evolution of interstitial waters along the passive margin of the southeast basin of france: welcom...

P e r g a m o n Applied Geochemistry, Vol. 9, pp. 657--675, 1994

Copyright ~) 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0883-2927/94 $7.00 + 0.00 0883--2927(94)E0032--8

Evolution of interstitial waters along the passive margin of the Southeast Basin of France: WELCOM (Well Chemical On-fine

Monitoring) applied to Balazuc-1 well (Ard~che)

L u c AQUILINA* and JACQUES BOULEGUE Laboratoire de G6ochimie et M6tallog~nie, Universit6 Paris VI, 4 place Jussieu, 75252 Paris C~dex 05,

France

JEAN-FRANCOIS SUREAU BRGM, Service G6oiogique National, BP 6009, 45060 Ori6ans C6dex 02, France

THIERRY BARIAC Laboratoire de Biog6ochimie Isotopique, Universit6 Paris VI, 4 place Jussieu, 75005 Paris, France

and

THE GPFTEAM

(Received 9 July 1993; accepted in revised form 27 May 1994)

Abstract--As part of the G6ologie Profonde de la France [GPF (Deep Geology of France)] Programme investigating the passive margin of the Southeast Basin of France, studies are being made on the possibility of modelling mass transfer. In the summer of 1990, when the first hole in the area (Balazuc-1) was being drilled along a basement-rooted fault, chemical monitoring of the drilling fluids (WELCOM) enabled the composition of the interstitial fluids contained within the intersected rocks to be reconstructed. The computations were compared to on-core leaching carried out during the drilling, following which the WELCOM results were calibrated. The interstitial fluids from the drilled formations show very contrasted compositions. From the top to the bottom of the hole it has been possible to distinguish: diagenetically evolved seawater within the Hettangian limestone, modified continental fluids within the upper part of the Triassic sandstone reservoirs, and evaporated seawater within the Middle Triassic evaporites. Several zones of intensive fluid circulation are also indicated by: dolomitization of the basal Hettangian limestone, circulation of fluids expelled from the evapontes within the upper part of Triassic sandstone reservoirs, and continental-derived hydrothermal fluid flow within the Lower Triassic sandstone reservoir and Carboniferous silty shale. The results show that chemical logging of drilling fluids can provide valuable data concerning interstitial fluids where other techniques are not available.

INTRODUCTION

THE FouItrn theme of the G6ologie Profonde de la France [GPF (Deep Geology of France)] Pro- gramme, is the study of mass transfer along the passive margin of the Southeast Basin of France (GxoT, 1987; GIOT et al., 1989). Past periods of extensive fluid circulation are documented by the mineralization in the upper part of the basin margin (Largenti~re district), whilst formations such as the dolomitic Hettangian and Triassic formations are recognized as regional pathways for basin fluids. In order to compare hydrothermal and basin fluids, two boreholes were planned; one in the centre of the basin, the other near the edge of the passive margin, along a fault that may have acted as a drain for hydrothermal fluids.

The western borehole (Balazuc-1), drilled on the

*Present address: BRGM, Direction de la Recherche, BP 6009, 45060 Orleans Cedex 02, France.

passive margin during the summer of 1990, included a subprogramme focused on recent and palaeofluids (core leaching, water-inflow sampling; AQUILINA et

al., 1991b; SUREAU et al., 1993). Following experiments carried out during the drilling of the first GPF well (VUATAZ, 1987) and a European geothermal well (VUATAZ e t al., 1990), drilling of the Balazuc-1 well was accompanied by intensive chemical logging of the drilling fluids using the WELCOM (Well Chemical On-line Monitoring) system.

Unfortunately, no water inflow was recorded in Balazuc-1. The porosity of the intersected rocks is only between 1 and 3% (SizuN et al., 1993) as a result of extensive sulphate diagenesis and the formations known as regional aquifers no longer contain mobile water. The WELCOM method, under these circum- stances, is of particular interest since it was the only way to obtain information concerning the interstitial waters and the soluble phases contained in the rocks.

A description of the WELCOM equipment and techniques, and an explanation of the signal process-

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1270.

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L. Aquilina et al.

1720

Middle

Hettan~,ian ~

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Rhaetian

Norian

Carnian

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Carboniferous

- 1 1 -

alternating limestone and marly layers

dolomitized limestone

black shale

-do/omifi zed]imestc~c . . . . . .

sandstone alternating with dolomite or dolomitic shale

sandstone -w it h -bari ffc cenlent . . . . .

sandstone alternating with dolomite or dolomitic shale

-do-lomite ,~ r -d o/o m it-i cshal e I ay er s - ~ alternating with sandstone

anhydrite

anhydritic shale • anh-yd~it~ - - - . . . . . . . . i

!

halite bed ( 1.3 m l sandstone ] ~breccia layers alternatingdolomite orWith

dolomitic shale (faults)

siltstone and black silty shale

- 111 -

FIG. 1. Stratigraphy and sedimentology of the cored section of Balazuc-1 well (1220-1720 m). Stratigraphy (1) from ELMI et al. (1991). Sedimentology (II) from STEINBERG et al. (1991). III represents the limits of

section used within this study, the dashed zone represents a lack of chemical data.

ing are given by AQUILINA et al. (1993a,b). The present paper deals with the results of the processing by which it is possible to compute the concentration of interstitial waters and estimate the soluble phase. Through comparing the W E L C O M results to on- core leaching it is possible to calibrate the W E L C O M reconstruction of the interstitial waters. The drilled formations are then examined and their water type characterized. A global pattern for the possible fluid path and its history is then presented.

B A L A Z U C - I W E L L ( A R D E C H E , F R A N C E )

Geolog ic set t ing

The Southeast Basin of France is a sedimentary basin of the Alpine system. It is connected to the Paleozoic Massif Central basement (northern edge of the Tethys) to the northwest by a passive margin controlled largely by N5-20°E-trending regional faults that were reactivated during Triassic rifting, and which had a major influence on the Alpine sedimentation.

The Balazuc-1 well is sited next to one of these regional faults, the Uzer fault (Gtox et at. , 1991), and

the bottom of the bole (1725 m) lies about 200 m east of it. Part of the Triassic series, however, has been truncated by satellite faults at 1626 and 1669 m. From ground level top to a depth of 1218 m, the borehole intersected Jurassic carbonates and marls. From 1218 m down to 1725 m, which is the subject of this study, the hole was cored and passed through the following succession (Fig. 1).

(1) The Lower Lias (1220--1350 m), consisting of the base of the middle Hettangian carbonates and the lower Hettangian dolomites. The fractured and dolomitized sections envelope about 30 m of black marl with a high organic matter content.

(2) The Rhaetian (1350-1378 m), which marks a transition between the Liassic marine sedimentation and the underlying Triassic continental deposits.

(3) The Triassic, comprising an upper and a lower sandstone unit separated by an evaporitic formation. These are here termed Upper, Middle and Lower Triassic for simplicity, although these limits do not match the stratigraphy (ELr~t et al. , 1991).

(4) The Carboniferous basement, formed of black silty shale and siltstone.

The continental sedimentation of the Upper and Lower Triassic (respectively, 1350-1560 m and 1 6 2 0 -

Evolution of interstitial waters, Southeast Basin of France 659

1669 m) is represented by bedded formations in which sandstone (or conglomerate) alternates with clay or dolomite (or dolomitic marl). The thickness of the beds varies from several tens of centimetres to a few metres. Faulting in the Lower Triassic is marked by brecciation at 1626 and 1669 m. The Uppe r Trias- sic can be divided into two parts on the basis of the geochemical analysis of the series (AQuIUr~A et al . , 1992): the upper part of the section (1350 m to about 1420 m, termed Part I in the present study) is marked by thick clay layers underlain, at 1410 m, by two sandstone beds with baritic cement ; in the lower part of the section (1420--1560 m, termed Part II in the present study) the sandstone beds are thicker and prevalent. The boundary, although not corresponding to a stratigraphic limit, represents a sedimentolo- gic discontinuity separating a major regressive trend from a transgressive one (DRouART et al . , 1992). The evaporitic Middle Triassic (1560--1620 m) comprises sulphate-bearing shale beds with a lower percentage of anhydrite in the middle part of the section. A halite layer and a carbonate bed form the base of the evaporites.

