Geochemistry and origin of formation brines from the Paris Basin, France: 2. Saline solutions associated with oil fields

Abstract

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 with oil is investigated, using the hypothesis that Cl − and Br − are conservative in solution. In a diagram of Cl − Br − ratios vs. Cl −, the points of the Dogger and the Keuper aquifers lie in the zone of primary brines whereas those of the Rhaetian are located in the zone of secondary brines. The Na + Cl − ratio is very high in all of the saline solutions, suggesting either a relatively poorly evolved brine (at the beginning of halite deposition) or some halite dissolution. All of the saline solutions, especially those from the Dogger aquifer, are clearly diluted. Dilution by meteoric waters would not alter the Cl − Br − ratio. High concentrations of lithium, rubidium and boron are attributed to the presence of an extremely evolved brine of marine origin in mixtures of secondary brines formed by dissolution of halite and small amounts of sylvite and Ca-sulphate. Mass-balance calculations of the excess of Cl − with respect to Na + + K + allows the fraction x of water derived from the primary brine to be calculated. Assumptions are that all Na + and K + is derived from the secondary brine and that the Cl − content of the primary brine is constrained by bischofite saturation at ∼ 11,000 mmol kg −1. The typical proportion of water from the primary brine waters is ∼ 0.7% in the Dogger formation and ∼ 2.4% in the Keuper. Values of the concentration factor CF, expressed as the ratio of the molal concentration of a conservative ion in the evolved brine to its concentration in seawater have been calculated 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 Rhaetian aquifer, the linear relationship between heavy-isotope contents of the brine ( 2H and 18O) and the Cl − content indicates a mixture of two water sources. The first one is meteoric water as defined by the intersection of the formation water line with the global meteoric water line ( δ 2 H = −65‰ , δ 18 O = −9.3‰ ). The other end-member is enriched in heavy isotopes and in Cl −, and is attributed to seawater that has dissolved evaporites (mainly halite). However, the presence of small amounts of an extremely evolved brine is required to account for the bromide, lithium, rubidium and boron contents of the formation brine from the Rhaetian aquifer. As this contribution is very limited in amount, it cannot affect the heavy-isotope content of the formation water. Mass-balance equations are solved 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 ≈ 11,000 mmol kg −1) are applied. It is also 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 ≈ 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 calculations also indicate that the SO 4 2− content of the Rhaetian formation water is exclusively due to the secondary brine component. In the three saline solutions, the very high Ca 2+ contents and high Ca 2+ Mg 2+ ratios, may be attributed to reaction of the very evolved brine (free of SO 2− 4), with gypsum or anhydrite. The result is an exchange of Mg 2+ for Ca 2+, which causes a release of Ca 2+ to the solution and a precipitation of secondary Mg-sulphate. This process can also account for the high strontium contents of the analysed samples. Values of the ratio 87 Sr 86 Sr are very high in the Keuper formation water due to close contact of the solutions with the granitic basement or with granite-derived detrital deposits. Values in the Rhaetian and Dogger aquifers equal those of Upper Triassic marine deposits. The stable isotope ( 34S and 18O) content of the dissolved aqueous SO 2− 4 indicates a Triassic origin for the bulk of sulphate involved in the evolution of formation waters from the Rhaetian and from the Dogger. All geochemical indicators thus show a vertical migration of Triassic brines. However, aqueous SO 2− 4 from the Keuper has a Permian isotopic signature suggesting a reworking in solution of salts from this period.