An integrated petrophysical and rock physics characterization of the Mangahewa Formation in the Pohokura field, Taranaki Basin

The Mangahewa Formation in the Pohokura gas field, Taranaki Basin, is a key reservoir for gas production in New Zealand, yet its deep and heterogeneous nature presents challenges for accurate reservoir characterization. While prior studies have explored aspects of the Mangahewa Formation such as lithology, fluid composition, and petrophysical properties, the interrelationships between these factors and their impact on hydrocarbon production remain underexamined. This study integrates detailed petrophysical and rock physics analyses to overcome these challenges. Petrophysical evaluation, based on well log data from depths of 3200-4000 m, reveals net reservoir thicknesses ranging from 164 to 479 m, with total porosity between 17 and 21% and effective porosity between 8 and 19%. Shale volume and water saturation vary from 21-28 and 22-34%, respectively. Rock physics analysis was performed using Rock Physics Templates (RPTs) to model the elastic properties of the reservoir. The Mangahewa Sandstone exhibits elastic properties consistent with the stiff sand model, with compressional sonic velocities ranging from 4100 to 5000 m/s. High correlations were achieved between measured and modeled velocities, with 97% for V_P and 94% for V_S. These models enabled the estimation of porosity from seismic-derived acoustic impedance, providing valuable insights in areas with limited well control. Furthermore, the RPTs effectively differentiated between gas sand, water sand, and shale facies, minimizing uncertainties in fluid and lithology prediction. These results provide a comprehensive understanding of the Mangahewa Formation, enhancing hydrocarbon prospect evaluation and supporting further exploration and development in the Pohokura field.