Diagenetic And Provenance Studies Of Sandstones In Parts Of Anambra And Afikpo Basins Southeastern Nigeria

ABSTRACT

The study of the textural and structural properties of sandstone sediments outcropping in parts of the Anambra and Afikpo Basins formed a basis for evaluating the diagenetic properties and source areas of the Cretaceous formations (Ajali, Afikpo and Amaesiri Sandstones, Mamu and Nkporo Formations) in the area. Preliminary field studies showed that the studied sandstone sediments were friable to moderately consolidated. Detailed petrological studies in thin sections revealed early diagenetic effects including the precipitation of chamosite, pyrite and hematite minerals exhibiting point-floating contacts in some of the sandstone samples. Observed deeper burial effects included physico-chemical compaction, formation of Illite and quartz overgrowths. Important post-burial effects observed in some of the sandstones, included dissolution of ankerite, quartz and labile grains. An average porosity of 32.5, 27.0, 26.6, 22.1, and 20.8% were recorded for the Ajali, Afikpo and Amaesiri Sandstones, Mamu and Nkporo Formations, respectively. These values compare favorably with known hydrocarbon producing reservoirs of the world. The Ajali Sandstone sediments, however, had the best qualities having retained a significant portion of their primary porosity, underwent the least mechanical compaction as evident from their floating and point contacts and were observed to be virtually cement-free. Compaction and precipitation of authigenic quartz and clay minerals were some of the diagenetic changes responsible for porosity reduction, while the dissolution and replacement of framework minerals observed in some of the samples enhanced porosity. Major elemental oxide analysis revealed samples with extreme depletion of mobile oxides and ferromagnesian minerals, which is suggestive of granitic origin for the sandstone samples. The heavy mineral suites comprising both the ultrastable (zircon, tourmaline and rutile) averaging about 70% of the non-opaque minerals and the semi-stable (kyanite, sillimanite, staurolite), is suggestive of mixed provenance of recycled sediments and Basement Complex rocks. This was further substantiated by the westernly and southwesternly paleocurrent directions recorded for the Ajali and Afikpo Formations. The Cameroun Basement Complex, the Oban Massif and Abakaliki Anticlinorium are the likely source areas for deposits in the basins.