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Application of brine differentiation and Langelier–Ludwig plots to fresh-to-brinewaters from sedimentary basins: Diagnostic potentials and limits

Tiziano BoschettiDepartment of Earth Sciences, University of Parma, 157a Parco Area delle Scienze, 43100 Parma, Italy

a b s t r a c ta r t i c l e i n f o

Article history:Received 6 August 2010Accepted 6 December 2010Available online 13 December 2010

Keywords:Sedimentary basinsBrine watersSalinization processesMajor dissolved constituents

Classical chemical classification plots that use major anions and cations can discern between different waterfacies but they do not offer sufficient discriminatory power for salt waters from sedimentary basins, whoseorigin is therefore frequently misunderstood.The Brine Differentiation Plot (BDP) was proposed by Hounslow (1995) in order to investigate the brinegenesis, principally evaporite dissolution, alkali lakes and oilfield brines. However, its diagnostic potential hasbeen undervalued so far.In this paper, the potential of BDP was tested and compared with the classical Langelier–Ludwig plot usingconcentration of major dissolved constituents of fresh to brine waters from different sedimentary basins(Northern Apennine Foredeep, Italy; Provence Basin, Western Mediterranean; Caucasus; Trinidad). Mixingprocesses between different water types as evaluated by these diagrams would seem to be constrained by theboron–chloride plot.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Chemical classification of subsurface waters from sedimentarybasins aids in interpreting the complex processes involved inproducing their dissolved solids, to better understand their originand evolution, and the origin of water that is more likely to beassociated with hydrocarbon accumulation (Collins, 1975). From ageneral point of view, hydrocarbon accumulations are primarilyrelated to Ca-chloride and secondarily to Na-bicarbonate waters(Sulin, 1946). The relative arrangement of the major constituents on asquare plot was first proposed by Tolstikhine at the end of 1930, andwas applied to distinguish between hydrocarbon-bearing Ca-chlorideand Na-bicarbonate waters (Chilingar, 1957; Chilingar et al., 2003). Asimilar plot but with inversion of the axis was proposed by Langelierand Ludwig (1942), and is nowadays universally employed in waterclassification (hereafter LLP). However, classification plots that usemajor dissolved constituents are unsuccessful to distinguish betweensalinization mechanisms like seawater evaporation and salt dissolu-tion because waters of different origin fall in the same Na-chloridefield; therefore other elements like Br and I coupled with Cl in binarydiagrams should be used (e.g. Richter and Kreitler, 1993). Recently, aBrine Differentiation Plot (hereafter BDP) was proposed as a tool todiscriminate betweendifferent brineorigins (Hounslow, 1995). Theplotuses molar Ca/(Ca+SO4) and Na/(Na+Cl) on the vertical andhorizontal 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 andNa-chloride waters from evaporite dissolution

Ca-chloridewaterswerefirst defined aswaterswith qNa/qClb1 andq(Cl-Na)/qMgb1, where q is the equivalent % (Sulin, 1946); mostrecently, thedefinitionof thiswater classwas revisedas rNa/rClb0.86±0.05 (seawater ratio) and rCa/r(SO4+HCO3)N1, where r is the mEq/Lconcentrations (Rosenthal, 1997). In the brine waters from NorthernApennine Foredeep (NAF) sodium and chloride are the most abundantions. Therefore these waters should be classified as Na-chloride.However, they are also definable as Ca-chloride waters using theabove ratios (Boschetti et al., 2011). Chemical and isotope compositionrevealed that Ca-chloride brines derive from a Miocene seawaterevaporated up to a stage between gypsum and halite saturation, thendiluted by Miocene or present-day waters of meteoric origin andmodified by water–rock interaction (Boschetti et al., 2011). Contrary,Picotti et al. (2007) concluded for a meteoric origin of these watersfollowed by interaction with rocks. In Fig. 1B, major chemistry ofCa-chloride brines from NAF is plotted on LLP along with waters ofmeteoric origin dissolving evaporite minerals (Poiano springs, ~6 g/L;Forti et al., 1988 and our unpublished data). All of them are displaced inthe Na-chloride area and no genetical distinction is possible becausewaters from evaporite could be interpreted as diluted Ca-chloridewaters.

