Comparing the Porosities of Upper Red Sandstone and Asmari Limestone using Petrographic Image Analysis and Saturation-Buoyancy Methods

This study compares rock porosities measured using petrographic image analysis (PIA) and saturation-buoyancy (SB) methods. To this end, the fresh block samples from sandstone and limestone were collected from Upper Red and Asmari Formations, respectively. Next, cylindrical core specimens from those were prepared using a coring machine. Afterward, the porosity values of sandstone and limestone samples were measured using PIA and SB methods. In the next step, the physical and mechanical properties of samples, including water absorption (W), P wave velocity (PWV), uniaxial compressive strength (UCS), Young’s modulus (E), point load strength index (PLSI), and Brazilian tensile strength (BTS), were determined. Results showed that PIA underestimates the porosity in both rocks. These conservative results can be attributed to the incomplete filling of micropores with blue-dyed epoxy resin in sandstone cement and limestone matrix or petrographic microscope limitation to differentiate micro-pores in cement and matrix. Furthermore, results demonstrated that SB in upper red sandstone has a good correlation with W, E, and BTS and a moderate correlation with PWV, UCS, and PLSI. Besides, porosity of limestone samples showed strong relation with W, PWV, and BTS, while UCS and E data versus porosity showed a scattering pattern. Finally, a strong relation was established between PIA porosity and UCS, E, and PLSI, a poor correlation was established in the cases of W, PWV, and BTS. Overall, PIA is a strong method for investigating the interior pore system of rocks, especially isolated pore spaces, although it underestimates rocks porosity compared to the SB method.