Microscopic characteristics and fractal Representation of limestone reservoirs in the Taiyuan Formation, Ordos Basin

The Lower Permian Taiyuan Formation in the central-eastern Ordos Basin possesses favorable near-source accumulation conditions. Since 2020, natural gas exploration in limestone formations has achieved promising results. Research on tight limestone reservoir conditions and microscopic characteristics is critical for guiding exploration and development. In this study, we measured the porosity of 20 samples using the long-time equilibrium porosity tests. Seven representative samples were analyzed using high-pressure mercury injection and low-temperature nitrogen adsorption–desorption to investigate pore structure parameters. The pore fractal dimension was calculated using the Frenkel-Halsey-Hill (FHH) model, and its relationships with reservoir storage and seepage capacity were examined, along with the controlling factors. The results showed the following. (1) Increasing the porosity measurement time significantly increased porosity. The 20 samples exhibited an average porosity of 2.7% and permeability of 0.13 mD. (2) The mercury intrusion results showed low mercury removal efficiency (<10%), curves with long flat plateaus and gentle slopes, a mean variation coefficient of 0.0652, and a mean sorting coefficient of 0.876, indicating a concentrated pore throat distribution. The fractal dimensions ranged from 2.099 to 2.738. (3) Nitrogen adsorption data indicated Brunauer–Emmet–Teller (BET) specific surface areas of 0.56–1.79 m 2 /g (average 1.081 m 2 /g). Micropores and mesopores accounted for 61.24%–82% of the pore volume, with fractal dimensions of 2.391–2.812. (4) Primary pore development was closely related to calcite recrystallization. Higher recrystallization reduced surface roughness and pore abundance; thus, the fractal dimension from nitrogen adsorption data was positively correlated with pore development. (5) The fractal dimension was highly correlated with pore throat parameters; higher values indicated more complex systems. (6) Scanning electron microscopy (SEM) observations demonstrated that calcite occurred primarily as pore-filling material in intercrystalline pores. The fractal dimension was positively correlated with the calcite content. These results provide vital data and theoretical support for the exploration and development of limestone gas reservoirs in the Taiyuan Formation.