Pore structure and fractal characteristics of transitional shales with different lithofacies from the eastern margin of the Ordos Basin

Abstract

AbstractTo better characterize the heterogeneity of transitional shale pore structure and understand its impact on shale gas enrichment, fine characterization of the pore structure of different shale lithofacies was performed by field‐emission scanning electron microscopy, high‐pressure mercury injection, low‐temperature N2 adsorption, and low‐pressure CO2 adsorption. Fractal theory was used to obtain the fractal dimension of the shale pores at different scales and reveal the relationships among the pore structural characteristics, mineral composition, total organic carbon content, and fractal dimension of the shale and their geological significance. The results showed that organic pores, intergranular pores, intragranular pores, and microcracks were generally developed in the transitional shale samples from the study area; elliptic or irregular organic pores were mainly developed in the argillaceous shale lithofacies and siliceous shale lithofacies, and wedge‐shaped or irregular intragranular pores were mainly developed in the calcareous shale lithofacies. The pore size distribution showed a multipeak pattern, and mesopores were the main contributors to the total pore volume (PV), while micropores and macropores contributed little to the total PV. The PV and specific surface area of the siliceous shale were lower than those of the argillaceous shale but higher than those of the calcareous shale, indicating that the change in lithofacies had a significant effect on the shale pores. The common influence of clay minerals, quartz content, and total organic carbon content results in strong heterogeneity and complex pore structure characteristics of shale reservoirs. The pores in the transitional shale from the study area had obvious multiscale fractal characteristics, and the fractal dimension characteristics of different lithofacies and pores at different scales were different, which reflected that the pore structure of the shale had strong heterogeneity. The heterogeneity of the shale pores mainly originated from the macropores and mesopores, while the heterogeneity of micropores was relatively low. The heterogeneity of the pore development and structure at different scales was controlled by the mineral composition matter and organic matter abundance to varying degrees.