Resistivity cutoff of low-resistivity and low-contrast pays in sandstone reservoirs from conventional well logs: A case of Paleogene Enping Formation in A-Oilfield, Pearl River Mouth Basin, South China Sea

Abstract

Abstract Deep sandstone reservoirs in the Paleogene Enping Formation of A-Oilfield, Pearl River Mouth Basin, are significant offshore petroleum targets for this basin. Compared with adjacent water zones and shales, the pays are of low resistivity and low contrast (LRLC). Without new unconventional well-logging techniques, conventional well-logging interpretation has encountered difficulties in these LRLC pays. In this article, based on the Simandoux water–oil saturation equation, influence factors of formation resistivity in sandstone LRLC pays are simulated theoretically. Resistivity cutoff is calculated at average reservoir conditions and proved by oil-testing results. It is applied to oil and water in LWD, and the possible genesis of LRLC is discussed. In sandstone LRLC pays, the resistivity of the oil zone is larger than the resistivity cutoff; the resistivity of the water–oil zone approximates the resistivity cutoff and the resistivity of the water zone is lower than this threshold. At average shale volume and porosity of pays in Enping Formation, the resistivity cutoff is referenced at 4.2 Ω·m overall. In sandstone reservoirs, low formation resistivity in the Enping Formation of A-Oilfield might result from high formation water salinity, complicated pore-throat structure, well physical properties, and the presence of conductive and clay minerals. The resistivity cutoff offers important information in real-time tracking of LWD. The proposed method is demonstrated to be beneficial for identifying hydrocarbons and monitoring trajectory in LRLC pays.