Subsurface facies analysis of the paleogene formations, nahal field, melut basin,south sudan

Abstract

The present work highlights the results of the facies study conducted to identify lithologic heterogeneity and determine petrophysical properties from six wells in Nahal oilfield in South Sudan with the aim of assessing the quality of reservoirs. A set of well log data was used for the analysis that involved identification of log facies, hydrocarbon zones; and determination of petrophysical parameters such as shale volume, porosity, fluid saturation and permeability. Two sand bodies were identified using pattern-matching technique and their correlation based on well data revealed lateral facies changes. Hydrocarbon indication was observed based on resistivity log curve in some zones in the wells from which petrophysical parameters were estimated to assess the quality of these reservoirs. Average shale volume ranges from 10 to 24% and effective porosity from 15 to 33% indicating fair to good porosity sandstone while permeability ranges from 10 to 1000 millidarcy. Determination of porosity and its water content has an impact on reserve estimation thus; water saturation in these reservoirs were calculated using Archie and Indonesia equations. Water saturation was found higher with Archie model; and this leads to a considerable difference in reserve estimation. Based on the integration of well log- and petro-facies, four depositional environments were deduced: funnel-shaped depositional trends in Samma and lower part of Yabus indicating braided channels deposits; bell shapes in the upper sections of Yabus reflecting deposits of meandering channels; irregular trends suggest floodplains and lacustrine deposition; and cylindrical trends representing channel-fills. The well log facies were extended to the entire study area by the help of seismic facies analysis. The study revealed that the Nahal field has high hydrocarbon potential due to excellent petrophysical characteristics that are favorable for hydrocarbon accumulation but short distance (abrupt) sedimentary facies changes have impact on fluid flow and production. Therefore, early production of water can be avoided by placing wells within good sand areas with favorable effective porosity and water saturation for easy flow.