鄂尔多斯盆地西峰地区延长组陡坡带三角洲前缘储层砂体成因

Genesis of Sandstone Reservoirs at Delta Front of Steep Slope in Yanchang Group of Xifeng Distict,Ordos Basin

以鄂尔多斯盆地延长组长8段为例,运用高分辨率层序地层学的短期基准面旋回原理,详细分析了以水下分流河道和河口坝沉积为主体的辫状河三角洲前缘厚层储集砂体的成因。研究表明,各种成因砂体的发育受沉积基准面升降、可容纳空间(A)与沉积物补给通量(S)比值变化的控制。在基准面上升过程中,当A/S<1时,沉积以叠加式水下分流河道砂体为主,当A/S>1时,沉积则为完整式河道砂体;在基准面下降过程中,当A/S<1时,沉积以完整式河口坝砂体为主,当A/S>1时,沉积则以叠加式河口坝为主。在此基础上,在西峰地区长8段辫状河三角洲前缘砂体中识别出3种类型和6种亚类型的成因砂体,并且从湖岸线到湖泊中心方向,从该套砂体中可明显地区分出3个微相组合区:水下分流河道组合区、水下分流河道与河口坝混合区及河口坝组合区。

As an example of Chang 8 member of Yanchang Formation in the Ordos basin,we use the short term base-level cycle theory of high-resolution sequence stratigraphy to discuss the genesis of the reservoir sandstones of braided delta front in the submarine distributary channel and mouth bar. The various types of sandstones are controlled by base-level change and A/S ratio which is the ratio of tolerable space and sedi ment supply flux. In the processing of base level rising,if A/S（1 ,the sandstone is formed in the multi-o- verlied submarine channel;if A/S〉 1, the sandstone is deposited in the integrated channel. In the processing of base level falling,if A/S〈1, the sandstone is formed in the integrated mouth bar if A/S〉 1, the sandstone is formed in the overlied mouth bar. Based on sandstone formation, three types and six subtypes of sandbodies are identified at the braided delta front of Xifeng area,three microfacies areas are obviously divided into submarine distributary channel area, mixed area of submarine distributary channel-mouth bar, and mouth bar area from the shore line to the lake center.