**5. Conclusions**

In the presented study, we implemented an integrated approach to characterize the ultralow-permeable carbonate reservoir rocks of the Tlyanchy-Tamakian Formation, Volga-Ural oil-gas province in Russia.

The suite of laboratory methods included both bulk measurements and multiscale void space characterization. Bulk techniques included gas volumetric, NMR, LS porosity and PDP, and PSS permeability, while imaging consisted of thin-section petrography, CT and μCT, and SEM. MICP was a proxy technique between bulk measurements and imaging. The target set of rock samples included whole cores, core plugs, mini cores, rock chips, and crushed rock.

The study resulted in the following significant findings for the target rock samples:


The observed experimental features are essential for understanding the measured porosity and permeability properties of tight carbonates similar to the target formation and are critical during the planning of petrophysical core analysis.

**Author Contributions:** Conceptualization, A.M., A.K., and A.C.; methodology, A.M., A.K., N.B., A.C., and M.S.; investigation, A.M., A.K., and T.K.; supervision, N.B., S.M., M.S., and A.C.; formal analysis, A.M. and A.K.; resources, N.B., A.C., M.S., and S.M.; data curation, A.M., A.K., T.K., and A.C.; writing and editing, A.M., A.K., T.K, and A.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received funding within the framework of an industrial project with LUKOIL Engineering LLC.

**Acknowledgments:** The authors would like to express gratitude to the PermNIPIneft branch of LUKOIL-Engineering LLC for providing the core material for testing. The authors thank their colleagues from Skoltech's Center for Hydrocarbon Recovery: Mikhail Yu. Spasennykh for the opportunity to conduct the research; Victor Nachev, Evgeny Shilov, Philipp Denisenko, and Denis Orlov for help in conducting experiments and discussions. We appreciate the major contribution of Maxim S. Mel'gunov (Boreskov Institute of Catalysis SB RAS), as well as Ivan V. Myakshin (LUKOIL-Engineering PermNIPIneft) with planning and conducting mercury injection porosimetry experiments. Finally, the authors owe to anonymous peer reviewers for their time and effort to make this paper even better.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Appendix A. Supplementary Figures**

**Figure A1.** 3D view photo of the whole core #545.

**Figure A2.** Fractures occurred after extraction on an example of the Darcypress probes.

#### **Appendix B. Common Shared Techniques**

The core analysis workflows employed the main recommendations for rock analysis [6]. We determined the open porosity of the target rock samples using the standard liquid saturation or gravimetric method. The technique consisted of saturating a rock sample with a liquid (usually kerosene or water) and determining its volume by immersion in the saturating fluid utilizing precise laboratory scales A&D GH-202GH with an AD1653 gravimetric console.

The core saturation procedure consisted of vacuuming the samples, capillary imbibition, and injection of a saturating fluid under a pressure of 15 MPa using an automatic saturating unit Geologika PIK-SK.

Core crushing and probe preparation employed the crushing machine ASCS Scientific Jaw Crusher JC-300-ST-Q. We separated the core fractions by mesh size using the vibratory sieve shaker RETSCH AS 450 control.

Core cleaning (extraction) included cleaning the rock samples (core plugs and core chips) with chloroform in a Soxhlet apparatus. We controlled the extraction quality by both visual inspection with a UV lamp and by measuring the TOC content on rock specimens every 24 h. The time of extraction averaged 150 h.

After extraction, we dried the rock samples at a temperature of 70 ◦C in the laboratory heating oven Memmert VO400 until attaining a constant weight.

The Wildcat Technology HAWK RW instrument provided Rock-Eval pyrolysis measurements for source rock geochemical analysis and data interpretation [68].

Optical microscopy is the most common and universal method for studying mineral composition and textural features of sedimentary rocks. We imaged a set of thin sections (10–20 μm) using the Carl Zeiss Imager A2m polarizing microscope with transmitted light (12 V halogen lamp, 100 W). The resulting spatial resolution depended on many factors related to sample preparation. However, we managed to achieve the image pixel size of about 10 μm.
