**3. Results**

#### *3.1. Porosity and Permeability*

Table 2 shows the porosity data from gas porosimetry, liquid saturation, and NMR, as well as gas-injection and Darcypress permeability. Because of the limited amount of the core material, as well as the specific probe requirements for each method, we observed the gaps in the received dataset at multiple depths. It complicated the integrated analysis of reservoir properties and disabled a direct comparison of the utilized methods. Nevertheless, Table 2 describes each technique in terms of measuring range and average numbers for the target rock samples.


**Table 2.** Porosity and permeability of the core plugs (including a priori Darcymeter data) and core chips.

During the study, we faced another essential issue—the necessity for core cleaning or solvent extraction. To identify the role of extraction in the core analysis workflow, we repeated some of the tests (particularly NMR for identifying the porosity and Darcypress for permeability) before and after the extraction (Figure 6). The obtained results show the core cleaning did not adversely a ffect either NMR porosity or permeability. The permeability of the cleaned core radically increased for only three samples, which can be explained by the presence of artificial fractures that occurred during sample preparation (Figure 6b). TOC was around 5 wt.% and less than 10 wt.% for the target rock samples [54]. Thus, we continue the core analysis and interpretation of the results for the core in a non-extracted (non-cleaned) state.

**Figure 6.** NMR porosity (**a**) and pseudo-steady-state permeability (PSS) (Darcypress) permeability (**b**) for the as-received versus extracted samples.

In addition to the core plugs and chips, two whole core plugs passed through to the proposed laboratory workflow (Figure 5).

## *3.2. Microstructural Characterization*

According to the Dunham classification of carbonate rocks [36], sample #545 was a crystalline limestone with detritus (Figure 7) because it was composed of predominantly crystalline calcite (sometimes with biogenic structure) and calcite remains of tentaculite shells. Sample #551 was an organic-rich wackestone (Figure 7) because the matrix held the immersed calcite detritus.

**Figure 7.** Petrographical analysis of thin-sections located at the closest depth to samples #545 and #551.

Our experience and observations showed that the petrographical description of the formation distinguished lithotypes, but did not provide reliable information on the void space structure [55,56]. Following down along the scale bar, we attempted to characterize the void space using whole-core CT and mini-plug micro-CT. We found that, similarly to the petrographic analysis of thin-sections, CT did not reveal natural voids and, thus, turned out to be a meaningless tool for the target rock (Figure 8).

**Figure 8.** Visualization of 3D X-ray density models resulting from whole-core CT of the target rock samples.

The whole-core CT enabled the assessment of the degree of sample preservation, including the characteristics of artificial (technogenic) fractures, single large inclusions of relatively less dense organic matter, and the spatial inhomogeneity of the X-ray density. The studied rock samples featured a low contrast of the X-ray density of the rock-forming minerals. The mineral composition of the target rock samples made the three-dimensional tomographic models much less contrasting in X-ray density than they looked as real samples in daylight or petrographic thin sections.

We obtained similar results and drew the same conclusion from an analysis of mini-core micro-CT data. Micro-CT at a voxel size down to 3 μm did not fully resolve the natural-genesis void space within the density models. We separated distinct pores without revealing the connection between them (Figure 9).

**Figure 9.** Representative micro-CT slices of mini-cores taken from the whole core #551: (**a**) detritus (tentaculites) in calcite matrix with organic matter—organic-rich wackestone; (**b**) contact of organic-rich wackestone (dark) and crystalline limestone (light); (**c**) crystalline limestone.

The micro-CT results only complemented the description of rock heterogeneity and a spatial distribution of the potential organic matter (OM) inclusions. The analysis of mini-core micro-CT data revealed several structural and textural features at the microscale. In general, the studied rock samples represented various combinations of a relatively dense finely-crystalline mass of limestone and a relatively less dense carbonate-siliceous mass (Figure 9). However, the identified features did not pertain to the void space structure; therefore, their detailed description fell out of the paper's scope.

The void space structure characterization at the highest resolution involved SEM imaging. Magnification of ×10k allowed us to identify pores due to its tiny size. Pores with specific dimensions of less than 1 μm, in the majority of cases, resided in the OM. In rare (individual) cases, the mineral matrix also contained voids (Figures 10 and 11). SEM images for samples #545 (Figure 10) and #551 (Figure 11) illustrated the multiscale variation with the maximum magnification on the last picture in the sequence.

**Figure 10.** SEM images for samples #545-1 and #545-2 (white arrows indicate voids).

**Figure 11.** SEM images for sample #551-1 (white arrows indicate voids).

Sample #545 was composed of predominantly crystalline limestone with biogenic texture (Figure 7). The OM filled the space between calcite crystals with tiny rare voids less than 1 μm (Figure 10). Sample #551 encompassed a significant number of crystals immersed in a calcite matrix with OM (Figure 7). SEM images showed that there were two major types of voids (Figure 11). The first type had specific dimensions of less than 500 nm and resided in the OM. The second type was sporadic voids in the mineral matrix (biogenic clasts), with the size rarely larger than 250 nm.

#### *3.3. Pore Size Distributions*

PSDs derived from MICP data reduction for both whole cores showed the actual independence of the differential properties of the void space structure on the rock fraction size (Figures 12 and 13). Sample #545 (taken from the upper formation interval) showed almost identical PSDs before and after extraction, while sample #551 (taken from the lower BF interval) demonstrated the substantial difference in void space structure while maintaining the size range.

**Figure 12.** Mercury injection capillary pressure (MICP) pore size distributions (PSDs) for the whole core #545 depend on the rock fraction size (**a**) before and (**b**) after extraction.

 **Figure 13.** MICP PSDs for the whole core #551 depend on the rock fraction size before and after extraction.

For both samples, the pore throat size spanned in the range of 10–100 nm. The whole core #545 (from the upper formation interval) showed a unimodal distribution with a median of around 50 nm. In comparison, the sample #551 from the lower formation unit showed a bimodal distribution with modes at approximately 50 and 8 nm correspondingly.
