渤海湾盆地埕北凹陷古近系沉积体系及砂体成因类型

Depositional systems and genetic types of sand bodies of the Paleogene in Chengbei sag,Bohai Bay Basin

埕北凹陷是渤海湾盆地渤中坳陷内部的次级构造,其勘探潜力一直受制于对区内砂体成因类型及储集层分布规律的认识。利用钻井岩心、测井曲线及地震反射剖面等资料,对该凹陷古近系沉积特征、砂体成因类型和分布规律进行分析。研究区古近系发育扇三角洲、辫状河三角洲和湖泊等沉积相,扇三角洲主要发育在凹陷东北侧的陡坡带,岩性以具块状构造的含砾粗砂岩为主,地震反射特征主要表现为楔状的前积退积和杂乱反射,测井相主要表现为低幅齿型、齿状箱型和钟型;辫状河三角洲主要发育在凹陷西南侧的缓坡带,岩性以具交错层理的砂岩为主,地震反射特征主要表现为S形前积和叠瓦状前积,测井相主要表现为箱型、钟型和漏斗型;湖底扇受重力流控制,主要发育在凹陷中心的半深湖-深湖区,地震反射特征主要表现为蠕虫状前积反射;滩坝主要发育在湖泊边缘或中央局部隆起区,因受波浪作用控制,沉积特征表现为席状砂质沉积被泥岩包围,地震反射特征主要表现为透镜状反射,测井相主要表现为指状和漏斗型。纵向上,沙三段以辫状河三角洲、扇三角洲和湖泊沉积相为主,沙一二段以辫状河三角洲、扇三角洲和湖泊沉积为主,东三段以辫状河三角洲、扇三角洲和湖泊沉积为主,东二段以辫状河三角洲和湖泊沉积为主,扇三角洲发育规模较小。埕北凹陷古近系砂体的生成和发育主要受构造活动、源区物源供给、微古地貌形态和湖平面变化等因素控制,主要发育扇三角洲砂体、辫状河三角洲砂体、湖底扇砂体和滩坝砂体。

Chengbei sag is a half graben basin in the Bohai Bay Basin.Its exploration potential is limited by the understanding of the genetic type of sand bodies and the distribution of reservoirs.This paper has analyzed the sedimentary characteristics,sand body genetic types and reservoir distribution patterns of the Paleogene in the Chengbei sag,using the drilling cores,log curves,and 3D seismic reflection data.The Paleogene in this sag has been developed with fan delta,braided river delta,sub-lacustrine fan sedimentary facies.The fan delta is developed in the steep slope zone on the northeast side of the sag,with lithology dominated by massive gravelly coarse-grained sandstones.The seismic reflections show lenticular and wedge progradation and retrogradation morphologies and chaotic and the logging facies mainly show low-amplitude jag-shaped,jagged box-shaped,and bell-shaped.The braided river delta is developed in the gentle slope zone on the southwest side of the sag,with lithology mainly consisting of cross-bedded sandstones.The seismic reflection shows S-shaped progradation and imbricated progradation patterns.The logging facies mainly show box-shaped,bell-shaped and funnel-shaped patterns.The sub-lacustrine fan is controlled by gravity flow and developed in the deep lacustrine area of the sag center.The seismic reflection is featured by worm-like progradation.The lacustrine shoal-bars are developed at the edge or central uplift of lakes,controlled by wave action,and are characterized by sheet-like sandy deposits surrounded by mudstone.The seismic reflection is featured by lens-shaped reflection,and the logging facies mainly show finger-shaped and funnel-shaped.Vertically,the Member 3 of the Shahejie Formation is mainly composed of braided river delta,fan delta,and lacustrine sedimentary facies.The Members 1 and 2 of Shahejie Formation are mainly composed of braided river delta,fan delta and lacustrine sedimentary facies.The Member 3 of Dongying Formation is mainly composed of braided river delta and fan delta sedimentary facies,while the Member 2 of Dongying Formation is mainly composed of braided river delta sedimentary facies,with the scale of fan delta development being small.The generation and development of Paleogene sand bodies in the Chengbei sag are mainly influenced by tectonic activity,sediment supply,palaeo-microgeomorphologies,and lake level changes.The predominant genetic types of these sand bodies include fan delta sand bodies,braided river delta sand bodies,sub-lacustrine fan sand bodies,and lacustrine shoal-bar sand bodies.