柴达木盆地台南气田低阻气藏成因机理及测井评价

柴达木盆地台南气田第四系下更新统涩北组气层普遍具有低电阻率特征,在高矿化度地层水环境下,复杂的地质条件使得低阻气层与中、高阻气层的识别难度加大。为此,利用岩心分析化验、录井、测井、构造及试气等资料,结合低阻气层的测井响应特征,从地质及工程因素入手,对该区低阻气层成因机制进行了系统深入的剖析。结果表明:涩北组低阻气层的主要地质成因是由于水动力条件较弱的浅湖沉积环境下发育的岩性粒度细、黏土矿物含量高以及低幅度构造背景下的高束缚水饱和度,而地层水矿化度高、复杂的孔隙结构、极强的非均质性和微裂缝发育进一步降低了气层电阻率;钻井液侵入与地层浸泡时间对电阻率测井造成的影响是外部工程因素。在此基础上所建立的变孔隙结构指数m和饱和度指数n的方法,能有效地提高含水饱和度的计算精度,"侵入因子"与"镜像反映"的交会图以及电阻率—孔隙度—含气饱和度交会图能够有效地识别气水层。该方法为气田的有效开发提供了技术保障。

Genetic mechanism and logging evaluation of low resistivity gas reservoirs in the Tainan Gas Field,eastern Qaidam Basin

The gas layers in the Quaternary Lower Pleistocene Sebei Fm in the Tainan Gas Field, eastern Qaidam Basin, are commonly low in resistivity and are difficult to differentiate from medium and high resistivity gas layers with high salinity formation water and complex geologic conditions. In combination with the logging responses of low resistivity gas layers, we systematically studied the genetic mechanisms of the low resistivity gas layers by integrating various data such as core, logging, structural and gas testing data. Geologically, the major genesis of the low resistivity gas layers in the study area are the fine grain size and high clay content of reservoir rocks deposited in shallow lakes with relatively low energy and the high irreducible water saturation formed in a low amplitude structural setting. In addition, the high salinity of formation water, complex pore structure, strong heterogeneity and highly developed microfracture further lower the resistivity of gas layers. From the engineering point of view, drilling mud invasion and soaking time have significant influences on resistivity logging. The variable pore structure index (m) and saturation index (n) proposed based on the analysis can effectively improve the calculation accuracy of water saturation. The cross plots of "invasion index" vs. "mirror image" and resistivity vs. porosity vs. gas saturation can be effectively used to identify gas and water layers.