鄂尔多斯盆地加里东期不整合面类型及分布特征*

加里东期不整合面是早古生代华北板块内一个重要的盆地性质转换界面。鄂尔多斯盆地奥陶系风化壳岩溶型油气藏和二叠系本溪组、太原组铝土质泥岩气藏的发现,促进了地质学家对该地区古生代加里东运动性质的探讨。基于大量的野外露头、钻井和地震资料,对加里东运动形成的不整合面的识别标志、不整合类型及其空间分布、不整合结构和时间变量进行深入解析。研究表明: (1)加里东运动后,鄂尔多斯盆地的古地貌形态大致可分为鄂托克旗—定边一带的高地貌区、神木—靖边—富县以及吴忠地貌过渡区、中东部地势低缓东倾区; (2)奥陶系顶部不整合面相邻地层呈角度各异的接触关系,具有“削截(下)—整一(上)”、“削截(下)—超覆(上)”的结构,整体表现为盆地边缘为具削截和褶皱的高角度不整合、盆缘向盆内过渡地区的低角度不整合及盆内大范围分布的平行不整合; (3)不整合面也是上、下古生界之间形成的主要风化壳界面,长期的沉积间断和风化剥蚀为该区铝土质泥岩发育奠定了环境背景,而铝土岩不仅可作为下古生界风化壳气藏的理想盖层,同时也是陇东勘探新区天然气大规模聚集成藏的有利场所。

Types and distribution characteristics of the Caledonian unconformities in Ordos Basin

The Caledonian unconformity is an important interface for transformation of basin types in the Early Paleozoic North China plate. The discovery of Ordovician weathered crust and paleokarsts as the oil and gas reservoirs and aluminous mudstone gas reservoirs in the Benxi and Taiyuan formations in the Ordos Basin has promoted geologists to explore the Caledonian orogeny in the Paleozoic in this area. Based on a number of field outcrops,drilling and seismic data,the following understandings are obtained with respect to the identification marks,geography,structures and time variables of the unconformity formed by the orogeny. (1)The paleogeomorphic formed after the Caledonian orogeny can be roughly divided into the highland area of Etok Banner-Dingbian area,the geomorphic transition area of Shenmu-Jingbian-Fuxian and Wuzhong areas,and the low-lying and gently eastward inclined area in the Middle East. (2)The adjacent strata of the unconformity surface at the top of the Ordovician are in contact with different angles,and have the structure of “truncation(bottom)-integration(top)”,“truncation(bottom)-overlap(top)”,which is characterized by a high-angle unconformity of cutting and folding at the basinal edge,a low-angle unconformity from basinal edge to the basin,widely distributed parallel unconformity in the basin. (3)The unconformity in this area is also the main weathering crust interface formed between the Upper and Lower Paleozoic. The long-term sedimentary discontinuity and weathering and denudation have laid an environmental background for the development of aluminous mudstone in this area. Bauxite is not only an ideal caprock for the Lower Paleozoic gas reservoir,but also a favorable place for large-scale accumulation of natural gas in the Longdong exploration new area,which is worthy of attention.