A correction factor for the resistivity log in gas reservoirs with low resistivity pay zone in the Poti Formation, Parnaiba Basin, Brazil

Abstract

The Parnaiba Basin, even being a new frontier basin, it’s already one of the most profitable for natural gas in Brazil. The shallow marine sandstones of the Poti Formation are the main gas reservoirs at Park of the Hawks. These reservoirs hold anomalously low resistivities which directly impacts water saturation calculations and leading to underestimations of their true potential as gas reservoirs. We propose a method for the characterization and mitigation of the problem and for the correction of the resistivity log based on the case study of well PGN-5, seeking to obtain more adjusted results for the reservoir evaluation. The methodology used is based on the integration data, where the main inputs were drill cuttings samples, well logs, gamma-spectral data and geochemical analyzes through X-rays fluorescence. We observed that the presence of pyrite and clay minerals are the causes of the low resistivities, still in the presence of gas-charged intervals, attested by crossover between density and neutron logs and through pressure gradients data. We propose a correction factor for the resistivity log in the anomalous low resistivity zone, allowing a new evaluation of the well, leading to a significant reduction in water saturation and a remarkable net pay gain.