鄂尔多斯盆地西南部寒武系风化壳天然气成藏特征

Characteristics of natural gas accumulation in the Cambrian weathering crust in the southwestern Ordos Basin

鄂尔多斯盆地早古生代中央古隆起及其周缘形成的寒武系风化壳一直都是该盆地油气勘探重点关注的目标,并取得了重要的油气勘探突破。为了进一步加快该领域天然气勘探的进程,在系统分析该盆地西南部寒武系风化壳源储配置及圈闭类型的基础上,评价了该盆地西南部寒武系风化壳天然气勘探的潜力以及有利的勘探区带。研究结果表明:①该盆地内上古生界煤系烃源岩是主要的气源岩,与寒武系储层形成“上生下储”型源储匹配关系;②寒武系风化壳储层的储集空间类型以白云岩晶间(溶)孔型及岩溶缝洞型为主,其发育分别受控于颗粒滩沉积微相和风化壳期岩溶作用;③寒武系风化壳圈闭类型主要为岩性—地层圈闭,形成于燕山期鄂尔多斯盆地东部抬升的构造背景,上倾方向存在着岩性及地层遮挡的地质条件,有利于天然气近距离运移与聚集成藏。结论认为,该盆地西南部中央古隆起东侧环县—正宁地区是寒武系风化壳气藏的有利勘探区带,中寒武统张夏组及上寒武统三山子组是该区油气勘探的有利层段。

The Cambrian weathering crust formed in the Lower Paleozoic central paleouplift of the Ordos Basin and its periphery has always been a key target for oil and gas exploration in this basin,and important breakthroughs in oil and gas exploration have been made there.In order to further accelerate the process of natural gas exploration in this field,this paper systematically analyzes the source-reservoir configurations and trap types in the Cambrian weathering crust in the southwestern Ordos Basin.Then,natural gas exploration potentials and favorable exploration zones in the Cambrian weathering crust inf the southwestern Ordos Basin are evaluated.And the following research results were obtained.First,in the Ordos Basin,the Upper Paleozoic coal-measure source rocks are the main gas source rocks,which together with the Cambrian reservoir form a source-reservoir pattern of"upper source and lower reservoir".Second,the types of the reservoir spaces in the Cambrian weathering crust reservoir are mainly dolomite intercrystalline(dissolution)pores and karst fractures and vugs,whose development is controlled by grain shoal microfacies and karstification in the weathering crust stage,respectively.Third,the traps in the Cambrian weathering crust are mainly lithologic-stratigraphic traps,which are formed in the tectonic setting of the eastern Ordos Basin uplifting in the Yanshanian.There are geological conditions of lithologic and stratigraphic barriers in the updip direction,which is beneficial to the short-distance natural gas migration and accumulation.In conclusion,Huanxian-Zhengning area on the east side of the central paleouplift in the southwestern Ordos Basin is a favorable exploration zone of Cambrian weathering crust gas reservoirs,and Middle Cambrian Zhangxia Formation and Upper Cambrian Sanshanzi Formation are favorable intervals of oil and gas exploration.