柴达木盆地东坪地区基岩风化壳特征

摘要

Large-scale distributed gas fields are discovered in frontal slope of the Altun Mountain in Qaidam Basin and the role of the basement weathering crust becomes the concerns.Basing on element analysis,X-ray diffraction analysis,cores and slices observation,conventional logging and image logging,structures of the basement weathering crust are identified and each structure owns different reservoir properties.The result shows that the structure of the basement weathering crust of Dongping area in Qaidam Basin can be divided into three layers,soil layer,completely weathered layer and semi-weathered layer which can be further divided into two zones,dissolution weathered zone and fracture weathered zone.The thickness of soil layer is 0-2 m.The thickness of completely weathered layer is 4-15 m.The thickness of dissolution weathered zone is 36.5-164 m.The thickness of fracture weathered zone is 300-1,000 m.The semi-weathered layer is the reservoir of the basement weathering crust.The reservoir property of the dissolution weathered zone is better than that of fracture weathered zone.The dissolution weathered zone with porosity range from 2％-16％ develops dissolved pores and map fractures with high degree of dissolution,and the fracture weathered zone with porosity range from 2％-8％ develops tectonic fractures and joint fractures with low degree of dissolution.Large-scale development of the basement weathering crust in Dongping area provides the basis for finding hydrocarbon in the slope area of Qaidam Bain far from the source rock.%柴达木盆地阿尔金山前斜坡带发现了规模分布的东坪基岩气田,其基岩风化壳的作用成为关注的问题.依据元素分析、X-射线衍射分析、岩心薄片观察,常规测井响应和成像测井响应特征,识别出东坪地区基岩风化壳发育不同结构层,且不同结构层的储集特征有很大差异.研究表明,基岩风化壳结构可划分为土壤层、完全风化层和半风化层,而半风化层又可进一步分为溶蚀带和崩解带;其中土壤层厚度0～2m,完全风化层厚度4～15 m,溶蚀带厚度36.5～164 m,崩解带厚度300～1 000 m.基岩半风化层是储层发育带,其中溶蚀带储集物性好于崩解带,溶蚀带发育较多的溶蚀孔洞和溶蚀加宽的网状裂缝,孔隙度范围2％～16％;而崩解带发育弱溶蚀构造缝和节理缝,孔隙度范围2％～8％.东坪地区大规模发育基岩风化壳为柴达木盆地远离烃源岩灶的斜坡地区寻找油气提供了借鉴依据.