THE EVOLUTION OF FAULTED-BASIN AND DEVELOPMENT OF HYDROCARBON SOURCE ROCKS IN EOGENE OF THE JINXIAN SAG, THE JIZHONG DEPRESSION

The mud rocks occurred mainly in Eogene Shahejie-Kongdian Formation of the Jinxian sag,the Bohaiwan Basin.The second member of Kongdian Formation(Ek²) and the first member of Kongdian Formation(Ek¹) to the fourth member of Shahejie Formation.(Es⁴) were the most important hydrocarbon source rocks.During the time of early deposit of Kongdian and Shahejie Formation(Ek³ to Es⁴),the rift active center transferred from south to north,as a result,the subsidence center transferred to north likewise,which took on ‘seesaw' deposition pattern.In the early stage(Ek²),subsidence center of the area occurred in the south of the Jinxian sag,and moved to the north in later(Ek¹ to Es⁴).Therefore,correlating to both the depositional thickness and mudstone thickness,it was apparently that the southern depression prevailed over the northern part.However,as far as Ek¹ to Es⁴ were concerned,it was conversed.In the northern depression,there was a set of evaporated saline-hypersaline lacustrine formation of halite,sulfate,carbonate and sand-mudstone alternating layer.The evaporite had a thickness over 1,000 m,the gypsolith of which exceeded 400 m.The gypsiferous mudstone,dolomitic mudstone,calcareous mudstone and dark mudstone in the formation were high organic-rich,buried at the depth of 2,500 to 3,500 m,which was the major hydrocarbon source rocks in the north Jizhong oilfields while Ek² dominated the south area.Oil and source rock correlation demonstrated that oil in the north oilfields was featured in high phytane,high gammacerane and high pregnance sterane abundance that differed from oil generated from southern formation.It was believed Ek¹-Es⁴ source rocks made significant contribution to the hydrocarbon input for the north oilfields when oil in the south oilfields generated from the southern Ek² source rock(or else the Ek¹-Ek⁴).