ICE 2022


The determination of accurate in-situ oil (hydrocarbon) content is critical for shale oil plays to determine total resource and their recovery in-place. Recently, although the correction of light hydrocarbons to S1 has been studied via the sealed or pressure-preserved cores, in the process of S1 detection using Rock-Eval and TD-GC methods, some hydrocarbons are still lost due to the crushing of samples. However, this has not received enough attention. Here, we present a methodology applied to pristine cores to determine the in-situ oil content per gram of rock (mg/g) by combining geophysical and geochemical techniques, which include four parallel methods: Dean-stark, NMR T1-T2 mapping, TD-GC-SE, and Rock-Eval-SE. The former two methods are conducted on the small block shales with 3-5 cm size pieces, while the latter two methods are TD-GC and Rock-Eval experiments (powders, 60 mesh) combined with solvent extraction (SE) to determine total oil content. In addition, a series of experiments such as NMR, TD-GC-SE, and Rock-Eval-SE are performed on the pristine shales with different storage time to investigate fluid changes. Examples of 109 lacustrine pristine (pressure-preserved) shale examples reveal that both the total oil and water contents measured by the Dean-stark and NMR methods show good agreement. A novel approach and workflow, namely NMR T1-T2 mapping is proposed, and its advantages in determining the total oil content and oil saturation of the pristine shales are discussed. The total oil content detected by the four experiments follows the order of Dean-stark (av. 14.74 mg/g) ≈ NMR (15.54) > TD-GC-SE (10.24) > Rock-Eval-SE (8.12). Sample crushing and crucible waiting process of Rock-Eval 6 results in a loss of one-third to one-half of the total oil in pristine shales. To the best of our knowledge, this is the first time to notice that the total oil content of the pristine shales is underestimated by the Rock-Eval and TD-GC method, and the previously proposed correction coefficient of S1 seems to be 50 percent underestimated. During the storage (open system) of pristine shales, both water and free oil/light hydrocarbons (C14-) evaporate quickly, while adsorbed oil remains unchanged. Thermal maturity and TOC are the main control factors affecting S1 evaporation. The understanding of in-place fluids such as hydrocarbon content, components, and their occurrence in pore space can provide basic data to obtain accurate phase characteristics (GOR).