Volcanic hydrothermal fluid activity and its influence on carbonate reservoirs in Bohai Sea area

Volcanic activity is frequent in the Bohai Sea. Hydrothermal fluid activity and its reformation effect on Paleozoic carbonate reservoirs were studied based on core, cast thin section, scanning electron microscopy, carbon and oxygen isotopes, strontium isotope and rare earth elements in combination with fluid inclusion analysis to study the influence of volcanic hydrothermal activity on carbonate reservoirs. The hydrothermal fluid in the Bohai Sea was controlled by fractures. The main ingredients of hydrothermal fluid include a large amount of CO₂ and H₂S gas from the earth mantle. Properties indicating hydrothermal fluid activity included induced joints by hydrothermal fluid, typical rock heat bleaching and typical hydrothermal minerals such as saddle dolomite (first discovered in the Bohai sea area), siliceous nodules, columnar authigenic quartz and veined pyrite. The calcites growing under the influence of hydrothermal fluid are featured by low δ¹⁸O value, high ⁸⁷Sr/⁸⁶Sr, light rare earth enrichment and Eu positive exception, and the homogenization temperatures of salt water inclusions are obviously higher than the formation temperature. When volcanic hydrothermal fluids flowed along faults, a large number of hydrothermally induced fractures were formed in carbonate reservoirs, and hydrothermal dolomitization took place in large intergranular pores. Hydrothermal minerals such as cladding of apatite and siliceous nodules supported and protected the previous apertures. Volcanic hydrothermal fluids flowed along the apertures, forming a large number of dissolution pores in carbonate rocks, which improved carbonate reservoir porosity.