This study aimed to reveal the depositional and hydrothermal-diagenetic history for the sampled carbonates of the Macabu Formation, in Campos Basin, offshore Brazil. It also investigated the possible alteration conditions affecting these rocks, including: temperature, water pH, pCO2, and fluid origins. The carbonates of the Macabu Formation and their equivalent formations are commonly reported with lithologies composed of stevensite, spherulitic calcite, shrub-like calcite, dolomite matrix, and different forms of silica formed from hydrothermal alteration. However, no spherulite and shrub-like calcite have been observed in the samples investigated herein from well S2. The lithologies of the Upper and Lower units are primarily composed of: (a) depositional minerals: dolomite matrix, and minor detrital materials (quartz, mica, and feldspar); (b) diagenetic minerals: chert, chalcedony, euhedral mega-quartz, and blocky calcite; and (c) minor diagenetic minerals: siderite, illite + kaolinite, dolomite II, solid hydrocarbon materials, pyrite, fluorapatite, and phosphate. Dolomite is the matrix phase to the rock, and the crystal sizes and habits differ between the Upper and the Lower unit cores. The Upper unit core consists of fine grained, laminated dolomite, that is subhedral, whereas the Lower unit core comprises euhedral dolomite, lacking significant sedimentary structures but with distinct crystal zonation; three distinct zones are recognized. Secondary Ion Mass Spectrometry (SIMS) analysis was designed to measure the δ18O and δ13C of the three zones, providing insight into the lake water chemistry and temperature changes from the surface to bottom during the deposition of dolomite and the early burial. Diagenetic calcite only occurs in the Upper unit. A limited number of samples were chosen to analyze the blocky calcite and the fine laminated dolomite within for their δ18O and δ13C signatures. The isotopic data for the Upper unit dolomite were compared with those of the Lower unit dolomite to investigate the reasons for an observed change in the alkalinity of lake water. Both the Upper and the Lower unit cores had been hydrothermally altered, principally by silicification. Silicification in the Lower unit was more intensive than for the Upper unit. There are recognized three phases of silicification in core samples of both the Upper and Lower units, and these phases favor the formation of chert, chalcedony, and euhedral mega-quartz, respectively. Chert acted as a replacement mineral, after the original dolomite. Chalcedony occurs as replacement and cementing mineral, and is associated with dissolution of earlier-formed chert and its re-precipitation. The chalcedony developed within one cavernous void has been observed with a maximum number of seven bands, which may represent repeated pulses of hydrothermal recharge, that were responsible for chalcedony precipitate. Euhedral mega-quartz from the Upper unit core were observed in a close association with diagenetic blocky calcite. The SIMS analysis was designed to examine the three phases of silicification, and a set of δ18O and δ30Si data for each phase were obtained. These data have provided evidence of the precipitating fluid temperatures, and of the geochemical character of fluids thought responsible to help understand their possible origins.