Characterizing thinbeds for petroleum potential: a case study of the Ngaji prospect of Lake Edward Basin, South Western Uganda

Abstract

Thinbed reservoirs are widely neglected as exploration targets due to conventional exploitation techniques that focus on thick beds.Thin layers of sand between thick Formations trap hydrocarbons and can be delineated once interpreted well. During Formation evaluation, these thinbeds are neglected or missed out because of the poor resolution of conventional logging tools. These tools only give the average properties of these layers which gives an unsatisfactory prediction of the amount of oil or gas in the layers. However, with more efficient interpretation techniques like cross plots and seismic attributes, subtle details that could otherwise not be identifiable with conventional methods can be solved. Consequently, more reliable results are obtained to detail the realistic reserve estimates in thin bed reservoirs. In this research, thinbeds were identified at specified depths that is between 948 to 953 m, 1174 to 1182 m, 1331 to 1338 m, 1589 to 1595 m and 1615 to 1617 m of the Ngaji prospect using both seismic attributes and interpretation tools with better resolution. Cross plots of neutron-density and log combinations of neutron-density and neutron-sonic were used to generate Litho-M and Litho-N parameters (M-N cross plots) in order to identify the lithologies within the delineated thinbeds . Using Thomas Stieber methods, porosity computations were conducted using petrophysical logs including gamma ray, density, sonic and neutron logs to determine the pay zones within the thin bed intervals. Hydrocarbon saturation of oil was derived from water saturation using Archie’s equation to determine the reservoir quality. The neutron-density cross plots show presence of sandstone, dolomite and shales. The dorminant lithology is sandstone of which are potential reservoirs. The M-N cross plots show the presence of calcite to confirm secondary porosity in the Ngaji prospect. The effective porosity of thinbeds ranges from 0.3 - 0.35 which is an indication of good reservoirs. Hydrocarbon saturation of oil within thinbeds observed in Lake Edward Basin is 25% suggesting fair to good reservoir quality and pay.