断陷湖盆水下扇沉积特征:以南大港断层下降盘典型断块为例

Sedimentary Characteristics of Subaqueous Fan in a Faulted Lacustrine Basin: A Case of Typical Blocks in the Downthrown Block of the Southern Dagang Fault

南大港构造带下降盘沙河街组沙三段岩石类型为砾岩、砂岩、粉砂岩及泥岩组合。岩心中发育块状构造、交错层理、变形层理等重力流和牵引流沉积构造,炭屑、植物茎干、同生断层、泥质砂岩团块等现象也常见。粒度概率累计曲线为一段式和二段式,测井曲线可分为箱形、钟形、漏斗形和平直形4类。地震剖面上也可识别出水下扇的反射特征。综合岩性、粒度、电性和地震反射特征,明确了南大港构造带发育水下扇沉积,并可进一步分为内扇主水道、中扇分支水道、中扇水道间、水道前端朵叶体、水下天然堤及决口扇6种微相。自沙河街组沙三段沉积开始,南大港断层活动强烈,在五断块形成了远源水下扇;而在歧南9X1断块发育近源水下扇。两种类型的水下扇在物源、岩性、电性、地震反射特征和形态上都表现出不同特征。近源水下扇以扇形为主,规模小而数量多,以古隆起物源、沉积物粒度粗、电性整体箱形、地震反射杂乱振幅强为特征;远源水下扇以朵叶形为主,规模大而数量少,以三角洲前缘物源、沉积物粒度细、电性整体钟形、地震反射杂乱振幅弱为特征。

Lithology of downthrown block of the southern Dagang fault consists of conglomerate,sandstone,siltstone and mudstone of the 3 ^（rd） member of Shahejie Formation. Sedimentary structures include massive structure,cross bedding,soft sediment deformation and some special features,with the presence of charcoal,plant stem,syndepositional faults and argillaceous sandstone clasts. The grain-size cumulate probability curves can be grouped into one-segment and two-segment type. Besides,four types of well-logging-curve patterns,including box-shaped,bell-shaped,funnel-shaped and straight,were indentified. Reflective characteristics of subaque-ous fan can be identified in seismic profile. Depending on grain size,electrical and seismic reflections,we concluded that the southern Dagang structure belt has developed subaqueous fans,which can be subdivided into the inner-fan main channel,middle-fan distributary channel,middle-fan inter-distributary bay,the lobe of channel front,subaqueous levee and crevasse. Since the deposition of the 3^（rd） member of Shahejie Formation,strong activity of the the southern Dagang fault has led to the development of the distal subaqueous fan in the 5 thfault block. Meanwhile,the proximal subaqueous fan existed in the Qinan 9 X1 fault block. These two types of subaqueous fans have different sedimentary characteristics in provenance,lithology,electrical property,seismic reflection and morphology. The proximal subaqueous fan is characterized by its fan-shape,paleo-uplift provenance,coarse-grained sediments,box-shaped log curve,strong amplitude and random seismic reflection,and small size and large quantity,while the distal subaqueous fan is featured by its bifurcation shape,delta provenance,fine-graind sediments,bell-shaped log curve,weak amplitude and chaotic seismic reflection,and large size and small quantity.