil and gas reservoirs;4) In the central and west sags such as Jinhu,Gaoyou and Qintong,sedimentary volcanism is significant for major hydrocarbon source rocks of Fu2 Formation.Surface thermal environment and abundant inorganic nutrients help grow rich lower hydrobiont in this area and develope premium hydrocarbon source rocks.Meanwhile,magmatism stages for the 3 sags are 3-4,with high geothermal gradient,which is conducive to high degree of thermal evolution and conversion yield.