PETROPHYSICAL ANALYSIS AND ROCK PHYSICS OF THE MOKI FORMATION, NEW ZEALAND

Understanding and applying rock physics fundamentals are essential steps to predict reservoir behavior from geophysical studies. In this work, petrophysical analysis was performed for a formation in two wells offshore of New Zealand. The main objectives are to identify the physical properties of the rocks, such as lithology, velocity, and water saturation; to compare the results of two Gassmann fluid substitution approaches in shaly sands; and to study the dry bulk modulus trend with respect to changes in porosity and depth. First, sandstone bearing zones that are considered as a potential reservoir were selected in the Moki formation in two wells. A routine petrophysical analysis, including mineral identification and crossplots, was applied to identify lithology, porosity, and water saturation. In addition, some empirical relations were performed to predict seismic velocities and compared with the logged values. Second, using parameters obtained from the petrophysical analysis, two different Gassmann fluid substitution approaches were employed to predict the acoustic response due to different fluids in the pore spaces. Besides new velocity behaviors of the formation, Gassmann fluid substitution provides us different seismic properties of the rock. The dry bulk modulus was computed as part of that analysis, and was used to compare with changes in porosity and depth while considering variations in shaliness. Porosity values tended to decrease with increasing shaliness and depth. On the other hand, dry bulk modulus trend decreases with increasing effective porosity and it raises with increasing depth. Thus, it is concluded that dry bulk modulus shows inverse proportion with increasing effective porosity, but it shows direct proportion with increasing depth in cemented rocks.