LAUSR

Abstract This study's main goal was to recognize pore types, sizes, and abundance within three carbonate mudrocks units (parts of Upper Marrat, Tuwaiq Mountain Limestone, and Hanifa formations) located in Central Saudi Arabia. The methods used for this study were petrographic thin section analysis, helium porosity measurements, and X-ray diffraction. Backscattered scanning electron microscopy with energy dispersive scanning, micro-computed tomographic scanning, Total Organic Carbon “TOC” analysis, nuclear magnetic resonance, N2 adsorption analyses were used for qualitative and quantitative pore type characterization. Three carbonate mudrocks lithofacies were specified in Hanifa Formation Hawtah Member: LFH1, LFH2, and LFH3. Two lithofacies were defined in the Tuwaiq Mountain Limestone mudstone: LFT1 and LFT2. Two mudstone lithofacies were identified in the Upper Marrat Formation: LFM1 and LFM2. Intra-skeletal, intraparticle, and secondary OM-hosted pores are found in the Hanifa mudstone lithofacies. Intra-skeletal, small scale interparticle (pyrite framboids), and OM-hosted pores were predominant in Tuwaiq Mountain Limestone mudstone lithofacies. Small scale interparticle pores (euhedral calcite) and secondary OM-hosted pores were found in both mudstone lithofacies of the Upper Marrat Formation. All pore types were linked to corresponding diagenetic processes and burial events. NMR-relaxation time curves and pore size distribution curves revealed that LFH1 and LFH3 lithofacies has higher pore abundance than in the LFH2 lithofacies. LFT1 and LFT2 lithofacies have similar bimodal relaxation time distribution with slightly higher abundance observed in the LFT1 lithofacies. LFM1 and LFM2 lithofacies have similar bimodal relaxation time distribution with slightly higher abundance observed in the LFM1 lithofacies. These results can be used as input parameters to generate fluid flow simulation to determine these carbonate-rich mudrock reservoirs' potentiality.