摘要： 在加里东期构造作用下,鄂尔多斯盆地中晚奥陶世整体抬升,直至晚石炭世再次接受沉积,期间经历了近1.5×10⁸年的风化剥蚀,发育对天然气聚集具有重要意义的奥陶系碳酸盐岩风化壳。文中选取鄂尔多斯盆地南缘1口典型井,依据矿物成分、主微量元素和测井响应特征分析该套风化壳的垂向结构、风化蚀变程度和古气候环境,进而复原其形成过程并探讨相关保存机制。研究结果表明: (1)风化壳自上而下可分为风化残积层、强风化带、弱风化带和基岩层,整体风化程度较强,各层段的测井响应特征均十分显著。(2)在该风化壳剖面中,TiO₂、Al₂O₃和Fe₂O₃富集,SiO₂轻微亏损,Ca、Mg和Na较强亏损。(3)根据古纬度数据,结合相关地球化学指标,认为鄂尔多斯盆地南缘晚古生代发育海陆过渡相沉积,风化壳形成时的年平均气温为28.15 ℃,处于潮湿—半干旱的环境,属热带—亚热带气候。(4)风化壳各层段裂缝普遍发育,表明鄂尔多斯盆地南部地区存在岩溶裂缝-洞穴系统,虽后期因地貌高差影响导致风化壳各结构层厚度不一,但在地势较为平缓地区,风化壳结构层的保存机率更大,因而是后期风化壳气藏勘探的有利场所。上述结果可为今后鄂尔多斯盆地马家沟组岩溶风化壳型油气藏和本溪组含铝岩系的勘探提供一定的借鉴。

关键词: 鄂尔多斯盆地, 奥陶系, 风化壳, 地球化学, 古岩溶

Abstract: During the Caledonian tectonic movement period,the Ordos Basin was uplifted as a whole in the Middle and Late Ordovician,and underwent deposition again in the Late Carboniferous. It has experienced nearly 150 million years of weathering and erosion,resulting in the development of an Ordovician carbonate weathering crust,which is highly important for natural gas accumulation. In this paper,a typical well was selected from the southern margin of the Ordos Basin. Based on the mineral composition,major and trace elements,and logging response characteristics,the vertical structure,degree of weathering and alteration,and paleoclimate environment of the weathering crust were analyzed,and the formation process and preservation mechanism were reconstructed. The results indicate that: (1)The weathered crust can be divided into a weathered residual layer,a strongly weathered layer,a weakly weathered layer,and a bedrock layer from top to bottom. The overall weathering degree is strong,and the logging response characteristics in each layer segment are very significant. (2)In this weathering crust profile,the main elements TiO₂,Al₂O₃,and Fe₂O₃ are enriched,with slight depletion of SiO₂ and strong depletion of Ca,Mg,and Na. (3)Based on paleolatitude data and relevant geochemical indicators,it is believed that the southern part of the Ordos Basin developed transitional sedimentary facies between sea and land during the late Paleozoic. The average annual temperature during the formation of weathered crust was 28.15 ℃,and it was in a humid-semi-arid environment,belonging to a tropical-subtropical climate. (4)The widespread development of cracks in each layer of the weathered crust indicates the existence of a karst fracture-cave system in the southern region of the Ordos Basin. Although the thickness of each structural layer of the weathered crust varies due to the influence of terrain elevation differences in the later stage,the probability of preserving the weathered crust structural layer is greater in areas with relatively flat terrain,suggesting that this location is favorable for later exploration of weathered crust gas reservoirs. These results can provide a reference for the exploration of oil and gas reservoirs in karst weathering crusts in the Majiagou Formation and aluminum-bearing rock series in the Benxi Formation in the Ordos Basin in the future.

Key words: Ordos Basin, Ordovician, weathering crust, geochemistry, paleo-karst

中图分类号: 

• P512.1+3