Analysis of Seismic Time-Depth Conversion Using Geostatistically- Derived Average

Abstract

Geostatistical techniques were used for seismic time-depth conversion over “Labod” field offshore Western Niger Delta Basin with the aid of surface seismic and borehole data. Gamma ray and resistivity logs in four exploratory wells were utilized to delineate formation lithologies (facies) and formation fluid content. The target horizon B (top of sand 2) was selected for seismic structural mapping. Two major faults (F1, F2) and three minor faults (F3, F4 and F5) were identified and interpreted on the seismic sections. Average velocities were first calculated from sonic logs. Geostatistical techniques - Kriging with External Drift (KED) and Simple Kriging - were employed to generate average velocity models used to convert the target B seismic time surface to depth in and away from well locations and depth maps were generated. The results of maximum percentage deviation computed among field-observed and computed depths were lower than 5% suggesting that the average velocity values estimated away from wells were reliable and applicable especially in areas where there is scarcity of well information. The Kriging with External Drift (KED) technique is particularly recommended because of the higher accuracy and denser data coverage. Finally, the method is cost effective because only few wells are required to achieve the expected results.