东营凹陷沙三中亚段沉积特征及复合砂体沉积过程——以牛庄洼陷南坡为例

东营凹陷沙三中亚段整体处于湖泊扩张至收缩过渡期,湖平面变化震荡频繁,砂体发育类型多样,沉积相类型及连片砂体分布成因有待明确.综合区域地质、测井和岩心等资料,系统开展了沉积相厘定及复合砂体沉积过程重建研究.研究表明:牛庄洼陷南坡沙三中亚段发育三角洲和半深湖滩坝两种主要沉积环境;滩坝多为孤立分布,河道及河口坝多期复合使得砂体呈连片分布;目的层d小层自下而上对应湖平面上升又下降的完整旋回,由三角洲前缘-前三角洲沉积过渡为半深湖;而后又演化为三角洲前缘,砂体展布由d4单层条带状过渡为d3单层孤立状;而后过渡为d2和d1单层近连片状;d2单层近连片砂体由4期沉积过程各异的河口坝叠置而成.第1期为两分流河道携沉积物进入研究区东南部形成初期河口坝;第2期分流河道及河口坝进一步前积向湖盆发展,同时侧向扩展;第3期分流河道大幅度前积,使得河口坝发育范围亦大幅度扩展,同时分流河道进一步分叉并控制形成4个河口坝;至第4期分流河道前积有所减弱,形成三个次级河口坝;最终使得河口坝连片分布,分流河道呈镶嵌状分布其上.研究区复合砂体成因多是由多期三角洲砂体(尤其是河口坝砂体)垂向叠置、侧向拼接而成,构成了研究区主力的储层.

Sedimentary characteristics and sand body sedimentary process of the middle third member of Shahejie Formation in Dongying Depression: A case study of south slope of Niuzhuang Sag

The middle sub-member of Shahejie Formation in Dongying Sag developed in the transitional period of lake expansion to contraction with frequently fluctuated lake level. Different types of sand body are developed, the facies type and mechanism of continuous sand distribution need to be further studied. Integrated with regional geology, well logging and core data, sedimentary facies types were identified and deposition process was reconstructed. The research shows that south slope of Niuzhuang Sag developed delta and semi-deep lake beach bars in the third middle of Shahejie Formation. Beach bars were mostly distributed in isolation, and the multi-stage distributary channels and mouth bars continuously distributed. From bottom to top, sub-layer d corresponded to a complete cycle of lake level rising and falling. The facies transformed from delta front/prodelta to semi-deep lakes, and then to delta fronts. The sand body in d4 had striped distribution, d3 had isolated distribution, d2 and d1 had nearly continuous distribution. The continuous distributed sand body in d2 was formed by 4 stages. During the first stage, two distribution channels carried sediments into basin to form the original mouth bar. When it comes to the second stage, distributary channels and mouth bars further prograded and imposed to basin. The third stage distributary channels had large scale progradation and controlled the following four mouth bars. During the fourth stage distributary channels weak progradation and formed three mouth bars. Finally, the mouth bar distribute continuously and distributary channel located in the top of mouth bar.