Three levels within the cored succession are re- gionally recognized as aquifers: the faulted Het tangian dolomite, and the Uppe r and Lower Triassic sandstone sequences. However , no water inflow was recorded in the Balazuc-1 well. The Het tangian dolomite no longer contains flowing water due to cemen- tation (principally CaSO4) of the faults; similar cementat ion has also affected the Triassic sandstone sequences, strongly reducing the permeabil i ty (K ranging from 0.2 to 0.4 millidarcys at 10 bars; SIZUN et al . , 1993). Nevertheless, these levels probably acted as aquifers for past fluid circulations from which the sulphate was precipitated.

Dri l l ing characteris t ics

From ground level down to 1218 m, the Balazuc-1 well was drilled in destructive mode. From 1218 to 1725 m, the well was cored (96 cores, each 6 m long) in 4 inch diameters (DEGOUY, 1991).

F rom 1218 to 1610 m, the mud was made up of surface water plus bentonite (30--40 kg/m3), C M C and polymers. N a O H was also added to stabilize the pH at basic values (7.5-12.5). A halite bed (1.3 m thick) encountered at 1610 m, required mud to be changed to a NaCl-saturated fluid (300 kg/m 3), which is why no values for Na and Cl are reported by the W E L C O M process below that depth.

DETERMINATION OF INTERSTITIAL WATERS

Determination by the W E L C O M method

The WELCOM process, described in detail by VUATAZ et al. (1990) and AQUILINA etal. (1991a,b, 1993a,b), consists of

chemically logging the concentration of the major elements and some trace elements in the drilling fluids. The fluids are continuously pumped and filtered through a tangential filter device (PIRANA, BRGM TM) at both the inflow and outflow of the well, and the element concentrations measured throughout the drilling operations in a field laboratory. After processing the signal (depth computation, corrections for mud additions, and homogenization problems in the mud tank), the difference between the inflow and outflow concentrations is measured (f in mold); this difference is attributed entirely to the influence of the formation crushed at the bottom of the borehole. From the evolution of element concentrations in the drilling fluids of the mud tank, an input of chemical species, integrated on a 5 m core interval, can be computed (Nfin mol/m cored). Comparison o f f and Nf does not reveal large discrepancies (AQUILINA et

al., 1993a), which constitutes an internal test. When the origin of the chemical input can be assumed to be interstitial waters (see later), the chemical input (Nj0 can be used to compute the composition of the interstitial fluids.

The concentration of a species Ce is defined as:

Ce = (Nf* Vm)/(V r * P), (I)

where N f = chemical input to drilling fluids during drilling of 5 m V m = volume of drilling fluids for the integrated step (5 m) V r = volume of rock crushed (hole volume - core volume) P = porosity (volume %) from neutron tools.

Monitoring drilling fluids can be compared to a leaching experiment done on the drilled rock column. However, several parameters which differ strongly from laboratory leaehings have to be taken into account. (1) Dilutions are made with surface water containing bentonite, polymers and NaOH (high pH), rather than with distilled water (pH 6), and so the saturation state of the diluent towards the minerals will be different. (2) The interaction time is slightly shorter, i.e. for the large particles, the time it takes for the drilling mud to return from drillhole bottom to the surface (about 45 rain). However, part of the fines remains in the drilling fluid for a longer duration. (3) Drilling strongly crushes rocks intersected, whereas for on-core leaching a 1- 10 mm fraction was used. (4) Another parameter which may be important is the very low solid/liquid (S/L) ratio in drilling, estimated to vary between 1/100 and 1/25.

The chemical input from formations can originate from mineral dissolution and/or from interstitial water contribution. With such parameters one could expect important dissolution during drilling. Nevertheless, the evolution of the Ca and SiO 2 concentrations in the drilling mud indicate that no intensive dissolution has taken place (AQUILINA et

a/., 1993a). On the other hand, the influence of clay, although no large ionic exchange seems to occur, is more difficult to regulate and the use of an external control is required to ensure the prevailing contribution of interstitial waters. The O and H isotopes in the drilling fluids were measured using an interval of about 30 m; since no H is contained in the rocks drilled (mainly carbonates and sandstone), water-rock interactions will be marked by minor ~2H variations in a ~2H--~lSo diagram. On the other hand, even a small amount of mixing with interstitial water will create trends along mixing lines since the proportion of interstitial fluid increases as drilling progresses. The results for the Balazuc-1 well are presented in Fig. 2: the value of the drilling fluids at the beginning of the hole is that of surface water and thus lies on the meteoric line; in the Hettangian carbonates a line with a gentle slope is observed which probably results from the combination of interaction with carbonates and mixing of interstitial waters; for all the other zones, the influence of fluids contained in the rock is clearly demonstrated since significant trends are expressed in each of them. The characteristic fluids of the different formations are discussed in more detail with the interpretation (see below).

660 L. Aquilina et al.

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. . . . ¢ . , . . . . . , °18° -6.5 -6 -5.5 -5 -4.5 -4 -3,5

Fro. 2. Oxygen and hydrogen isotopic evolution of drilling fluids. Isotopic composition of the drilling fluids is, at the beginning of drilling B, that of the meteoric water forming the mud. As drilling progresses, the fluids are progressively modified by the contribution of interstitial waters and a mixing line is drawn. Black squares, Hettangian limestone; black triangles, Upper Triassic Part I; black circles, Upper Triassic Part II; black crosses, Middle Triassic evaporite; black diamonds. Lower Triassic and Carboniferous sandstone. White arrows indicate the evolution with depth. Line is the CRAm (1961) global meteoric line. Data are in per mil vs SMOW. Error is 0.1 for oxygen and 0.5 for hydrogen.

Extraction by leaching process

Several methods are currently used for extracting interstitial waters, although it is always very difficult to control the origin of the chemical elements derived from the sediments, particularly when clay-rich strata are leached. For unconsolidated sediments, such as deep-sea cores, a press extractor can be used (MANHmM, 1966; SAVL~ and MAN- nmM, 1975), although TouaEr (1988) has noted variations in the concentration of the extracted fluid with pressure. For consolidated rocks with a high porosity, simple centrifugation can be used (SHOLKOVrrz, 1973; EDMtmDS and BATH, 1976; SW^LAN and MURI~V, 1983). For consolidated rocks that do not contain a high water content (non-aquifer formations), leaching appears to be a more suitable method (MANGER and WmtTMANN, 1967; MURky and F~RR~LL, 1972; SCmMDT, 1973).

For the leaching method DEVINE et al. (1973) tried to characterize the cation exchange with clays through the evolution of the diluent concentration with the percentage of days leached. Several parameters of the leaching methodology were also tested for the present studies prior to drilling, in order to define a field process. The full results of the experiments are presented by FaEVSSlNET and DEGV, ANGES (1989).

The mineralogy of clay fraction material tested (St Palais, France) is kaolinite 40%, illite 20% and interstratified illite (40%)--smectite (60%) about 15%.

The parameters tested were: ----shaking time (0.5, 1, 1.5, 2, 3, 6, 9, 12, 18, 24 and

168 h); --shaking method (rotation and ultrasonic); --solid/liquid ratio (1/1, 1/2, 1/5, 1/10 and 1/20); --sample hydration (hydrated and dehydrated samples).

Only one parameter was varied in each experiment which included:

---sample weighing; ---moisture measurement (24 h dehydration at 100°C); ---sample fractioning;

--liquid-solid melting; --shaking; -----centrifugation: 3000 rev/min for 30 min; --filtration: 0.45/~m.

