Physical and chemical controls on the composition of waters in sedimentary basins

Physical and chemical controls on the composition of waters in sedimentary basins* Jeffrey S. Hanor Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA Received 6July 1992; revised 30April 1993; accepted2 May 1993 The composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion. The south-west Louisiana Gulf Coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence. Most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition. Dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section. The simultaneous systematic variation in dissolved Na, K, Mg, Ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate-carbonate mineral assemblages is a first-order control on fluid compositions. The chemical potential of chloride, or alternatively, total anionic charge, appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region. This variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper, mudstone-dominated sediments. Where the composition of the fluid is largely rock-buffered, the ultimate origin of the fluid and its pathway of chemical evolution may be obscured, at least in terms of major solute composition. Non-buffered components, such as CI and Br, or isotopic compositions are more likely to retain information on original end-member fluid compositions and reaction pathways. Keywords: formation waters; physical controls; chemical controls

Introduction The composition of aqueous fluids in sedimentary basins is of intrinsic scientific interest because of the potential information fluid compositions can provide on the geochemical, hydrological, thermal and tectonic evolution of the earth's crust. The composition of formation waters also provides an insight into a number of important applied problems specifically related to hydrocarbon exploration and production, including potential well scaling problems, the effect of variations in salinity on bulk sediment properties such as resistivity and seismic velocity, and problems of determining reservoir continuity and compartmentalization. The purpose of this paper is to provide a brief overview of the major physical and chemical processes which control pore water compositions in sedimentary basins. The overall subject is broad and complex and the present discussion will focus only on the behaviour of major dissolved species in some deep saline waters. An introduction to other aspects of the geochemistry of deep subsurface waters and a bibliography up to 1987 are given in notes for a short course on the origin and * Presented at the Petroleum Group of the Geological Society of London meeting 'North Sea Formation Waters: Implications for Diagenesis and Production Chemistry', London, UK, 15 January 1992

migration of subsurface sedimentary brines (Hanor, 1988). Historical reviews on the development of thought on the origins of subsurface waters are presented in two earlier papers (Hanor, 1983; 1987). The general review of BjOrlykke (1988) of sandstone diagenesis includes a useful introduction to processes operating in shallower and less saline systems; Bath et al. (1987) and Downing et al. (1987) provide well documented field examples of such systems. The structure of this review reflects the increasing interest by the geological community in the potential importance of large-scale fluid flow and solute transport in the evolution of the earth's crust and of the complex coupling and interaction which must be occurring between chemical reactions and physical transport (e.g. Goff and Williams, 1987; National Research Council, 1990; Torgersen, 1990). The basic strategy to be followed in this discussion is to treat the problem of the evolution of the composition of waters in sedimentary basins as a mass balance problem involving chemical and physical processes equally. Field examples for this paper will be drawn primarily from the US Gulf Coast, a mature hydrocarbon-producing province where large amounts of non-proprietary data are available. Many of the general principles, however, should apply to other deep sedimentary basins.

0264-8172/94/010/031-15 ©1994 Butterworth-Heinemann Ltd Marine and Petroleum Geology 1994 V o l u m e 11 N u m b e r 1

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Controls on composition of waters in sedimentary basins: J. S. Hanor Ultimate controls on pore water composition

The composition of pore water contained within a given volume of sediment in a given sedimentary basin is a direct function of several factors: (1) the composition of water physically included in the pore spaces of the sediment at the time of sediment deposition; (2) the net effects of diagenetic exchange of components between the water and (a) the ambient solids which make up the matrix of the sediment and (b) any other fuids, such as gases or liquid hydrocarbons, which may be present; (3) the net physical transport of material into and out of the sediments by bulk flow and the mixing of waters. All of these factors can be accounted for in a basic equation for the conservation of mass of a dissolved species

9Ci - Z R i j + ( - v ' V C i ) + [ V ' ( D V C i ) ] ~t

(1)

(gCi/St) is the change in the aqueous concentration, C i (kg/m3), of solute i with time, t (s).

where

Each of the three mass balance terms on the right has SI units of kg/m3 s. The first mass balance term, ZR~j, describes the net effects of diagenetic reaction on the concentration of solute i. R~j is the rate of the jth diagenetic reaction which results in the addition or removal of i from solution. The next term, ( - v . V Ci), defines the net addition or removal of dissolved i as a function of time as a result of bulk fluid flow through the sediment. The vector v (m/s) is the fluid velocity field, and V Ci, or grad Ci, is the three-dimensional concentration field. The negative sign accounts for the fact that if Ci is less in the influent entering the volume than in the effluent leaving, there will be a net loss of solute from the volume. The last term on the right-hand side of the equation, [ V ' ( D V C i ) ] , describes the mass balance resulting from diffusional and dispersive mixing. D (mZ/s) is a dispersion tensor which describes the magnitude of mixing which occurs in response to a given concentration gradient and fluid velocity. The term (DVCi) (kg/m2/s) is the flux of i resulting from dispersion, and the divergence of (DVCi), that is, V .(D V Ci), is the net rate in change in composition due to mixing. The initial composition of the fluid, Cg(0), comes into play when we begin to integrate Equation (1) with respect to time

Ci(t) = Ci(O) -1- f(XRij)dt + f ( - v " V Ci)dt + I[V.(DVG)]dt

(2)

Equation (1) can be solved for a given point in a sedimentary basin if ZRq, v, V Ci and D are known for that point at the time of interest. In theory, a suite of such equations describing the evolution of the concentration field for i could be solved for an entire sedimentary basin over its entire geological history if the spatial and temporal variation in all of the above parameters is known. However, because variations in pressure and temperature influence rates of diagenetic reactions and fluid velocities, the temporal and spatial evolution of the pressure and temperature fields must also be considered. Rarely, if ever, of course, is such information available. Even in numerical simulations where we can make some educated guesses as to 32

reasonable values to assume for boundary and internal conditions, the quantitative evaluation of the concentration field is highly complex because Ci, Rij, v, D, P and T are to some degree coupled functions of each other (e.g. Burrus et al., 1991). Although Equations (1) and (2) can often not be solved quantitatively in specific problems because of a lack of sufficient information, they still provide a useful and necessary inventory of various processes and variables which we must consider, at least qualitatively, when we attempt to interpret the meaning of temporal and spatial variations in pore fluid compositions. The various terms in Equation (2) will serve as the outline for subsequent sections in this paper. Field areas

