Study on the pore structure, fluid mobility, and oiliness of the lacustrine organic-rich shale affected by volcanic ash from the Permian Lucaogou Formation in the Santanghu Basin, Northwest China

Lacustrine organic-rich shale with extremely heterogeneous pore structure is widespread in the second member of the Permian Lucaogou Formation (P(2)l(2)) of the Santanghu Basin. However, information on the pore structure, fluid mobility, and oiliness characteristics is still lacking, as well as the interaction between them in the organic-rich shales remains controversial. To address these problems, this study analyzed the influencing factors of pore structure, oiliness, and fluid mobility of the P(2)l(2) shale and constrained the lower limit of movable fluid pore throat radius based on thin sections, routine core analysis, whole rock X-ray diffraction (XRD), scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), constant-rate mercury injection (CMI), and nano-CT scanning data. The P(2)l(2) shale was characterized by fine-grained minerals of felsic (tuffaceous material) and carbonate (dolomite), with poor reservoir physical properties and strong oil-bearing heterogeneity. Micro- and nano-scale pores are the main type of storage space. Among them, most of the microfractures emit green to yellow hydrocarbon fluorescence regardless of whether they are filled or not, as do the edges of dolomite grains. While the edges of the felsic minerals are characterized by stronger orange yellow fluorescence of organic matter. According to the morphology and parameters of HPMI and NMR curves, the pore structure can be divided into four types. From type 1 to type 4 pore structure, the maximum mercury saturation decreases from 95 to 47 % (avg. 70 %), indicating poor pore connectivity, and the pore throat radius that mainly contributes to permeability is > 0.04 mu m. Meantime, the NMR T-2 spectrum gradually transitions from right unimodal to left unimodal behavior, with a gradual decrease in the saturation of movable fluid and a transition of the mineral type from carbonate to felsic. Fluid mobility is not only influenced by macropores, but also related to the content of small pores (T-2 < 1 ms) and the mineral type. On this basis, the lower limit of the movable fluid pore throat radius in the P(2)l(2) organic-rich shale is finally determined to be 40 nm.