Na, K, Ca, Mg, CI, SO4, NH 4 and NO 3 were analysed in the filtrate obtained at the BRGM.

Elemental chemical concentration of interstitial water is computed as:

Ce = Ca * (V~,+ VO/V I, (2)

where Ce = element interstitial water concentration Ca = concentration analysed in the filtrate Va = added water volume Vi = interstitial water volume (moisture).

General observations from these tests are as follows. (1) All other parameters stated as equivalent, no differ-

ence was noted between the tests using different shaking methods.

(2) Dehydration of the sample does not affect the composition of the calculated interstitial waters with a solid/liquid ratio (S/L) = 1/2 and gives a higher value at S/L = 1/10.

The results are illustrated in Fig. 3 which presents the kinetics of extraction for Ca, Mg, Na, K, CI and SO4 at several S/L ratios (NH4 and NO3 were found close to the detection limit).

Two other general trends that can be inferred from the tests are:

(3) An increase of all the chemical elements is noted with a decreasing S/L ratio, which seems to be the most influent parameter.

(4) Since the concentrations obtained are stronger for all the elements after 7 d than after 24 h, regardless of the S/L ratio, it can be concluded that an increase in shaking time increases the concentration of the leachate.

Details of the above general observations are discussed below.

Influence of S /L ratio. The increase in concentration with decreasing S/L ratio is particularly visible with Na and K. With other elements, it is best seen during the first 10 h of leaching, after which a relative homogenization is noted; the concentrations obtained with different S/L ratios after 24 h shaking are nearly equal for Ca, Mg and SOn. On the other hand, with Na and K, the S/L = 1/20 does not give larger concentrations than S/L = 1/10, in spite the fact it is 0.5 times greater than other S/L values for other elements.

Influence o f time. The increase in concentration with time can be seen during the first 24 h with S/L = 1/10 for Ca, Mg, CI and SO4; it is accompanied by an instability of the leaching process. With S/L = 1/2, after 3 h of shaking, the element concentrations remain relatively constant until 10 h then, at 12 h, a sharp increase is followed by a more gentle increase of concentration with increasing shaking time. For Na and K, after 3 h of shaking, concentrations remain relatively constant.

Beyond the general conclusions, these results are quite puzzling. It is interesting to note that CI and SO 4 behave in a similar way to Ca and Mg, indicating that exchange with interlayer cations cannot be the only effective process operating. The general trend of these results is that the more "aggressive" the leaching process, the more concentrated the leachate. From these experiments, it was concluded that a 4 h leaching with S/L = 1/2 would be the least aggressive process and would prevent leaching due to cation exchange problems. This, therefore, was the method that was applied to the core samples during the drilling of Balazuc-l; a detailed description of the method and the analytical technique is given by AQUILINA et al. (1991b).


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FiG. 3. Kinetics of element extraction during leaching at different solid/liquid ratios (ppm).

Comparison of&aching and WELCOM results

Figure 4 shows a comparison between moisture measured by desiccation of core samples at I00°C and porosities given by neutron logs. The discrepancy can be partially explained by the fact the moisture is a discrete measurement of about 100 g of solid, whereas the physical log integrates a much larger volume of rock. Nevertheless, a factor exists between the measurements that cannot be accounted for by this effect. The physical measurement can be divided into combined water and free water, although the correlation is not improved when using the free water alone.

The data used in Fig. 4 are grouped principally along the Carb line, which represents the linear regression for carbonates. Two other groups, characterized by a better agreement of the data (1:1 slope), fall along the Sand (sandstone) and Clay (claystone) regression lines. The three groups of points indicate that the rock facies have a different behav- iour for each measurement type. The fact that none of the

lines has a 1/1 slope indicates that the two measurements never correlate. Nevertheless, the carbonate line and the claystone line are closer to a 1/1 slope when the on-core moisture is plotted, respectively, against free and combined water, which implies that the neutron log separation between free and combined water does not correlate with the type of water taken into account by the moisture measurement. For this reason, the total water measurement was used for computing the interstitial water concentration by the WELCOM method [P of Eqn (1)].

Another feature of the moisture-physical measurements relationship is that the lower values obtained from moisture suggests water contained in the rocks is lost during drilling and/or during the operations carried out prior to desiccation, in spite of efforts made in the field to process the samples as swiftly as possible once the core was extracted.

To further understand the influence of mineral dissolution during drilling, a comparison was made between the interstitial waters deduced from core leaching (Fig. 5A) and those computed by the WELCOM process (Fig. 5B). Be-


Y water wl % 4

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Fro. 4. Comparison of humidity and porosity measurements. Humidity was determined by core sample dessica- tion at 100°C during 24 h. Sigma water (=combined water + free water) was determined from Schlumberger neutron logs. Square, carbonates; circles, sandstone; stars, claystone. Regression lines are Carb, carbonates (R=0.61); Sand, sandstone (R=0.53) and Clay, claystone (R=0.61).

tween 1220 and 1560 m, the concentrations obtained for all the elements from core leaching are generally very high, ranging from 0.01 to 5.0 mol/l. In this section the Na concentration may have been affected by contamination from the addition of NaOH to the drilling fluids by the drillers, which could account for up to 30% of the Na value. This is also true for Na and C1 below 1620 m depth. From 1220 to 1560 m, especially for Ca and Mg, concentrations measured in the leachate are often close to detection limits, thus giving large uncertainties. Within the evaporitic zone (1560-1620 m), the dissolution of anhydrite would have contributed high concentrations of dissolved Ca and SO4, therefore these values could not be used for computing the interstitial waters. However, the other elements are also highly concentrated within the same zone, with the dominant species being Na, CI and SO4. Comparison of these figures with the WELCOM computations (Fig. 5B) shows no important discrepancy. The same effect of anhydrite dissolution is noted within the evaporitic zone, the values are of the same magnitude, and the tendencies are clearly expressed in both records (this is not apparent on Fig. 5 since the large increase in concentrations within the evaporitic zone mask the trends in the upper section). However, except for Na, all the element concentrations from the WELCOM determinations are higher than those from the core leachings. It is noteworthy that at several depths (1250- 1270 m, 1450-1470 m and 1560-1590 m) the core leaching concentrations become equal to or slightly greater than monitoring concentrations for all elements. The similar curve-trends for the two methods suggests differences between the WELCOM and the core-leaching values are not due to significant mineral dissolution.

Chloride is generally assumed to be an inert element and can be used as an internal reference to indicate possible attack of the rock matrix, which would give an increased element/Cl ratio. The element/CI ratios for both the WEL- COM and the core-leaching computations were determined for the Balazuc-1 well (Fig. 6 shows only the 1220-1570 m section for better readability). Except at 1620-1630 m, the core-leaching ratios are equal to or higher than the WELCOM ratios, indicating that the factor separating the

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FIo. 5. On-core leaching and WELCOM chemical input. (A) On-core leaching (tool/I), (B) WELCOM leaching (mol/l). Interstitial water compositions are reconstructed by reporting chemical elements inputed from the rock to diluant (deionized water for on-core leaching and drilling fluids in the case of WELCOM) to the porous volume. No correction has been made for rock dissolution, which explains the very high values obtained in the evaporitic zone

(black zone).

Na/CI KICI ( 'a / ( ' l Mg/CI SO4/('1 5 , - , . - ";

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FIG. 6. On-core and WELCOM chemical input to chloride ratios. Black dots, on-core leaching; white dots, WELCOM

leaching.


WELCOM values from the on-core values cannot be attributed to an increase in drilling fluid concentration due to mineral dissolution. On the contrary, the on-core values which exceed those of the WELCOM technique indicate a possible water-rock interaction during the on-core leaching, especially with the carbonates.

