The Characteristics of Lacustrine Mixed Deposit in Lower Xiaganchaigou Formation and its Effect on Reservoir Property: A Case in the Central Areas of Northern Qaidam Basin
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摘要: 柴达木盆地北缘的古近系下干柴沟组下段是油气勘探的重要目标，查明该区储层发育的控制因素是深入勘探的重要基础。根据岩屑录井、普通薄片以及铸体薄片等资料对该区冷湖七号和南八仙构造的储层特征进行了对比分析。研究表明，该区大量发育陆源碎屑—碳酸盐的混合沉积，但是由于水体咸度的不同：在冷湖七号的滨浅湖混积岩中碳酸盐和硬石膏含量较高，颗粒主要为基底式胶结；而南八仙构造地处辫状河三角洲前缘相带，因河流的稀释作用，混积岩中硬石膏含量很少，颗粒为孔隙式－基底式钙质胶结，原生孔喉较为发育，有利于后期酸性流体的溶蚀改造。两种类型混积岩的初始孔隙度以及孔喉演化模式的不同，是最终引起两个地区含油气丰度巨大差异的一个原因。Abstract: The lower part of Xiaganchaigou Formation is an important prospecting object in the central areas of northern Qaidam basin. To find out the factors controlling the reservoir property is the basis of prospecting. Based on the normal thin sections, cast thin sections and drilling data, the rocks characteristics between Lenghuqihao and Nanbaxian were correlated. The mixed deposits were found very common in the central part of northern Qaidam basin，but the cementaion type and the cement composition vary obviously in different sedimentary facies. Because of the high salinity of Lenghuqihao area which located in littoral facies，the cements contains a lot of anhydrite and the sandstones are mainly basal cemented. The salinity of Nanbaxian located in delta front facies is lower because of the great mounts of water input by the rivers, so there is little anhydrite in the sandstones which are porous cemented to basal cemented. The initial porosity in Nanbaxian is much higher than Lenghuqihao, and the sandstones were much more easily dissolved. The abundance of petroleum in two aeras is very different as the great difference in initial porosity and porosity evolution model.
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Key words:
- Qaidam basin /
- mixed deposit /
- reservoir property /
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- initial porosity
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