Abstract:

Study on the chemistry of the Ordovician,Silurian and Carboniferous formation waters in Tazhong area suggests that these waters are of CaCl₂ type with high TDS and contains mainly cations K⁺+Na⁺ and anion Cl.Their average sodium-chlorine coefficient is in the range of 0.73-0.87,the de-sulfuric acid coefficient averages at 1.54-9.29,and the metamorphic coefficient ranges from 5.54 to 12.80.The chemical composition and ionic ratio of these waters present positive,negative and steady variations along with depth.While horizontally,the chemical compositions of Carboniferous formation water remain relatively stable,but the Ordovician formation water is complicated in chemical changes.Generally speaking,the Ordovician,Silurian and Carboniferous formation waters are in a closed fluid dynamic environment now.However,the Ordovician,Silurian and Carboni-ferous unit C_Ⅲ are quite different in their fluid dynamic environments and conditions of oil & gas accumulation and preservation.While the C_Ⅰ and C_Ⅱ units have similar fluid dynamic environments and are favorable for oil & gas accumulation and preservation.The TDS of the formation waters is low and the fluid dynamic environment is less closed in the paleo-uplift and areas near faults that cut into the shallow formations.The Ordovician and Silurian formation waters suffered from strong leaching of meteoric water in the geologic history,thus their fluid dynamic systems were open.Especially,the Silurian formation water was exposed to significant open fluid dynamic environment,extremely unfavorable for hydrocarbon accumulations and preservation,while the Carboniferous formation water was,in the contrary,excellent for hydrocarbon accumulation and preservation due to its closed fluid dynamic environment.

Key words: TDS, sealing capacity, fluid dynamic environment, formation water chemistry, Tazhong area