Textural Characteristics and Post Depositional Effects on the Reservoir Rock: A Case Study of Core Samples from Wells GX1 and GX2 Located on the Western Offshore of the Niger Delta, Nigeria. – A. N. Ugbaja1, I. M. Akaegbobi, A. O. Ola-Buraimo, and G.O. Otosigbo
Textural parameters in terms of grain size and sorting play a vital role in determining the sandstone reservoir characters such as porosity and permeability. Core samples of two wells were described lithologically in terms of grain size, sorting, colour, structures, bioturbation and post depositional effects. A total of 101 samples were plugged and oil was removed within using methanol and chloroform. Pore volume measurements on porosity and permeability were carried out in standard format and the flow test was carried out under steady state. Textural properties exhibited by the lithofacies are resultants of energy of deposition which yielded various lithofacies associations which are dominantly tidal channel, tidal flat, deltaic shale, lower shoreface and marine shale. Reservoir genetic unit is an upgrade of lithofacies association whereby the reservoir genetic units are map able over distance and across wells based on peculiar characteristics. Therefore, the core GX1 is categorized into channel fill and lower shoreface units while core GX2 is mainly lower shoreface and the underlying marine shale. Reservoir quality shows that the channel reservoir genetic units possess moderately high porosity and excellent permeability values compared to lower shoreface reservoir genetic units of higher porosity values and relatively lower permeability values. These differences are as a result of textural properties (grain size and sorting), and post depositional effects (structures and bioturbation) which tend to increase pore throat size and intergranular connectivity for channel deposits’ high permeability while finer grain size and good sorting are responsible for high porosity in lower shoreface deposits.