Logging identification of fluid properties in low porosity and low permeability clastic reservoirs: A case study of Xujiahe Fm gas reservoirs in the Anyue Gas Field, Sichuan Basin

The 2nd member of the Upper Triassic Xujiahe Fm in the Sichuan Basin is the major pay zone in the Anyue Gas Field. The reservoirs in this member are characterized by strong heterogeneity, complex pore structure, low porosity, low to ultra-low permeability, and high irreducible water saturation. In addition, these reservoirs are strongly influenced by tectonic structures and lithological facies, the degree of differentiation of gas and water is rather low, the differences in the logging responses of gas layers, intermediate gas-water layers and water layers are ambiguous. Therefore, the identification of fluid types is challenging and the success ratio of formation test is low. In this paper, logging data and formation test data in combination with the characteristics of the Xujiahe Fm reservoirs are used to systematically analyze the logging responses of the gas and water in the reservoirs. Various methods, such as the saturation superposition method, the resistivity-porosity cross-plotting method, the lateral-induction resistivity ratio method, and the compressional-shear wave velocity ratio method are used to identify reservoir properties and reveal the differences between gas, water and dry layers, so as to comprehensively determine the properties of reservoirs in the 2nd member of Xujiahe Fm in the Anyue field. Through tracking and analyzing the new wells drilled during 2011 - 2012, the saturation superimposition and resistivity-porosity cross-plotting methods were finally selected as the major technologies for the discrimination of fluid types. They were applied to identify fluid types of 46 layers. The coincidence rate of logging interpretation was raised from 60% to 83%, and the success ratio of formation test was also improved significantly. These technologies have strong applicability and operability, and can be applied in other blocks in the Middle Sichuan Basin.