Study on reservoir characteristics and evaluation methods of altered igneous reservoirs in Songliao Basin, China

To accurately evaluate altered volcanic reservoirs in the Songliao Basin of China and study the effect of alteration on pore structure, this paper established a methodology for log evaluation based on conventional log data. The volcanic reservoir in the Songliao Basin, China is variable in lithology, complex in pore structure and widespread in alteration phenomena. Because of these problems, the conventional charting method is not good at lithological identification, and the reservoir interpretation conclusions based on interpretation of the conventional logging method are not consistent with the actual logging data, making it difficult to carry out accurate logging evaluation. To solve such problems, this study adopted the particle swarm optimization-support vector machine (PSO-SVM) for carrying out lithological identification. Then, alteration factors were established to evaluate the alteration degree of volcanic reservoirs and obtain the altered clay content by fitting to correct the effect of alteration. Designing the volume model that considered altered clay and selecting the Simandoux model that could correct the influence of clay to calculate relevant petrophysical parameters such as porosity and water saturation, thus formed a methodology of logging evaluation suitable for the altered volcanic reservoir in the study area. At the same time, the study found that for different volcanic rocks, the effects of alteration on pore structure are different. An analysis, in combination with alteration factors and T2 NMR spectra, showed that for the volcanic reservoir of nonsevere alteration, the higher the alteration degree, the greater the total porosity and the void porosity. However, with increasing alteration degree, the effective porosity of andesite was higher, while that of tuff worsened. The findings may provide a new idea for the analysis of the pore structure of altered volcanic reservoirs.