We will examine formation water compositions from three areas, the Cenozoic siliciclastic section of the south Louisiana Gulf Coast, USA, the Mesozoic carbonate section of the south Arkansas Gulf Coast, USA, and the Mesozoic section of the Norwegian Shelf, North Sea basin. The first of these areas, south Louisiana, will be discussed in the greatest detail. The 2300 km 2 area of Calcasieu Parish in south-western Louisiana, US Gulf Coast (Figure 1), provides an instructive study area for assessing the net effects of diagenetic reaction and solute transport on pore water compositions in a siliciclastic sequence on a regional scale. The US Geological Survey (Wallace et al., 1978; Wallace, personal communication) compiled 127 brine analyses for 17 producing oil and gas fields in the parish from several company sources. Even though some of these previously unpublished analyses are old and some are incomplete, as a group they provide useful insight into some of the first-order regional processes of subsurface diagenesis in south-west Louisiana. The upper 3-3.5 km of the sedimentary section in Calcasieu Parish consists of hydropressured, sanddominated, fluvial-deltaic sediments ranging from Oligocene to Recent in age (Curtis and Picou, 1980; Cossey and Jacobs, 1992). Older marine sediments of Eocene age immediately below are mudstonedominated and overpressured. Seven salt domes penetrate the hydropressured section within the parish, and four additional shallow salt diapirs occur immediately to the south. Depth to the top of salt at these 11 domes ranges from 200 to 2500 m (Halbouty, 1979). A contrast in geological setting is provided by formation waters from Jurassic carbonate strata of the Smackover Formation of the south Arkansas shelf of the Gulf Coast basin. A number of studies have provided evidence to support the hypothesis that waters here are genetically related in part to an evaporatively concentrated precursor (Carpenter and Trout, 1 9 7 8 ; Moldovanyi and Walter, 1992). Moldovanyi and Walter (1992) provide a detailed description of the geochemistry and geological framework of these waters. Egeberg and Aagaard (1989) have published a series of analyses of formation waters from Upper Triassic/Jurassic clastic reservoirs and Cretaceous carbonate reservoirs in the Norwegian shelf of the North Sea. We will simply look at some first-order compositional trends for these waters. A more

Marine and Petroleum Geology 1994 Volume 11 Number 1

Controls on composition of waters in sedimentary basins: J. S. Hanor

Calcasieu Parish

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Figure 2 Variation in dissolved Na, K, Mg and Ca as a function of dissolved chloride, Calcasieu Parish, south-west Louisiana. The solid line on each plot is the seawater evaporation-dilution curve for that cation (Carpenter, 1978), and the cross is the composition of seawater. Open circles represent single data points, black circles represent two data points and asterisks represent multiple data points. Note the general increase in concentration of each cation with increasing chloride Marine

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1994 Volume

11 N u m b e r

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Controls on composition of waters in sedimentary basins: J. S. Hanor complete discussion and interpretation is given by studies to have been deposited in fluvial, deltaic and Egeberg and Aagaard (1989). normal marine environments (e.g. Curtis and Picou, 1980). It can thus be concluded that few of these sediments contained fluids significantly more saline Original pore water compositions, Ci(0) than approximately 35 g/l, normal seawater salinity, at Most clastic, biogenic and evaporitic sediments now the time of their deposition. Most of the connate fluid buried in sedimentary basins contained pore water at compositions in this region would plot on the left-hand the time they were deposited. There are a few segments of the seawater curves shown in Figure 2. In exceptions, of course, such as aeolian sediments and contrast, fluids entrapped in the Louann Salt, the glacial deposits. The fluid buried at the time of Middle Jurassic evaporite unit which underlies much of deposition is the connate fluid of the sediment. The the Gulf Coast, including the area of the Upper Jurassic term connate has often been misused to describe any Smackover Formation (Moldovanyi and Walter, 1992), pore fluid, even one which has been profoundly were certainly halite-saturated at the time of modified chemically, or physically transported after deposition, and thus had connate salinities of 350 g/1 or sediment deposition (see discussion by Hanor, 1988). more. The true meaning of the term connate becomes clear, at least for an English-speaking audience, when we Present pore water compositions, C(t) translate and rearrange the roots of this Latin-derived word into their four-letter Anglo-Saxon equivalents, The problems of obtaining accurate information on the born-with. composition of subsurface fluids can be significant. Connate fluids in most fluvial, deltaic and open These problems include knowing whether produced marine sediments had salinities ranging from a few water is truly representative of ambient formation milligrams per litre to normal seawater salinities of water (Coleman, 1992), the problem of maintaining the 32-37 g/l. The major solute composition of seawater is chemical integrity of the sample from in situ conditions today nearly uniform on a global basis and has varied to the laboratory, and problems of establishing suitable only slightly during the Phanerozoic (Berner and analytical techniques (Lico et al., 1982). These Berner, 1987). The composition of continentallyproblems, though critically important; will not be dealt derived fluvial waters, however, is much more variable with here. This discussion will focus on the types of (Berner and Berner, 1987). Fluvial waters normally conclusions which can be drawn if suitable information represent a mix of meteoric waters derived from rain, on pore fluid compositions is available. snow and ice, and diagenetically-altered shallow The composition of formation waters provides groundwater introduced by base discharge. immediately useful information, even if the ultimate Both continental and marine surface waters can be processes controlling the composition are not known. modified in terms of their solute and isotopic For example, water analyses are needed to assess composition by subaerial evaporation and by the potential well scaling or brine disposal problems removal of solutes through the precipitation of salts. (Osborne et al., 1992). In addition, variations in salinity The evolution in composition of evaporated marine directly affect bulk sediment properties, such as water is well known and reasonably predictable resistivity and seismic velocities, which may be (Carpenter, 1978; McCaffrey et al., 1987). Figure 2 important factors in exploration. Spatial variations in shows scatter plots of the variation in major solutes as a chemical composition, or even just simply in salinity, function of chloride concentration in formation waters can be used to deduce the degree of hydraulic from the Calcasieu Parish study area. The large cross marked 'seawater' on each of the plots is the solute-chloride composition of normal seawater. The lines marked 'evaporation-dilution curve' show the sea water CalcasieuParish, LA compositions formed by diluting seawater with TDS freshwater and by concentrating seawater through iooo subaerial evaporation and precipitation of evaporite E ° ..~ minerals (Carpenter, 1978). "2000 • ooQ • " •• • t• ql~ The evaporation of continentally-derived waters produces a much more widely variable suite of .o , • "_ 8" /~ h y d r o p r e s s u r e d compositions because of the more widely varying range i t S o / f -ao00 ========================================================= of initial fluid compositions and pathways of chemical ILl "''l, precipitation (Eugster and Hardie, 1978). Further .i,, i • ~, overpressuredfluids variations in composition can be produced by -4000 1 evaporating mixes of continental and marine water, by 1 introducing fresh meteoric water, and by redissolving 1 -5000 . . ............. , .... earlier chemical precipitates. 0

