南堡凹陷滩海地区不同级别断裂活动及其对沉积的影响

Different Levels of Rift Activity and Its Impact on Deposition in Offshore Area,Nanpu Sag

南堡凹陷是渤海湾盆地中一个小型裂谷成因的陆相沉积盆地,盆地内不同级次的断裂活动影响着砂体的展布。所以在研究区中进行层序地层以及沉积相研究须以构造演化研究作为轴线,通过对不同级次断层活动史的分析解释沉积砂体在盆地内部的充填特征。基于580 m岩心的观察以及覆盖整个工区的三维地震资料,对南堡凹陷滩海地区进行了详细的沉积相与断层演化的研究。古近系沙河街组到东营组沉积时期该地区发育有七个裂陷亚幕。每个裂陷亚幕之间的沉积具有继承性,而裂陷内部砂体的垂向分期主要受湖平面的升降变化控制。湖盆边缘的断层影响朵叶体的迁移摆动以及沉积相的发育,湖盆内部次级断层则控制砂体的再次分配。明确不同级别断裂活动对沉积砂体展布的影响,为后续的勘探提供了新的思路。

Nanpu sag, located in Bohai Bay Basin, is a small rift lacustrine basin. In study areas, the sand distribu tions were effected by the fault activity in different levels and thus study of sequence stratigraphy and sedimentary should be guided by tectonic evolution as a principal axis. Through the analysis of faulting histories in different levels, the filling characteristics of sand body inside the basin can be explained. Combining 580 m core observation and 3D seismic data which covered the entire study area, sedimentary facies and fault evolution in offshore area of Nanpu Sag were defined. Periodic rifting activity in Nanpu sag could be classified into two periods from 31 Ma to 23.5 Ma, one is Shahejie Formation and the other is Donying Formation. Shahejie Formation can be divided into three sub-episodes which correspond with three third order sequences and only two sequences in Shahejie Formation would be discussed because of the overlying strata of study area were eroded in the course of the First member of Shahejie Formation and four third sub-episodes, which correspond to 4 third order sequences, were identified in Dongying Formation. Fan delta and nearshore subaqueous fan were formed in the hanging wall of boundary fault, and the braided river delta de posits were developed in the gentle slope belt and the gravity flow deposits were found in deep lake and semi-deep lake area. In the same tectonic unit, steep region or gentle slope, for example, though the feature of sedimentary facies e volution is inheritance, vertical stages are obvious, by analyzing the filling sequences evolution sections. Active inten sity of tectonic remains constant in each sub-episode, so vertical stages of sand bodies in the form of foreset and retro grade were controlled mainly by lake-level fluctuations which also have some impact on vertical stages between each sub-episode. After calculated and analyzed the fault activity velocity of Bogezhuang Fault, Gaoliu Fault and the faults in the transition zone coupled with the plans of sedimentary facies, to make clear the control of the fault to the deposi- tion and filling, figured out the distributions of detrital material in catchment basin are strongly controlled by different level of faults which were classified into boundary fault and secondary fault. The activity of boundary fault with time shared and segmentation control the migration and swing of fan delta deposit around steep slope through controlling the swing of transfer zone around the margin of basin. Some gravity flow deposits caused by earthquake or delta frontal in- stability occurred re-distribution along the transfer zones, which belong to secondary fault between the edge and inner of the basin. As the secondary fault movement is a multi-stage process, the inner transfer zones control the redistribu tion of autogenic deposits. All in all, the deposition and filling of basin were controlled by the fault evolution, activity intensity and tectonic styles and structural associations. The lake-level variation and climate exchange also play a very important role in determining the vertical stages in sub-episode. The research on fault activity in different levels and its impact for the depositional sand distribution can provide a new idea for the subsequent exploration.