鄂尔多斯盆地东北部侏罗系延安组漂白砂岩成因和古风化壳形成环境探讨

摘要

Bleached white sandstone represent the evidence of hydrocarbon migration in the northeastern Ordos Basin.Previous studies only focus on the chemical bleaching from the perspective of hydrocarbon-rock interaction,but ignoring its forming environment.Bleached sandstones in study area only distribute in the weathered rock under the Jurassic unconformity,indicating unconformity played important role in the formation of bleached white sandstone.Combining field,petrographic,and geochemical analyses,the characteristics of unconformity structure were deeply investigated.The results show that the weathered clay zone is not developed in the Jurassic unconformity.The unconformity structure can be classified into two types:weathered rock zone developed (type Ⅰ) and non-weathered rock zone developed (type Ⅱ).Sandstone in the upper part of weathered rock is composed of quartz and kaolinite,marked by secondary pores by dissolution,kaolinization of biotite,and loss of alkali and alkaline earth elements(K,Na,Ca,and Mg).K-feldspar content is higher in sandstone of the lower part of weathered crust,and biotite was partially weathered to hydro-biotite,K and Mg were retained in K-feldspar and hydro-biotite.Pore fluid exchange was restricted in the fine-grained sediments,with low porosity and permeability,causing weaker weathering process and hydrocarbon-induced alteration.Kaolinite in white sandstone was the product of weathering process,rather than product of hydrocarbon-rock interaction.Weathering enhanced the porosity and permeability of sandstone and oxidation during weathering led to the formation of iron oxides grain coatings,which changed the colour of sandstone from gray/green to red.Hydrocarbon reduction caused migration of Fe and bleaching of red weathered sandstone,which further decreased the degree of cementation after weathering.Hydrocarbon-induced sandstone bleaching is a key record of hydrocarbon migration and accumulation in paleo weathered clastic rock.%鄂尔多斯盆地东北部侏罗系延安组顶部的砂岩漂白现象是油气运移逸散的重要证据,但前人仅从油气蚀变角度对漂白砂岩进行研究,而忽视了其形成地质环境.漂白砂岩分布在不整合面之下的风化壳中,指示不整合结构是其形成的重要地质背景.本文结合野外观察、矿物学和地球化学分析,对该区侏罗系不整合结构特征进行重点剖析,并对漂白砂岩成因进行探讨.结果表明,研究区侏罗系不整合风化黏土层不发育,不整合结构可划分为具半风化岩石和缺失半风化岩石两种类型.半风化层上部砂岩矿物成分以石英和高岭石为主,溶蚀孔隙发育,黑云母普遍发生高岭石化,砂岩中碱、碱土金属元素(K,Na,Ca,Mg)大量流失.半风化层下部砂岩钾长石含量偏高,黑云母不完全蚀变为水黑云母,砂岩中残留K和Mg.风化壳内细碎屑沉积岩物性条件差,与外部流体沟通有限,早期风化蚀变和后期烃类蚀变均较弱.漂白砂岩中的高岭石主要为风化淋滤的产物,砂岩因风化作用物性得以改善而成为重要的油气运移通道,同时富集铁氧化物而呈红色.风化壳内红色砂岩后期受到烃类的还原蚀变而发生褪色漂白,由于先后经历了风化淋滤和烃类还原两种蚀变作用,延安组顶部砂岩表现为胶结程度差、易遭受流水侵蚀的特性.碎屑岩古风化壳中的漂白现象是识别不整合结构参与油气运移成藏的重要标志.