Lithological characterization of the Alter do Chão Aquifer using geophysical well logging in Manaus, Amazonas, Brazil

ABSTRACT Groundwater is a resource of high economic, strategic, and social value, and its sustainable management relies on accurate subsurface characterization. The Alter do Chão Aquifer, the primary groundwater source in Manaus, Amazonas, Brazil, represents the saturated portion of the Alter do Chão Formation and serves as the main reservoir of the Amazon Basin and one of the most significant aquifers in the country. Despite its importance, geological characterization of the aquifer is often constrained to data from isolated well points, limiting the spatial resolution and continuity of subsurface models. This study focuses on lithological characterization derived exclusively from well log data. Geophysical logging from 15 wells - utilizing gamma ray, spontaneous potential, and electrical resistivity profiles - was interpreted to differentiate between sandstone and claystone lithologies, supplemented by drilling mud samples. The interpreted data were used to generate a 3D block model in Datamine Studio RM, representing 26.96 km³ of rocks (77.6% sandstones and 22.4% claystones) through 126,720 parent cells, each measuring 250 x 250 x 5 m. The block model delineates lithological zones (upper, middle, and lower) extrapolating well locations. The model shows general agreement with existing 2D geological mapping and stratigraphic correlations, underscoring the need for integrating geophysical datasets to enhance subsurface characterization of the Alter do Chão Aquifer.