冀中坳陷晋县凹陷下第三系断陷湖盆的演化与烃源岩的形成

渤海湾盆地冀中坳陷晋县凹陷的泥质岩类主要发育在下第三系孔店组—沙河街组,其中孔店组二段(Ek2)和孔店组一段—沙河街组四段(Ek1-Es4)是最重要的2套烃源岩。在孔店组和沙河街组早期(Ek3至Es4)沉积时期,断陷活动强度由南向北转移,沉降中心也随之向北迁移,使晋县凹陷南北呈现出“跷跷板”式沉积,即早期(Ek2)沉降中心在凹陷的南部,而后期(Ek1-Es4)沉降中心则转移到北部。因此在凹陷南部Ek2的沉积厚度和泥岩厚度都明显大于北部,而Ek1-Es4发生了反转,北部的沉积厚度和泥岩的发育程度都远大于南部。其中在凹陷北部Ek1-Es4为一套岩盐、硫酸盐岩、碳酸盐岩和砂泥岩互层的蒸发咸湖—盐湖沉积建造,蒸发盐厚逾千米,其中膏盐厚度超过400 m,与岩盐、膏盐互层的含膏泥岩、白云质泥岩、钙质泥岩和暗色泥岩有机质丰度较高,目前埋深在2 500~3 500 m左右,为北区油气田的主力源岩;而Ek2为南部的主力烃源岩。油源对比表明,北区原油以明显的植烷优势、高伽马蜡烷含量、高孕甾烷含量等特征明显区别于南区各套烃源岩,油源应来自北部的Ek1-Es4烃源岩;而南区原油中伽马蜡烷和孕甾烷含量极低、姥鲛烷含量较高等特征的原油应来自于南部的Ek2烃源岩,否则是来自南部的Ek1-Es4烃源岩。 

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(Ek~2) and the first member of Kongdian Formation(Ek~1) to the fourth member of Shahejie Formation.(Es~4) were the most important hydrocarbon source rocks.During the time of early deposit of Kongdian and Shahejie Formation(Ek~3 to Es~4),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(Ek~2),subsidence center of the area occurred in the south of the Jinxian sag,and moved to the north in later(Ek~1 to Es~4).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 Ek~1 to Es~4 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 Ek~2 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 Ek~1-Es~4 source rocks made significant contribution to the hydrocarbon input for the north oilfields when oil in the south oilfields generated from the southern Ek~2 source rock(or else the Ek~1-Ek~4).