准噶尔盆地南缘八道湾组扇三角洲露头基准面旋回与储层的响应关系

The response relationship between base-level cycle and reservoirs of fan delta in Badaowan Formation, Southern Junggar Basin

准噶尔盆地南缘八道湾组四棵树剖面发育扇三角洲沉积,可划分为2个长期基准面旋回,并细分为3个中期旋回。基于露头精细解剖,将扇三角洲成因砂体分为叠加型、过渡型及孤立型砂体3种类型,分别对应于辫状水道、辫状水道向分流水道过渡及分流水道成因,其宽度、厚度及宽厚比由叠加型向孤立型逐渐减小。3类砂体的特征、形成与变化与基准面旋回变化及所处的位置有关。A/S比值由低到高依次发育叠加型、过渡型、孤立型砂体。水道砂体的规模与A/S比值呈负相关关系,即随A/S比值的增大而减小。随着基准面的上升,八道湾组水道砂体出现的频率减小,叠加厚度减薄,砂体之间的连通性变差,水道砂体的粒度变细,发育于陆地的古土壤减少,且宽厚比也逐渐减小。

Fan delta sediments are well developed in Badaowan Formation along Sikeshu section in Southern Junggar Basin. Two long-term base-level cycles were identified in Badaowan Formation, which can also be subdivided into three medium-term cycles. Based on the dissection of outcrops, the authors divided fan delta sand bodies into three types, i.e., superposition, transitional and isolation types, corresponding respectively to the genesis of braided channel, transformation of braided channel to distributary channel, and distributary channel. The width, thickness and width-to-thickness ratio decrease from superposed to isolated sand bodies gradually. The characteristics, formation, and variation of sand bodies are controlled by the variation and the relevant position. Superposed, transitional and isolated sand bodies are developed under the conditions of low, middle, and high A/S ratios. Furthermore, the dimension of sand bodies is in inverse proportion to the A/S ratio, i.e., the dimensions decrease with the increase of the A/S ratio. With the rise of base level, the proportion, thickness, interconnection, grain size and dimension （width-to-thickness ratio） of channel sand bodies and paleosols decrease accordingly.