Geochemistry and genesis of the formation water in Huagang Formation of the Tiantai Inversion Zone, the Xihu Depression of the East China Sea Basin

Based on the geochemical features of the formation water in Huagang Formation collected from 20 wells on the Tiantai structure of Xihu Depression, the genesis, source and preservation conditions of the formation water are discussed in this paper. The water is dominated by Cl⁻, Na⁺, Ca²⁺ and HCO₃⁻. And the concentrations of Na⁺ and Cl⁻ show an obvious linear relationship with TDS which suggests a kind of highly concentrated formation water. The water is mainly of types CaCl₂ Ⅳ and Ⅴ. r(Na⁺)/r(Cl⁻) and 100×(rSO₄²⁻)/r(Cl⁻) are lower than those in seawater; while r(Ca²⁺)/r(Mg²⁺) are higher in deep water. All of these features indicate that the Huagang Formation has effective sealing capability and is deposited in an alternative stagnation zone, which is conducive as well to oil and gas accumulation and preservation. The slight loss of Na⁺ is mainly caused by albitization. In addition to the formation of sodium feldspar, organic acids associated with the maturation of organic matter and the dissolution of feldspar and calcium-bearing minerals, also promoted the enrichment of Ca²⁺ in the formation water. The deficiency of Mg²⁺ may be closely related to the formation of kaolinite and chlorite in addition to dolomitization. The formation water of Huagang Formation originally came from land water and therefore is controlled by sedimentary environment, water-rock reaction, evaporation and concentration and fluid mixing characterized by rich Ca²⁺, poor Mg²⁺ and slightly poor Na⁺.