Evaluation and Significance of Porosity Measuring techniques for Laminated Shales in the Dongying Sag

Laminated shales are extensively developed in the fourth member of the Paleogene Shahejie Formation (Es₄) within the Jiyang Depression, Bohai Bay Basin. Studies suggested that accurate measurement and effective evaluation of porosity in shale reservoirs are crucial for shale oil resource assessment and sweet spot prediction. This study focuses on the upper sub-member of Es₄ (Es₄ˢ) laminated shales in the Dongying Depression. Building upon an evaluation of their basic geological characteristics, we employed an integrated experimental approach including scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), low-temperature nitrogen adsorption (LTNA), heliμm porosity, and high-pressure mercury intrusion capillary pressure (MICP) analyses. This study clarifies compares different porosity testing methods and the influence of experimental conditions on porosity measurements. Shale in this study area is predominantly composed of mixed shale facies, including organic-rich laminated clay-rich mixed shale, organic-rich laminated carbonate-rich mixed shale, and organic-bearing laminated clay-rich mixed shale. The main pore types observed in the Es4 shales of the Dongying Depression are intergranular pores, intercrystalline pores, clay interlayer pores, and dissolved pores. Shale oil primarily occurs within intergranular pores, intercrystalline pores, and micro-fractures. The comparison of multiple porosity measurement methods shows that: crushed-sample helium density method > GRI method > core-plug helium density method ≈ water-saturated NMR method ≈ oil-saturated NMR method > alcohol method > high-pressure mercury intrusion method. Among them, the crushed-sample helium density method yields the highest total porosity, and its experimental conditions are optimized. For porosity determination, rock samples should be crushed to 200-mesh and subjected to ternary solvent extraction for at least 3 days. The extracted samples should be dried at temperatures ≥100°C, with drying at 140°C or above being implemented to ensure thorough removal of residual fluids. The equilibration time for heliμm pycnometry measurements should be set at 10 minutes. The comprehensive application of the above experimental conditions can obtain the maximum total porosity value. This study provides an effective methodological basis and data support for the accurate determination of total porosity in continental laminated shale reservoirs.