Mineralogical and petrophysical characterization of stromatolite from Salgada lagoon, Campos dos Goytacazes (RJ), Brazil

Approximately 60% of oil and 40% of gas world reserves are in carbonate reservoirs. These reservoirs are heterogeneous with a textural variety and typically fractured, leading to low recovery factors and also complex relationships between rock properties and geophysical data (Vanorio et al, 2008). Studies of these reservoirs have become very important in Brazil after 2005, when Petrobras announced a large carbonate reserve in the pre-salt region of Santos basin. The reservoir characterization requests a better understanding of the relationship between geology, petrophysical and acoustic properties. In this work, we made a mineralogical and petrophysical characterization of a stromatolite from Salgada lagoon. Measurements of X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that the stromatolite is composed mainly by high-magnesian calcite (HMC), with a small amount of aragonite and quartz. The thin section observation revealed the presence of skeletal material (shells), quartz grains and some regions that suggest recrystallization of high-magnesian calcite into low-magnesian calcite (LMC). The porosity of this sample is approximately 20%, as measured with a gas porosimeter, and these pores have different shapes and sizes, as observed with a polarizing petrographic microscope. Using a rock physical system it was possible to measure the traveltime of P wave and thus determine the velocity, which is slightly higher than 3000 m/s.