Density and reservoir properties of Cretaceous rocks in southern Benue Trough, Nigeria: implications for hydrocarbon exploration

Abstract

Densities and reservoir properties of rocks were determined using 167 representative fresh rock samples of Cretaceous sedimentary rocks outcropping in the southern portion of the Benue Trough, Southeastern Nigeria, through the application of the Archimedes principle and grain size analysis. Result of the analysis indicates that the mean bulk densities for the rocks in the study area are as follows: Asu River Group (1.65 g/cm3), volcanic rocks (1.60 g/cm3), Eze-Aku Group (2.44 g/cm3), Awgu Formation (1.83 g/cm3), Nkporo Group (1.77 g/cm3), Mamu Formation (1.79 g/cm3), Ajali Sandstones (2.20 g/cm3), Nsukka Formation (1.74 g/cm3). Porosity and permeability values of the sandstones are as follows: Asu River (6.81%; 98.36 mD), Amasiri Sandstone (11.12%; 120.30 mD), Afikpo Sandstone (8.90%; 133.11 mD), Owelli Sandstone (11.15%; 659.37 mD), Mamu Sandstone (14.53%; 1596.45 mD), Ajali Sandstone (18.52%; 759.61 mD), Nsukka Sandstone (14.64%; 3838.61 mD). Porosity of the pre-Santonian shales is controlled by age and burial history, whereas those of the post-Santonian shales are more controlled by their physical properties than age and depth of burial. Porosity of the pre-Santonian sandstones is controlled by texture, depth of burial, and rock matrix, whereas that of the post-Santonian sandstones are controlled by depth of burial and rock matrix. While compaction and diagenetic alterations have influenced the permeability of the pre-Santonian sandstones, degree of sorting cum depth of burial has been a major factor influencing the permeability of the post-Santonian sandstones. The reservoir properties of the Asu River Group sandstones indicate that they cannot make good reservoirs. The reservoir properties of the Amasiri, Afikpo, Owelli, Mamu, Ajali, and Nsukka sandstones indicate that they can make good reservoirs. However, aside from the Amasiri, Afikpo, and Owelli sandstones, the sandstones of Mamu, Ajali, and Nsukka formations are not stratigraphically positioned to serve as hydrocarbon reservoirs in the study area.