Pre-salt microbialites from the Campos Basin (offshore Brazil): image log facies, facies model and cyclicity in lacustrine carbonates

This study uses a borehole image log, supported by limited sidewall-core samples, and conventional wireline logs to erect facies and stratigraphic models for the Aptian pre-salt microbial carbonates (Macabu Formation (Fm)) of the southern Campos Basin, Brazil. This supports the widely held view that these reservoirs are very significant, unusual, partly microbial, and lacustrine in origin. This single-well study penetrates 220 m of microbialite facies from the pre-salt Lagoa Feia Group non-marine carbonates. Continuous borehole images, available sidewall-core data, and gamma-ray and sonic wireline logs are used to identify and characterize borehole image facies. These facies are interpreted to have formed in four lacustrine depositional environments: deep subaqueous, intermediate subaqueous, shallow subaqueous and subaerial. The borehole image facies commonly show shallowing-upward facies trends topped by emergent surfaces. Such trends are interpreted as metre-scale, high-frequency cycles that are grouped into lower order depositional sequences interpreted from the gamma-ray log. A Fischer plot of the high-frequency cycle suggests that the entire Macabu Fm is represented by a decrease in the accommodation space followed by increase in the accommodation space; consistent with this suggestion are trends in delta O-18 values from an adjacent well that indicate an initial trend of increased evaporation followed by increased freshwater inflow (lighter delta O-18 and thicker cycles).