The Geological Evidences and Impacts of Deep Thermal Fluid on Lacustrine Carbonate Reservoir in the Actic Area of the North Part of Bozhong Depression, Bohai Bay Basin

Several high-yielding lacustrine carbonate rock and mix-deposit reservoirs with the total proven reserves of more than 1×10⁸ cubic metres have been found in the actic area of the north part of Bozhong depressoin, with the reservoir depth of 3 200~3 800 meters. The research shows that there are strong dolomitization and dissolution in the lacustrine carbonate rock and mix-deposit rock, and the porosity can be over 25%. Early research suggests that the dissolution of biodetritus is the decisive factor to improve reservoir physical property. But the further research shows obvious deep thermal fluid activity evidences in this belt, and deep thermal fluid plays a decisive role on forming the high quality reservoirs. Based on the Laser Raman analysis, there are a large number of inclusions which contain CO₂, N₂, H₂S and H₂, and the high content of CO₂ can be over 70%; There are 3 CO₂-bearing structures found at the Actic Area of Shijiutuo Uplift, the δ¹³C CO₂ values, R/Ra values values show that CO₂ was derived from the mantle. There are a lot of pyrite in the lacustrine carbonate rock, and the Co/Ni ratio of most pyrite is greater than 1, which indicates most of the pyrite derived from the thermal fluid. Saddle dolomite are found at the fractures and big pores, which have the typical arc-shaped crystal and wavy extinction characteristics, and the crystal size is about 0.05~0.10 mm. Dense growth striations are observed under the SEM. According to the thermal history and inclusion homogenization temperature drop points, the thermal fluid activity time is 3~0 Ma. The CO₂ thermal fluid activities produced strong dissolution of calcite cements and feldspars in reservoir, which played an important role on the increasing porosity. With the dissolution of the calcite cements and the volcano lithic fragments and gravels, the PH value and salinity of fluid increased, which resulted in strong dolomitization. The higher inclusion homogenization temperature of late stage dolomite and the saddle-like dolomite show that the thermal fluid promote the dolomitization. The mentle-derived thermal fluid is from the deep fault and the exhaust and drainage of the early volcano.