Logging Evaluation on the Effectiveness of Karst Fractured-Vuggy Reservoirs in the Maokou Formation, Sichuan Basin

Logging evaluation was carried out to solve the problems of high levels of heterogeneity, complexity with respect to typing reservoir spaces, and difficulty in evaluating the reservoir effectiveness in carbonate reservoirs in the Maokou Formation, Sichuan Basin. By analyzing the pore structure of the fractured-vuggy reservoirs of the Maokou Formation in the working area, the reservoir space was classified based on the tri-porosity model and the relationship between bond index and total porosity, connected fracture-vug porosity and isolated fracture-vug porosity. The information was then combined with core calibration logging, the mean value and variance of apparent porosity spectrum and apparent formation water resistivity spectrum extracted from micro-resistivity scan imaging logging data, and fracture porosity and other sensitive parameters that reflect pores, vugs and fractures, as well as the data from well testing. Integrating above information, and evaluation standard for reservoir effectiveness was established as follows: For porous and vuggy reservoirs, the mean value of electric imaging apparent porosity spectrum of Class I reservoir is greater than 1.9 and the corresponding variance is greater than 1.2, and those of Class II reservoirs are greater than 1.7 and 0.9, respectively. For fractured reservoirs, the fracture porosity of Class I reservoir is greater than 0.30 while that of Class II reservoir is between 0.05 and 0.30. Enhancing the connectivity of fractures can obviously improve the effectiveness of the reservoir. For the porous-fracture and vuggy-fracture reservoirs, the mean value of apparent formation water resistivity spectrum of Class I reservoir is greater than 700, and the variance is greater than 300, and those of Class II reservoirs are 500–700 and 100–300, respectively. According to the evaluation standard, a secondary interpretation of 20 exploration wells in the working area was carried out, which effectively improved the interpretation coincidence rate of fractured-vuggy reservoirs, and resulted in achieving a good application effect.