Pore typing using nuclear magnetic resonance, an example with samples from cretaceous pre-salt lacustrine carbonates in the Santos Basin, Brazil

Abstract

The Brazilian sub-salt play carbonate reservoirs present highly complex and heterogeneous pore systems, making it difficult to define their petrophysical properties, which is essential for the reservoir characterization. Nuclear magnetic resonance (NMR) has emerged as an effective tool in the petrophysical characterization of carbonates, both on the core and log scales. This work uses plug samples from the Barra Velha Formation in the Santos Basin, SE Brazil, which is characterized by unconventional lacustrine carbonate deposits dominated by shrub-like forms and spherulitic allochems. The porosity types were defined based on NMR laboratory analysis associated with petrographic descriptions and high-resolution X-ray computed tomography. From the NMR results it is possible to define irreducible water saturation and calculate porosity and permeability for each sample. The results obtained through the resonance technique showed high correlation with the porosity and permeability values obtained by gas injection. Additionally, the interpretation of the data enabled the division of samples into porous and tight categories and the definition of groups with characteristics of tight, good and excellent reservoirs. The interpreted data contributes to a better understanding of the petrophysical properties of these complex and heterogeneous samples.