Connate water compositions, G u l f Coast, USA A first-order estimate of the connate composition or at least the connate salinity of fluid within a sedimentary sequence, C/(0), can often be obtained from simply knowing the environment of deposition. Cenozoic sediments of the south Louisiana Gulf Coast are known from detailed sedimentological and palaeontological 34

100

200 TDS, g/L

300

400

Figure 3 Variation in formation water salinity with elevation (= negative depth), Calcasieu Parish. The vertical broken line marked seawater TDS represents normal sea water salinity. Waters in the hydropressured, sand-dominated section above 3 km are hypersaline. Salinities decrease with depth into the overpressured, mudstone-dominated section below. The horizontal dash-dot lines mark the transition between pressure regimes,

Marine and Petroleum G e o l o g y 1994 V o l u m e 11 N u m b e r 1

Controls on composition of waters in sedimentary basins: J. S. Hanor are far more saline than seawater (Figure 3). Maximum 10e l Calcasieu Parish, L~ pore water salinities of 125 g/l are typical over much of the depth range between 1 and 3 km. Several waters near a salt dome in the eastern part of the parish, the Iowa dome, have salinities which exceed 300 g/l. Below j aters near a depth interval of approximately 3-3.5 km there is a E'~ 104 ' ~ s a l t dome progressive decrease in salinity with depth as we proceed downward into the underlying overpressured O o , mudstone-dominated section. Here, the pore water 1 0 3. • -~ salinities actually decrease below connate marine 0 values. 1 0 2.

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Major cations. As shown in Figure 2, dissolved Na systematically increases with increasing C1. Sodium values plot parallel to and are nearly coincident with the evaporation-dilution curve. Potassium also increases with increasing C1, but the values are depleted by a factor of three to five relative to the seawater evaporation-dilution curve for K. Magnesium is also strongly depleted. Variations in dissolved Mg and Ca can be described by two trends. In trend 1, which represents most of the analyses, there is a significant increase in Mg and Ca with increasing C1. In the second trend, which includes the > 300 g/1 waters from the vicinity of the Iowa dome in the easternmost part of the parish, there is a much less significant increase in these two cations with increasing salinity. In contrast with Mg, most of the Ca values, particularly those for the more saline waters in trend 1, plot well above the seawater evaporation-dilution curve. Figure 4 is a combined plot showing the change in all four cations as a function of chloride. In terms of mass of solute, the waters are dominated by Na, then Ca, and then Mg and K in subequal concentrations.

106

CI, mg/L Figure 4 C o m b i n e d c a t i o n - c h l o r i d e p l o t for formation waters f r o m Calcasieu Parish. The circled fields represent waters f r o m t h e immediate vicinity o f t h e I o w a salt d o m e

continuity or compartmentalization (Smalley et al., 1988) and to derive PTX-related fluid properties, such as fluid density and viscosity, which are required for the analysis of hydrodynamic forces (Hanor and Sassen, 1990). A longer tern] interest may be in trying to deduce what present pore fluid compositions tell us about the diagenetic, hydrological, thermal and tectonic evolution of a sedimentary basin. Is it possible, for example, to identify continental, marine, basinal, metamorphic, or magmatic components in formation waters (Grant et al., 1990)? Such questions may be difficult to resolve because there are not necessarily unique chemical and physical pathways to a particular end pore water composition. However, it may be possible to at least put constraints on reasonable interpretations.

Major anions. Chloride is the most abundant anion by mass, followed in general by alkalinity, and then sulphate. The plot of titration alkalinity versus chloride shows a trend unlike any of the major cation trends (Figure 5). Low salinity waters are enriched in alkalinity relative to seawater and typically have values of 1000-2000 mg/1 total alkalinity. As salinity increases, however, alkalinities as a group decrease. At the highest salinities, values of 100 mg/1 are typical. The values of titration alkalinity included in the US Geological Survey database presumably include both

Present formation water compositions, south-western Louisiana Salinity. Most of the formation waters in the Calcasieu Parish study area above a depth of 3-3.5 km I

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Controls on composition of waters in sedimentary basins: J. S. Hanor 1 i

10 6.

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Figure 6 Variation in total dissolved solids (TDS) as a function of elevation, formation waters from the Jurassic Smackover Formation of the southern Arkansas Gulf Coast USA. From data of Moldovanyi and Walter (1992) carbonate alkalinity, predominantly contributed by bicarbonate, and organic acid anion alkalinity, represented by such species as acetate and propionate. In a study of the distribution of organic acids and bicarbonate in hydropressured waters in the south-central Louisiana Gulf Coast, Hanor and Workman (1986) found that between 1 and 50% of the titration alkalinity was contributed by organic acid anions. The proportion of organic acid alkalinity to carbonate alkalinity in the Calcasieu Parish waters is unknown, but there is little evidence here and elsewhere (Land et al., 1988; Macpherson, 1992) to support the notion that Gulf Coast brines invariably contain large (> 1000 mg/1) concentrations of organic acid anions. Sulphate shows a large scatter in concentration and no well defined trend with respect to chloride (Figure 5). Most waters are depleted by one or more orders of magnitude relative to marine values, most likely as the result of reduction of sulphate to sulphide.

Isotopic composition. The isotopic composition of formation waters in Calcasieu Parish is largely unknown. However, work by Gonthier (1989) on

105

10 6

CI, m g / L

Figure 8 Variation in major dissolved species as a function of chloride, formation waters from the Jurassic Smackover Formation of the southern Arkansas Gulf Coast, USA. From data of Moldovanyi and Walter (1992) similar waters in south-eastern Louisiana has shown these waters to be generally enriched in 6180 and depleted in 6D relative to probable meteroric and marine precursors. The connate isotopic composition of these waters has been modified by burial diagenesis and mixing.

