Microscopic fluids occurrence characteristics in different oil layers of Qingshankou Formation in Gulong Sag: Insight from nuclear magnetic resonance

Insight into the microscale distribution and abundance of pore-contained oil and water are essential for shale oil resource evaluation. This study investigates continental shales from various oil layers of the Qingshankou Formation in the Gulong Sag, Songliao Basin. Nuclear magnetic resonance (NMR) T1–T2 and T2 measurements were employed to characterize fluid occurrence and quantify oil and water contents. Integrated analyses using scanning electron microscopy, x-ray diffraction, total organic carbon (TOC), and Rock-Eval pyrolysis (conventional and multi-stage) were conducted to evaluate the controls of mineral and organic components on fluid distribution. The shales are mainly composed of clay minerals, quartz, and feldspar. Reservoir space is primarily associated with intergranular pores in quartz and intragranular pores in clay minerals. The NMR T1–T2 spectrum enables the identification of adsorbed, capillary-bound, and movable oil in pores corresponding to T2 peaks p1, p2, and p3, respectively. Capillary-bound water in p1 pores restricts adsorbed oil. Adsorbed oil increases with TOC below 2% but decreases at higher TOC levels. Capillary-bound water correlates with clay content. Vertically, Q7–Q9 layers show high TOC, abundant meso–macropores, and elevated capillary-bound oil. These findings improve understanding of pore fluid behavior and support sweet-spot targeting in K2qn shale oil reservoirs.