歧口凹陷北大港潜山南北向走滑断裂的河道化作用分析

通过北大港地区重力异常资料、三维地震资料分析,证实在北大港潜山上发育具有左旋走滑性质的红7-1断裂,其夹持于大张坨断裂与港西断裂之间,呈弧形沿南北方向展布。该断裂在古近系早中期均有活动,对多个层位砂体分布具有明显的控制作用,在该走滑断裂弧形带南侧,砂体厚度突变增大,呈"点源式"分布,从而形成明显的、且具有继承性的"沟-扇"对应关系。结合重矿物分析及北大港潜山上下切河道追踪发现,在走滑断裂顶部发育一系列下切河道;这些河道下切深度可达230～280ms,它们可以沟通板桥次凹与北大港潜山南翼,在过补偿阶段将板桥次凹的沉积物沿下切河道搬运到北大港潜山南侧的歧北次凹。可见,北大港潜山上近南北向的走滑断裂可以形成下切河道并起到搬运沉积物的作用(即断裂的河道化作用),进而在过补偿阶段控制板桥次凹与北大港潜山南翼的物源分配形式,所以,板桥次凹和歧北次凹在一定程度上均隶属于沧县隆起物源体系。

The Channelization of the NW Striking Slip Fault in the Beidagang Buried Hill of the Qikou Sag

The provenance of the southern Beidagang buried hill in the central Qikou sag has been a matter of controversial,which directly inflict upon the oil and gas exploration program.Based on analyses of the gravity and 3D seismic data of the Beidagang area,we recognized a sinistral strike-slip fault(Hong 7-1) in the Beidagang buried hill,the Hong 7-1 occurs as a NW striking curved fault in-between the Dazhangtuo and Gangxi faults.This fault was active in the early and mid-Paleogene,and controlled obviously the Paleogenic sand distribution.The thickness of sandbody increases abruptly in the south end of the strike-slip fault exhibiting point-sourced distribution with distinct successive " channel-fan" characteristics.Heavy minerals analysis and incised channel tracking show that a series of incised channels with depth up to 230 ～ 280ms developed above the strike-slip fault in the Beidagang buried hill area.These channels joined the Banqiao sag and the south part of the Beidagang buried hill,and transported sediments to the latter.Therefore,the NW strike-slip fault in the Beidagang buried hill led to the formation of the incised channels and channel ways for sediment transportation,and thus,controlled the provenance in the Banqiao sag and south limp of the Beidagang buried hill during the period of over compensation.It can be inferred that the Banqiao and Qibei sags were somewhat controlled by the same provenance system,the Cangxian uplift.