Formation water compositions, Arkansas and Norwegian Shelf Salinities in the southward-dipping Jurassic Smackover Formation, southern Arkansas Gulf Coast, progressively increase downdip to values in excess of 300 g/l, and then reverse (Figure 6). The reason for this reversal is not yet known, but may reflect lateral hydraulic compartmentalization (Moldovanyi and Walter, 1992). These waters as a group are far saltier than typical waters from the south Louisiana Gulf Coast (Figure 3) and from the Norwegian Shelf, North Sea basin (Figure 7). Figures 8 and 9 are simple scatter plots showing the variation in major cations for the Smackover 106

Norwegian Shelf

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Marine and Petroleum

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Ca

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1994 Volume

Figure 9 Variation in major dissolved species as a function of chloride, formation waters of the Norwegian Shelf, North Sea basin. Values calculated from analyses of Egeberg and Aagaard (1989) 11 N u m b e r

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Controls on composition of waters in sedimentary basins: J. S. Hanor Formation (Moldovanyi and Walter, 1992) and for the Exchange with other fluid phases. Other fluids in Norwegian Shelf. Mass concentrations for the latter the system, such as free gas phases and hydrocarbon have been calculated from the original analyses of liquids, have the potential for exchanging Egeberg and Aagaard (1989), which were given in diagenetically reactive components with aqueous fluids. molality. No attempt has been made here to reconcile Important examples include the introduction of organic the differences between the concentration units g/1 and acids through thermal maturation of organic matter and g/kg, which become increasingly significant with the loss of reactive volatiles through degassing increasing salinity. For example, a 260 g/kg NaC1 solution is equivalent in concentration to a 320 g/1 NaC1 CH3COOH(org) ~ CH3COO- + H + (9) solution (Hanor, 1988; p. 41). It will suffice to note for our purposes here that each of the three data sets H + + HCO~- ~ H20 + CO2(g) 1' (10) document systematic increases in each of the major cations with increasing chloride. There is a general Acid-base reactions. Most of these reactions can decrease in alkalinity and a wide scatter in sulphate be written with the hydrogen ion as a reactant or with increasing chloride similar to that for south-west product and are hence acid-base reactions. Any Louisiana (not shown). process which has the capacity for altering the pH thus has the capacity for shifting many aspects of fluid chemistry and mineral diagenesis simultaneously.

Modification by diagenetie reactions (ZRu) Types of reactions

The composition of sedimentary mineral phases and pore fluids can be profoundly modified after deposition by spatially and temporally variable and complex chemical reactions. There are numerous ways in which these reactions can be classified, but the abbreviated scheme in the following is adequate for the present discussion.

Redox reactions. Most solid phases and waters deposited in continental and open marine sedimentary environments contain elemental components in relatively high oxidation states. Examples include Fe n~ in Fe-bearing mineral phases produced by weathering and Sv1 in dissolved sulphate ions. Notable exceptions are most biologically derived carbon compounds other than CO> The nominal valence state for C in most such compounds and their burial derivatives varies from - I V to 0, and organic carbon thus exists as a potential electron source for the biologically mediated or inorganic reduction of other elemental components in the system. Examples of such redox reactions include CH20 +

0 2 ----> H + -t-- HCO~

(3)

2CH20 + SO 2- -~ H2S + 2HCO~-

(4)

Mineral hydrolysis. Mineral hydrolysis in its most general sense simply refers to reactions between solid mineral phases and an aqueous solution. Important subsets of this general type of reaction in the subsurface include the dissolution of evaporite minerals, such as halite and anhydrite NaC1 ~ Na + + C1-

(5)

CaSO4 ~ Ca 2+ + SO 2-

(6)

and the dissolution and precipitation of silicates and carbonates NaA1Si308 + 4H + + 4H20 = Na + + AI 3+ -4- 3H4SIO4°

(7)

CaCO3 + H + = Ca 2+ + HCO~-

(8)

Rates of reaction. The absolute rates of diagenetic reactions, R, in basinal settings are in general not well known. Recent summaries of work on a number of important chemical systems are included in the collection of papers edited by Kharaka and Maest (1992). Evidence will be presented here, however, which suggests that the rates of some hydrolysis reactions involving major species are sufficiently rapid, even at modest sedimentary temperatures, that the drive towards thermodynamic equilibrium between formation waters and ambient silicate and carbonate mineral phases is an important control on the composition of formation waters. Diagenetic modification of salinity in south Louisiana The most important problems to resolve regarding pore water compositions in the south Louisiana Gulf Coast are the origin of increased salinities in the fluvial-deltaic part of the section and the cause of the decreased salinities in the deeper marine section. This translates largely into the problem of the controls on the concentration of chloride, the most abundant anion in these waters. The burial or infiltration of subaerially produced brines is an unlikely cause of increased salinities in the upper part of the section because the entire Cenozoic sedimentary section in this study area was deposited under normal fluvial-deltaic-marine conditions. Other than a superficial coincidence of Na values with the evaporation-dilution trend, the major solute compositions do not resemble in any way those of evaporated marine waters (Figure 2). There are other sequences within the Gulf Coast, however, such as the Jurassic, where a subaerial brine precursor probably does represent a significant component of the overall salinity (Moldovanyi and Walter, 1992). Hydraulic forces of sufficient magnitude apparently exist in the overpressured section of south Louisiana to induce membrane filtration (Graf, 1982). The regional hydraulic gradient is causing fluid flow up through this sequence (Hanor and Bailey, 1983; Hanor and Sassen, 1990), and if membrane filtration is a significant effect, pore water salinities should increase downward as a result of electrostatic retardation of charged solutes. Salinities, however, actually decrease down through the mudstones (Figure 3). Either these clay membranes are inefficient, and/or fluid flow is preferentially occurring through sands and fracture porosity rather

Marine and Petroleum Geology 1994 Volume 11 Number 1 37

Controls on composition o f waters in sedimentary basins: J. S. Hanor C D 0 5 m

2

=_

10

r~

-3 "r

4

15 20

Figure 10 Fence diagram showing regional variation in formation water salinity, southern Louisiana Gulf Coast, USA; see F i g u r e

I for

location. Values calculated from spontaneous potential response (modified from Hanor, 1988)

than through mudstone matrix porosity (Hanor and Sassen, 1990). As a result of a series of studies begun by Hanor and Bailey (1983), it is now clear that the increased salinities in the Cenozoic section of south Louisiana are the result of the subsurface dissolution of salt domes. The laterally and vertically extensive plumes from these domes have coalesced to form a regional zone of high salinity waters (Figure 10). The single most important diagenetic reaction occurring in this part of the section is arguably the dissolution of halite. Two possible explanations for the lower than marine salinities in the deeper part of the section are: (1) the introduction of meteoric waters at some stage in the burial history of these sediments; and/or (2) the generation of free H20 by dehydration reactions (Morton and Land, 1987). The latter seems more likely, at least on a regional basis. This section is so hydrologically tight that it has not been pervasively invaded by brines, even though it has been penetrated by at least as many salt domes as the overlying sand-dominated section. If these hydrological characteristics have prevailed over the burial history of these sediments, it is difficult to imagine the pervasive introduction of meteoric water into this sequence by topographic recharge. Further work on the isotopic 12

T

9 -

composition of these waters (Gonthier, 1989) and mass balance considerations may help to resolve these problems.

