用于计算火成岩储层基质孔隙度的首选测井曲线

火成岩储层具有岩性复杂、储集空间复杂、孔隙结构复杂和蚀变程度高等特征,使得其声波、中子、密度、核磁和地层元素测井的响应与沉积岩有较大的差别,通常适用于沉积岩基质孔隙度计算的测井曲线并不完全适应于火成岩储层,为此进行了深入研究。结果表明:火成岩储层具有孔径差异大,孔隙分布不均匀、各向异性强的显著特征,理论分析得知该类储层的声波测井值往往偏低,同时,大量实际测井资料也揭示声波对火成岩岩性反应不敏感;火成岩蚀变程度高,蚀变后的一些矿物含氢指数高,使补偿中子测井值显著增加;对理论模拟、测井资料和实验资料进行综合分析后认为,火成岩的强磁化率特征使岩石孔隙内部产生强磁场梯度,使T2谱前移明显、幅度降低,核磁孔隙度明显偏低;地层元素测井孔隙度在部分井误差大,反映该方法在火成岩地层中使用的局限性;而密度测井却能有效反映地层的总孔隙度,实际资料显示密度孔隙度与岩心分析孔隙度误差较小。因此,火成岩储层基质孔隙度的计算应首选密度测井曲线。

The preferred logging curve for the calculation of matrix porosity of volcanic reservoirs

Volcanic rocks are characterized by complex lithologies, reservoir space and pore structure as well as strong alteration, making their responses of sonic logging, neutron logging, density logging, NMR logging and strata element logging quite different from those of sedimentary rocks. Such logs commonly suitable for the calculation of matrix porosity of sedimentary rocks are not completely suitable for volcanic reservoirs. Volcanic reservoirs have large variations of pore diameters, uneven distribution of pores and strong heterogeneity. Theoretical analysis shows that their sonic logging porosity is generally lower than the normal value. In addition, a large amount of real logging data also shows that sonic wave is not sensitive to the lithology of volcanic rocks. Volcanic rocks are high in alteration and some altered minerals have a high hydrogen index (HI), causing the significant increase of compensated neutrons. Theoretical simulation and comprehensive analyses of logging data and experimental data reveal that the strong magnetic susceptibility of volcanic rocks generates a strong magnetic field gradient within pores, resulting in significant forward moving ofT₂ spectrum and lowering of T₂ amplitude, thus their NMR porosity is significantly lower than the normal value. Great errors of the strata element logging porosity were found in some wells, indicating this method′s limitation in application to volcanic rocks. In contrast, the density logging can effectively reflect the gross porosity of reservoirs, and real data show that the density logging porosity matches well with the porosity data by core analysis. Therefore, the density log should be the first choice for matrix porosity calculation of volcanic rocks.