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In Fig. 2 waters are shown on BDP. In this type of diagram, seawaterhas Na/(Na+Cl)=0.46 and Ca/(Ca+SO4)=0.26,whereasNa-chloridewaters from evaporite dissolution arrange exactly in the center of theplot because dissolution of gypsum-anhydrite and halite is the mainsource of dissolved constituents. In the same diagram, Ca-chloride brinewaters have 0.38bNa/(Na+Cl)≤0.46 and 0.9bCa/(Ca+SO4)≤1, thusfalling in the oil-field brine space (Fig. 2B). However, in spite of theoriginal BDP representation in which the oil-field waters are connectedwith the area of evaporite dissolution and their great distance fromseawater point, no contribution from evaporite dissolution is manifestin Ca-chloride brines from NAF. In fact, they derive from evaporatedseawater up to gypsum precipitation followed by variations in thecationic concentration due to deep burial diagenetic processes(Boschetti et al., 2011; Davisson and Criss, 1996). In particular,plagioclase albitization

CaAl2Si2O8anorthite

+ 4 SiO2aq + 2 Naþaq = 2NaAlSi3O8albite

+ Ca2+ aq

and/or zeolitization

0:995CaAl2Si2O8anorthite

+ 1:02SiO2aq + 2:647H2O + 0:676Naþaq =

= Na0:676Ca0:657Al1:99Si3:01O10�2:647 H2Oð Þ

mesolite+ 0:338Ca2+ aq

involve a 2:1 exchange of Na for Ca, whereas sulfate drop is ascribed tosulfate reduction. Therefore, seawater-derived brines may be wellrepresented by a reversed triangle having Ca-chloride waters as baseand seawater as vertex (Fig. 2A). In fact, its worthy of note that

seawater and evaporated seawater solutions up to gypsum precipi-tation have an almost constant Na/(Na+Cl) ratio of about 0.46, that issimilar to the maximum ratio revealed in the NAF Ca-chloride brines(Fig. 2B).

In order to confirm the potentiality of the BDP to describe theseawater origin and the diagenetic processes, interstitial waters fromProvence Basin have been added to the plot (grey triangles in Fig. 2C;site 372, Western Mediterranean; McDuff et al., 1978). Similar to NAFwaters, they were misinterpreted as evaporite-dissolving waters(McDuff et al., 1978), but most recently their evaporated-seawaterorigin has been confirmed by chemical and boron isotope investiga-tions (Vengosh et al., 1994, 2000). In fact these waters, classifiable asCa-chloride brines [1brCa/r(SO4+HCO3)b16], are displaced in thereversed triangle field of the BDP, and have Na/(Na+Cl)b0.46 and0.55bCa/(Ca+SO4)b0.95, that is they evolve from seawater towardstypical oil-field brines because of sulfate loss (reduction) and Caincrease (albitization and/or zeolitization) related to an early burialdiagenesis (eogenesis).

3. Na-bicarbonate waters and terrestrial mud volcanoes

Fresh Ca-bicarbonate is the dominant facies of the shallowgroundwaters and its origin, i.e. shallow meteoric water interactingwith calcite-bearing sediments, is well known and common also inother places of the world, therefore it is not discussed further.Na-bicarbonate waters are less common than Ca-bicarbonate but theyare considered as a favourable, shallow indicator for the presence ofhydrocarbons (e.g. Chilingar et al., 2003; Collins, 1975). In the NAFand elsewhere, these waters have fresh to brackish salinity, are

A

B C

Fig. 1. Langelier–Ludwig plot, where (Na+K)=(ΣrNa+K/Σrcations)×50 and (HCO3)=rHCO3/Σranions)×50, where r is the mEq/L concentration of the constituents. The (Ca+Mg) and(SO4+Cl) are complementary values. (A) Reading key; dashed lines represent binary mixings. (B) Waters from Northern Apennine Foredeep (NAF); brine, saline and brackishwaters from mud volcanoes: Boschetti et al. (2011); Martinelli and Ferrari (1991). Bold cross: brackish groundwater enriched in Na and depleted in Ca due to overpressure (SALV,Boschetti et al., 2011); black crosses (Lesignano waters, Boschetti et al., 2011) and grey crosses (Porretta waters; Bencini and Duchi, 1980; Ciancabilla et al., 2007): Ca-chloride andoverpressured waters mixed with Na-bicarbonate waters, respectively. Fresh Na-bicarbonate waters (sp, f10 and f14 are brackish samples mixed with hydrocarbon-bearing waters):Duchi et al. (2005); Venturelli et al. (2003); our unpublished data. Water of meteoric origin dissolving evaporates: Forti et al., 1988; our unpublished data. Double arrow in the alkali-bicarbonate field represents mixing with or evolution from Ca-bicarbonate fresh waters. (C) Saline to brine interstitial waters from Provence Basin (site 372, WesternMediterranean; McDuff et al., 1978); mud volcanoes from Caucasus (Lavrushin et al., 2003) and Trinidad (Dia et al., 1999).