Thermodynamic buffering of major solutes Although there is up to an order of magnitude spread in the absolute concentrations of some of the major cations in the Calcasieu Parish waters at any given chloride concentration, the first-order trend for each cation is that of increasing concentration with increasing chlorinity. On the log-log scatter plots, the monovalent cations show an approximately 1:1 slope and the divalent cations an approximately 2:1 slope with respect to chloride, at least for the moderately saline waters. Such observations led me to suggest in an earlier discussion (Hanor, 1988) that the first-order controls on pore fluid compositions in this region are the dissolution of salt and subsequent buffering of pore water compositions by multiphase silicate-carbonate mineral assemblages. Dissolution of salt alone would produce a progressively NaC1 dominated fluid. Consider Figure 11, which shows two generic phase diagrams which relate mineral stability and aqueous fluid composition. Equilibrium with respect to the hypothetical mineral assemblage, quartz, Na-smectite, 14

1

Ab C~J

4 " 10 -r v

+ "1-

% Z

6

{M

c~

- Sm

Sm

v

O

[33 O

3

0

6

K

2 0

3

6

9

2

log a (K+/H +)

I 4

6

8

log a (Na+/H + )

Figure 11 Phase diagrams showing stability relations between albite (Ab), smectite (Sm), K-feldspar (Kf), illite (I), kaolinite (K) and chlorite (CL) at quartz saturation as a function of the activities of dissolved Na +, H + and Mg 2+ at 25°C and 1 bar. Black circle represents buffered composition discussed in text

38

Marine and Petroleum Geology 1994 Volume 11 Number 1

Controls on composition of waters in sedimentary basins: J. S. Hanor kaolinite, illite and chlorite fixes the activity ratios 106 Calcui (aNa+/aH+), (aK+/aH +) and [aMg2+/(aH+)2] at any Calculated F}uid Compositions given temperature and pressure. An increase in the ~--- N a ~ 105 0 activity of Na + of one order of magnitude as a result of halite dissolution requires an increase in aH+ of one "1- 104 ~ 7--:.>" I K order of magnitude, i.e. the solution becomes more acid. The stoichiometries of the coupled hydrolysis E 1°3 reactions buffering fluid compositions require an "~=~ 102 ~ . ~ increase in the activity of monovalent ions, such as K +, of one order of magnitude, and the activity of divalent o 10 ~ cations, such as Mg z+, of two orders of magnitude. Some of the Na released by salt dissolution is tO 100 -I ,~.e incorporated in authigenic Na-bearing silicates to maintain the charge balance with chloride as other cations are released into solution by silicate and 103 104 10 s 10 e carbonate hydrolysis. There is therefore a decrease in Chloride, mg/kg H20 the Na/C1 ratio of the solution as CI increases. Even though the relations between the concentrations and Figure 12 Calculated composition of waters in equilibrium with activities of the individual cations are strongly nonthe mineral assemblage q u a r t z - m u s c o v i t e - a l b i t e - K 4 e l d s p a r linear at increased ionic strengths, broadly similar calcite-dolomite at an arbitrary temperature of 100°C, a, pressure of 1 bar and a C02 fugacity of 10-3 bar trends are reflected in the concentration scatter plots, particularly for the south Louisiana and North Sea waters. virial equation of state for aqueous solutions (Harvie et The idea that the compositions of subsurface brines al., 1984; Pitzer, 1987). are at least partially thermodynamically buffered has There are gross similarities with the computed been invoked as far back as the work of Carpenter and compositional trends and the observed compositional Miller (1969) on saline water in the Ozark Dome and variations in natural waters. Chief among these are the has also been proposed for Gulf Coast waters (e.g. roughly similar relative proportions of major dissolved Land et aL, 1988; references cited therein). The close species at any given chloride concentration, and the coupling between water chemistry and diagenetic progressive increase in dissolved Na, K, Mg and Ca, mineral assemblages is, in general, an important and the decrease in alkalinity with increasing chloride. component of most contemporary thinking on burial Note also that the differences between the rates of diagenesis (Warren, 1987). What is new here, however, increase of Na and K and the rates in increase of Mg is the emphasis on the idea that the concentration of and Ca with chloride are roughly similar to many of the chloride is a master variable which can influence an observed values in south Louisiana and the North Sea. entire suite of mass exchange reactions involving most Lower salinity waters are Na-dominated, but divalent of the major solutes in sedimentary formation waters. cations, particularly Ca, become much more important Testing this hypothesis by evaluating the saturation constituents as salinity increases. By changing the state of pore fluids with respect to common aluminobuffer system, fco~, and/or T, it is in fact possible to silicate and carbonate minerals is complicated by the produce model wafers dominated by Ca. fact that reliable data for dissolved aluminium and for in situ pH are often lacking, as is the case with the It is highly unlikely on the basis of the complex and present data sets. One technique for partially obviating variable petrology of sedimentary rocks and the these problems is to invert the process and calculate the observed range in formation water compositions at any bulk solution compositions which should exist in the given chloride value and temperature that a single presence of known or assumed mineral buffers and buffer system is responsible for the range of compare these with observed compositional trends. compositions observed in natural waters. Scatter in the The results of one such calculation are presented field data sets presented here presumably reflect in Figure 12, which shows the variation in the varying PT conditions, different buffer assemblages, concentrations of major dissolved species in fluids as a and departure from metastable equilibrium. Egeberg function of dissolved chloride in waters in equilibrium and Aagaard (1989), for example, have documented with the mineral assemblage quartz-muscovitedifferences in composition between waters from albite-K-feldspar-calcite-dolomite at an arbitrary carbonate and sandstone reservoirs in the Norwegian temperature of 100°C, a pressure of 1 bar, and a COz Shelf. The Smackover waters are distinct in their fugacity of 10 -3 bar. These constraints, plus the potassium signature. Some waters may not be buffered concentration of chloride, are sufficient to uniquely fix at all, including the most saline waters from south-west the chemical potentials of dissolved species in the nineLouisiana, whose compositions can most simply be explained by congruent dissolution of bulk salt dome component system N a z O - K 2 0 - M g O - C a O - A I z O 3 S i O 2 - C O 2 - H C 1 - H 2 0 (or, alternatively, NaCI-KC1evaporites (Figure 4). Work is in progress to determine MgCI2 - CaCl2 - A1C13 - SiO2 - CO2 - HC1 - H20). more exactly what constraints can be put on identifying Allowing CO2 to be mineral-buffered (c.f. Smith and possible natural buffer systems using this approach. Ehrenberg, 1989) requires adding an additional solid The necessity for having a large number of silicate phase, for example, chlorite, to the system. Hydrolysis mineral phases to buffer a multicomponent fluid system constants for mineral phases and for COz(g) were taken may mean in some settings that formation waters in from Bowers et al. (1984). Conversion between mineralogically simple sandstones and carbonates, aqueous activities and concentrations was made using which are the usual waters sampled, are influenced by an in-house computer program which utilizes a Pitzer reactions occurring in nearby mudstones and shales. Marine and Petroleum Geology 1994 Volume 11 Number 1 39

