鄂尔多斯西缘冲断带南段构造特征及其对古生界天然气成藏演化的影响

鄂尔多斯盆地西缘处在阿拉善地块、鄂尔多斯地块及北祁连褶皱带之间的特殊构造位置，因而也是构造-沉积演化特征明显不同于盆地本部的一个复杂构造区。本文试图在西缘南段与盆地本部沉积演化特征对比分析的基础上，结合构造演化与古生界天然气成藏关系的研究，评价优选西缘南段古生界天然气成藏的有利区带和目标。主要形成以下几方面的认识：1）相对盆地本部而言，西缘南段早古生代沉积结束的晚、晚古生代沉积开始的早，表现出拉张、裂陷为主的早期构造活动特征，形成上、下古生界两套有利的成藏组合；中生代（尤其是侏罗纪）以来，则由于受冲断推覆构造活动的影响，西缘南段侏罗系及其后续构造层系的地层厚度在横向上变化较大，各构造层系的厚度差异可达千余米以上；2）西缘南段在大规模冲断推覆构造前即已基本成藏，燕山中-晚期以来的强烈冲断推覆构造，使部分地区古生界地层抬升剥露至近地表附近，导致古生界先成气藏遭受大面积破坏；3）在推覆体系的宽缓向斜区、以及主冲断层下盘的原地岩体中，古生界地层保存相对完整，受断裂破坏程度低，仍具有相对较好的保存条件；4）综合分析认为石沟驿向斜、韦州向斜、红寺堡向斜及银洞子向斜等4个宽缓向斜构造区的上古生界，以及处在惠安堡-沙井子主冲断层下盘原地岩体中的奥陶系台缘相带，是西缘南段古生界岩性圈闭气藏勘探的有利目标。

The structural characteritics of south segment of western Ordos thrust belt,and its influence on gas reservoir forming of Plaeozoic

The west rim of Ordos Basin is located in the special tectonic position between Alashan block, Ordos block and Qilian fold belt. Therefore, it is also a complicated zone in tectonic and sedimentary evolution, which is different from the central Ordos Basin. On the basis of comparative analysis of sedimentary evolution characteristics between the south segment of western Ordos and central Ordos Basin, this paper focus on the study on the relationship between tectonic evolution and gas-pool forming in Paleozoic, evaluating the favorable zones and targets of Paleozoic gas accumulation in the south segment of western Ordos. The main understanding has been made as follows: 1）Relative to the central basin, the Lower Paleozoic sedimenting ended late in the western margin, and the Upper Palaeozoic depositing began early in the western margin. It shows the characteristics of early tectonic movement mainly stretching and rifting, forming two sets of reservoir-forming assemble as for Upper Paleozoic and Lower Paleozoic. The stratigraphic thickness of the Jurassic and its subsequent tectonic layer varies greatly in the lateral direction in the western margin, affected by thrusting activity since the Mesozoic（especially the Jurassic）. The thickness difference of each tectonic layer can be more than 1 000 meters. 2）The reservoir in Palaeozoic had formed in south segment of the western margin, before the large-scale thrusting had begun. Because of the strong thrusting since middle and late Yanshan Movement, the Paleozoic strata in some areas are uplifted and stripped near the surface of the earth; It results in extensive destruction of Paleozoic gas reservoirs. 3）In the wide and gentle syncline area in nappe system, and the in-situ rock masses of the footwall of the main thrust fault, the Paleozoic strata are relatively well preserved, the fracturing degree is low, still having relatively good preservation condition. 4）Comprehensive analysis showed that the Upper Paleozoic in Shigouyi syncline, Weizhou syncline, Hongsipu syncline, and Yindongzi syncline these 4 broad synclines, as well as the Ordovician platform margin facies belt in the in-situ rock masses in the footwall of the main thrust fault of the Huianpu-Shajinzi are favorable targets for Paleozoic lithologic trap exploration in south segment of the western margin of Ordos Basin.