Application of brine differentiation and Langelier–Ludwig plots to fresh-to-brine waters from sedimentary basins: Diagnostic potentials and limits Tiziano Boschetti Department of Earth Sciences, University of Parma, 157a Parco Area delle Scienze, 43100 Parma, Italy abstract article info Article history: Received 6 August 2010 Accepted 6 December 2010 Available online 13 December 2010 Keywords: Sedimentary basins Brine waters Salinization processes Major dissolved constituents Classical chemical classification plots that use major anions and cations can discern between different water facies but they do not offer sufficient discriminatory power for salt waters from sedimentary basins, whose origin is therefore frequently misunderstood. The Brine Differentiation Plot (BDP) was proposed by Hounslow (1995) in order to investigate the brine genesis, principally evaporite dissolution, alkali lakes and oilfield brines. However, its diagnostic potential has been undervalued so far. In this paper, the potential of BDP was tested and compared with the classical Langelier–Ludwig plot using concentration of major dissolved constituents of fresh to brine waters from different sedimentary basins (Northern Apennine Foredeep, Italy; Provence Basin, Western Mediterranean; Caucasus; Trinidad). Mixing processes between different water types as evaluated by these diagrams would seem to be constrained by the boron–chloride plot. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Chemical classification of subsurface waters from sedimentary basins aids in interpreting the complex processes involved in producing their dissolved solids, to better understand their origin and evolution, and the origin of water that is more likely to be associated with hydrocarbon accumulation (Collins, 1975). From a general point of view, hydrocarbon accumulations are primarily related to Ca-chloride and secondarily to Na-bicarbonate waters (Sulin, 1946). The relative arrangement of the major constituents on a square plot was first proposed by Tolstikhine at the end of 1930, and was applied to distinguish between hydrocarbon-bearing Ca-chloride and Na-bicarbonate waters (Chilingar, 1957; Chilingar et al., 2003). A similar plot but with inversion of the axis was proposed by Langelier and Ludwig (1942), and is nowadays universally employed in water classification (hereafter LLP). However, classification plots that use major dissolved constituents are unsuccessful to distinguish between salinization mechanisms like seawater evaporation and salt dissolu- tion because waters of different origin fall in the same Na-chloride field; therefore other elements like Br and I coupled with Cl in binary diagrams should be used (e.g. Richter and Kreitler, 1993). Recently, a Brine Differentiation Plot (hereafter BDP) was proposed as a tool to discriminate between different brine origins (Hounslow, 1995). The plot uses molar Ca/(Ca+SO 4 ) and Na/(Na+Cl) on the vertical and horizontal axes, respectively. On this diagram, field characteristic of oilfield brines, evaporite solutions and seawater is separate and distinct. Despite its clarity, it is still practically neglected by geochemists. 2. Distinction between seawater-derived Ca-chloride brines and Na-chloride waters from evaporite dissolution Ca-chloride waters were first defined as waters with qNa/qCl b 1 and q(Cl-Na)/qMg b 1, where q is the equivalent % (Sulin, 1946); most recently, the definition of this water class was revised as rNa/rCl b 0.86 ± 0.05 (seawater ratio) and rCa/r(SO 4 + HCO 3 ) N 1, where r is the mEq/L concentrations (Rosenthal, 1997). In the brine waters from Northern Apennine Foredeep (NAF) sodium and chloride are the most abundant ions. Therefore these waters should be classified as Na-chloride. However, they are also definable as Ca-chloride waters using the above ratios (Boschetti et al., 2011). Chemical and isotope composition revealed that Ca-chloride brines derive from a Miocene seawater evaporated up to a stage between gypsum and halite saturation, then diluted by Miocene or present-day waters of meteoric origin and modified by water–rock interaction (Boschetti et al., 2011). Contrary, Picotti et al. (2007) concluded for a meteoric origin of these waters followed by interaction with rocks. In Fig. 1B, major chemistry of Ca-chloride brines from NAF is plotted on LLP along with waters of meteoric origin dissolving evaporite minerals (Poiano springs, ~6 g/L; Forti et al., 1988 and our unpublished data). All of them are displaced in the Na-chloride area and no genetical distinction is possible because waters from evaporite could be interpreted as diluted Ca-chloride waters. Journal of Geochemical Exploration 108 (2011) 126–130 E-mail address: work@tizianoboschetti.com. 0375-6742/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2010.12.002 Contents lists available at ScienceDirect Journal of Geochemical Exploration journal homepage: www.elsevier.com/locate/jgeoexp