Impact of lithologic heterogeneity on acoustic velocities in the Bornu basin, Nigeria

Abstract

Six exploratory wells were analyzed and interpreted with the aim of determining the effect of lithologic heterogeneity on acoustic velocities using the petrophysical properties calculated from the logs. The wells include Murshe-1, Tuma-1, Ziye-1, Krumpta-1, Gubio SW-1 and Herwa-1. Gamma ray logs were used for the lithological delineation; the computation of porosity and compressional (acoustic) wave velocity was achieved utilizing sonic logs while the sediments bulk density was determined from density log. The analysis of compressional wave velocity with depth confirms a general trend of non-linear increase in velocity with depth across the wells, and there exists a marked lithological variation between 3000 m and 3200 m depth across the wells in the basin. Also, analyses of the results depict a non-linear increase in density with depth. The relationship between permeability and depth showed permeability increasing in high porosity areas. Permeability range value of 116 to 9238 MD and porosity of 1to 60% computed in this study is an indication that the reservoir sand in Bornu Basin has the potential to accumulate significant amount of extractable hydrocarbon. In general, the trends of relationship amongst acoustic wave velocity, porosity, density and permeability with depth have produced positive but non-linear variation. Ideally, if the formation is isotropic and homogeneous the trends would have been linear. The non-linearity which exists amongst these parameters could be as a result of the presence of a geologic structure, especially at the southern part of the basin.

Key words: Heterogeneity, Acoustic velocity, compressional wave, Bornu basin