西湖凹陷中南部花港组浅水三角洲辫流型-曲流型分流河道沉积特征

Sedimentary characteristics of braided-meandering distributary channels in shallow-water delta of Huagang Formation in the central and southern parts of Xihu Depression

西湖凹陷中央洼陷带中南部花港组上段是重要的构造-岩性油气藏勘探层系,研究区以浅水三角洲沉积为主,但纵向和平面上砂体相类型和规模存在较大差异。通过对花港组古水体、古地貌、古气候等沉积背景的分析,认为花港组SQ3层序发育浅水三角洲平原分流河道沉积,并且受中期基准面、微古地貌、物源供给等因素控制,平原分流河道呈现出辫流型和曲流型特征。研究结果表明:①中期基准面上升期和下降期,东北侧物源供给为主,西北侧物源补充,可容纳空间较小,地貌低势区以辫流型分流河道为主,粒度粗、发育多期冲刷面和滞留砾石、心滩砂体,测井相以箱形为主,横剖面呈现厚而宽的槽状;②中期基准面转换期,西北侧物源供给为主,东北侧物源供给减弱,可容纳空间较大,地貌低势区以曲流型分流河道为主,粒度细,冲刷面见撕裂泥砾,测井相以钟形为主,横剖面呈现不对称透镜状。

The upper section of Huagang Formation in the central Xihu Depression is an important exploration layer series for structural-lithologic oil and gas reservoirs.The study area is mainly composed of shallow-water delta sedimentation,where significant differences exists in the types and scales of sand body facies in the vertical and horizontal directions.Through the analysis of sedimentary backgrounds such as ancient water bodies,paleogeomorphology,and paleoclimate of Huagang Formation,it is believed that the SQ3 sequence of Huagang Formation developed shallow-water delta plain distributary channel sediments,which were controlled by factors such as mid-term base level,micro ancient landforms,and sediment supply.The distributary channels in delta plain exhibited braided and meandering characteristics.The research results indicate that:①During the rising and falling periods of the mid-term base level,the sediment is mainly provided by the northeast source,with a small amount from the northwest.The accommodation space is relatively small.The low terrain areas are mainly dominated by braided distributary channels,with coarse sediment grain size and development of multi-stage scouring surfaces and retained gravels,as well as the development of sand bodies of channel bars.The logging facies are mainly box shaped,and the cross-section presents a thick and wide trough shape.②During the transition period of mid-term base level,the southwest source is dominant,while the northeast is weakened,and the accommodation space is relatively large.The low terrain areas are mainly characterized by meandering distributary channels,with fine sediments and tearing mud and gravel on the erosion surface.The logging facies are mainly bell shaped,and the cross-section presents an asymmetric lens shape.