Reformation of hydrothermal fluid activity on the Lower Paleozoic carbonate rock reservoirs in the Dongying Depression

Recent petroleum exploration and development in the Dongying Depression of eastern China have focused on Paleozoic carbonate buried-hill reservoirs, which are typical fault-block reservoirs strongly influenced by hydrothermal activity. To clarify the impact of hydrothermal processes on reservoir quality, petrographic observations, stable and radioactive Sr isotope analyses, trace and rare earth element geochemistry, and fluid inclusion studies were carried out on hydrothermally precipitated minerals in the Lower Paleozoic carbonates. Extensive hydrothermal brecciation and thermal fading, together with a variety of hydrothermal minerals, are observed in the carbonate rocks. These hydrothermal minerals are characterized by high Mn and Fe contents, negative δ¹⁸O values, high ⁸⁷Sr/⁸⁶Sr ratios, and positive Eu anomalies. Hydrothermal dissolution during the early diagenetic stage generates secondary pores and preferentially improves the physical properties of high-permeability dolomite and granular limestone reservoirs. The development of the final porosity depends on late-stage hydrothermal mineral precipitation, because excessive crystal growth clogs previously formed pores and adversely affects reservoir physical properties. In granular limestone reservoirs, ferroan dolomite with high surface porosity can be regarded as a primary target for petroleum exploration.