An integrated study of low resistivity hydrocarbon zones : a study of the two "dual resistivity" gas reservoirs onshore Mozambique Basin

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.