Using the porosity exponent (m) and pore-scale resistivity modelling to understand pore fabric types in coquinas (Barremian-Aptian) of the Morro do Chaves Formation, NE Brazil

Abstract

The recent major discoveries of petroleum in Pre-Salt carbonate reservoirs, of mainly lacustrine origin, offshore Brazil has increased interest in studying these complex non-marine reservoirs, which present many production-development challenges, largely due to their heterogeneous nature. Some of the reservoir carbonate rock-types present in Brazil are limestones composed predominantly of bivalve shells, which are known as ‘coquinas’.The coquinas show a variety of pore types, pore sizes, pore shapes and pore connectivity which define their porosity and strongly influence the permeability and resulting electrical resistivity but this aspect of these relationships in these carbonates has been relatively poorly characterised (particularly in a quantitative manner) in the literature. The petrophysical characterisation of the coquinas of the Morro de Chaves Formation (Barremian-Aptian), exposed in a quarry in NE Brazil, was carried out using the outcrop-selected samples, plugs and petrographic thin-sections to investigate these aspects. Porosity, permeability, grain density and porosity exponent (m) measurements were carried out on the plugs. X-ray microtomography was performed to identify key characteristics of the pore system and for the basis of 3-D modelling.Significant variation in m was observed across the plug data set, which is not unexpected in carbonates, but it suggested that a range of fabrics and pore topology were present in these rocks, which are described only as calcirudites. Modelling of the resistivity using 3D pore scale models was used to understand the role of disconnected macro-pores and dissolution seams in controlling the more extreme variation in petrophysical properties observed in the coquinas. This understanding of the controls on resistivity pathways in this outcrop coquina will help in understanding the pore types in the subsurface and the estimation of saturations in these complex rocks where they are found to be oil-bearing.