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sulfide-bearing and represent evolved Ca-bicarbonate waters ofmeteoric origin interacting with albite, clay and silica phases resultingin the acquisition of Na and loss of Ca by calcite deposition (Toran andSaunders, 1999; Venturelli et al., 2003). These waters are character-ized by a rNa/rCl≫0.86 and they are easily identified in the specificupper right side sector of the LLP (Fig. 1A and B). In this diagramshould be also noted that most of Na-chloride brackish waterassociated to CH4-rich mud volcanoes cluster in a distinctive ternaryshape area, revealing intermediate characteristics between Ca-chloride and Na-bicarbonate waters but with higher ΣrNa+K/Σrcationsratios. In fact, fluids from mud volcanoes consist of formation watersdiluted by clay dewatering, caused by overpressure that producesdeep fluids with Na excess (rNa/rClN0.86) combined with a Cadecrease, and shallow groundwaters (Boschetti et al., 2011). A Ca forNa exchange between water and clay could be supposed as the easierexplanation for this shift. However, the origin of the Na-excess/Ca-

deficit in Na-bicarbonate waters from sedimentary basins is mostprobably due to an interaction with Na-silicate like albite and calciteprecipitation (Cheng et al., 2006; Du et al., 2010). In NAF, a brackishgroundwater shows a water isotope composition quite similar to mudvolcanoes and an extreme Na-excess thus representing a typical end-member for overpressured waters (SALV in Fig. 1B; Boschetti et al.,2011). In the LLP, a mixing triangle between this point, a Ca-chloridebrine with the highest ΣrNa+K/Σrcations value and a pure Na-bicarbonate end-member identified in the left corner of the diagramcontain terrestrial mud volcano waters from different countries(Fig. 1C). Most probably, in the mud volcano waters the Na-bearingsilicate hydrolysis is enhanced by hydrocarbon oxidation–biodegra-dation, a process characterized by heavy δ13C(CO2) up to +20‰ andoccurring at shallow depth (b2000 m) and low temperature (b70 °C)(Capozzi and Picotti, 2002; Feyzullayev and Movsumova, 2010;Pallasser, 2000).

Fig. 2. Brine Differentiation Plot (BDP), where Ca/(Ca+SO4) and Na/(Na+Cl) are molar ratios of the constituents. (A) Reading key (inset redrawn from Hounslow, 1995); G and H aregypsum and halite precipitation points after seawater evaporation (Pierre, 1982). (B)Waters fromNorthern Apennine Foredeep (NAF; samples references as in Fig. 1). (C) Saline to brineinterstitial waters from Provence Basin, mud volcanoes from Caucasus and Trinidad (samples references as in Fig. 1 plus other data from Azerbaijan from Mazzini et al., 2009).

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In the BDP, mud volcanoes and Na-bicarbonate waters plot on theright side, the former with 0.50bNa/(Na+Cl)b0.55, the latter with0.9bNa/(Na+Cl)b1.0, both spanning most of the Ca/(Ca+SO4)range due to calcite precipitation and sulfate reduction (Fig. 2). Theshifts shown in the LLP by some Na-bicarbonate samples towardsalkaline-chloride field and vice versa (Fig. 1B) are magnified in theBDP (Fig. 2B), but in this latter plot it is impossible to distinguish theend-member. However, the linking betweenNa-bicarbonate and deepalkali-chloride waters supposed in the LLP may be confirmed usingchloride versus bromine or iodine, as often observed in deep waters(e.g. Richter and Kreitler, 1993). In NAF, the maximum content ofbromine and iodine was revealed in brines and overpressuredbrackish waters like mud volcanoes, that is in fluids that underwentactive decomposition of marine organic matter owing to burialdiagenesis and compressional tectonics, respectively (Boschetti et al.,2011). Same conclusions could be derived using boron content.

