Residual oil saturation investigation in Barra Velha Formation reservoirs from the Santos Basin, Offshore Brazil: A sedimentological approach

Abstract

Residual oil saturation (Sor) is a petrophysical parameter of great relevance, as it is used for flow simulation, production forecast, and reserve estimation. However, oil trapping is rarely investigated from a geological point of view. This work integrates petrography, RCAL, SCAL, and X-ray μCT to understand how lithological characteristics control Sor in the lacustrine carbonate reservoirs of the Pre-salt section of the Santos Basin, offshore eastern Brazil. The in-situ facies, composed of calcite shrubs and spherulites, and magnesian clays, have Sor directly related to the diagenetic evolution of the Mg-clays, including processes of replacement by dolomite and dissolution and their relationships to the calcite shrubs and spherulites. The intense replacement of the clay matrix by dolomite, followed by dissolution, developed intercrystalline porosity with a small pore-to-throat ratio, favoring water sweeping. On the other hand, the preservation of primary porosity among shrubs or the development of large matrix dissolution pores favors oil trapping. In intraclastic rocks made by redeposited fragments of shrubs and spherulites, the main control of Sor is depositional texture: rocks with larger grain size tend to have greater porosity heterogeneity and, consequently more oil trapped. On the other hand, fine-grained calcarenites tend to have a more homogeneous porous medium, which enables oil displacement. Finally, the imaging of Sor in reservoir conditions indicates mixed wettability. However, there is no clear relationship between Sor and predominant mineralogy, which should be investigated in future works.