Comparative Evaluation Of Sandstones Properties In Parts Of The Anambra Basin, Southeastern Nigeria.

ABSTRACT

The evaluation of sandstone properties in parts of the Anambra Basin was undertaken to identify the spatial distribution of textural, petrophysical and mineralogical properties of sandstone bodies in the area in order to assess their potentials to host hydrocarbons. Sandstone units from Nkporo Formation, Mamu Formation, Ajali Sandstone and Umuna Sandstone were subjected to textural analysis using sediment granulometry, thin-section petrography and geochemical analysis. Following the preliminary field studies which showed that the sandstone are friable to moderately consolidated, the results of the granulometric analysis confirmed that the sediments in the study area are fine to medium grained. Results show that Ajali Sandstone samples have a value range of 0.78-2.3phi, samples from Mamu Formation showed a value range of 0.85- 1.83phi, samples from Nkporo Formation recorded values range of 0.07- 1.38phi, while a value range of 0.27-2.4phi was recorded for the Umuna Sandstone. Detailed petrographic studies in thin sections revealed that Ajali Sandstone is poorly sorted and are angular to subrounded grains with predominance of quartz which is an indication of textural maturity. Compaction is minor, evident from the point contacts of the grains. The samples from Mamu Formation are poorly sorted, coarse to medium grained and subrounded quartz dominated sandstone showing floating contact. The average composition of quartz grains in Nkporo Formation is moderately high. This is also an indication of textural maturity of the sediments. Umuna Sandstone show subrounded grains, moderately sorted with unstable fragments dominating the grains which indicate textural immaturity. The result of the major elemental oxides distribution for sandstones from the study area showed a dominance of silica. Ajali Sandstone recorded an average percentage composition of 92.18% silica; Mamu Formation showed a value of 86.7% silica, Nkporo Formation showed a value of 85.7% silica while Umuna Sandstone has a value of 75.03%. The Al2O3/TiO3 ratio of sandstones in the study area indicates an intermediate to felsic igneous rock origin for the samples. Comparative evaluation on maturity, porosity and permeability which was based on the calculated result reveals that Sandstone bodies from Ajali, Mamu and Nkporo Formation are sub-mature and are poorly sorted while Umuna Sandstone porous and immature.