Evaluation of depositional environments and reservoir quality of sediments in “OLI” field, offshore, Niger Delta, Nigeria

The evaluation of environments of deposition and reservoir quality of sediments in the “OLI” field, offshore Niger Delta, Nigeria, was done using a suite of wireline logs from five (5) different wells, one analogue well, and core data. The “OLI” field is an important hydrocarbon-producing area in the Niger Delta region of Nigeria. The objective of this study is to analyze the depositional environments and reservoir quality of the sediments. This will aid in the optimization of hydrocarbon extraction and the identification of new exploration targets. The lithofacies were interpreted by a systematic description of their petrographic features from core data, while the inferred lithology and depositional environments were deduced from the characteristics of the log motifs. The reservoir quality of the different sand bodies was determined from the petrophysical parameters’ interpretation obtained from both log and core data. The results revealed that the rock properties were variable and were controlled by successive depositional environments that occurred during the Oligocene–Late Miocene. Three lithofacies (sand, silt, and shale) and six sub-lithofacies (ranging from coarse-grained sand to shale) were delineated based on the relationship between grain size and the bulk volume of water. The results of gamma ray log motif and core analyses revealed the sandstones to have been deposited in a broad environment of fluvio-deltaic plain, deltaic front, and open-shelf margin/slope. Five reservoir sand units were identified. Reservoir sands were found to occur from 1800 to 4000 m. The porosities of reservoir sands, which ranged from 14.29 to 22.5%, were interpreted as excellent for hydrocarbon reservoirs. Their permeabilities have an average field range value of 43.95 to 121.68 mD and were interpreted as excellent for hydrocarbon reservoirs. Hydrocarbon saturation was high in all the reservoir sands, ranging from 70.65 to 80.28%, with corresponding water saturation also ranging from 19.72 to 29.35%. Water saturations were irreducible for reservoir sands C, D, and E. The field is predominantly an oil field because gas only occurs in sand E. The reservoir quality of the “OLI” field was found to be excellent for hydrocarbon reservoirs.