The Monte Alegre Formation outcrops, Amazonas Basin, occur on the north and south basin edges. Previous work classified sandstones on the north-south edges, however, little was related to diagenesis and how this affected the quality of reservoir rocks. Thus, the objective was to classify the sandstones of the Monte Alegre Formation in the central part of the basin and on the southern edge of the Amazonas Basin, to understand how diagenesis affected these rocks and the influence on the quality of the Monte Alegre reservoir. The sandstones of the central part, close to the city of Monte Alegre, were classified into fine, well selected, closed framework, with silica cement subarkose and reach up to 5.4% porosity, while on the south edge, the sandstones range from fine to- coarse, moderately selected, open framework subarkose to quartzarenites with quartz and iron cement, in addition to matrix with a maximum porosity of 17.7%. It was observed the eo to- mesodiagenetic diagenetic stage was reached in the central part, while in the southern edge, telodiagenesis occurred, which led to a large amount of iron cement and matrix, generation of secondary porosity and opening of the rock framework. These variations directly affected the quality of the Monte Alegre reservoir, leading to a porosity variation correlated with the geographic position in the basin as well as environments and sedimentary facies.
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