松辽盆地营城组玄武岩流动单元测井响应特征

流动单元是玄武岩地层的最基本组成单元,其内部分带性控制储层的储集性能和有效储层的分布位置.运用钻井岩心资料建立了玄武岩流动单元分带地质模式,单个流动单元由上而下依次为上部气孔带、中部致密带和下部气孔带.依据自然伽马(GR)、声波时差(DT)、补偿密度(RHOB)、深侧向电阻率(LLD)和中子孔隙度(NPHI)分析流动单元测井响应特征,判别蚀变带、气孔带和致密带3种测井相类型;自然伽马能谱(K、Th、U)用于判别玄武岩地层中自然伽马高值的界面性质.蚀变带具有高伽马、高声波时差、高中子和低阻、低密度的特征,致密带呈低伽马、低声波时差、低中子和高阻、高密度,气孔带介于两者之间.流动单元分带的测井响应特征反映的是孔隙、裂缝发育和蚀变作用在纵向上的渐进式和韵律性变化.K-GR交会图用于区分蚀变带和气孔带,DT-RHOB交会图用于区分致密带和蚀变带、气孔带.单个或多个流动单元构成喷发单元,喷发单元之间具有明显间断,发育蚀变带、风化带、沉积夹层和凝灰岩层4类界面,由于K含量高于喷发单元内部玄武岩而呈现高自然伽马,Th-U交会图、K-Th/U交会图用于进一步判别界面类型.

Characteristics of well-logging response to lava flow units of the Lower Cretaceous basalts in Songliao Basin

Flow units are the most elementary constituents of basaltic lava strata, and the zonation of pores and fractures determines the capability and distribution of basaltic reservoirs. Both outcrop and borehole information are used to build the geologic model of basaltic lava flow units, indicating that each unit is composed of three parts such as the upper vesicular zone, the central massive zone and the lower vesicular zone from top to bottom. Three electro-facies including alteration zone, vesicular zone and massive zone were identified by combining the natural gamma log (GR), acoustic log (DT), density log (RHOB), electrical resistivity (LLD) and neutron log (NPHI). Natural gamma-ray spectral logging (K, Th and U) were also utilized to distinguish intervals with abnormal high gamma values. Alteration zone is characterized as high in GR, DT and NPHI values while low in LLD and RHOB values, contrarily does the dense zone, and vesicular zone lies in between. Logging response to the zonation of basaltic flow units indicates that porosity, fracturing and alteration changes gradually and cyclically in the vertical profile. Alteration and vesicular zone are differentiated by K-GR cross-plots, and massive zone is further discriminated with them by DT-RHOB cross-plots. Eruption units are composed of single or several flow units with long volcanic quiescence in between, four types of boundaries with abnormal high GR values as a result of enriched K contents are distinguished including alteration and weathering zone, sedimentary and tuff interbeds. Moreover, the four boundaries are differentiated with Th-U and K-Th/U cross-plots depending on their different origins.