Low-to-medium maturity shale oil resources hold significant potential, but their economic accessibility is limited by low porosity, low permeability, and a low proportion of movable oil. In-situ conversion technology can crack organic matter and in-place oil into lighter molecules, enhancing oil and gas mobility and improving recovery rates. The success of this approach depends on dynamically evaluating the amount of movable shale oil during in-situ conversion. This study targets the lower submember of the fourth member of the Eocene Shahejie Formation (Lower Sha4 Member) in the Damintun Sag, Bohai Bay Basin, China. Through thermal simulation experiments, organic geochemical experiments, and nuclear magnetic resonance experiments, shale residual oil evaluation, organic matter hydrocarbon generation process evaluation, and dynamic evaluation of immovable oil were carried out. By integrating numerical simulations of the temperature field with the experimental results, a dynamic evaluation method for movable resources in shale during in-situ conversion was established. The findings indicate that the conversion rates of kerogen-to-oil and kerogen-to-gas first increase and then gradually stabilize as thermal maturity increases, with oil generation reaching its peak when the vitrinite reflectance reaches 1.0%. Long-term preserved shale samples were identified to contain immovable oil, the content of which increases with maturity before peaking and then declining. In-situ conversion of low-to-medium maturity shale in the upper part of the model can significantly increase movable oil resources in a year, potentially reaching the levels of extractable medium-to-high maturity shale. This work presented a crucial approach for assessing and improving In-situ conversion technology, providing a means of maximizing economic feasibility.

Shunbei oil/gas field is of the ultra-deep carbonate type controlled by multi-stage strike-slip faulting within the craton. The significant differences on the oil/gas property and abundance are shown in different fault zones even different segments in a fault zone. An integration of the core and thin section observation, cathodoluminescence, in-situ trace and rare earth element (REE) analysis, systematic analysis of fluid inclusions, is applied to study the sequence of fracture vein formation, sources of vein-forming fluids and their coupling relationships with hydrocarbon charging in the Middle-Lower Ordovician reservoir of the No. 1 and 5 strike-slip fault zones in Shunbei area. We also investigate the factors controlling differential hydrocarbon accumulation in Shunbei area while understanding the evolution of strike-slip faults and the hydrocarbon accumulation pattern. The results show that there are at least four phases of calcite vein development, that is, Cal-1, Cal-2, Cal-3, and Cal-4, respectively, in the Middle-Lower Ordovician reservoir of the No. 1 and 5 strike-slip fault zones in Shunbei area. The vein-forming fluids of Cal-1 are mainly sea-sourced fluids, while that of Cal-2, Cal-3 and Cal-4 are derived from diagenetic fluids in the reservoir where the veins occur. The formation of Cal-2, Cal-3 and Cal-4 calcite veins is associated with tectonic activities and hydrocarbon charging in the Late Caledonian to Early Hercynian, the Late Hercynian to Indosinian, and the Yanshan-Himalayan, respectively. The strike-slip fault zones are different in hydrocarbon charging, as shown by that the main charging phase of the No. 5 fault zone was during the Late Hercynian to Indosinian, while the main charging period of No. 1 fault zone was during the Late Hercynian to Indosinian, and Yanshan-Himalayan. The differential hydrocarbon charging controlled by strike-slip fault structural style and evolution history is the key to differential hydrocarbon accumulation in Shunbei area.