Exploration and Development Research Institute, Jidong Oilfield Company, CNPC, Tangshan 063000, Hebei, China

Alteration is a common phenomenon in volcanic reservoir,which has great influence on the evaluation of the reservoir. Through the analysis of logging data and clay mineral data of wells A, B, and C in the study area, the logging curves sensitive to alteration are selected and the alteration index I_A is defined, and the degree of alteration is quantitatively evaluated using I_A. Based on the analysis of logging series, according to five logging curves including gamma ray, deep dual resistivity, sonic, density and neutron, the clay mineral content of volcanic reservoir after alteration is predicted by regression. The results show that the higher I_A, the higher the alteration degree and clay content. By fitting the experimental data of core-electrical, the saturation index n after alteration is obtained, and is evaluated according to Archie formula. The results show that the value of n after alteration is bigger than the value of n when it is not altered, because the alteration products chlorite and illite are oil-wet minerals, which increase the saturation index n.