柴北缘地区走滑反转构造及其深部地质因素分析

Strike slip faults reversion and deep geologic setting in north Qaidam Basin

过去普遍认为柴达木盆地北缘地区的冷湖—南八仙构造带的深部为向南逆冲的燕山晚期构造,浅部为向北逆冲的喜马拉雅晚期构造。但通过详细层位对比和构造分析,发现冷湖5号、6号、7号构造与南八仙构造基本为喜马拉雅晚期定型的走滑压扭构造,局部构造主体发育多条高角度走滑断层,构造变形集中于断裂带附近,而断裂带之间的凹陷则未发生大的变形,以沉积、沉降作用为特征。深部地质与地球物理特征揭示,柴达木盆地深部地壳存在低速层,地壳块体可能沿深部断裂带发生东西向的拆离作用。构造分析认为,在侏罗系烃源岩的排烃高峰阶段,冷湖6号构造并没有形成圈闭,而冷湖7号构造的高点与现今不一致,因而现在的构造高部位不具备原生油气藏形成条件。冷湖构造带附近为侏罗系断陷分布区,侏罗系厚度较大,原生油气藏可能分布在该带的周缘地区,包括北部斜坡、喜马拉雅运动早中期的构造圈闭与岩性-地层圈闭,继承性凹陷的深部以及由深凹陷内侧断裂带所围限的断块也是油气运聚的有利部位。

It was considered that the deeper part of double-layer thrust structures developed in Lenghu-Nanbaxian, northern Qaidam Basin, thrusted southward at later Yanshan Movement and the shallower part thrusted northward during the later Himalaya Orogeny. Based on seismic data, landsat images and well data, the paper indicates that the structures of Lenghu 5, 6, 7 and Nanbaxian are compressional strike slip faults developed during later Himalaya Orogeny. The deep geology shows that there are a few low velocity layers under the basin and the dips of major surrounding faults are nearly vertical, the possible detachment direction of deep crust is east-westward. Structural deformation concentrated along the strike slip faults, and the subsidence within the faults kept on developing with no or less deformation. During the maturation of Jurassic source rocks, no trap was formed on the Lenghu 6 structure, and the tops in the Lenghu 7 structure were different from the present ones. The Jurassic fault depression was distributed along the Lenghu-Lingjian belt, the primary accumulations may be distributed around the belt, including northern slope, structural traps developed during earlier and middle Himalaya Orogeny, and lithologic and stratigraphic traps. The deep parts of succeeding sag and their surrounding blocks controlled by deep faults are also favorable for petroleum accumulation.