Logging evaluation for low permeability reservoirs under different mud systems in East China Sea

The East China Sea has rich oil and gas resources, in which the low porosity and low permeability reservoirs account for two-thirds of the total and hence are of great importance for exploration. It is occasionally found during exploration that some of the reservoirs have good physical properties, but low production. Efforts have been made to find out the reasons for such a phenomenon and the ways to improve the productivity. Our study found that water-based mud has irreversible pollution to the oil and gas reservoir, and thus oil-based mud is recommended. In the new mud system, the environment for well logging and mud logging are opposite. In the environment of oil-based mud, the invasion phenomenon is completely different for oil-gas reservoir and water reservoir. For oil and gas reservoirs, there is no mud invasion. For water reservoirs, however, mud invasion is common. Besides, the mud environment has great impact on the component of gas logging and fluorescence data. Some of the components may be absorbed by oil based mud and the fluorescence disturbed. In conclusion, the influence of oil based mud can not be ignored, when identification and quantitative evaluation are made for oil, gas and water reservoirs. By comparing some well logging and mud logging interpretation methods in different mud environment, we have established in this paper a fast and effective method for identification of oil and gas reservoirs in the East China Sea.