Petroleum Prospects of Benue Trough, Nigeria

Abstract

Exploration activities in the Benue trough have been minimal over the years, mainly because of large petroleum deposits found in the adjoining Niger delta and early gas finds in the Anambra basin, south of the Benue trough. The recent increase in exploration activities in the trough has necessitated a reevaluation of the petroleum potentials of the basin. In this study, the time-temperature index (TTI) method was used to evaluate petroleum prospects of the basin, assuming a mean surface temperature of 80°F (27°C), negligible compaction effects, and various values of geothermal gradient (1.5°F/100 ft, 1.9°F/100 ft, 2.3°F/100 ft, 2.7°F/100 ft; 2.7°C/100 m, 3.5°C/100 m, 4.2°C/100 m, 4.9°C/100 m). An increase in geothermal gradient resulted in a decrease in depth to the oil window, with the sediments maturing earlier at higher geothermal gradients. At geothermal gradients of 1.5-1.9°F/100 ft (2.7-3.5°C/100 m) and maximum TTI values, the Asu River Group and the Eze-Aku Group of sediments are still within the gas-generating stage. The Awgu Shale and the Nkporo Shale are capable of g nerating gas at geothermal gradients of 2.3-2.7°F/100 ft (4.2-4.9°C/100 m). The Benue trough is essentially a gas-condensate basin with little oil, which occurs stratigraphically higher than gas. Therefore, exploration for oil in the basin is less attractive. The deep basins, flanking the central uplifted blocks in the trough, contain thick sedimentary piles with mature source rocks and reservoir rocks. Exploration targets in the basin include both the sub-Santonian and super-Santonian sediments, with the Eze-Aku Group, Awgu Shale, and Nkporo Shale being more prospective than the stratigraphically lower Asu River Group. In general, the middle Benue trough is considered to be the most prospective area within the trough because depths to the mature zones are moderate (6,600-13,000 ft; 2-4 km). These depths are variable, decreasing northeastward and increasing southwestward toward the Niger delta.