Another factor that could explain the WELCOM/on-core ratio is a "volume effect" in the WELCOM computation of the interstitial waters. In the computation of chemical input (and then of chemical concentrations) the volume of rock used is the drilled (crushed) rock volume so as the chemical inputed are related to the leached rock volume i.e. volume penetrated by the drilling fluids. The drilled rock volume (Vr) / leached rock volume (Vr') ratio < 1 leads to overestimated concentrations.

The chemical input is attributed to a rock volume which is defined for I m as

V r = (R 2 - R 2) * x/lO, (3)

where Vr = litres per metre cored Rw = well half diameter (in cm) Re = core half diameter (in cm). In the case of Balazuc-1, Rw is 5.0 cm, Re is 3.2 cm and Vr = 4.8 l/m. Measurement of fluoresceine (a tracer in the drilling fluids) in the leachate of the external part of the core showed that drilling muds penetrate at least the outer 1-2 cm ring of the core. This must be also the case in the immediate external ring surrounding the well. The diameter of the well is also generally larger than the tool diameter. Since the well was enlarged prior to physical logging, we do not have the hole diameter measurement. However we can compute the volume that can reasonably be taken into account as the rock volume penetrated by the drilling mud (Re', Rw' and V / ) . If we assume R c' = R e - 1 .5andR w' =Rw + 2 , V r' becomes 12.5 l/m and V / / V r ratio is 2.1. For a value of Rw" = R w + 4, Vr" is 22.5 l/m and Vr"IV r is 4.2. Thus the "volume effect" can compensate for at least a factor 2-5 the difference between the on-core and WELCOM leaching. In the case of such effect, all elements at a same depth should be affected by the same factor.

In order to test this effect, when comparison at the same depth was possible, a CI leaching/C1 monitoring ratio (CIt/CI,~) was computed. The other WELCOM elements were then multiplied by the CIj/CIw ratio, which varies from 0.8 to 12 with a mean value of 6. A comparison of the leaching and monitoring values before and after normalization is presented in Fig. 7A and B. Before normalization, except for several Ca and Mg and a few K and Na points, nearly all values are in the upper part of the diagrams. As was seen with the element/Cl ratio, for the points which are in the lower part of the diagrams, slight dissolution of carbonates (calcite, dolomite), feldspar or illite during the on-core leaching experiments must be invoked. After bal- ancing the concentration by the CIj/CI w ratio, the other elements are better grouped along the 1/1 slope. This result shows that, when corrected for the "volume effect", the interstitial waters determined by WELCOM and by leaching have very close compositions.

The computed "volume effect" is, however, less than the measured CI/CI w ratio. Another effect that could be a source of difference between the two types of experiment, although smaller than the "volume effect", is the "clay influence". During the leaching kinetics experiments, it was noted that a decrease in the S/L ratio leads to an increase in the quantity of elements released in the diluent. Similar results have been observed by MAN6ELSOORF et al. (1969) and Mtmrnv and FERRELL (1972), although only the cation evolution was considered. These authors and D~VINE et al. (1973) interpret this increase as due to the progressive mobilization of an absorbed layer equilibrated (Donnan equilibrium) with the clay particle, this layer is more concentrated than the outer solution. Figure 8 presents the

interstitial concentrations deduced from the different S/L ratio experiments after 24 h shaking: an increase can be seen for all elements. For Ca, Mg, Na, K and SO4, the ratio between the concentration for S/L -- 0.5 and the concentration for S/L = 0.05 varies from 1.8 to 2.3; for CI, this ratio is 2.7. The ion exchange capacity of the clays can hardly explain the increase of all the cations with the same ratio and cannot account for the anion increase. PoNs (1980) and TESSIER (1984) show that several kinds of water can be present within a clay formation. Interaggregate, interparti- cle and interfolia water can be distinguished. Clay has been demonstrated to act as a semi-permeable membrane (KHA~- AKA and BElfRY, 1973; HANSHAW and COPLEN, 1973; KHAR- AKA and SMALLEY, 1976). MANrtEIM (1970) questions the possibility of such phenomena under geological conditions since lateral movement is more likely through clay layers than vertical flow which is required in order to obtain a retention effect. However, when sediments are subjected to burial, the expulsion of water from the innermost pores of clay strata may force the interstitial fluids to pass through the clay films and so cause enrichment of the residual fluid. The semi-permeable property of clays may, in such a case, influence the composition of the interstitial waters in the inner pores. Both explanations (removal of absorbed fluids along the particles and of absorbed fluids within the smaller pores) can account for the enrichment shown by the experiments if the leaching at lower S/L ratios reaches waters more firmly linked to clay particles. In the case of the WELCOM process, complete crushing and the very low S/L ratio may emphasize this effect.

Precision o f the W E L C O M method

The fairly good agreement between on-core leaching and the WELCOM computations indicates a good degree of confidence for the WELCOM method. As the WELCOM process samples the whole drillhole rock volume, the results are more comprehensive and more sensitive than the on- core leaching. The WELCOM computed concentrations may be enriched as a result of the "volume effect" and the "clay influence". In this first field application of the method, the "volume effect", although important, does not seem to vary greatly along the Balazuc-1 well; in addition, it does not affect the interelement relationships,. The evolution of the interstitial waters with depth during drilling may give the more accurate results, especially since the variations are very large (three orders of magnitude).

INTERPRETATION OF RESULTS

The evolut ion of total chemical input acquired by the drilling fluids (Nf) and of es t imated total dissolved solids in the interst i t ial waters (corrected for "vo lume effect") are plot ted against a dep th in Fig. 9. In the anhydri t ic zone es t imated values for Ca, Na and SO 4 have been used, thus the sharp increase of Nfwi th in the su lpha te -bea t ing shale due to anhydr i te dissolut ion is not seen in the TDS curve. Since N f i s

used in the computa t ion of the interst i t ial waters , the two curves show a similar t rend and indicate a clear increase with depth . This p h e n o m e n o n has a l ready been no ted in sed imenta ry basins (GRAF et a l . , 1966; CLAYTON et al . , 1966; DICKEY, 1969; CARPENTER, 1978), where the most general cause is cons idered to be mixing of deep br ines with surface waters (HIT- CrION et al . , 1971; BATH and EDMONDS, 1981; FISHER and BOLES, 1990), a l though physical factors can


55

10-

0,1

0.02 0.02

CI monitoring , 4 i oo ~SO4 monitoring t . / / I i • ++ V

++ -•~• °7 l lO- •+• - /'~ • " • • * /,~ I ! C ~+ / •

" ; ' " : "

. . . . . . . ~ . . . . ,q co~y.,0,02 . ,., S,O4core 0,1 I 10 55 0.02 0,1 I 10 100

05

I0-

0,2

5 Namonito~ing •• * * / / / q

• • • / . ] ,

".".'--'Y - I '7

Z:" 1 ° . . . . . , n a c ? ~ y . O.Ol 0.2 ~ io 65 O.Ol o., ,

+ * K monitori~¢]

• " , ' . • / I : u e •P'o I

+ / / . • ~ ¢ •

~ , * K core , , , , , , H , , I , , H V , ,

100

10-

0.1

0.01 0.01

Ca$..m°nit°ring.++

. ! " /

Ca core ,~" ...... , , ' ' ~ , , , i

0 , 1 I

Mg monitoring /

I-~ . o • • , 0 / / ~

. . 2 , + +++ ; o y . . . . . . . , ........ o.ol . . . . . . . . , . . . . . m g , :pry. ,

I0 I00 0,01 0.1 I •

Fx6.7. (A) On-core leaching vs WELCOM leaching (data are in mol/l, line is 1/1 line).

create increased diffusion towards the lower part of the basin (MAr~GELSDORF et al., 1970). The deep brines originate in evaporitic formations either through the dissolution of minerals (CaSO 4 or NaC1) or through flushing of inner brines (HrrcxtoN et al., 1971; CARPENTER, 1978; MATRAY, 1989; THOMAS et al., 1989).

