Type and Size Distribution of Nanoscale Pores in Tight Gas Sandstones: A Case Study on Lower Cretaceous Shahezi Formation in Songliao Basin of NE China

Predominant nano scaled pores, complex pore structure and various types of pores in tight sands, combined with the limitations of current experiments, block the understanding of full-range pore distribution and the contribution of different types of pores to porosity and permeability. A case study in the Lower Cretaceous Shahezi tight gas reservoirs of Songliao Basin is presented to reveal the full-range pore distribution and distinguish different types of pores, by comparing and integrating various experimental data, such as nuclear magnetic resonance (NMR), pressure-controlled porosimetry (PCP) and N-2 adsorption. Due to different pore connectivity, the interparticle/dissolution pores and inter-crystalline pores can be distinguished based on the distinct mercury intrusion features and surface fractal dimensions. It is indicated that NMR has obvious advantages in detecting the interparticle/dissolution pores (connected with narrower throats) than PCP which is affected by the intrusion hysteresis; PCP has significant advantages in revealing the inter-crystalline pores (with treelike pores structure) due to their good inter-connectivity and non-hysteresis, whereas N2 adsorption is good at probing the smaller pores (<20 nm) beyond the detected range of PCP and NMR. Therefore, the full-range pore distribution is determined by combining of the advantages of various experiments. By increasing the content of illite in tight sand samples, the upper limit of inter-crystalline pores decreases from 980 nm to 85 nm in diameters, the content increases from 48% to 80%, and the contribution to permeability increases from 5.8% to 100%, indicating that the inter-crystalline pores play an important role in storage and percolation for tight sands with high clay content instead of interparticle/dissolution pores.