Shale oil and gas have been discovered in the lacustrine Zhangjiatan Shale in the southern Ordos Basin, China. To study the distribution of extractable organic matter (EOM) in the Zhangjiatan Shale ($R_o$ ranges from 1.25% to 1.28%), geochemical characterization of core samples of different lithologies, scanning electron microscope observations, low-pressure ${\mathrm{N}}_2$ and ${\mathrm{CO}}_2$ adsorption, and helium pycnometry were conducted. The content and saturation of the EOM in the pores were quantitatively characterized. The results show that the distribution of the EOM in the shale interval is heterogeneous. In general, the shale layers have a higher EOM content and saturation than siltstone layers. The total organic content and the original storage capacity control the EOM content in the shale layers. For the siltstone layers, the EOM content is mainly determined by the original storage capacity. On average, 75% of the EOM occurs in the mesopores, followed by 14% in the macropores, and 11% in the micropores. The EOM saturation in the pores decreases with the increase in pore diameter. The distribution of EOM in the shale pores is closely related to the pore type. Micropores and mesopores developed in the kerogens and pyrobitumens and the clay-mineral pores coated with organic matter are most favorable for EOM retention and charging.

Forty-six core samples were collected from a deep well that penetrated organic-rich layers of the Chang 7, 8, and 9 members of the Yanchang Formation (Fm) in the Ordos Basin. Tests for total organic content (TOC), Rock-Eval pyrolysis, X-ray diffraction (XRD) mineralogy, and molecular composition of gases released from rock crushing were conducted. Analytical results indicate that TOC and clay contents are elevated. The organic matter (OM)-rich mudstone in the Triassic Yanchang Fm suggests good-to-excellent source potential for oil generation. Its thermal maturity is in the oil window. Strong petroleum expulsion occurred from the upper part of the approximately 13 m (42.6 ft) thick Chang 7 member, and for the Chang 8 and Chang 9 members, resulting in low free oil and low methane (${\mathrm{CH}}_4$) concentration in these OM-rich intervals. A combination of sandstone and thin organic-rich mudstone layers is a perfect hybrid lithology stacking pattern for petroleum expulsion. The thickness for effective source rock, approximately 10–12 m (32.8–39.3 ft), varied with sandstone/mudstone lithology stacking pattern. In contrast, limited or no oil expulsion occurred in the lower part of Chang 7 member, a 25 m (82 ft) thick organic-rich interval, which is indicated by high free oil and high ${\mathrm{CH}}_4$ concentration. A ${\mathrm{CH}}_4$-TOC plot can be used to differentiate generated gas, retained gas in OM-rich mudstones, and migrated gas in permeable sandstone beds. We have developed a conceptual model for petroleum expulsion from OM-rich thin versus OM-rich thick layers. Compaction and thermal volume expansion of oil generated from OM may play an important role in petroleum expulsion in OM-rich mudstones. The estimated petroleum expulsion efficiency is approximately 70% and 35% for thin and thick OM-rich mudstone layers, respectively. The redistributed OM in clay-dominated rock assemblage likely forms the preferred migration path to petroleum expulsion.

A chemostratigraphic study of a 300 m long core recovered from the southeastern central Ordos depocenter reveals thick ($<10–65\mathrm{m}$) intervals of fine-grained, organic-rich lacustrine strata, interpreted to represent deepwater deposition under meromictic conditions (incomplete watermass overturn) during lake highstand phases, interspersed with thick (10–30 m) intervals of arkosic sandstones, reflective of fluvio-deltaic deposition during lake lowstand phases. Along with elevated concentrations of %Al, traditionally a proxy for clay content, maximum total-organic-carbon (TOC) values in the deepwater lacustrine facies reach 8%, with average values of approximately 3%. The fine-grained, organic-rich facies is also characterized by elevated S (up to 6%) and As concentrations, both proxies for pyrite, an indicator of more stagnant, reducing conditions in the hypolimnion (the volume of the lake below the surface mixing zone) during lake highstand phases. Enrichment factors for redox-sensitive trace elements (RSTEs) are not significantly elevated throughout the TOC-rich intervals, but they are punctuated in thinner intervals. Punctuated enrichments in RSTEs reflect episodes of enhanced suboxia/anoxia in the hypolimnion during lake highstand phases resulting from sustained meromixis (protracted episodes of incomplete watermass turnover). Although dramatic shifts between fluvio-deltaic and deepwater lacustrine deposition are recorded in the Yanchang strata, no evidence indicates that the lacustrine system ever built up a significant salt concentration, suggesting that an outlet was maintained throughout much of the depositional history. A chemofacies framework for the Yanchang Formation is developed based on hierarchical cluster analysis and ranking of major element chemostratigraphic results. The outcome of chemofacies analysis is similar to the lithofacies analysis, subdividing the stratigraphic record of calcite-cemented sandstone/siltstone lithologies (fluvio-deltaic facies) and silty to finer grained mudstone lithologies (deep-water lacustrine facies) based on changes in elemental concentrations that equate to shifts in the relative abundance of mineral contents (e.g., clays, quartz, feldspars, pyrite, and carbonates).

Identifying the microscopic mechanism of CO₂–oil miscibility is significant for the CO₂ enhanced oil recovery (CO₂–EOR) in tight sandstone reservoirs. In this work, the effects of oil composition, formation pressure, temperature, and methane content on the characteristics of the CO₂–oil miscibility were systematically studied by molecular simulation methods. According to the change of oil–gas centroid displacement, the CO₂–oil miscibility behavior was divided into four stages: rapid diffusion, CO₂ dissolution and oil swelling, competitive adsorption and oil film detachment, and complete miscibility or dynamic equilibrium stability. The results showed that light or medium component oil is more easily miscible with CO₂ under reservoir conditions. The changes in temperature and pressure will greatly influence the oil–gas miscibility. Increasing the temperature is conducive to reducing the adsorption energy between oil and quartz, thus improving the miscibility of CO₂ and heavy component oil. However, the static swelling effects of CO₂ alone cannot effectively displace the heavy component oil on quartz. The CO₂ diffusion coefficient perpendicular to the quartz surface does not increase continuously with the temperature increase due to the adsorption of oil and quartz. There is a critical temperature range of 320–340 K, which makes the miscibility effects the best. A small amount of CH₄ can enhance the interaction energy between the two phases of oil and gas, thus promoting the miscibility of CO₂ and oil at the interface. However, it is not conducive to oil film detachment, with the CH₄ content increasing.