The influence of hydrothermal activities on shale oil reservoirs during the burial period of the Upper Triassic Chang 7 Member, Ordos Basin

The hydrothermal activity of the Upper Triassic Chang 7 shale in the Ordos Basin has been extensively studied in previous studies, with a focus on hydrothermal sedimentation during the depositional period of Chang 7 Member. However, there is limited research on the impact of hydrothermal activities during the corresponding burial stage. This study aims to investigate the stages and ages of hydrothermal activities and their impact on shale oil reservoirs during the burial stage of the Chang 7 shale. An integration of multiple techniques, including optical/electronic microscopy, electronic probe, micro-laser Raman spectrum and inclusion homogenization temperature analysis is applied to study the hydrothermal pyrite and solid phases and fluid inclusions in the Chang 7 Member. The results reveal that there contains many types of pyrite with Co/Ni greater than 1 exhibiting a variety of morphologies, including veins, lumps, lenticular shape as well as xenomorphic-hypidiomorphic scattered and hypidiomorphic-euhedral massive forms, indicating hydrothermal origin. Based on the analyses of microscopic observation, regional tectonic history, as well as the simulation of burial-thermal evolution history, it is inferred that there were at least two phases of hydrothermal activities during the burial period of the Chang 7 Member, with one of them occurring in the Early Cretaceous. The maximum temperature of the hydrothermal fluid injected into the Chang 7 shale in well Yy1 may reach up to 270.5 ℃ based on the homogenization temperatures of aqueous inclusions associated with pyrite inclusions. The calculation based on the Easy%R_o kinetic model indicates that the Chang 7 shale underwent a rapid cooling process after hydrothermal injection, which may be one of the important reasons for the lower-degree thermal evolution of shale organic matter (R_o = 0.70 %).