## 6 Conclusions  
Using X-ray micro-CT, the present work investigates the pore-throat structure of five marine biogenic carbonate samples and a terrigenous sedimentary sandstone sample. As the results show, the five carbonate samples, which came from the drill cores at five different depths of a single drilling, differed significantly in porosity. On the whole, the research results were consistent with [PERSON] et al. [35] which also confirmed that the sedimentary law of coral reefs is different from that of terrigenous strata. In terrigenous strata, the physical and mechanical properties of rocks are usually enhanced with the increase in depth. However, coral reef strata have significant sedimentary facies belt effect and facies belt cycles. In this manuscript, the depth of the sampling did not influence the rock porosity significantly. This demonstrates the complex forming process of carbonate rocks, as the randomness of the sedimentary particulates and the heterogeneity of the diagenesis process result in anisotropic pore structures.  
Cross sectional porosity was measured for the cross-sectional images of the samples in the direction of Z axis. Much higher cross sectional porosity was observed for the carbonate samples compared to the sandstone sample. Moreover, the deviation of cross-sectional porosity was more severe for the carbonate samples, indicating a more heterogeneous pore structure. In contrast, small and less deviative cross-sectional porosity was observed for the sandstone sample, indicating a relatively uniform and dense internal structure. How the cross-sectional porosity evolves along the Z axis reflects the change of porosity inside the specimen, which can help locate large pores in the sample in a non-disruptive manner.  
A 3D reconstruction was performed for each rock sample using the cross-sectional images from CT scanning. Representative elementary volumes (REVs) at fixed positions of each rock sample were placed in the 3D reconstructed rock samples. The maximum ball algorithm was then applied to extract the pore-throat network of the REVs. As the results indicate, the average pore radius, the average pore volume, the average throat radius and the average throat volume of carbonate are not only larger than the counterparts of sandstone, but also distributed in a wider range compared to those of sandstone. Yet, similar distributions of the pore shape factors and the throat shape factors were observed between carbonate and sandstone. Over 85% of the pores in the carbonate samples were classified as mesopores while more than 85% of pores in the sandstone sample were deemed micropores. The average coordination number was 1 for carbonate, while 0.1 for sandstone.  
As revealed in the present work, the average pore volume is larger than the average throat volume for both sandstone and carbonate. The average throat volume is approximately one order of magnitude smaller than the average pore volume. For all the scrutinized REVs, the number of throats were either almost equal to or outnumbered by the number of pores. It can be thus inferred that the porosity of the REVs is majorly related to the volume of the pores.  
Determine research goals and aims, creation of models, H.L. and C.Z.; data curation, writing--original draft, H.L. and C.M.; research funding support, H.L. and C.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key R&D Program of China grant number (2021 YFC3100604), the National Natural Science Foundation of China grant numbers (41877271, 4210720) and the Natural Science Foundation of Hubei Province of China grant number (2020 CFB243).  
Not applicable.  
The authors also wish to acknowledge Professor [PERSON], Nanjing University, for his help in interpreting the carbonate and sandstone microfacies.  
The authors declare no conflict of interest.