In the case of the Balazuc-1 well, the top of the studied section is at 1220 m depth and the upper part of the well intersected rocks with low permeability (STExNBER6 et al., 1991), therefore, the presence of surface water is unlikely. On the other hand, influence of evaporitic layers is almost certainly due to the presence of 50 m of anhydrite at 1560 m depth. The very high TDS values are further evidence of this in that at 1220 m, they vary from 30 to 200 g/1 which is in the range of saline fluids. In the lower part of the

section, TDS values reach 500 g/l which is in the order of brines associated with evaporites.

Since the lithologies and sedimentological settings (continental to marine) of the different formations are highly variable, the evolution of the interstitial waters within each type of formation will be examined before analysing whether some tendencies transcend stratigraphic boundaries, thus indicating diffusion and/or circulation.

Hettangian carbonates (1220-1350 m)

The components in the interstitial waters of the Hettangian carbonates show relative abundance in the order Na, Cl, SO 4 >K >> Ca, Mg. The evolution of Ca, Mg, Cl and SO4 in the interstitial waters from


55,

10,

I,

0,1,

0,02 / , . . . . . . . . . . . . . , . . . . cor ,.,

100

10-

I 1 0,1

0,01 0,02 0,1 I

100 5

10-Na mol to r ~

I . /X ' . t • • I ~ • 4

' Na core 4 •

0 , 1 . . . . . . . . ~ o , . ; . . . . . . ~ . . . . . . . .

0,1 I 10 100 I00

so4 m n.oring

y..,. / SO4 core

10-

0,1

W o

Ca core | i i H i n | | l i t | ° l ° I i i i i i i i J I I i i | | w l

0,1 1 10 100

0,01

0.001 0,01

10 55 0,01 0,1 1 10 100

K monitoring . / l -. / , , i • ° / o -

O,l~ * ~ , °°°

O,Ol. J . ": :con

0,001 0,01 0,1 1 5

5 1~. q Mg monitlring . ~

0'1! ° ° J ~ " I

~,0] -~

/ . . . . . . . . , . . . . . . . . , , I

0,001 0,01 0,1 I 5

Fro. 7. (B) On-core leaching vs chloride normalized WELCOM leaching (data are in mol/I, line is 1/1 line). A correction factor (WELCOM Cl/On-core CI) has been applied to other elements in order to correct

concentrations for a difference of rock volume leached.

1220 to 1350 m depth (Fig. 10) show a dependence of derived compositions on lithology. In the upper carbonates, Ca concentrations are very low and negli- gible Mg is acquired by the drilling fluids. Upon penetration of the dolomitic formation, Mg appears in the water composition, and an increase in Ca, Mg, CI and SO4 characterize both the upper and the lower dolomites. The presence of a non-dolomitized zone (1305-1335 m) is clearly reflected by a lower Mg value in the water composition. These observations are also reflected by the Ca/Mg ratio; in the upper and lower dolomites, this ratio reaches values greater than 1, whereas in the preserved zone the ratios are nearer 1/1 equilibrium.

Since the initial interstitial water of the marine limestone (1220-1280 m) was seawater, it is interesting to compare the obtained values to this reference.

Ca, CI and Na concentrations are very close to IAPSO standard: Mg is strongly depleted whereas K and SO4 are enriched by one order of magnitude. Since the limestone has a low permeability and no large water flow can have passed through the rocks, modification of the seawater composition by water- rock interaction is more likely. Also, the most com- mon feature of diagenesis is Mg and SO4 depletion and Ca enrichment (HITCHON et al., 1971; SAYLES and MANnEIU, 1975; LAND and P~ZmNDOWSIa, 1985; KnAr.AKA et al., 1987); K is generally slightly depleted although some enrichments have been noted (MAN- OELSDORF et al., 1969). SO4 enrichments will be discussed below.

The isotopic record shows a shift to the right from the meteoric line (Fig. 2), which could be caused by a slight interaction of the drilling fluids with the car-


300

200.

I00.

8O

:1 20-]

01 0 0 5 I0 15 20 5 10 15 20

100 150

2 0 . . . . . , . . . . , . . . . , . . . . . O / o . . . . 5" . . . . , '0 . . . . , '5 . . . . 30 O, 5 10 15 20 2500, 2O

~o 10002 d

0 . . . . i . . . . i . . . . i . . . . 5 0 0 . . . . i . . . . i . . . . I . . . . 0 5 10 15 20 0 5 10 15 20

Solid/Liquid ratio (1 / S/L)

FIG. 8. Evolution of interstitial waters concentration vs solid/liquid ratio (concentrations in ppm).

bonates. A slight enrichment of deuterium is also visible, but is difficult to quantify precisely due to the addition of mud during the drilling of this formation. The spacing of the isotope measurements does not allow corrections to be made as was done for the chemical record. Mixing with interstitial waters, comparable to seawater in their isotopic composition, will produce a shift towards SMOW (origin). Both effects (carbonate interaction and interstitial solution mixing) could produce the evolution shown by the "Hettangian drilling fluids" in the 6180--~2H graph. From the deuterium increase, it is possible to compute a mixing percentage.

We used the equation:

62Hs = 62Hi * x + 62HSW * (1 - x), (4)

where 62Hs represents the isotopic composition of the mud at the end of drilling the Hettangian carbonates, 62Hi the isotopic composition at the beginning of drilling the carbonates and 62HSW the seawater composition; x and 1 -x are, respectively, the percentage of mud and interstitial water.

Because of the poor precision of the shift determination with respect to the very low quantity of interstitial fluid, it is only possible to estimate that the interstitial water represents at least 2% of the mud volume which varies 8 to 16 m 3. The crushed rock volume is not sufficient to account for this interstitial water volume--a factor of about 8 is required. Thus the isotopic data also show that a rock volume greater

than that extracted from the hole is leached by the drilling fluids, although the volume indicated by the isotopes seems higher than that indicated from the core leaching-monitoring comparison.

Upper Triassic, Part 1 (1350-1420 m)

The Upper Triassic succession shows a large del- taic fan derived from continental sedimentation. Sandstone beds alternate very rapidly with dolomite- rich marly layers reflecting oscillations of sea level; the sandstone and claystone corresponding respectively to the continental and marine extremes. In this context, the interstitial waters may be highly variable, ranging from a relatively meteoric composition within the sandstone to a more marine composition in the marly layers. In addition, the sandstone may also have drained fluids that modified their initial interstitial water composition. Logs of the interstitial water compositions are plotted in Fig. l l A ; the waters are of the Na, CI> SO 4 >> K > Ca, Mg type with very high values (up to 12 mol/l for Na and CI) that are undoubtedly overestimated as discussed for some of the Hettangian values. Except for Na, which tends progressively to higher values, and Ca, which gradually decreases, no trend with depth can be observed. However, compositional variations seen in the logs seem to be related to lithology although the process of smoothing and integration makes logging

Evolution of interstitial waters, Southeast Basin of France

Total Chemical Input Total Dissolved Solids o ~ tooo ~s, oo 0 200 4o0 6oo

.~,,,~,, Nf mol/m cored ~ TDS g/I

113a cemem~ I ~ .

:rO[_,~.~ "F. B. U.' I

° + L _ _ .... doll'mlltlllC

Stdl>l~ltC ~ 'Z7 . . . :

667

FIG. 9. Logs of total chemical input and estimated total dissolved solids (1220-1620 m).

