Calibration of Gardner coefficient for density–velocity relationships of tertiary sediments in Niger Delta Basin

Abstract

The relationships between density and velocity are important in many geological analyses that involve rock property parameters. However, the accuracy of the results is often limited when there is a verbatim application of generalized rock property relationships. Many studies in the Niger Delta Basin suffer from this limitation. As a way forward, well logs in the Niger Delta were used to derive lithology-specific coefficients that can be applied in density–velocity transforms that make use of the Gardner equation. Whereas the default coefficient (α) and exponent (β) in the original Gardner equation are 0.31 and 0.25, respectively, fitting the Gardner curve to local data results in a coefficient value of 0.33 and 0.29 for shales and sands, respectively, when the exponent is kept at the default value. Comparing measured density data with estimates obtained from sonic velocities using the original Gardner equation gives a regional mean absolute deviation of 0.13 g/cc while those of the newly derived local coefficients do not exceed 0.05 g/cc giving an improvement of over 60% in the accuracy of estimated rock properties.