1. Introduction
Mozambique (Fig. 1) has entered its post-war recovery period and the country is on the brink of industrial revolution. The economic growth is largely dependent on the development of the energy sector, such as building of gas-to-electricity power stations. The overall local demand for gas in Mozambique is projected to grow from 3.5 Petra Joules per annum (PJ/a) to approximately 35 PJ/ a by 2025 (IESE, 2012). To meet the growing energy demand of local industry and that of the neighbouring countries such as South Africa, hydrocarbon production from the Mozambique basin must be significantly increased in the next decade. Technical and geological
difficulties however, cause problems that need fast and successful solutions.
Most hydrocarbon (gas) bearing clastic reservoir formations in sedimentary basins are characterised by an increase in electric resistivity contrast between the hydrocarbon-bearing part of the reservoir and the water-bearing zone. The resistivity of oil and gas is generally higher than that of formation water, since dissolved solutes in the formation water provide an effective pathway for electrical current to flow. However, in many basins including the Mozambique basin, gas-bearing reservoir formations may display low-resistivity response and as a result, conventional wire-line log analyses give inaccurate water saturation (Sw) results. This might
* Corresponding author.
E-mail addresses: vincent.mashaba1@sasol.com (V. Mashaba), wlady. altermann@up.ac.za (W. Altermann).
http://dx.doi.org/10.1016/j.marpetgeo.2015.05.016 0264-8172/© 2015 Elsevier Ltd. All rights reserved.
often be the cause of overlooking potentially productive hydrocarbon accumulations.
The calculated water saturation using conventional log analysis for the J- and K-Reservoirs of the Maastrichtian Lower Grudja Formation discussed herein, contradicts water saturation calculated using the Nuclear Magnetic Resonance (NMR) tool, which gives
Fig. 1. Schematic outlines and location of the major onshore sedimentary basins of Mozambique. The approximate location of the fields A and B, with the discussed boreholes is
shown in the rectangle marked as “Study Area”.
accurate water saturation (Sw) results in low-resistivity pay reservoirs. The conventionally calculated water saturation values also contradict the Drill Stem Test (DST) to determine the flow of hydrocarbons and water before the reservoirs can be set to a full field gas development and production.
The basis of Archie's equation (Archie, 1942), conventionally used to calculate the water saturation of a formation, is that the conductivity of the formation is a function of the conductivity of the fluid content in its pore space and therefore the equation can be written in terms of conductivity as:
Sw ¼ ½ða*RwÞ=ð4mRtÞð1=nÞ (1)
where: Sw equals water saturation; Rt equals total resistivity as measured by the resistivity logs; 4m equals total porosity and Cw, the conductivity of the formation water is defined from 1/Rw with m ¼ Cementation factor and; n ¼ Saturation exponent.