Ca Mg CI SO4 Ca/Mg 0 O.2 0 0.2 0 4 0 4 0 I 2 1230~

+'I I ( F

1 3 3 0 ~ 13~)

FXG. 10. Concentration of interstitial waters vs depth in the Hettangian carbonates (1220-1350) (data are in mol/l). 1220-1280 m, alternating limestone-marl; 1280-1310 m, dolomitized limestone; 1310-1340 m, preserved black marl;

1340-1350 m, dolomitized limestone.

of very thin layers difficult. Dolomitic shales increase in C1, Na and SO 4 (dashed arrows), whereas sandstone is marked rather by an increase of Ca, Mg and K (solid arrows).

Element ratios (Fig. l l B ) show that Ca/Mg = 1 within the dolomitic shales and increases within the sandstone; Na/Cl is also close to 1 within the marl and higher within the sandstone. The Na/Cl ratios are in good agreement with the hypothesis of a seawater origin for the interstitial waters of shaly levels, whilst the Ca/Mg ratios indicate that the waters have become equilibrated with the dolomite crystals contained in the marl. The high Na and Cl values, however, imply a concentration effect. The semi- permeable membrane role of clays retarding cations when a flow of solute is forced through them has already been invoked; however, the clays may also have simply acted as a porous filter, retaining ions when the solution was expulsed through burial of a shale layer. To preserve the Na/C1 ratio, the filter effect must be favoured over the osmosis one. SO4


( 71 Na SO4 Ca Mg K 0 t~ I} 7 0 2 0 0~3 0 0 2 0 0,6

(~) Ca/Mg Ca/SO4 Ca/CI Mg/CI NaJCI O fl 0 .15 0 q

1

14~X) I

(?a/Mg Ca/S04 Ca/C1 Mg/CI Na/CI I) I 0 0,3 II 0~(17 0 0,08 0 I 2

~ sandstone I dolomitic shale Io dolomite

- - - 4 > inllucnce ol sandstone la~ers - - - - --1> influence of marly layers

Fro. 11. Concentration of interstitial waters vs depth in the Upper Triassic (Part !) (1350-1420 m) (data are in tool/I).

II 7 0 1 4 1 ( ) ~ I i J

1 14~5

L 4 ~ , )

/ ~ 5 2

1510-

1 5 q ÷

1561) - -

SO4 ( "a Mg Na K (~ {) ~ ~1 I i) 2O fl O.N

"~ 0 0 V I/ 0.6

~andstorlt I dolomit ic shale ILl dolomite

~> ~ in l luence ~ t re:My levels

FIG. 12. Concentration of interstitial waters vs depth in the Upper Triassic (Part II) (1420-1560 m) (data are in mol/l).

and K are also strongly enriched when compared to seawater. Dissolution of up to 25% of the feldspar observed in the cores (CRos et al., 1986; VINCHON et al., 1992) is the most likely process to explain the K enrichments.

The origin of the interstitial water within the sandstone is more problematic. The Ca/Mg ratio increase is similar to that in the dolomitized zones of the Hettangian and may indicate that similar fluids were drained within the silty layers. However, no support- ing argument is available for this interpretation. The clearest effect would seem to be contamination of the sandstone waters by chemicals from interbedded shale, which prevents them from being clearly characterized.

The isotopic record of the drilling mud is very surprising. A very clear decrease of both 6180 and 62H is seen in Fig. 2. This shift is interpreted as mixing between interstitial water and drilling fluids, and an end-member for the interstitial waters can be computed. Assuming about 2% interstitial water, gives very low values (62H -~ -80 , 6180 ~- -9 .5) for this end member which, however, lies close to the present meteoric line. CLAYTON et al. (1966) found such an end-member in the Alberta and Michigan basins, but invoked very cold waters from the Pleisto-

cene glaciations. Although the influence of meteoric waters in the interstitial waters from the Upper Triassic is clearly established from isotopic trend, its significance is unclear.

The operational replacement of a large part of the drilling fluids from 1405 to 1420 m strongly modified the mud composition, significantly decreasing all the elements except C1 and (to a lesser extent) Na. Thus it was not possible to compute the interstitial waters for this section of the drilling. This is unfortunate since baritic sandstone beds were intersected at this level, and information on the sulphate and/or barium transport would have been of great interest since sulphate enrichment is visible all along the cored section, both in the interstitial waters and in the rocks (anhydrite deposits).

Upper Triassic, Part II (1420--1560 m)

The logs of interstitial water concentrations from 1410 to 1560 m are presented in Fig. 12A. The concentrations are very high, all in the order of 1 mol/l which yields total dissolved solids of between 500 and 1500 g/l. Once again, these values are probably overestimated by a factor 3 or 4. In spite of a


possible "volume effect", the higher Na concentrations (both from WELCOM and on-core leaching) are close to being unreasonable; this had already been suspected in the overlying section and continues in the underlying evaporitic formation. It is difficult to compare leaching results for low-porosity rocks with the composition of waters from an aquifer zone or with leaching results for high-porosity rocks, i.e. porosities of 40% (EDMONDS and BATH, 1976) or 12- 20% (ScnMIDT, 1973). The possibility of ions being strongly bonded to clay particles was considered when comparing WELCOM to core leaching, and this stratigraphic section provides another reason for considering that this kind of pollution effect may substantially increase the concentration of the K and especially Na monovalent cations in the interstitial fluid computation.

The interstitial water compositions at the top of the profile are similar to those of the previous section (Na, Cl > SO 4 >> K > Ca, Mg). They become modified with depth, and the dominant species at 1550 m is Na > C1, SO 4 > Ca > Mg. The most striking feature of logs of Fig. 12A is a strong increase of all the elements along this profile, particularly from 1450 to 1540 m, followed by a slight decrease from 1540 to 1555 m; the sharpest increase is related to a thick, relatively high porosity sandstone section. Helium anomalies, characteristic of fluid circulation (MARaV et al., 1987), appear below 1440 m (AQUILINA et al., 1991a). These features indicate intense circulation within the sandstone beds, and likely expulsion of fluid from the underlying Middle Triassic evaporitic formations.

The element to element ratios (Fig. 12B) disguise the increase of the species affected by this phenom- enon, but do emphasize the relationship between species and lithology as was seen in the Upper Trias- sic (Part I). The interbedded shales at 1430-1435, 1450-1460, 1480, 1495-1500 and 1530-1545 m show a decrease in the Ca/Mg and Ca/SO4 ratios, as in the previous section. However, the Na/CI, Mg/C1 and (to a less extent) Ca/CI ratios increase at these shaly levels, which implies a selective enrichment of cations in the claystone, probably due to a shale membrane effect.

The 61So and 62H isotopic record of the drilling fluids (Fig. 2) in this section of the well shows a shift which is opposite to that in the Upper Triassic Part I section; it globally displaces the points to the upper right part of the diagram. The inflection to the right of the last point reflects the presence of more carbonate (1530-1560 m). This is in good agreement with a mixing of drilling fluids and interstitial waters. The compositional end member of these must be to the upper right of the isotopic space, which may agree with composition of an evaporated seawater (see below). An assumed 3% interstitial fluid would give strong values (about +65 for 62H and +26 for 61SO), which is compatible with evaporation of seawater in arid zones (FoNrES and GONnArCrINI, 1967).

Middle Triassic, evaporitic formation (1560-1610 m)

In this section of the well, all the element concentrations, except C1, show a very strong increase, as can be seen in Fig. 5B. Anhydrite dissolution elimin- ates direct use of the Ca and SO4 input, and so it is impossible to compute values in the same way as for the upper sections of the well. It is, however, possible to compare other elements to what could have been the original interstitial waters of the evaporitic formation, i.e. evaporated seawater. Several laboratory measurements have been published on the evolution of element concentration when evaporating seawater (HERRMANN and KNAKE, 1972; COLLINS, 1975; HAR- VaE et al., 1980). We used the values of AMDOUNI (1990) because they result from analysis of real brines from solar salt works on the East coast of Tunisia. In Fig. 13A, the interstitial compositions are plotted for CI against Mg and K together with seawater evaporation curve. The values plot close to the line, near the beginning of halite precipitation in the Mg--C1 diagram. A factor of 2-3, indicated in the previous sections, would plot the data at the end of the anhydrite control of seawater evaporation (although with a higher Mg concentration). Several points in the K--CI diagram are slightly enriched in K; a diagenetic effect, as recorded in the Hettangian, could explain this enrichment. It seems very likely that the interstitial waters contained in the Middle Triassic beds are connate waters produced by the evaporation of Triassic seawater close to halite precipitation. Since a salt bed, 1.3 m thick, is found at the base of anhydrite formation, the evaporation of the paleosea may be considered to have reached the halitic zone.

