Estimation of reservoir properties from well logs and core plugs to reduce uncertainty in formation evaluation: a case study from the Kohat-Potwar Geologic Province

Abstract

Formation evaluation and rock physics are powerful techniques to link the physical properties of rocks and pore fluids measured at boreholes with petrophysical, elastic, and seismic properties at boreholes or faraway from boreholes. However, several sources of uncertainty in the measurements of these properties can affect the strength of this link. A complete statistical workflow is proposed for obtaining petrophysical properties such as porosity, permeability, volume of shale, and water saturation at the well location. This workflow is based on the wireline logs and core plugs and is applied on the lower Jurassic siliciclastic reservoirs of Kohat-Potwar Geologic Province, Pakistan to determine its applicability, the advantages of the new integrated approach, and the value of uncertainty analysis. The linear regression relations are developed between several petrophysically derived parameters measured from core samples and calculated from well logs data. All these parameters are then used as input constraints in rock physics modeling to calculate seismic properties such as bulk and shear moduli, compressional and shear wave velocities etc. A linear relationship is established between porosity and seismic velocities obtained from rock physics model. The well logs predicted rock physics properties such as seismic (P and S-wave) velocities, effective densities and elastic moduli are in close agreement to those measured by using rock physics analysis. Statistical regression analysis revealed significant similarity in the porosity values obtained from geophysical well logs and core samples. The permeability of reservoir intervals show fairly strong linear relationship with the porosity, indicating that the reservoir interval of Datta sandstone is highly permeable and porous thus having large potential of hydrocarbon accumulation and production.