Spatial Variation Modeling of Geothermal Gradient and Heat Flow in Eastern Parts of Niger Delta Sedimentary Basin, Nigeria

Abstract

The aims of this research are to carry out statistical and geostatistical analysis of geothermal data and to determine the spatial variation of geothermal gradient and heat flow of parts of Niger delta sedimentary basin. Corrected Bottom hole temperatures from 18 deep oil wells were used for the computation of geothermal gradient and heat flows. The data were compiled from previous work. Both statistical and geostatistical analysis involving histogram, variogram, and ordinary kriging were applied to the data. The computed geothermal gradient values range from 16 – 33 oC/Km while the heat flow ranges from 23 to 70 mW/m2. The statistical analysis of the geothermal gradient shows that the minimum, maximum, median, mean and standard deviation are; 15.57, 33.66, 21.27, 22.51 and 5.07 respectively. The most occurrence geothermal gradient interval in the histogram is 20-24°C/Km with a frequency of 9. For the heat flow values, the minimum, maximum, median, mean and standard deviation are: 21.99, 70.31, 44.19, 45.44, and 13.92. The most occurring heat flow interval is 51-61 mW/m2 with a frequency of 5. The experimental variograms for the geothermal gradient and heat flow were modelled with spherical and Gaussian functions respectively. The heat flow variogram has nugget effect. The values of variances are assumed to be the values of the sill of the geothermal gradient and heat flow in the study area. The omnidirectional variogram provides a good interpretations for the spatial relationships for the two data sets. The ordinary kriging maps obtained from the variogram models for the geothermal gradients and heat flows provided a very good estimate for the un-sampled points. The geothermal gradient in the study area from the ordinary kriging ranges from 18 to 34.0°C/km, with an average value of 25.5°C/km. The geothermal gradient shows that the southeast region is associated with high values while northwest and central parts have low geothermal gradient respectively. The estimated heat flow from the ordinary kriging analysis ranges from 24 to 70 mW/m2, with an average of 47.0 mW/m2. The maximum heat flow occurs in the eastern and parts of the southwest regions. The lowest heat flow which trend in the northwest to southeast directions is found in the central region of the study area.