The chemical input of Na, Ca and SO4 is extremely high in the evaporitic zone. Although it is reasonable that dissolution of anhydrite occurs and contaminates the drilling fluids for Ca and SO4, the very sharp increase in Na, unsustained by C1, indicates that another mineral may be dissolving within this zone. The well intersection from 1560 to 1610 m is characterized by two anhydritic zones (CaSO4 >90% of the total rock) enclosing an anhydritic shale layer. This zoning is reflected in the diagrams of Fig. 13B which plot the Na-Ca-SO4 relationship. Because of the constantly increasing mud concentration, the upper- most point is in the lower left-hand corner of the diagrams and evolution with depth passes up to the top right-hand corner. The three parts of the evaporitic formation are characterized in all the diagrams by changes in slope: stratigraphically downward, the slopes are 0.5, 1, 0.5 in the SO4-Ca diagram, approxi- mately 1, ? , I in the SO4-Na diagram, and 0.5, ?, 0.5 in the Ca-Na diagram. In the middle part of the formation, the element relationships are dominated by anhydrite dissolution, which is indicated by the Ca/SO4 ration of 1. In the upper and lower parts, on the other hand, anhydrite dissolution is balanced by another effect which, nevertheless, is related to Na, Ca and SO4; as is indicated by the Na/Ca and Na/SO 4


,o K ( M / l ) .........

¢

, . % - < + - - - . +

OA- , +' • ,

0,01- Seawater

CI (M/I) 0.001 . . . . . . .

0 . 5 . . . . [ I 0

<5 M g ( M / l )

O A - ~

0,02 Seawater e l (M/l)

0.5 . . . . I 10

25- ( 'a (mM/I)

/ 211. ~ . , . / 4 , -4, J

15" / ~ ¢

111-. i ~

5 . : - , ' 4 1 , - 4 ,

SO4 (mMll) O + i . . . . i . . . .

I 0 21) 31) 411 I I X )

,)5-Na (mM/I) * * * / ~

tR).

SO4 (mM/I) g 0 . . . . j . . . . ~ . . . .

m 2o 3o 4o 25 Ca (mMll)

2O. 41-

15- •

I 0 - •

5-: ~ • ~ I

Na (mM/I) ( ) . . . . I . . . . I . . . . 1 . . . .

g ( ) 85 9 0 95 1 0 0

Fro. 13. (A) Comparison of concentration of evaporating seawater and interstitial water in the evaporites. Black square, seawater; concentration of evaporating seawater from AraDOUm (1990); white circles, interstitial water from the evaporitic zone (1560--1610 m). (B) Concentration of drilling fluids through the evaporitic formation (1560-1610 m). Plain line indicates 1/1 relation; dashed line indicates 1/2 relation. Evolution with depth is from lower left to upper

right in the diagrams.

ratios which respectively tend to 2 and 1. This could be accounted for by the dissolution of glauberite, Na2Ca(SO4), a mineral commonly encountered within evaporitic formations (BRArrscn, 1971; HAR- VIE et al., 1980). Although it was not detected by X- ray analysis, it was observed at Balazuc, within the fluid inclusions (EDON, 1993). From the chemical input (determined from the increased concentration in the mud tank), expressed as moles per metre cored, it is possible to compute the mass of each mineral necessary to account for the enrichment analysed. Assuming that, in the upper and lower parts of the evaporitic formation, an equal quantity of glauberite and anhydrite is dissolved (which would be sufficient to produce the ratio observed), the average of 4.4 mol Ca/m cored added to the drilling fluids implies dissolution of 300 g of anhydrite and 400 g of glauberite. If a core rock volume (Vr) of 4.8 l/m cored is taken, then the percentage of total rock dissolved would be 2% for anhydrite and 2.8% for glauberite; using an estimated leached rock volume (Vr') of 14.5 1/m cored would decrease these figures to 0.7 and 1% respectively. These percentages indicate that only a very small fraction of the anhydrite is dissolved, and that more than half of the glauberite possibly contained in the rock (< 5%) is dissolved. The equilibrium index of the drilling fluids, computed with an EQ3 equilibrium model (AQUILINA et al., 1993a), shows that they are equilibrated with respect to anhydrite and below saturation for glauberite; thus the saturation state of the drilling fluids allow a balanced rock mass dissolved with anhydrite (which constitutes the majority of the rock) being only very slightly attacked and glauberite (present as a few percent) being highly dissolved.

Although only two isotopic analyses were done within the evaporitic formation (Fig. 2), the indicated trend appears to be parallel to the Upper Triassic (Part If) trend and provides support for the hypothesis of the same interstitial water origin for both sections.

Lower Triassic and Carboniferous (1610-1720 m)

The Lower Triassic sandstone and Carboniferous shale are considered together since their interstitial water concentrations are surprisingly similar but very different from the overlying formations. Because the drilling mud was saturated with NaCI these elements were not analysed in the samples. The Ca, SO4, K and Mg logs (Fig. 14) exhibit very high concentrations for all these elements. K and Mg values in the Lower Triassic and the Carboniferous value are similar, about 2 mol/l for K and 0.5 mol/1 for Mg with increases at the 1625--1630 m and 1669 m fault levels. Again, since the computed Ca and SO4 concentrations are not realistic and the Ca/SO 4 ratio is close to 1, dissolution of anhydrite must be suspected. The CaSO4 rock content in the Lower Triassic is about


SO4 Ca K Mg Ca/SO4 n 20 0 20 o 3 0 I 0.6 1.2 1.8

1620

,., "t >

17(10.

1720

sand~lone / dolomilic shale ~ breccia

~illslone / dolomilic sillslone - - 1 > I'a.llx

FIG. 14. Concentration of interstitial waters vs depth in the Lower Triassic and Carboniferous (1620-1720 m) (data are

in mold).

C! Hettangian limestone ~ ~ e ~ shale

T C a evolution along a Ca=Mg line Mg

F~G. 15. Composition of interstitial water in the Hettangian (data are in mol/1). Marks indicate 10%: CI from 70 to

100%, Ca and Mg from 0 to 30%.

8% (as against about 2% in the Upper Triassic) and about 3% in the Carboniferous shale; this change is reflected in the Ca and SO4 mud content which is higher for Lower Triassic than for the Carboniferous. The Ca/SO 4 ratio is very close to 1 in the Lower Triassic, but shows inflexions associated with the faults; in the Carboniferous, where anhydrite dissolution is probably more limited due to a lower proportion of CaSO 4 rock and a higher Ca and SO4 content in the mud, it is slightly greater than 1. If it is assumed that, within the Carboniferous shale, the entire Ca-SO 4 difference is due to a contribution of interstitial waters and that SO4 is very low compared to Ca, then the calcium concentration is 2.7 mol/1. This value is of the same magnitude as K and Mg in the same zone. The most likely hypothesis, although no indications of the Na and C1 concentrations are available due to NaCI spiking of the drilling fluids, is that the interstitial waters are CaC12, MgCI 2 and KCI type brines. These types of brine are generally encountered in basement aquifers.

