Paleo-fluid evolution in the Lower-Middle Ordovician carbonate reservoirs of the Shunbei area, Tarim Basin

Deep carbonate reservoirs affected by prominent strike-slip faults represent crucial targets in oil and gas exploration owing to their immense resource potential. However, the complex geological environments and poorly understood histories of the associated paleo-fluid activity have hindered the development of robust theories regarding pore formation and preservation mechanisms, resulting in suboptimal exploration strategies. Leveraging the extensive well deployment by the China Sinopec Group in the Shunbei area of the Tarim Basin, this study addresses these challenges by establishing a comprehensive framework for the evolution of diagenetic fluids within the Middle-Lower Ordovician carbonate formations. Using core samples, thin-section analysis, and cathodoluminescence observations, this study employs high-resolution geochemical methodologies, including isotopic analyses, rare earth element profiling, fluid inclusion studies, and uranium-lead dating, as primary tools for identifying and interpreting paleo-fluid characteristics across various rock types and calcite cement varieties within this stratigraphic interval. The findings reveal several key insights: (i) both RFC and C1 cements are derived from seawater, with C1 forming under burial conditions; (ii) C2, C3, and VC cements result from distinct tectonic events, specifically during the first and third episodes of the Middle Caledonian movement, with meteoric water infiltrating fault systems independently of orogenic belts or paleo-karst systems; and (iii) previous conclusions are challenged, as the influence of hydrothermal activity in this area is found to be minimal. Furthermore, the model presented here serves as a valuable reference for understanding fluid activity events at distal locations within orogenic belts under compressive stress, while accurately capturing fluid variations over different temporal scales within fault zones plays a decisive role.