Identification of Proton Populations in Cherts as Natural Analogues of Pure Silica Materials by Means of Low Field NMR

Abstract

Recent theories about the sources of silica in bedded and nodular cherts do not fit the origin of cherts from the Kraków–Częstochowa Upland. Since siliceous sponges as a single source of silica is questionable, assumptions about additional sources have to be verified. In order to do so, three samples of nodular cherts and one representative sample of bedded chert were studied by means of 1 H LF-NMR 1D and 2D relaxometry and complementary geochemical methods. The results were compared with the literature and standard silica materials which helped to identify five types of 1 H signal. The very distinct 1D- T 2 spectra of the dried samples indicated the existence of closed pores which, after comprehensive analysis, were identified as inclusions filled with different types of siliceous materials. Saturation revealed the differences between nodular and bedded cherts that were visible mainly in the amount and size of open porosity. The principal component analysis of NMR parameters showed the excellent separation of these two groups of samples and this is well visible on the biplots. Additionally, the estimated pore size distribution revealed that the total porosity of around 2% consisted primarily of mesopores (2–50 nm in diameter) and macropores (diameter >50 nm). In bedded cherts, open porosity is dominated by macropores, while the share of mesopores and macropores is similar in nodular cherts.

Prospects and Challenges for the Spatial Quantification of the Diffusion of Fluids Containing 1H in the Pore System of Rock Cores

“…After the elimination of all pixels with intensity equal to the background values, the basic images were generated (Figure 4). 3D (for all slices) porosity distribution in the sample was normalized to the value of 13.90 ± 0.01% obtained from 1D‐ T 2 experiment conducted in a low magnetic field (0.05 T; see Section S1, Table S1 and Figure S4 in Supporting Information S1), which is the method of choice for the accurate determination of total porosity (Krzyżak, Habina‐Skrzyniarz et al., 2020; Mukhametdinova et al., 2021). The images in Figure 4 additionally depict slice‐to‐slice porosity distribution, which after normalization to 1D‐ T 2 , can be characterized quantitatively (see Section 4.2) There is only one reservation regarding such normalization, that the contribution to the total porosity of 0.425% is underestimated via DTI due to considerable relaxation mechanisms in proton populations with T 2 ≤ TE .…”

“…Thus, this workflow allows typical geophysical parameters to be determined using diffusion coefficients: PSD, pore geometry, permeability, wettability, fluid structure, etc (Maerki et al., 2004; Mitra et al., 1992; Mutina & Hürlimann, 2008; Watanabe et al., 2020; Zhang et al., 2020), with much greater precision and regardless of the location of the sample in the laboratory frame. The challenge is to use the workflow for nanopore rocks such as shales (Krzyżak, Mazur, et al., 2020; Saif et al., 2017). This issue is described in more detail in the following paragraphs.…”

“…Furthermore, none of these works shows the actual meaning of DTI, that is, the registration of spatial maps of the diffusion tensor (most often water) in an examined object (see the literature review below). This is related to the considerable difficulties in using magnetic resonance imaging (MRI), and especially DTI in these areas, due to the shortening of T 2 relaxation times and local differences in magnetic susceptibilities in samples, with greater complications with increases in the magnetic field (Hürlimann, 1998; Krzyżak, Mazur, et al., 2020). This work documents the use of DTI to examine a carbonate rock core for the first time.…”

Enhanced Resolution Analysis for Water Molecules in MCM-41 and SBA-15 in Low-Field T2 Relaxometric Spectra

“…In the immediate vicinity of the value of 0.06 ms, there was a signal from strongly bound silanol OH groups, which is a separate problem that has already been studied using 2D relaxometry elsewhere [ 2 , 24 ]. It is also the area of possible ILT artifacts due to the time t E used in the measurements and, due to this value itself, limits the effectiveness of the analysis in this area.…”

Water Interactions in Hybrid Polyacrylate-Silicate Hydrogel Systems

“…Generally, in aqueous silicates, proton ( 1 H) populations (except free water) occur in the form of OH hydroxyl groups or strongly bound molecular water [41,59]. The degree of binding of water molecules depends on the structure and quantity of silica, in synthetic [41,60] and natural systems [61]. This is a phenomenological explanation.…”