Deposition, diagenetic and hydrothermal processes in the Aptian Pre-Salt lacustrine carbonate reservoirs of the northern Campos Basin, offshore Brazil

Abstract

The discovery of large oil accumulations in the rift and sag Pre-Salt sections of the Campos and Santos Basins has revived interest in the exploration of the lacustrine carbonate reservoirs in the Brazilian and African marginal basins. More than half of Brazilian oil production originates from the Pre-Salt reservoirs of these offshore basins. A study integrating systematic petrography, cathodoluminescence, scanning electron microscopy, microprobe and X-ray diffraction was performed on seven wells in the northern Campos Basin. This study highlights the major primary, diagenetic and hydrothermal features of the Pre-Salt succession, with the aim to improve our understanding of the factors that influence the porosity and permeability distribution in these important lacustrine carbonate reservoirs. The Pre-Salt deposits correspond to bioclastic grainstones and rudstones, syngenetic crusts of fascicular calcite, and intraclastic grainstones and rudstones of reworked crust fragments and calcite spherulites. Magnesian silicates are frequently associated with carbonate deposits. In the sag phase, stevensitic laminations constitute the substrate for the precipitation of calcite crusts and spherulites, which displace and replace the syngenetic magnesian clay deposits. In the rift section, stevensitic ooids are mixed with bioclasts or form ooidal arenites. Pre-Salt carbonate reservoirs have undergone a complex and heterogeneous diagenetic evolution. Eodiagenetic processes include the dissolution, neomorphism and cementation of bivalve bioclasts in the rift, as well as the dissolution of magnesian silicates and their replacement by calcite spherulites, silica and dolomite in the sag section. Burial alterations are commonly associated with hydrothermal fluids carried through faults and fracture systems. These fluids promote dolomitization, silicification, and dissolution at varying degrees and intensities. Eodiagenetic precipitation and dissolution owing to variations in the lake water chemistry and the episodic flow of hydrothermal fluids under burial conditions control the creation, redistribution, and obliteration of porosity in the Pre-Salt reservoirs.