Evaluation Of 3D small-scale lithological heterogeneities and pore distribution of the Boda Claystone Formation using X-Ray Computed Tomography images (CT)

Abstract

This study was undertaken to quantify and evaluate the density and porosity characteristics of a Boda Claystone Formation (BCF) core sample using medical CT. Each voxel of the 3D CT volume was described with three variables: dry CT number, saturated CT number, and effective porosity. Disparity pore voxels were revealed using the genetic groups’ algorithm of data-mining techniques. The K-fold cross-validation algorithm has been applied to determine the number of the most stable cluster. The 3D spatial distributions of voxel-porosity by rock constituents, as well as the 3D distribution of porosity clusters by rock components, were found by Boolean function implementation. The terrigenous detrital fragments had the lowest porosity mean (0.16%) and highest coefficient variation value (1039.39%). While the Fine siltstone component had the highest porosity mean (3.39%) and lower coefficient of variation (134.99%). The difference in the variation of coefficient proportions is related to the outlier ratios in each rock component. Independently of both the rock types and the sedimentary structures, two clusters could be defined: one for the micro-porosity and one for the macro-porosity regimes. The former showed a continuous 3D spatial appearance, while the latter appeared in patches. These patches may also be connected, at least partly, to some local smectite aggregates. These clay minerals could lose their structured water content during vacuuming and swell when adsorbing water during sample saturation. In each rock type, the micro-porosity regime could be related to low-density rock fragments. The mean effective porosity of the micro-pore regime was about 0.02, which corresponds to the petrophysical core measurements. For the macro regimes, the average was 0.1.

Assessing the representative elementary volume of rock types by X-ray computed tomography (CT) – a simple approach to demonstrate the heterogeneity of the Boda Claystone Formation in Hungary

“…In contrast, the clay in L16, which might be allogenic or/and authigenic and even be collapsed during the drying process, would not influence the inherent pore space ratio because of its low percentage. Studying the porosity distribution of the current BCF core sample, Abutaha et al (2021) managed to confirm collapses of smectite/illite structures during the drying/saturation process and postulated sedimentary structures, i.e., convolute structures, to act as significant porosity-improving factors.…”

“…Various illite and illite/ smectite-mixed assemblages constitute the dominant clay minerals of the Boda Claystone Formation (Németh & Máthé, 2016). Clay minerals are present as part of rock fragments, "allogenic clays" or authigenic clays (Abutaha et al, 2021). Partial to complete albitisation of detrital plagioclase of the BCF yielded Ca 2+ to react with HCO 3and Mg 2+ (+Fe 2+ ) to produce carbonate cement (Table 2; the positive relationship between albite and carbonate is ~0.7).…”

“…Each CT number can be assigned a real-world distance (or depth), allowing CT number profiles to be constructed so that depth and geometrical measurements can be calculated. DICOM images are be easily read by 'classical' 3D volume rendering softwares (Abutaha et al, 2021).…”

“…For the identification of rock-forming components of the core sample, CT HU intervals defined by PURAM for characteristic rock types of the BCF were used as follows: detrital fragments (coarse siltstone): <2700 HU, fine siltstone: 2700-3150 HU, claystone: 3150-3300 HU, calcite and /or dolomite: 3300-3600 HU, and albite: >3600 HU (Abutaha et al, 2021). The reality of the rock-forming components was compared with macroscopic core descriptions.…”

Studia Quaternaria

“…As the studied rocks represent a pool of the RFCs, in this pioneering work we adopted mixture analysis as a non-hierarchical clustering method for the segmentation of each RFCs using the CT data. We have found only two references that reported the CT-based density data for carbonates: limestone (2765 HU; Bolliger et al, 2009), secondary formed marine dolomite in claystone of the Permian Boda Claystone Formation (3300-3600 HU; Abutaha et al, 2021). These could have been used only as tentative guidelines in our work as the density values are highly dependent on the complexity of the studied rock sample, i.e.…”