GEOCHEMISTRY AND ORIGIN OF FORMATION BRINES FROM THE PARIS BASIN, FRANCE .2. SALINE SOLUTIONS ASSOCIATED WITH OIL-FIELDS

Oil fields from the Paris Basin are located in three geological levels : Dogger (Middle Jurassic), Rhaetian and Keuper (Upper Triassic). The origin of water and dissolved salts in saline solutions associated wit h oil is investigated, using the hypothesis that Cl- and Br- are conse rvative in solution. In a diagram of Cl- /Br- ratios vs. Cl-, the poin ts of the Dogger and the Keuper aquifers lie in the zone of primary br ines whereas those of the Rhaetian are located in the zone of secondar y brines. The Na+/Cl- ratio is very high in all of the saline solution s, suggesting either a relatively poorly evolved brine (at the beginni ng of halite deposition) or some halite dissolution. All of the saline solutions, especially those from the Dogger aquifer, are clearly dilu ted. Dilution by meteoric waters would not alter the Cl-/Br- ratio. Hi gh concentrations of lithium, rubidium and boron are attributed to the presence of an extremely evolved brine of marine origin in mixtures o f secondary brines formed by dissolution of halite and small amounts o f sylvite and Ca-sulphate. Mass-balance calculations of the excess of Cl- with respect to Na+ + K+ allows the fraction x of water derived fr om the primary brine to be calculated. Assumptions are that all Na+ an d K+ is derived from the secondary brine and that the Cl- content of t he primary brine is constrained by bischofite saturation at approximat ely 11,000 mmol kg-1. The typical proportion of water from the primary brine waters is approximately 0.7% in the Dogger formation and approx imately 2.4% in the Keuper. Values of the concentration factor CF, exp ressed as the ratio of the molal concentration of a conservative ion i n the evolved brine to its concentration in seawater have been calcula ted for lithium. Solutions from the Dogger and Keuper aquifers have CF (Li+)-values of 1.8.10(3) and of 7.7.10(3), respectively. In the Rhaet ian aquifer, the linear relationship between heavy-isotope contents of the brine (H-3 and O-18) and the Cl- content indicates a mixture of t wo water sources. The first one is meteoric water as defined by the in tersection of the formation water line with the global meteoric water line (deltaH-2 = - 65 parts per thousand, deltaO-18 = - 9.3 parts per thousand). The other end-member is enriched in heavy isotopes and in C l-, and is attributed to seawater that has dissolved evaporites (mainl y halite). However, the presence of small amounts of an extremely evol ved brine is required to account for the bromide, lithium, rubidium an d boron contents of the formation brine from the Rhaetian aquifer. As this contribution is very limited in amount, it cannot affect the heav y-isotope content of the formation water. Mass-balance equations are s olved for ternary mixtures of meteoric water, seawater and brine. The same constraints as above (no Na+ and K+ supplied by the primary brine and Cl(Brine) almost-equal-to 11,000 mmol kg-1) are applied. It is al so assumed that the seawater component has the same ionic contents as modern seawater and that the salt content of the meteoric component is negligible. The fraction of primary brines is x almost-equal-to 1.4% according to the Cl- balance. This estimate agrees with that obtained from the lithium balance assuming that the primary brine component was the same as that of the Keuper formation water. Mass-balance calculat ions also indicate that the SO42- content of the Rhaetian formation wa ter is exclusively due to the secondary brine component. In the three saline solutions, the very high Ca2+ contents and high Ca2+/Mg2+ ratio s, may be attributed to reaction of the very evolved brine (free of SO 42-), with gypsum or anhydrite. The result is an exchange of Mg2+ for Ca2+, which causes a release of Ca2+ to the solution and a precipitati on of secondary Mg-sulphate. This process can also account for the hig h strontium contents of the analysed samples. Values of the ratio Sr-8 7/Sr-86 are very high in the Keuper formation water due to close conta ct of the solutions with the granitic basement or with granite-derived detrital deposits. Values in the Rhaetian and Dogger aquifers equal t hose of Upper Triassic marine deposits. The stable isotope (S-34 and O -18) content of the dissolved aqueous SO42- indicates a Triassic origi n for the bulk of sulphate involved in the evolution of formation wate rs from the Rhaetian and from the Dogger. All geochemical indicators t hus show a vertical migration of Triassic brines. However, aqueous SO4 2- from the Keuper has a Permian isotopic signature suggesting a rewor king in solution of salts from this period.