Study on the chemistry of the Ordovician

Silurian and Carboniferous formation waters in Tazhong area suggests that these waters are of CaCl

2

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.