Analysis of the Controlling Factors of Resistivity Fluctuations in Tight Sandstone Reservoirs of the Hangjinqi Gas Field, Ordos Basin

In the central region of the Lower Shihezi Formation within the Hangjinqi Gas Field, Ordos Basin, China, gas-bearing tight sandstone reservoirs exhibit a coexistence of low-resistivity and medium-to-high-resistivity characteristics. These reservoirs are characterized by significant resistivity fluctuations, posing challenges to the accurate calculation of gas saturation using the classical Archie equation and its derivatives. Consequently, both qualitative and quantitative evaluations of gas-bearing tight sandstone reservoirs are often hindered. To address these complexities, a comprehensive analysis was conducted utilizing an integrated dataset comprising mercury intrusion porosimetry (MIP), conventional thin-section petrography, whole-rock and clay mineral analyses, scanning electron microscopy (SEM), natural gamma ray (GR), spontaneous potential (SP), deep and shallow lateral resistivity (LLD and LLS), array induction resistivity (HDIL), acoustic travel time (AC), density (DEN), neutron (CNL), and nuclear magnetic resonance (NMR) logging data. The results demonstrate that among various factors influencing resistivity—including sedimentary characteristics, clay mineral distribution, porosity, pore structure, and gas content—porosity within distinct pore size ranges is the primary controlling factor driving resistivity variations. Specifically, analysis of NMR T2 spectrum data reveals that low resistivity is predominantly associated with porosity in the 3–12 ms pore size range, while high resistivity is primarily controlled by porosity in ranges exceeding 24 ms and below 12 ms. These findings enhance the understanding of the mechanisms underlying resistivity fluctuations in gas-bearing reservoirs. This understanding is essential for improving the accuracy of reservoir identification and optimizing development strategies, offering valuable insights for the exploration and exploitation of similar reservoirs worldwide.