Abstract:
The overall demand for gas in Mozambique is projected to grow from 3.5 PJ/a to
approximately 35PJ/a by 2025, as a result of growing extractive industries such as
petroleum exploration and mining as well as non-extractive industries such as
agriculture. The potential for gas to supplement the national energy supply and to
drive economic development in the country is large. With growing energy demand
and conversion of gas to electricity, integrated reservoir studies to optimise recovery
of gas are becoming increasingly important. This study presents an integrated
reservoir study of the low-resistivity sandstone gas reservoirs in the Mozambique
Basin. The two investigated reservoir formations are the J-Reservoir and KReservoir
of the Lower Grudja Formation of Maastrichtian age, which host two
separate gas accumulations.
Conventional water saturation (Sw) calculations such as Archie’s equation fail to give
accurate results that are consistent with the production figures and well test results.
The results of the water saturation calculated using this method are also inconsistent
with the water saturation calculated using Nuclear Magnetic Resonance (NMR), the
only tool that gives accurate calculation of the water saturation (Sw) in the lowresistivity
reservoirs. The primary objective of the research was to understand the
cause of the low-resistivity phenomena by evaluating the reservoir characteristics
such as textural fabric, mineralogy and clay content of the J- and K-Reservoirs. The
presence of clay-bound water and the conductive minerals in the reservoir formation
are known to be some of the main causes of low-resistivity phenomena in
sedimentary basins around the world.
The clay volume in the two reservoirs was evaluated using the gamma ray (GR) log
and the results were compared well to clay calculated from core data using X-ray
Diffraction (XRD) and Laser Grain Size Analysis (LGSA). The study showed that the
clay volumes ranged from 6% to 30% from the top towards the bottom of the JReservoir
and 2.4% to 18% from the top towards the bottom of the K-Reservoir. The
calculated silt volumes for the J-and K-Reservoirs ranged 0-70% with a general
increase in silt volume towards the bottom of the reservoirs.
