Zhigang Wen

Fluid type and content directly control fluid mobility in tight reservoirs. At present, there are two ways to classify fluid types. One is to classify fluids into movable fluids (MF), capillary-bound fluids (CAF), and clay-bound fluids (CLF). The other is to classify fluids into free fluids and adsorbed fluids. However, the intrinsic relationship between the two fluid classification schemes is still unclear. In order to investigate the pore structure and fluid type characteristics, a series of experiments were performed on the Chang 7 tight sandstone in the Longdong area. The full-scale pore size distribution (PSD) can be obtained by combining low-temperature nitrogen adsorption (LTNA) with nuclear magnetic resonance (NMR). The PSD of Chang 7 tight sandstones primarily ranges from 1 nm to 20 μm. Based on the fractal characteristics, pore system is divided into macropores (mainly >150.8 nm), and micropores (mainly <150.8 nm). MF, CLF, and CAF constitute 11.1–49.7% (avg. 34.6%), 17.3–40.1% (avg. 26.7%), and 32.9–48.8% (avg. 38.7%) of total fluid, respectively. Additionally, macropores are positively correlated with MF and negatively correlated with CAF and CLF, whereas micropores show the opposite trend. By integrating NMR data with theoretical modeling, a clear correspondence between the two fluid classification approaches was established: MF and CAF closely correspond to free fluids, while CLF is strongly associated with adsorbed fluids. Notably, in samples with low porosity and extremely fine pore throats, the combination of centrifugation and theoretical modeling may underestimate the actual free fluid content. MF shows a weak negative correlation with quartz and clay minerals; CAF and CLF are weakly positively correlated with quartz and clay but negatively correlated with feldspar. The occurrence patterns of different fluid types within various pore-throat structures were established, revealing the relationships among mineral composition, pore size, pore-throat structure, and fluid distribution. These findings provide valuable insights into the pore structure and fluid distribution of tight sandstone reservoirs, enhancing the understanding of fluid behavior in unconventional systems.

The interglacial period of the Cryogenian glaciation is a pivotal interval in geological history, marked by two “Snowball Earth” events and the emergence of early animals. Currently, there is considerable debate regarding the paleo-oceanic environment and the dominant factors controlling organic matter enrichment. Here, based on inorganic geochemical data and mineral composition from the Datangpo Formation in Xiangtan (South China), combined with previous research, we have analyzed the paleo-climate, redox condition, seawater restriction, and primary productivity across different sedimentary facies during this critical interval. The results exhibit that the Datangpo Formation can be divided into three members (Da1–Da3) based on lithology. Paleoclimatic proxies suggest the environment was relatively cold during the deposition of the Da-1 Member, while it was relatively warm and humid during the deposition of the Da 2–3 members. Compared to shallow water areas, deep-water areas experienced a more rapid transition in paleotemperature following the Sturtian glaciation event. Combining Mo-U elements, Ce_N/Ce*_N, and C_org/P ratios, the environment was characterized by an oxic environment during the early deposition period of the Datangpo Formation, then gradually transitioned to suboxic, and finally anoxic conditions. Furthermore, the decompression of terrestrial magma chambers led to intense volcanic/hydrothermal activity during the deglaciation period. Hydrothermal activity was most intense during the Da-1 depositional period, followed by Da-2, and gradually declined during Da-3 depositional period. Hydrothermal activity not only provided essential materials for the formation of Mn carbonate ores but also significantly enhanced the primary productivity by introducing large amounts of nutrients in the paleo-ocean. The primary productivity indicators (Ni/Al, Cu/Al) exhibited an obvious coupling with Ce_N/Ce*_N and C_org/P ratios in the Datangpo Formation, indicating that oxygen-rich environments were favorable for biological proliferation, thereby providing abundant organic matter. Anoxic conditions further facilitated the preservation of organic matter, which may be the primary factor driving organic matter enrichment in the Datangpo Formation.

Organic geochemistry experiments, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), N₂ adsorption, CO₂ adsorption, and two-dimensional nuclear magnetic resonance (2D NMR) were performed on ten Chang 7 shale samples (Longdong area, Ordos Basin) to elucidate their pore structure and the characteristics of oil occurrence. Moreover, the factors influencing free oil were discussed, and an occurrence model was established. FE-SEM analysis reveals that the pore types include interparticle pores, intraplatelet pores within clay aggregates, rare fracture pores, and organic matter (OM) pores. The pores are predominantly slit-shaped. The development of micropores was mainly contributed to by OM. Quartz and clay minerals influence the development of macropores and mesopores, feldspar mainly controls macropore development, and pyrite most strongly affects micropore development. Micropores and mesopores constitute the main total pore specific surface area, while mesopores and macropores are the main contributors to the total pore volume. Pores > 2 nm are the main storage spaces for shale oil, and free oil mainly occurs in pores > 20 nm. Adsorbed oil and free oil were assessed by NMR T₁–T₂ mapping. The adsorbed oil signal intensities range from 7.5–23.4 a.u. per g of rock, and the free oil signal intensities range from 4.4–23.2 a.u. per g of rock. The free oil proportions are 15.9–70.6% (average of 44.2%). The free oil proportion is negatively correlated with the clay mineral content and total organic carbon (TOC) content but positively correlated with the saturated hydrocarbon content and volume of pores > 20 nm. The results of this study could help optimize favorable shale oil target areas.