4. Mixing trend elucidation by boron–chlorine diagram

In Fig. 3, mixing between Na-bicarbonate waters and Ca-chloridebrines or mud-volcanoes is confirmed by boron–chloride binary plot.In the same diagram it is possible to discern the end-member of mixedNa-chloride brackish groundwater (see also Figs. 1 and 2). Boronadsorbed to clay minerals is preferably released to the fluid in a deep

environment, where either tectonic stress (vertical and/or lateralcompaction) or temperature increases, with a maximum enrichmentin fluid phase where fault roots are deepest and deformations arestrongest (Kopf and Deyhle, 2002). The B released during thesmectite–illite transformation is accompanied by salinity reductiondue to dehydration, thus explaining the water isotope composition ofthe mud volcanoes (Boschetti et al., 2011; Fitts and Brown, 1999;Moore and Vrolijk, 1992). For TN120 °C the partition coefficientsediment–water for boron is 0, that is the element is full partitioned influid (You et al., 1996). This is in accord with the temperature up to150 °C reached by the deep source inferred for mud volcanoes(Capozzi and Picotti, 2002) and some brine waters (Boschetti et al.,2011) in which highest boron contents were revealed (up to 138 and400 mg/L, respectively). At lower temperature, clay uptake could beimportant as shown in particular by deep-sea interstitial waters(Fig. 3C; Vengosh et al., 2000). Waters with low chloride and highboron content could be related to geothermal degassing and/or phaseseparation followed by fluid condensation at shallow depth (e.g.Arnórsson and Andrésdóttir, 1995; Truesdell et al., 1989) as shown insome Caucasian mud volcanoes (Fig. 3C; Lavrushin et al., 2003).However, an investigation using boron isotopes could be useful tobetter determine the effect of increasing burial temperatures at depth,the different fluid sources (clay dewatering and formation water) andsmectite-to-illite transformation.

Fig. 3. Boron versus chlorine, mg/L contents. Symbols as in Figs. 1 and 2. Condensate field depicts shallow condensation of geothermal fluids (e.g. Arnórsson and Andrésdóttir, 1995;Truesdell et al., 1989). Dashed lines represent binary mixings. (A) Reading key. (B)Waters fromNAF (see Fig. 1 captions for sample references); the evaporite-dissolving waters fromNAF were not shown because of their quite low B contents (mean 0.05 mg/L). (C) Saline to brine interstitial waters from Provence Basin, mud volcanoes from Caucasus and Trinidad(sample references as in Fig. 2 plus other data from Azerbaijan from Planke et al., 2003).

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5. Conclusions

Seawater evaporation and evaporite mineral dissolution are themain salinization processes associated with the origin of brine andsaline waters from sedimentary basins. The Brine Differentiation Plotproposed by Hounslow (1995)may help in the differentiation of theseprocesses. In that diagram, evaporite dissolving waters are clusteredtowards the center of the plot, Ca/(Ca+SO4)=Na/(Na+Cl)≈0.5,whereas seawater-derived Ca-chloride brines have Ca/(Ca+SO4)≈1and Na/(Na+Cl)≤0.46. Saline to brine waters from differentsedimentary basins (Northern Apennine Foredeep, Italy; ProvenceBasin, Western Mediterranean) cluster between these latter valuesand the seawater. This is the result of the diagenetic reaction asalbitization, zeolitization and sulfate reduction that modify theoriginal seawater signature.

The classic Langelier–Ludwig diagram allows a clear subdivision andclassification of the three hydrocarbon-bearing water types commonlyfound in sedimentary basins (Ca-chloride brines, overpressured/mudvolcanoes and Na-bicarbonate waters) and may help to discern thepossible mixing between them. In particular, terrestrial mud volcanoesfrom different countries (Northern Apennine Foredeep, Caucasus,Trinidad) describe a mixing triangle between these water types.

Mixing trend and relative end-members could be supported bychloride–boron plots that represent an effective alternative to thehalogen–halogen binary plots.

Obviously, investigations on waters from sedimentary basincannot prescind by use of stable isotope and minor and trace elementanalysis (e.g. boron and halogens). However, a conscientious use ofthese (LLP and BDP) and other diagrams (e.g. Basin Fluid Line plot;Davisson and Criss, 1996) that use major constituents could clarifyorigin and processes behind the chemical composition of waters fromsedimentary basins and rationalize the analysis and use of otherminor-trace elements and isotope ratios.

Acknowledgments

The manuscript benefited from the constructive comments of twoanonymous reviewers and Editor-in-Chief, Prof. B. de Vivo. Specialthanks to Prof. G. Cortecci for his review of an earlier version of thismanuscript.

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