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Conceptual models developed from modern distributive fluvial systems (DFSs) show a predictable spatial variation of facies progression from the margin to the interior of continental sedimentary basins. The Upper Cretaceous Serra da Galga Member of the Marília Formation has recently been interpreted to be a result of DFS deposition in the intracratonic Bauru Basin. This paper describes the spatial variations in stratigraphic architecture from the proximal to medial regions of the studied DFS and presents the implications of the recorded changes for fluvial reservoir characterization. The proximal areas are dominated by amalgamated channel-fill deposits and weakly developed, well-drained palaeosol profiles with only sparse floodplain deposits. The medial areas display a reduction in channel-fill deposits, an increase in the preserved thickness of floodplain deposits and the occurrence of sheetflood deposits and well-developed, poorly drained palaeosols. This spatial trend is accompanied by changes in the net : gross ratios of the geological bodies, forming possible high net : gross reservoirs in the proximal areas and reservoirs with very variable net : gross ratios in the medial areas. This down-system change in preserved stratigraphic architecture is evidenced for the first time in the studied sedimentary succession and follows the same pattern of facies progression described from modern examples of DFSs.

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ABSTRACT:The petrologic analyses of 40 thin sections from two wells located in Atalaia High, offshore of Sergipe Sub-Basin, allowed the identification of primary composition and diagenetic patterns of Barra de Itiúba Formation. The original detrital compositions included arkoses, sublithic, and lithic sandstones. The main diagenetic processes observed were: compaction of metamorphic rock fragments and mud intraclasts, generating pseudomatrix; precipitation of quartz and feldspar overgrowths and outgrowths, cementation and grain replacement by kaolinite; dolomite and ferrous dolomite/ankerite; pyrite; iron oxides and hydroxides; and diagenetic titanium minerals, in addition to dissolution phases during eo-, meso-and telogenesis. The macroporosity in the two studied wells is primary intergranular, but there is secondary porosity due to dissolution of primary and diagenetic constituents, as well as fracture porosity. Thirteen reservoir petrofacies were defined and grouped into four reservoir petrofacies associations. They reflect the reservoir quality in microscale: good, medium and low-quality and non-reservoir. The good-quality is characterized by average total porosity greater than 15%, whereas the medium shows average total porosity greater than 7%. Lowquality presents average total porosity between 1 and 4%, and the non-reservoir has an average total porosity consistently less than 1%. Overall, the studied reservoirs consist on low-quality and non-reservoir rocks, which are intercalated with levels of mediumand good-quality. The loss of original porosity was mainly due to mechanical compaction (generating pseudomatrix), and cementation by kaolinite and dolomite. Preservation of primary porosity was favored by the presence of quartz overgrowths. KEYWORDS:Reservoir Quality; Sergipe-Alagoas; Cretaceous; Diagenesis; Petrography. RESUMO: As análises petrológicas de 40 lâminas em dois poços localizados no Alto de Atalaia, offshore da Sub-Bacia de Sergipe, permitiu a identificação das composições primárias e dos padrões diagenéticos para a Formação Barra de Itiúba. As composições detríticas originais das rochas são subarcósios, sublitarenitos e litarenitos. Os principais processos diagenéticos observados foram: compactação dos fragmentos de rocha metamórfica e intraclastos lamosos