Assessing grain size distribution and ichnofossils impacts on the reservoir performance of Paleocene Eastern Offshore Dahomey Basin, Southwestern Nigeria

Abstract

Paleocene Offshore Eastern Dahomey Basin is petroliferous and was earlier abandoned and tagged “dry” after the discovery of the Niger Delta basin in 1958. There is need to identify the characteristic features that caused the poor reservoir performance. This study was carried out using core samples from a well located in the offshore section of the basin to determine how depositional characteristics, textural analysis and ichnofossils impacted the reservoir quality. The well was cored from a depth 2096 to 2150m. One third section of the slabbed cores were described for lithological characteristics, ichnology and Bioturbation Index (BI) estimation. Horizontal core plug samples were drilled from the ⅔ section at positions to reflect bioturbated and non-bioturbated section of the core. A total of 170 plugs were cleaned, dried to stability and run for porosity and air permeability using porosimeter and permeameter respectively. Sieve analysis was carried out on 10 end trim samples to determine the grain size distribution. Offshore Eastern Dahomey Basin comprises a repetition of fluvial -transitional/marginal marine deposits successions with minor variations. The lithologies are mostly alternations of thick bedded sandstone and inclined heterolithic stratifications (IHS) with frequent sandwiches of heterolithic facies. The IHS basically bear trace fossils in resonance with tidal fluctuations. These ichnofossils are Macaronichnus, Ophiomorpha, Palaeophycus, Planolites, Phycosiphon, Rosselia, Rhizocorallium, Skolithos, Teichichnus, and Thalassinoides. About 70% of the permeability values fall below 10mD. Bioturbation at low BI was observed to enhance permeability while increased bioturbation reduced permeability. Porosity had been reduced by ichnofossils. The grain size distribution reveal that the sediment are basically fine grained (2.45- 2.7Φ) and poorly sorted (1.0-2.0Φ) sediment. The persistent heterolithic sandwiches within reservoirs, high percentage of clay particles, poor sorting and increased bioturbation may have contributed to poor performance and dryness of the reservoir.