Petrophysical approach for estimating porosity, clay volume, and water saturation in gas-bearing shale: A case study from the Horn River Basin, Canada

Shale gas exists partly as a gas adsorbed to clay mineral and partly as a free gas within the pores. To evaluate a shale gas reservoir and calculate total gas content, it is essential to accurately analyze porosity, clay volume, and water saturation. In this study, we estimate these factors for the Horn River Basin using various types of well log data such as density log, sonic log, resistivity log, and neutron porosity log. Because a simple density porosity equation results in unreasonable fluid densities, we estimate porosity using total organic carbon. Based on brittleness, an empirical equation for clay volume is defined. Because the correlation coefficient between core-tested clay volume and water saturation is greater than 0.9, the empirical equation for water saturation is also defined in terms of brittleness. For the shale gas reservoir in the Horn River Basin, porosity can be calculated by using a linear equation with the density log, and clay volume and water saturation can be calculated by using a linear relationship with Young's modulus and Poisson's ratio. This study suggests that porosity, clay volume, and water saturation models can be established using the elastic model built on seismic inversion.