鄂尔多斯坳陷湖盆缓坡型三角洲前缘沉积微相分带及成因分析——以定边-安边地区延长组长6油组为例

THE SEDIMENTARY MICROFACIES ZONING AND ITS GENETIC ANALYSIS OF DELTA FRONT IN DEPRESSED-TYPE LACUSTRINE BASIN

运用沉积相分析理论对鄂尔多斯盆地定边-安边地区晚三叠世延长组坳陷湖盆缓坡型三角洲前缘沉积微相组合进行系统研究,共识别出其组合形式有:水下分流河道叠加组合、河口坝叠加组合、水下分流河道与河口坝叠加组合以及河口坝与远砂坝、席状砂的叠加组合等4大类9种类型。并深入研究三角洲前缘沉积微相组合的时空分布规律,以三角洲前缘坡折带为界,将三角洲前缘分为"台型前缘"、"坡型前缘"和"盆型前缘"三部分,各部分有独特的沉积微相组合形式。高分辨率层序地层学的基准面旋回原理和沉积物体积分配原理表明:短期基准面的规律性变化影响着储层砂体的成因类型、叠加组合形式和空间分布规律,据此建立坳陷湖盆三角洲前缘不同沉积微相组合的成因分布模式。

Taking the Yanchang Formation of Triassic System in Ordos Basin as example, this paper studies the microfacies architecture and association types of depressed-type lacustrine delta fronts. The microfacies of delta fronts include truncated distributary channels and integrated distributary channels,river mouth bars,distal bars,front sheet sandstones and interdistributary bay. According to association type, the microfacies may be divided into four types of sandstones that consist of nine subtypes, including superimposed distributary channel, superimposed channel mouth bars, suhaqueous distributary channels and channel mouth bar associations and channel mouth bar, distal bar and front sheet association. The depressed type lacustrine delta fronts can be divided into three parts： platform delta front, sloping delta front and basin delta front by the bounds of slope break. Every part has unique microfacies association type. On the base of basic level cycle principle and sediment volume partition principle in high-resolution sequence stratigraphy,regular changes of short-term base-level cycle affect the genetic types,association types and distribution pattern of different genetic sandstone bodies. And distributional model of different sedimentary microfacies association is established.