Controls on composition of waters in sedimentary basins: J. S. Hanor Bromine is similarly non-buffered, and Br-C1 The importance of the chemical potential of chloride, systematics are thus more likely to provide clues to the or, alternatively, HCI, in the thermodynamic identity of saline end-member water types than other interpretation of processes involving natural aqueous pairs of solutes. The high Br/C1 ratios of waters in the solutions was recognized over 20 years ago by Helgeson Smackover, for example, support the hypothesis that (1970). This general concept has been utilized by Br-rich, subaerially produced brines are important Giggenbach (1984) and others as a charge balance saline end-members in this natural system (Figure 13). constraint in the analysis of high temperature The low Br/C1 values of waters in South Louisiana, in hydrothermal processes. With the exception of work by contrast, support the idea that high salinity is derived Hanor (1988) in the Gulf Coast and Michard and from the dissolution of halite-dominated salt domes, Bastide (1988) in the Paris Basin, it has not attracted as which typically have low bulk Br/CI ratios. The waters much attention in lower temperature basinal studies. In of the Norwegian Shelf have yet a third signature, one the examples discussed by Helgeson (1970), CI activity that supports the conclusion of Egeberg and Aagaard was held constant and the pH allowed to vary. In (1989) that there has been at least some contribution sedimentary basins, however, the case can be made from subaerially produced brines. that chloride is usually the more important master Sulphate is also not rock-buffered, at least in variable and that instead pH may be buffered. There south-west Louisiana, but for much different reasons. are many other important ramifications of this general As can be noted in Figure 4, sulphate shows little or no concept, including its application to fluid-solid mass systematic variation with chloride. Recent work by transfer problems and to brine geothermometry, that McManus and Hanor (1988) provides clues as to why are unfortunately beyond the scope of this brief paper. this is the case in south Louisiana. The isotopic The concept, however, is one that must be considered composition of massive carbonate and pyrite cemented along with other hypotheses, such as conservative sands near the West Hackberry salt stock just south of mixing, in explaining the suites of water compositions Calcasieu Parish supports the hypothesis that the typically found in sedimentary basins. sulphur in the pyrite was derived from salt stock anhydrite and the carbon in the carbonate is from a Non-buffered components methane source. The net reaction has involved the Not all components in sedimentary formation waters thermogenic reduction of sulphate and oxidation of are buffered by ambient mineral phases. The single methane and the consequent precipitation of iron most important example is chloride. Most subsurface sulphides and calcium carbonate. The concentrations of waters are undersaturated with respect to halite, sylvite dissolved sulphate in this area are thus controlled by and other chloride-bearing mineral phases, and relative rates of: (1) release of sulphate through salt chloride concentrations are controlled primarily by dissolution, (2) solute transport and (3) removal by mass transport processes, not by thermodynamic reduction. The latter process, of course, is further equilibrium with respect to one or more mineral dependent on the availability of suitable reducing phases. Even in the US Gulf Coast where a compelling agents. case can be made for the origin of chloride by subsurface dissolution of salt domes and by burial of once halite-saturated brines, most waters have salinities Advective mass balance ( - v- V C/) well below the 350-400 g/1 expected at saturation (Figures 3 and 6). An important exception is the Hydraulic flow Michigan Basin, where some pore waters are clearly Bulk fluid flow plays a critically important part in halite-buffered (Wilson and Long, 1992). determining the composition of fluids by simply transporting solutes. Funayama and Hanor (1992), for example, have reported the transport of brines derived 10 4by the dissolution of salt domes over lateral distances in excess of 100 km within the Eocene Wilcox Formation of central Louisiana. In the absence of bulk fluid flow, Wilcox formation waters would have salinities of no 103 O, more than 35 g/1. Most of these waters now have 0 ° salinities in excess of 100 g/l, even though they are 0 0.* E spatially far removed from the nearest salt. The e- 10 2 dido ~? -g_ transport of solutes through varying PT conditions also X TM has the potential for driving significant mass transfer E • Southern Arkansas o between fluid and solid sediment, particularly if the o North Sea (mg/kg) 03 fluid composition is partially buffered by equilibrium 101 "~ x Southwest Louisiana with respect to the ambient solid mineral phases. The terms advect (to carry to) and convect (to carry Evaporated Sea Water with) are sometimes used nearly interchangeably in the 100 hydrogeological literature (e.g. de Marsily, 1986; p. 10 s 106 104 230). Implicit in the term convection, however, is the Chloride, mg/L simultaneous transport of solutes, charge and heat by Figure 13 Variation in dissolved Br versus CI for waters from advecting water. It is important to note that the use of south-west Louisiana (this study), the Jurassic of southern the term convection includes, but is not limited to, Arkansas (Carpenter and Trout, 1978; Moldovanyi and Walter, highly organized closed cells, the existence of which in 1992), and the North Sea (Egeberg and Aagaard, 1989). Also sedimentary beds has been a subject of some debate shown is the variation in Br with CI in evaporated marine waters (Wood and Hewitt, 1982; Bj~rlykke et al., 1988). In a (McCaffrey et al., 1987)

•

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Marine and Petroleum Geology 1994 Volume 11 Number 1

C o n t r o l s on c o m p o s i t i o n o f w a t e r s in s e d i m e n t a r y b a s i n s : J. S. H a n o r

system where the total volume of fluid is conserved, flow in one part of the system may be highly focused, and the convective counterflow elsewhere diffuse. An excellent example is provided by the pattern of thermohaline convection in the world's oceans. Most of the downward flow in the oceans is focused in a few high latitude areas in the Atlantic. Most of the counterflow up, however, occurs as a diffuse process over a broad area (Broecker and Peng, 1982). Hydraulic flow is driven by the complex interplay of gravitational body forces, 9g, and fluid pressure gradients, VP. At low fluid velocities there is a linear relation between fluid flux and the magnitude of these forces, which is described by Darcy's law qd = - - k d / I I

