The paper introduces the complex approach in determining possible geological reasons causing rate decline in the wells of T field, Russia. Therefore, possible geological reasons are sequentially considered, which were divided into three main groups: 1) rate decline due to poorer reservoir quality; 2) rate decline due to facies lateral substitution; 3) rate decline due to active fault tectonics. The most appropriate facies models are constructed using all available data and in this work core, well logging, seismic and well test data were integrated for the fullest reservoir characterization. Core from several recently drilled wells was described in details determining clue features. Further, seismic data was interpreted: both structural interpretation, including faults and attribute analysis were implemented. Appropriate electrofacies models were chosen as well. On the final stage all previously-mentioned data was integrated with appropriate facies model construction. However, as turned out later, facies model is not the key factor affecting rapid decline appeared in some wells. It is suggested that very proximal faults can be the possible explanation. To confirm this suggestion, well test data was additionally used that show consistency of this theory.