鄂尔多斯盆地南缘上奥陶统平凉组浅钻中古岩溶洞穴的发现及其意义

Discovery and significance of the Ordovician paleo-karst caves in shallow drilling in the southern Ordos Basin

金晓辉 张军涛 孙冬胜 丁茜 杨佳奇

Xiaohui Jin Juntao Zhang Dongsheng Sun Qiang Ding Jiaqi Yang

鄂尔多斯盆地南缘（鄂南）奥陶系具有与盆地中东部完全不同的沉积、岩溶环境和岩溶储层特征，其中设计的一口浅钻井揭示了碎屑物质充填岩溶洞穴的存在。浅钻钻遇上石炭统本溪组和上奥陶统平凉组两套地层，本溪组岩性为铝土质泥岩，平凉组主要颗粒灰岩和藻灰岩，并夹有多套泥岩层。利用矿物成分、主量元素、微量元素和稀土元素特征，对比分析了两套泥岩的来源、风化蚀变程度、氧化还原环境、气候和古构造环境。结果表明，本溪组泥岩代表了更高的风化程度，平凉组泥岩沉积时所处的还原环境比本溪组泥岩更强，两套泥岩都反映了晚古生代海陆过渡的沉积环境、变化频繁的气候环境以及活动大陆边缘-大陆岛弧的构造背景。平凉组泥岩形成时间为鄂南隆升阶段湿润岩溶发育期，为风化蚀变的陆源碎屑，代表了前石炭纪岩溶期洞穴和暗河堆积。结合泥岩段上、下裂缝普遍发育的特征，认为鄂南地区曾经存在过岩溶裂缝-洞穴系统，虽然浅钻仅仅钻遇了代表着已经被碎屑堆积充填的洞穴，但可以推测在合适的古地貌位置可能存在残余的裂缝洞穴，是未来进行油气勘探值得期待的领域。

The Ordovician sedimentary and karst environment as well as karst reservoir characteristics in the southern margin are completely different from those in the central-east of Ordos Basin.A shallow well drilled into the southern margin revealed the existence of karst caves filled with clastic rocks.Two sets of strata were recognized as the Benxi and Pingliang Formations, with the former dominated by bauxite mudstone and the latter by grainstone and algal limestone interbedded with multiple sets of mudstone layers.Mineral composition, major element, trace element and rare earth element analyses were carried out to compare the source, weathering alteration degree, redox environment, climate and paleo-tectonic environment of the two sets of mudstones.The results show that the Benxi Formation mudstones are of higher-degree weathering and the Pingliang Formation mudstones were deposited in a more reductive setting.The sedimentary environment of both sets represents a transition from sea to land with frequent climate changes against tectonic backgrounds of active continental edge and continental island-arc in the Late Paleozoic.The Pingliang Formation mudstones of terrigenous clast origin that represents cavities and underground river deposits in the pre-Carboniferous karst period, deposited during wet karst development stage when the southern Ordos Basin were uplifting.Considering the widespread fractures at the upper and lower parts of the formations, we suggest that a karst fracture-cavity system once existed in the southern Ordos Basin.Although the caves revealed by the well are filled with clasts, it can be inferred that there may be residual fractures and cavities at certain paleomorphological locations in the southern Ordos Basin, which are of great value to future oil and gas exploration.

10.11743/ogg20210306