CONTROLS ON HALOGEN CONCENTRATIONS IN SEDIMENTARY FORMATION WATERS

Chlorine is the most abundant halogen in sedimentary formation waters with concentrations from <100 to >250000 mg/l. Bromine is the second m ost abundant halogen at <1 mg/l to >6000 mg/l with iodine from <0.1 mg /l to >100 mg/l and fluorine from <0.1 mg/l to 30 mg/l. Chlorine and b romine show a strong systematic covariation suggesting that they are s ubject to the same controlling mechanisms. Fluorine only shows relativ ely high concentrations at elevated chlorine and bromine concentration s showing that fluorine, chlorine and bromine are possibly controlled by the same processes. Iodine does not correlate with any of the other halogens indicating that unique processes control iodine. Key geologi cal parameters that influence chlorine and bromine (and possibly fluor ine) concentrations are the presence of salt in a basin, the age of th e reservoir unit and the kerogen-type within the main hydrocarbon sour ce rock in a basin. The presence of salt in a basin shows that sea wat er was evaporated to halite saturation producing connate waters with h igh concentrations of chlorine and bromine. The presence of salt also leads to high salinity waters through water-salt interaction during bu rial and diagenesis. Tertiary reservoirs typically have much lower chl orine and bromine concentrations than Mesozoic or Palaeozoic reservoir s. The age of the reservoir unit may simply reflect the different amou nts of time available for formation water to interact with salt. The d ominance of type II marine kerogen in a basin leads to higher bromine concentrations. This may reflect the dominance of a marine influence i n a basin which is more likely to lead to salt deposition than terrest rial depositional environments. Iodine concentrations are independent of all these parameters. Other geological parameters such as depth of burial, temperature, basin forming mechanism and reservoir lithology h ave no influence upon halogen concentrations. Key processes that affec t halogen concentrations are sea water evaporation and dilution, water -salt interaction and input from organic sources. Chlorine and bromine data lie close to the experimentally-derived sea water evaporation tr end showing that sea water evaporation may be an important general con trol on halogens. Sea water dilution is probably responsible for most low salinity formation water chlorine and bromine concentrations for t he same reason. Sea water dilution can occur either by meteoric invasi on during burial, or following uplift and erosion, or by diagenetic de hydration reactions. Water can interact with salt in a variety of ways : halite dissolution by congruent processes, halite recrystallization by incongruent processes, sylvite dissolution or recrystallization and halite fluid inclusion rupture. Halite dissolution will lead to high chlorine and relatively low bromine waters because halite contains lit tle bromine. In contrast, halite recrystallization will lead to bromin e-enhanced waters because NaBr dissolves preferentially to NaCl. The o ccurrence of dissolution or recrystallization will depend on the water rock ratio, greater volumes of water will lead to more dissolution an d waters with higher Cl/Br ratios. Sylvite is usually rich in bromine so dissolution will lead to bromine-enhanced waters. Primary aqueous i nclusions in halite contain high bromine concentrations so that ruptur e, during deformation or recrystallization, will lead to bromine-enhan ced formation water. A combination of these processes are responsible for the very limited range of Cl/Br ratios although congruent halite d issolution must have a limited role due to the absence of waters with high Cl/Br ratios. Iodine is strongly concentrated in organic material s in the marine environment; oils and organic rich-source rocks have h igh I/Cl and I/Br ratios relative to sea water or evaporated sea water . All formation waters are enriched in iodine relative to sea water im plying that there has been input from organic matter or interaction wi th oil. However, hydrocarbon source rock type in a basin has no discer nible effect upon iodine concentrations.