Qualitative and quantitative characterization of multiple factors that influence movable fluid saturation in lacustrine deep-water gravity-flow tight sandstones from the Yanchang Formation, southern Ordos Basin, China

Fluid mobility is one of the most important factors in evaluating the potential for recovering tight oil. However, quantitative effects of multiple factors that influence movable fluid saturation in different pore-throat combinations and their relationships with lithofacies in tight sandstones, especially for lacustrine deep-water gravity-flow deposits, remain controversial due to the strong heterogeneity and complex pore structure of tight reservoirs. Core samples obtained from the Upper Triassic Yanchang Formation in southern Ordos Basin were evaluated by using a variety of techniques, including nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), X-ray diffraction (XRD), pressure-controlled porosimetry (PCP), rate-controlled porosimetry (RCP), impregnated thin sections, and helium porosity and nitrogen permeability measurements. Based on NMR T-2 distributions under water-saturated and centrifugal conditions, four types of pore-throat combinations are identified by using the relaxation time thresholds T-s1 and T-s2 from NMR: large intergranular pore-dominated pore-throat combinations (LIP), small intergranular pore-dominated pore-throat combinations (SIP), intragranular pore-dominated pore-throat combinations (IAP), and micropore-dominated pore-throat combinations (MP). Among them, SIP is the dominant pore space favorable for fluid flow. Movable fluid saturation in SIP pore space varies among different lithofacies. The best fluid mobility commonly occurs in fine-grained, cross-bedded sandstones (Sc), whereas the worst fluid mobility usually appears in siltstones to very fine-grained sandstones (Ss). The quantitative effects of a total of 41 factors that influence movable fluid saturation of different porethroat combinations were investigated through the analysis of the Pearson correlation matrix. The results demonstrate that porosity and permeability mainly affect the movable fluid saturation in the pore space of SIP and MP. In addition, maximum mercury intrusion saturation from PCP (S-max) and tortuosity (lambda) are the critical pore structure parameters affecting the movable fluid saturation in the pore space of SIP and MP, whereas the average pore throat radius ratio (eta) is the critical factor affecting the movable fluid saturation in LIP pore space. Overall, movable fluid saturation in different pore-throat combinations is characterized mainly by different microscopic pore structure parameters. A general pore network model for different lithofacies with different fluid mobility is established to facilitate assessment of the heterogeneity of tight sandstones and to further guide hydrocarbon exploration and development in similar lacustrine deep-water depositional settings.