A study on water saturation predictions in igneous reservoirs based on the relationship between the transverse relaxation time and the resistivity index

Maintaining the accuracy of well-logging interpretations in igneous reservoirs has long been a difficult problem. The igneous gas reservoirs in eastern China are important exploration targets at the present time. It has been found that, when compared with sedimentary rock reservoirs, the properties of igneous rock change faster with depth, and the reservoir spaces tend to be more complex. In addition, the porosity and permeability are lower, making the evaluations of water saturation more challenging. Generally speaking, the Archie Equation parameters can be obtained through a large number of laboratory core measurements, and then applied to the logging interpretations of water saturation. However, the longitudinal heterogeneity of reservoirs is neglected by fixed parameters which are referred to as “static parameters”. In the current research investigation, in order to improve the accuracy of the logging interpretations, a method of segmentation characterization was used to invert the Archie's Equation parameters from NMR logging data, based on the relationships between the transverse relaxation times (T2) and the resistivity indexes (I). Subsequently, the dynamic Archie parameters which changed with depth were obtained using the aforementioned method, which had not only considered the longitudinal heterogeneity of the reservoirs, but also provided a new interpretation method for the water saturation of igneous reservoirs. In addition, this study also analyzed the relationships between the key parameters and pore structure parameters in the designed model. It was found that the data distributions of igneous rock and sedimentary rock had displayed opposite trends. Therefore, the results of this study confirmed that the conductivity of igneous rock is affected by the proportion of macropores. •We have verified the applicability of T2-I model (which we have validated in tight sandstone and shale) on igneous rocks.•According to the pore structure, the relationship between transverse relaxation time and resistivity index is established.•We provide a workflow for researchers to use our model to obtain dynamic Archie formula parameters for joint inversion of water saturation using resistivity and NMR logging data.•Finally, we found a very interesting phenomenon. The key parameters in the model are sensitive to diagenesis, and the data show a clear difference between sedimentary rocks and igneous rocks.