渤中凹陷南部东二下段沉积体地震反射特征及沉积模式

Seismic Reflection Characteristics and Sedimentary Model of Sedimentary Bodies of Lower ED2 of Dongying Formation,Southern of Central Bohai Sag

东营组沉积时期,渤南低凸起发育大面积的花岗岩,母岩类型好,供砂能力强。在郯庐大断裂活动的影响下,其坡折带及前方形成独具特色的沉积异常体。依据三维地震等资料,东二下段沉积早期,受继承性古沟谷与其前方断隆的共同限制,在渤南低凸起北部的坡折带古沟谷内发育4期限制性三角洲,以中弱振幅的平行-亚平行反射与杂乱的蠕虫状反射为主。东二下段沉积晚期,限制性三角洲将沟谷填满,由限制型向非限制型转化,坡折带上不发生沉积,其主要功能为沉积物的搬运过道,在靠近洼陷的中心部位发育3期辫状河三角洲,以中弱振幅的平行-亚平行反射与中强振幅的高角度前积反射为主。洼陷内部的辫状河三角洲的物源可以有2种来源:(1)限制性三角洲遭受侵蚀提供的再沉积物源;(2)渤南低凸起母岩带提供的粗碎屑,过道坡折带堆积在洼陷内部。渤中凹陷东二下段异常体的结构特征与空间展布与渤南低凸起北部坡折带的结构与演化密切相关,为坡折带的"源-渠-汇"体系研究、裂陷盆地演化过程中斜坡折带的沉积充填演化及渤海海域东营组三角洲的详细解剖提供了一个典型范例。

A large area of granite, exposed in the low uplift in southern Bohai during Dongying Formation deposition, has a powerful capacity to supply sand. Due to the influence of Tanlu fault, unique lithologic bodies are formed in its slope-break zone and front from the south to the north, The three-dimensional seismic data illustrate that four stages of confined deltas were developed in the ancient valley in the north- ern part of the southern Bohai low uplift slope-break in early deposition of Lower ED2 of Dongying Forma- tion because of the confinement of ancient valley and fault-uplift. The confined deltas are mainly formed with weak amplitude parallel to sub parallel reflections and disordered wormlike reflections. Southern Bohai Dongying Formation in the late deposition of Lower ED2, the confined deltas filled up the valley and trans- formed from confinement to unconfinement. In this time no deposition was found in the slope-break, which then worked as a passage for the transportation of sediments. Three braided river deltas were developed near the center of the sag, dominated by weak amplitude parallel to sub parallel reflection and moderately strong amplitude high angle progradational reflection in its front. Dongying Formation braided river deltas in the research area may have two sources： ①re-deposited materials provided by the confined deltas after e- rosion; and ②coarse clastic rocks supplied by the parent rock of Southern Bohai low uplift,which accumu- lated inside the sag after being transported through the slope-break zone. The structural characteristic and spatial distribution of the abnormal sedimentary bodies Central Bohai sag are closely related to the structure and in the Lower ED2 of Donging Formation in the evolution of the northern Southern Bohai low up- lift slope-break zone. This can be a good example for the research on the ＂Source-Conduit-Sink＂ system in the slope-break zone and for the analysis of the sedimentary filling process in slope-break zone during the rift basin evolution and detailed study of the deltas of Dongying Formation in Bohai Sea area.