Microdistributions of Pore Fluids and Lower Limit of Oil Accumulation in Shales of the Gulong Sag in China

The content and mobility of shale oil, as well as the ultimate recovery factor, are primarily governed by the occurrence behavior of fluids within shale nanopore systems. To investigate the microdistribution of pore fluids, a series of nuclear magnetic resonance (NMR) measurements was performed on shale samples from the Qingshankou Formation in the Gulong Sag, Songliao Basin, China. Complementary analyses, including total organic carbon (TOC), Rock–Eval, X-ray diffraction, low-temperature nitrogen adsorption–desorption, scanning electron microscopy, and mercury intrusion capillary pressure measurements, were conducted to elucidate the controlling factors. The T 2 spectra of pore water and oil in various states were deconvoluted from the T 2 spectrum after water and oil restoration using the Gauss decomposition. Shale oil content, particularly free oil, shows a strong positive correlation with TOC content, whereas capillary-bound water is closely associated with clay mineral abundance. Shale with TOC greater than 2% is considered a favorable interval for shale oil development, exhibiting high free oil content and oil saturation. The shale oil occurrence pore size is constrained by oil content (or oil generation capacity) and pore structure simultaneously. Higher oil content, coupled with reduced micropore and minipore volumes, leads to smaller occurrence pore sizes for shale oil, particularly for adsorbed oil. In contrast, the occurrence pore size of pore water is predominantly controlled by pore structure; more developed micropores and minipores generally result in water occupation in relatively larger pores. This study provides insights into the microscopic distribution characteristics of pore water and oil in shale oil reservoirs.