摘要： 奥陶系平凉组是鄂尔多斯盆地早古生代深水沉积最发育的地层,研究平凉组沉积体系类型和特征,不仅有助于深入理解平凉组沉积时期的古地理格局和构造特征,而且对于油气勘探也具有重要的价值。基于对盆地西南缘平凉组及其相同时代地层的野外调查和室内综合研究,从地层的空间分布、岩石类型、结构构造、沉积相、有机地化特征等方面,对鄂尔多斯盆地平凉组进行了系统研究。研究表明,平凉组发育深水原地沉积、牵引流沉积、重力流沉积、火山碎屑岩及震积岩,形成一套台地前缘斜坡和深水盆地沉积组合。结合构造背景和沉积组合,建立了平凉组的深水沉积模式: 平凉组沉积初期水体较浅,等深流沉积不发育,以重力流沉积为主;到中晚期,随着水体的加深,重力流减弱,先存沉积普遍受到等深流的改造。平凉组的深水沉积同样也有利于油气成藏。综合考虑地层厚度、岩石类型、沉积相、有机质类型和含量及热演化程度,优选出了平凉组的有利油气勘探区,主要分布于乌海以南、石嘴山—贺兰以东、鄂托克前旗以西之间,呈南北向条带状展布,面积约1×10⁴ km²。勘探核心区位于贺兰以东、天环坳陷内的部分区域。

关键词: 深水沉积, 重力流, 等深流, 平凉组, 鄂尔多斯盆地

Abstract: Deep-water deposits are well developed in the Early Paleozoic Pingliang Formation of the Ordos Basin. The investigation on the sedimentary system types and characteristics of the Pingliang Formation is not only helpful to understand the paleogeographic pattern and tectonic characteristics,but also has an important implication for oil and gas exploration. Based on the field survey and indoor comprehensive study of the Pingliang Formation and its contemporaneous strata in the southwest margin of the basin,we have systematically studied the Pingliang Formation in the Ordos Basin from the perspectives of stratigraphic spatial distribution,rock types,structure and texture,sedimentary facies,organic geochemical characteristics,etc. Our work indicated that there are deep-water autochthonous deposits,traction current deposits,gravity current deposits,pyroclastic rocks and seismites deposits in the Pingliang Formation,forming a sedimentary assemblage of front platform slope and deep-water basin facies. Combined the tectonic setting with sedimentary facies assemblage,the deep-water sedimentary model of the Pingliang Formation was established. The water depth of the Pingliang Formation was shallow at the initial stage of sedimentation with undeveloped contour current deposits,while the gravity flow deposits were dominant. Towards to the middle and late developed period,the gravity flow weakened with the deepening of water level,which leaded to rework of the preexisting deposits by contour currents. In addition,the deep-water deposits of the Pingliang Formation are conducive to oil and gas accumulation. Based on stratum thickness,rock type,sedimentary facies,organic matter type and content,as well as thermal evolution degree,favorable oil and gas exploration areas in the Pingliang Formation are selected in this research,which are mainly distributed in south of Wuhai,the east of Shizuishan-Helan and the west of Otog Qianqi. The area is about ten thousand square kilometers and is banded distributed in north-south direction. Finally,it is found that the key area of exploration is mainly distributed in part of the Tianhuan depression,east of Helan.

Key words: deep-water deposit, gravity flow, contour current, Pingliang Formation, Ordos Basin

中图分类号: 

• P534.42