Geochemical characteristics of paleo-fluids in thrust belt in the northern Middle Yangtze and its significance for shale gas preservation: a case study of well Baodi 1

Abstract

To study the geochemical characteristics of paleo-fluids in the Dahongshan thrust belt of Middle Yangtze and its significance for shale gas preservation, in this study, Silurian vein samples were collected from well Baodi 1 and fluid geochemistry and inclusions were systematically analyzed. Results show that the variation range of δ13C of calcite veins is -8.19‰ to 0.16‰, with the minimum value much lower than that of Silurian limestone interlayer and marine carbonate. The desulfurization coefficient of vein inclusion group is 23.53 to 87.90, with downward trend from bottom to top. It is indicated that the SO42- in paleo-fluid of Silurian increases because of the mixing of underlying Cambrian gypsum brine, as well as CO2 and H2S entering into Silurian along fracture system from Cambrian TSR productions, resulting in negative δ13C of calcite veins and H2S show in drilled formations. The inclusions at the bottom of Silurian Longmaxi Formation are mainly pure aqueous solution inclusions. The maximum homogenization temperature peak is about 110 to 120℃, and the minimum homogenization temperature distribute from 60 to 80℃. Compared with those in the eastern Sichuan Basin, the shale in Dahongshan thrust belt entered uplifting and denudation stage earlier, and the paleo-fluid formation stage had shallower burial depth, low temperature and low gas saturation. The range of homogenization temperature of inclusions near the detachment zone is wider, and the lowest homogenization temperature is developed in this section, indicating the detachment zone has more periods of tectonic activity and longer duration time, especially in the later stage, which has become a long-term channel for shale gas escape. Comprehensive analysis shows that there are many detachment layers in the Dahongshan thrust belt, and the thrust and deformation destroyed multi-layer shale gas reservoirs and conventional gas reservoirs.

Keywords

• paleo-fluids
• shale gas
• preservation condition
• inclusions
• geochemistry
• dahongshan thrust belt
• northern middle yangtze