(11)

( V P - pg)

where qd(m3/m 2 s) is fluid flux, kd is the intrinsic permeability tensor, ~1is fluid viscosity, V P is the fluid pressure gradient, P is fluid density, and g is gravitational acceleration. The absolute velocity of Darcian flow, v, is related to the magnitude of fluid flux by (12)

V = qd/d~

where qb is sediment porosity. Note that two compositionally dependent properties of fluid, viscosity and density, play significant parts in determining fluid velocity. The complex coupling between physical and chemical processes is evident in the mass balance term in Equation (1) which describes the change in the composition of pore water resulting from advection -

v. V Ci = (kj~p ~1) ( V P

- pg). V C~

(13)

The concentration field thus influences fluid velocity, and the velocity field influences composition.

Forces driving f l o w The term forced convection is used to describe flow

driven by pressure gradients in systems where the fluid density is a function only of pressure. The driving forces may arise by differences in the height of the water column in different portions of the system, as in topographic-driven flow, or because the fluid is bearing part of the weight of the overlying sediment column, as in compaction-driven flow. Good introductions to the subject in a large-scale basinal context are provided by Bethke et al. (1988), Garven (1989) and Demming and Nunn (1991). The term free convection refers to flow driven by density differences arising from spatial variations in temperature and/or salinity. Depending on the dominant control on fluid density, such flow may be termed thermal convection, haline convection, or thermohaline convection. There has been considerable work on the numerical modelling of fluid convection and solute and heat transport around salt domes, which has been stimulated in part by the field documentation of large salinity plumes extending laterally away from the top of some diapirs (Figure 14) (see Evans et al., 1991; and references cited therein). Fluid flow velocities are poorly known for many deep sedimentary systems. Numerical simulations of regional flow in sedimentary basins (e.g. Bethke et al., 1988; Garven, 1989; Evans et al., 1991) suggest that velocities can range over many orders of magnitude, from essentially zero to metres per year or more, depending on the hydrological setting. Even where the magnitudes of the driving forces are reasonably well known, however, often insufficient information is known on the scale-dependence of the permeability field to make accurate calculations of flux and velocity.

Dispersive mass balance [ V . (DV C/)] Importance o f mixing Dispersion is perhaps one of the more underrecognized processes influencing the composition of basinal fluids. The term dispersion refers to the mixing resulting from the differential velocity of components

NORTH

SOUTH

0A

A' 0 SALINITY, g/I •

4

' " '

40

2

i

14 Figure 14 Variation in pore water salinity in the vicinity of the Welsh salt dome, south-west Louisiana Gulf Coast, USA (reproduced from Hanor, 1988, by kind permission of Academic Press, Inc.) M a r i n e and P e t r o l e u m G e o l o g y 1994 V o l u m e 11 N u m b e r 1

41

Controls on composition of waters in sedimentary basins: J. S. Hanor in a fluid. Dispersion includes molecular diffusion, in where D is the coefficient of hydrodynamic dispersion. which solute ions or molecules move at a different net This term combines the relative effects of fluid-solute velocity to the water molecules, and mechanical interactions, fluid velocity and pore structure on the dispersion, in which small parcels of water of spatially efficiency of dispersion. In some settings, it can be varying composition move at different velocities defined as (de Marsily, 1986; Domenico and Schwartz, through a porous medium as a result of their being 1990) forced to take paths of varying length or by their being slowed by proximity to grain surfaces. The sum of these D = D ° / O = + ~1 vl (15) processes is known as hydrodynamic dispersion. Differential transport can also be driven by other The first term on the right-hand side describes the forces, such as gradients in temperature (Soret contribution to dispersive transport by molecular diffusion) and charge (electrokinetic diffusion), and by diffusion. D ° is the molecular diffusion coefficient in differences in the chemical potential of water in the free solution, and 0 is tortuosity, the relative length of presence of a semipermeable membrane (osmosis). the tortuous path followed by the diffusing solute. The The net effects of dispersion are of critical second term describes the effects of mechanical importance to our understanding of how formation dispersion, where ~ is a characteristic property of the water compositions evolve. For example, if there were medium known as dispersivity and has units of length, no mixing of formation waters in sedimentary basins, and ]v] is the magnitude of fluid velocity. Both then most subsurface waters today would either be tortuosity and dispersivity are tensors, and hence D is a fresh, have normal marine salinities, or be saturated tensor. with respect to halite. In fact, most waters in most The mass balance due to hydrodynamic dispersion is sedimentary basins have salinities (Figures 3, 6 and 7) simply the algebraic sum, or divergence ( V -), of all the and B r - C I systematics (Figure 13) which can only be dispersive fluxes into and out of the volume of interest accounted for by mixing various end-member water types. If the chemical potential of chloride fixes the (16) V . J = [ V - ( D V C/)] chemical potential of other majors solutes as described earlier, then we could argue that dispersion and At low fluid velocities, molecular diffusion is obviously hydrolysis are the two most important mechanisms the dominant mixing process; at high fluid velocities, controlling the major solute composition of subsurface mechanical dispersion is dominant. For a hypothetical waters. system where D°/O== 1 × 10 -5 cmZ/s (Lerman, 1979) A field scale example of dispersion is shown in Figure and 0¢ = 10 m (Domenico and Schwartz, 1990; p. 373), 14, which shows the variation in salinity in a vertical mechanical dispersion becomes dominant when fluid cross-section near the Welsh salt dome in velocities exceed 10 -8 cm/s, i.e. a few millimetres per south-western Louisiana. The physical system is year. transient and open with respect to chloride, and hence salinity. Chloride is continually introduced through the Field example of vertical dispersion dissolution of salt and removed by mixing with meteoric waters, which eventually discharge at the Dickey (1966) noted that there is a nearly linear earth's surface. increase in salinity with depth over a 3 km vertical It might be supposed that mixing should invariably section of northern Louisiana and the southern produce a compositionally homogeneous mass of fluid, Arkansas Gulf Coast (Figure 15). Hanor (1984) unlike the heterogenous fluids documented in Figure 14. Mixing in nature, however, is rarely so efficient and Total Dissolved Solids, g/I rapid that it produces complete homogeneity. The 00 50 100 150 200 250 300 350 world's oceans, which are actively convecting and mixing on a time-scale of several thousand years are not homogeneous with respect to the absolute concentration of any dissolved component (Berner and Berner, 1987). The only way that the salinities in Figure 14 could have been produced is by mixing of halite-saturated brines (TDS = 350-400 g/l) with fresh and marine o 4 waters having salinities of < 1-35 g/1. Determining that g mixing is occurring requires no more than determining that continuous compositional gradients exist. However, evaluating what dispersion processes are 6 operating and what their absolute rates are is more complex: Ottino (1989) provides a well illustrated o i 7 account of the heterogeneity of mixing for those oo interested in pursuing the matter further. 1

