Multi-technique approach to the petrophysical characterization of Berea sandstone core plugs (Cleveland Quarries, USA)

Abstract

Berea sandstone has been used by the petroleum industry as a representative model siliciclastic rock for a number of years. However, only incomplete data has been reported in the literature regarding its petrographic, geochemical, and petrophysical properties. In particular knowledge of the mineral distribution along the pore walls is particularly scarce, despite the fact that mineral exposed in the pore space will be crucial in determining the rock-fluid interactions that occur during core-flooding experiments. In this paper, four Berea sandstone samples (with 4 different permeability ranges from <50 mD, 50–100 mD. 100–200 mD, and 500–1000 mD) were subjected to a multi-technique characterization with an emphasis on determining the mineral composition, and distribution at the pore surface as well as pore structure and connectivity analysis. The mineral distribution was measured in two-dimensions by chemical mapping using energy dispersive X-ray spectroscopy–scanning electron microscopy (SEM–EDX). The bulk composition of the Berea sandstones was also measured by X-ray diffraction and micro-X-ray computed tomography. From this, it was found that authigenic minerals, especially clay minerals, make up a small portion of the bulk rock volume (3.3–8%) but are over-represented at the pore surfaces and in pore spaces compared to the other major mineral constituents of the rock (quartz and feldspar). The effective mineralogy, from the standpoint of rock-fluid interactions, is the mineralogy that predominates at pore surfaces. For the Berea sandstone samples studied, the effective mineralogy is represented, mainly, by kaolinite, illite, and chlorite. For 3 of the four permeability ranges studied, kaolinite is the predominant pore lining mineral observed. In the remaining sample (50–100 mD), illite is the predominant mineral. In addition to SEM, we used atomic force microscopy to show that the nano-sized particles with the shape and size of clay crystals are observed on the surface of recrystallised quartz grains in a Berea sample. Regardless of their origin and identity, the presence of these particles shows that the quartz grain surfaces in Berea sandstone are more heterogeneous than previously assumed. Carbonate cement was somewhat localized throughout two of the Berea sandstone specimens, however, quartz cement is common in all of the Berea cores studied and include both microcrystalline quartz and amorphous silica phases. The pore structure within the four different Berea samples was studied using a combination of X-ray computed tomography, mercury injection porosimetry and high resolution scanning electron microscopy. Results show that two Berea sandstone permeability ranges have a bimodal pore-throat-size distribution whereas the other two were dominated by a unimodal pore-throat size distribution. SEM imaging of the pore network showed that permeability is mainly controlled by pore connectivity in the clay mineral matrix. Next to the pore connectivity, three-dimensional pore space showing both pore-to-pore and pore-to-pore-throat-to-pore relationships are also important.