**6. Conclusions**

This study indicates that natural fractures are abundant in genetic types, including tectonic, pressure-solution, and dissolution ones, in the carbonate basement reservoirs in the Jizhong Sub-Basin, where tectonic fractures are the dominant type. Macroscopic fractures vary in height and length from several centimeters to meters, with a wide range of linear densities and dip angles. These fractures are found in three sets of NNE–SSW, NW–SE, and near E–W strikes. The apertures of microscopic fractures are concentrated below 60 μm, and more than 60% of them are opening-mode ones without mineral fillings.

Dolostones with higher dolomite content are more likely to develop tectonic fractures than limestones with higher calcite content, and tectonic fractures have the least development intensity in the mudstone. Most tectonic fractures are developed inside the mechanical units and end almost perpendicularly or with a higher angle to the mechanical unit interface. In a thinner mechanical unit, these fractures are more frequent and have a smaller height. The dominant orientation of fractures changes regularly and is sub-parallel to faults or oblique at a small angle.

The analysis of opening-mode fractures and well-testing data in perforated intervals reveals that oil production is related to fracture characteristics. In intervals where dolostone and limestone are

interstratified or dolostone is the main lithologic composition, fractures are generally more developed, and oil production is higher, unlike intervals where limestone is the main constituent. Moreover, tectonic fractures with larger dip angles present a bigger aperture and contribute more to oil production. Finally, it is concluded that natural fractures are the main controlling factor of oil production to create favorable reservoir conditions in these basements. This study provides reference and future guidance for a better understanding of natural fractures and reservoir heterogeneity in the basement reservoirs

**Author Contributions:** Conceptualization, G.L. and L.Z.; Data curation, C.H. and F.H.; Formal analysis, G.L.; Investigation, W.L. and J.Z.; Resources, C.H. and F.H.; Software, J.Z.; Supervision, L.Z.; Validation, W.L. and J.Z.; Writing—original draft, G.L.; Writing—review and editing, L.Z., M.O. and Q.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Major Science and Technology Projects of China, gran<sup>t</sup> number 2017ZX05008-006-002-005.

**Acknowledgments:** The authors greatly acknowledge the Petro China Huabei Oilfield Company for providing information and other support. The authors would also like to thank the work made by Li Jian and Lyu Peng in the China University of Petroleum-Beijing, which enabled our study to proceed successfully. We are also particularly grateful to Veljko Pajovic and three anonymous reviewers for providing constructive comments and suggestions, which improve our manuscript significantly.

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