Occurrence characterizations and controlling factors of lacustrine shale pore fluids in the Qingshankou Formation, Sanzhao Sag, Songliao Basin, China

The pore fluid occurrence predominantly restricts shale oil production. Few studies have addressed both pore oil and water concurrently. In an effort to delineate the distribution of pore fluids within shale oil reservoirs, this study collected diverse shale samples from the Qingshankou Formation in the Sanzhao Sag, Songliao Basin, China. The in-situ pore fluids were initially resurrected under equilibrium moisture conditions and subsequently saturated with light oil. A comprehensive suite of analytical techniques was employed in tandem, encompassing total organic carbon (TOC), Rock-Eval, X-ray diffraction (XRD), scanning electron microscopy (SEM), low-temperature nitrogen adsorption-desorption, and nuclear magnetic resonance (NMR). The occurrence characterizations of pore fluids were clarified by NMR T₁–T₂ spectra across various states, shedding light on the governing factors. A pattern of pore-fluid occurrence in shale oil reservoirs was proposed. Results indicate that NMR T₁–T₂ combined with water and oil restoration effectively assesses the distribution of in-situ pore fluids. Capillary-bound water primarily contributes to pore fluids, with nearly half being depleted at the as-received state. Shale oil mainly comprises capillary-bound oil, succeeded by adsorbed and movable oil. The alterations in shale oil occurrence characteristics are synchronous with the depletion of pore fluids. NMR T₁–T₂ primarily detects the adsorbed oil in shale pores, whereas Rock-Eval is capable of quantifying oil adsorbed on pore surfaces and absorbed within organic matter. NMR T₁–T₂ offers a more precise technique for quantitatively evaluating shale pore fluids. Micropores (<25 nm) and minipores (25–100 nm) are primarily saturated with capillary-bound water, accompanied by a minor fraction of adsorbed oil. Capillary-bound and movable oil are primarily distributed within mesopores (100–1000 nm) and macropores (>1000 nm), respectively. Consequently, adsorbed oil is significantly influenced by pore water, followed by capillary-bound oil, while movable oil remains largely unaffected. Felsic-rich (FR) shales may represent the optimal lithology for shale oil enrichment, characterized by the development of interparticle pores, a lower Brunauer-Emmett-Teller (BET) specific surface area, and abundant meso- and macropores. These insights into the characteristics of pore fluids in shale oil reservoirs could bolster shale oil exploration in the Sanzhao Sag.