Abstract: It is commonly believed that Cangdong sag belt is a dustpan-like fault -depression with hydrocarbons migrating towards its gentle slope.Therefore,the ge tle slope belt has been emphasized in exploration,while the center and steep slope belt of the sag have been ignored.Seismic sections and drilling data made ia the last few years revealed the complexity of the interior texture of the sag,which is composed of both Pre-Tertiary basement uplifts and basement terraces with related covering traps formed since the Tertiary.The texture of Cangdong sag belt is related to paleotectonic settings formed before the Tertiary deposits and blockfaulting occurred since the Tertiary.During the Indosiniasn-Yangshanian stage,anticlinal uplifts in NE direction were formed in response to the squeezing of the Pacific plate northwestward.The upwarped axea roughly corresponded to the present Cangdong fault belt.A tension fault system was formed in the top of the uplift by compressional uplifting,and the strata were severely disintegrated.Starting from Tertiary,the top of the uplift slid down dne to a tension from east to west along the system,and then,it was draped by contemporaneous sediments to form the present Cangdong sag belt.Obviously,the basement of the sag is closely related to the sedimentary formations above it.The basement of the sag gentle slope corresponds to the eastern flank of the paleouplift and is composed of Mesozoic-Paleozoic strata.During the Eocene,it was tensioned.The overlying Eocene strata formed the northwestern flank of the central uplift belt of Huanghua fault-depression.The Pre-Tertiary strata pinched out downdippingly.while the Tertiary strata thinned updippingly by overlapping on the fault plane.This is favourable for the formation of basement fault block reservoirs and anticline reservoirs in the upper formation.The basement of the center of the sag corresponds to the top of the paleouplift,and was severely disintegrated.In some parts,there is a lacuna of the Mesozoic and Paleozoic.It was tensioned during the Oligocene,and overlaid by the Oligocene strata to form faulted benches with older formation in the high and younger formation in the low.This is favourable for the formation of reservoirs in older formation with oil from younger source beds.The basement of the sag steep slope is composed of strata of different epoches.With a very complicated relief,the overlying strata overlap on the fault plane all the way.It may form paleogeomorphic reservoirs in the basement formation and structural-stratigraphic reservoirs in the overlying formation provided that sealing and blocking condition is good enough in the updip direction.The northwestern flank of the paleouplift is a part of the present Cangdong uplift and the basement of Wen'an slope.The new understanding of Cangdong sag belt texture not only widens the field of exploration in the gentle slope,but also shows good prospect in the center and steep slope.

摘要： 长期以来,人们普遍认为,沧东凹陷带属结构简单的箕状断陷,油气运移指向缓坡隆起,对缓坡带给予重视,而对凹陷中心及陡坡带则置于次要地位。这种倾向的产生、源于对凹陷内幕了解不够。事实上、据笔者结合近年地震剖面和有关钻探成果分析,证明其内幕亦很复杂,既有前第三系的基岩隆起,也有第三纪以来形成的基岩阶地和与之相关的盖层圈闭,为寻找多种类型油气藏奠定了基础。沧东凹陷带的结构同第三系沉积前的古构造背景及第三纪以来的块断活动有关,印支—燕山期因太平洋板块作北西向挤压,形成北东向的背斜隆起,隆起部位大致对应现今的沧东断裂带,隆起顶部因受压上隆产生张裂系,地层遭受剥蚀。第三纪始,沿顶部张裂系自东而西被拉张下滑,并被同期的沉积所披盖形成沧东凹陷带。显然这类凹陷的基底同沉积盖层关系密切;凹陷缓坡基底对应于古隆起东翼,由中生界—古生界地层组成,始新世被拉张,上覆始新统地层,构成黄骅断陷中央隆起带西北翼,前第三系地层下倾尖灭,第三系盖层上倾断超减薄,有利于基岩断块油藏和盖层背斜油藏的形成;凹陷中心基底则对应于古隆起顶,受剥强烈,中生界与古生界有的地区缺失,渐新世被拉张,上覆渐新统地层,形成古高今低的断阶结构,有利于新生古储油藏的形成;凹陷陡坡基底是由不同时代地层组成的断面斜坡,形态复杂。上覆地层自下而上断超于其上,若上倾方向封堵条件好,基底可形成古地貌油藏,盖层可形成构造岩性油藏;而古隆起西北翼则属于现今沧县隆起和文安斜坡基底的组成部分。沧东凹陷带的这个结构,不仅为凹陷缓坡开阔了找油领域,同时也为凹陷中心和陡坡带的勘探展示了美好的前景。