-

I

I

I

I

I

I

I

I

I

I

I

[

~o

8

Rates of hydrodynamic dispersion

9

The flux of solute i, Ji, due to molecular diffusion and mechanical dispersion can be described by Ji = - D V C~ 42

(14)

Figure 15 Variation in total dissolved solids as a function of depth for Upper Jurassic through Upper Cretaceous formations of the south Arkansas, north Louisiana Gulf Coast (from Hanor, 1984). Open circles represent single analyses, solid circles two or more analyses. Note nearly linear gradient

Marine and Petroleum Geology 1994 Volume 11 Number 1

Controls on composition of waters in sedimentary basins: J. S. Hanor chloride in these sediments to have produced the suggested that this profile represents an ongoing, increased salinities currently observed. Most of the steady-state mass transport of dissolved NaC1 from chloride has been generated instead by the subsurface Middle Jurassic evaporites at the base of the section to dissolution of salt domes and has been pervasively the base of the shallow meteoric regime. Hanor (1984) transported by convection and dispersion throughout further proposed that the mass transport processes most of the hydropressured section. Chloride in other responsible for this profile could include some parts of the Gulf Coast, such as the Jurassic section of combination of molecular diffusion, Soret diffusion and mechanical dispersion. southern Arkansas, appears to have been derived in part from burial of subaerial brines (Moldovanyi and Subsequent numerical modelling of this problem by Ranganathan and Hanor (1987) showed that molecular Walter, 1992). and Soret diffusion are simply not rapid enough to The dissolution of salt has done far more than simply account for the observed profile in the length of increase the Na and CI concentrations of the south geological time available to have produced it. They Louisiana formation waters. The simultaneous concluded that convection and mechanical dispersion systematic variation in dissolved Na, K, Mg, Ca and must have dominated. How can free convection occur, alkalinity with chloride is permissive evidence for the however, if there is a pronounced increase in salinity thermodynamic buffering of these water compositions with depth? The fluid column should be gravitationally by silicate-carbonate mineral assemblages. The stable. concentration of chloride appears to be a master More recent work by Sarkar et al. (1992) provides a variable which ranks in importance with such other possible driving mechanism. They show on the basis of variables as pressure, temperature and fco2 in driving theoretical grounds and previous laboratory diagenesis in this region. It is controlled largely by experiments that where there is an increase in salinity physical processes of convection and dispersion in the with depth in a dipping sedimentary sequence, there upper 3 km of the section and by dehydration reactions will be spatial perturbation of isohaline contours at the in the deep, overpressured section below. Field data boundary between two sediments of differing and model calculations indicate that the addition of molecular diffusivity, D ° q~/0 2, where q~ is porosity and chloride to a rock-buffered system produces saline the other terms are as defined in Equation (7). The waters with decreasing Na/C1 and increasing Ca/C1 constraints of conservation of mass reqmre that ratios. The occurrence of high Ca concentrations in otherwise horizontal isohaline contours cross sediment very saline subsurface waters may by itseff tell little boundaries at right angles, i.e. they dip towards the about the details of the chemical evolution of the sloping contact. Fluids in a sandstone immediately waters. adjacent to a dipping sand-shale contact are thus The waters of the Norwegian Shelf, North Sea basin slightly less saline and less dense than fluids at the same discussed by Egeberg and Aagaard (1989) are broadly elevation in the sand some distance away. The similar in major solute composition to the waters in magnitude of this lateral density gradient is sufficient, south-western Louisiana, even though the geological in theory, to induce gravitational instability near the histories of the two areas are different. The waters from boundary and induce fluid flow up the base of the bed the Smackover Formation of southern Arkansas are having the higher molecular diffusivity. Counter flow much more saline than either the North Sea or Gulf occurs downward along the top of the bed. Sarkar et al. Coast waters and are different in their proportions of (1992) have calculated that the lateral flux generated major cations, particularly K. The spatial variations in could be of the order of a few cm3/cmZ/yr. Given a salinity and the B r - C l systematics of each of these porosity of 0.2, this would be equivalent to a velocity three groups of waters show that dispersion has been a approaching 10 cm/yr. Convection and dispersion can first-order control on their composition. thus apparently occur if there is a dip to sedimentary Where the composition of the fluid is largely bedding even where there is an overall increase in fluid rock-buffered, its ultimate origin and its pathway of density with depth. chemical evolution may be obscured, at least in terms of its major element composition, by its most recent P T X environment. Non-buffered components, such as C1 and Br, or isotopic compositions (Smalley et al., 1988), are more likely to retain information on original Conclusions end-member fluid compositions and reaction pathways. The problem of determining the controls on the composition of waters in sedimentary basins is largely a mass balance problem involving physical and chemical Acknowledgments processes equally. The south-west Louisiana Gulf Coast provides a useful example of the net effects of I thank E. A. Warren, BP Research, for the invitation diagenetic reaction and solute transport on pore water to participate in the 1992 Geological Society compositions in a siliciclastic sequence on a regional symposium in London on the chemistry and origins of scale. Most of the formation waters in the south-west North Sea formation waters and for the suggestion to Louisiana Gulf Coast study area have compositions write this review article. My thanks to W. H. Wallace totally unlike the compositions of the connate meteoric for making available to me the unpublished US and marine fluids that were buried with their host Geological Survey database on Louisiana formation sediments at the time of deposition. The changes in waters. Discussions with J. A. Nunn and A. Sarkar and fluid composition which have taken place in the upper reviews by E. A. Warren and K. Bj0rlykke helped to 3 km of this section cannot be accounted for simply by improve this manuscript. The work reported was diagenetic exchange with the immediate surrounding supported in part by NSF Grant No. 90-19341 and sands and mudstones, because there is insufficient funds from the Shell Development Company. Marine and Petroleum Geology 1994 Volume 11 Number 1 43

Controls on composition

of waters in sedimentary

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