Trace elements analysed in the drilling fluids present similar characteristics. Their concentrations are identical within the Lower Triassic and Carboniferous formations and very much higher than in the Upper Triassic formations (the results are still being pro- cessed). Since fluid circulation along the regional faults are indicated by metal deposits in the upper part of the passive margin investigated by the Balazuc-1 well, the most likely interpretation for the Lower Triassic and Upper Carboniferous is that the Uzer fault was the site of intense hydrothermal fluid flow and that the Lower Triassic sandstone and Carboniferous siltstone drained the same fluids. Since no trace of such inflow is found in the upper part of the Balazuc-1 well, the Middle Triassic evaporitic formation must have behaved as an imper- meable barrier, restricting the circulations to the Lower Triassic and Carboniferous.

The isotopic record (Fig. 2) agrees very well with

the above hypothesis. The hydrothermal fluids would have been continentally derived and thus expected to lie upon the (present or past) meteoric line; as indicated by the evolution of isotopic composition in the drilling fluids of this section, they show a trend along the present-day meteoric line.

FLUID PATH

Although a chronology cannot be determined, several circulation paths can be defined. The interstitial waters for the Hettangian carbonates interval are represented in a ternary diagram (Fig. 15). Points from the 1220--1280 m limestone are similar to those of the 1305-1335 m interval (preserved zone) and close to evolved seawater composition as seen pre- viously. This is a good agreement with the fact that the 1305-1335 m interval represents limestones which were not, or only slightly subjected to water circulation. Between 1280 and 1350 (dolomitized zones), the carbonates have been subjected to water flow which resulted in a transformation of calcite to dolomite. Several of the interstitial water compositions plot along a Ca = Mg line. A lowering of Mg is also indicated where the Ca/Mg ratio is greater than 1. The interstitial waters along the Ca = Mg line are equilibrated with dolomite, which probably occurred after fluid circulation and dolomite precipitation. The lowered Mg points may result from a Ca > Mg fluid precipitating dolomite crystals; they could also result from diagenetic conversion of the remaining CaCO 3 to CaMg(CO3)2 (post-fluid dolomitization) which would increase the Ca percentage in the fluid. Both processes are tenable and not mutually exclus- ive.

The Upper Triassic (Part I) seems to have been relatively spared from important fluid flow, but the Upper Triassic (Part II) reflects channelling, within a thickness of about 70 m of highly concentrated fluids.


Ci CI SO4

Triassic ~5~ 0 ~ p CY / P v ' O K

Na Ca Ca Mg

16. Composition of interstitial water in the Upper Triassic (Part II) and the Middle Triassic (evaporites).

The evolution of water concentrations within the lower part (Part II) of the Upper Triassic are illustrated in the ternary diagrams of Fig. 16. The upper- most composition point is close to the composition in the Upper Triassic Part I section; the increase of concentration with depth then displaces the points towards higher concentrations of Ca, SO4 and Na. Influence of claystone levels can also be seen. In the ternary diagrams of Fig. 16, the values for the evaporitic formation are plotted together with those of the Upper Triassic (Part II). As was anticipated from the previous sections, the evolution of the interstitial waters within the sandstone overlying the anhydrite tends to join the poles defined by the evaporite fluids. It is interesting to note that the evolution of the Upper Triassic (Part II) interstitial waters moves towards the poles obtained without correction for the dissolution of sulphate-bearing minerals. Two explanations are possible: (1) the same minerals are also dissolved in the Upper Triassic (Part II) sandstone; and (2) the fluids that circulated within the conglomerate drains were constituted of interstitial waters from the anhydritic formation (as appears to be indicated by the Cl, K and Mg composition), enriched in Ca, Na and SO4 through dissolution of sulphate minerals. The second solution appears more likely since the Ca enrichment is very low when compared to SO4, since no clear relationship can be inferred between Ca, Na and SO4 within the Upper Triassic (Part II), and since the percentage of sulphate rock in the Upper Triassic (Part II) is similar to that in the Upper Triassic (Part I) and does not increase with depth. This feature is important for understanding the metaUogenesis of the sulphate deposits; it shows that fluids highly enriched in Na, Ca and SO4 may be produced and circulate in a sedimentary basin that contains evaporite beds. It should also be noted that the important hydrothermal circulation activity of Lower Triassic and Carboniferous is strictly restricted to that part of the well.

It seems surprising that, at Balazuc-1, very few effects can be attributed to vertical diffusion. Each formation has its own particular signature which indicates that, since lateral flow has taken place within the Hettangian dolomite and Triassic sandstone, the formations were rapidly cemented and then remained relatively unchanged. The importance of specific (and sometimes not very thick) levels in fluid evolution is underscored by these data.

Sulphate enrichment is strongly indicated by the data, as shown by the SO4-Ca-Na ternary diagrams for all the formations (Fig. 17). From the Hettanglan to the Upper Triassic (Part I), this enrichment is expressed by an evolution of the points along the same axis towards the SO4 pole. Within the evaporites, several points lie upon the Ca=SO4 line whereas others are close to the CaNa2(SO4) 2 pole, but the increase towards the SO4 pole can nevertheless be seen since many points have a higher sulphate percentage. Within the Upper Triassic (Part II), the evolution towards the SO 4 pole is also clearly defined.

The origin and the place of the sulphate-enriched fluids within a global fluid circulation pattern is difficult to define. Since sulphate enrichment is recorded throughout the cored section of the well, it can be assumed that a late migration affected the whole column. On the other hand, the lateral restric- tion of earlier fluid flow and the low porosities do not support this hypothesis. Could, therefore, the enrichment be due to a slow diffusion of sulphate from the rich reservoir constituted by the anhydritic formation or could it even be a relict of primary diagenesis ?

CONCLUSION

Although the reconstruction of interstitial water compositions from the chemical monitoring of drilling fluids is not easy, this approach has been quite

Evolution of interstitial waters, Southeast Basin of France

S 0 4 SO4

673

Hettangian limestone

A~Hettan~,ian dolomite

O

Upper Triassic (Part

Ca Na / " X Ca N a

evolution along a~ CaSO4 line / ~ ¢'

Upper Triassic (Part I!) ~evolution with depth

evaporated seawater

FIG.

Ca Na " ~

17. Evolution of SO4--Ca-Na in interstitial waters from the cored section of Balazuc-1 well (1220--1620 m).

successful along the cored part of the Balazuc-1 well. Uncertainties are inherent, however, since it is difficult to control the rock volume leached by the drilling fluids; the present data indicate that this volume is probably larger than the hole diameter. Other problems arise from interaction between drilling fluids and clay aggregates; cations absorbed on the clay particles are probably squeezed out of the interparti- de pores and thus increase concentrations within the drilling fluids---this is indicated in particular with Na. Although comparison of concentrations with other basins, or with known processes such as evaporation of seawater, is thus difficult, comparison with on-core leaching results for the Balazuc-1 well provided a high confidence level for the data and enabled simple hypotheses to be tested. Though the Balazuc-1 computations are the first practical use of the WELCOM method to determine interstitial waters compositions, they show interesting results. Further experi- ence in Well Chemical On-line Monitoring of drilling fluids will allow better control and calibration that in turn will lead to greater precision. Since the data are affected by the same error, internal comparisons are probably more fruitful for interpretation.

Within the Balazuc-1 well, the major results determined for each stratigraphic section of the cored column are:

(1) in the Hettangian limestone, the interstitial waters are modified connate seawater;

(2) the Hettangian dolomite [and probably part of the Upper Triassic (Part I) sandstone] have been subjected to fluid circulation related to the dolomiti-

zation of earlier limestone; (3) the Upper Triassic (Part I) sandstone shows a

contrasted pattern with the interstitial waters of the shale layers indicating a marine origin whereas the sandstone beds contain meteoritic waters contami- nated by ions expelled from the intercalated clay;

(4) the Upper Triassic (Part II) sandstone would appear to have drained brines originating from the underlying evaporitic formation; these fluids have dissolved CaSO 4 and CaNa2(SO4)2 minerals;

(5) the interstitial fluid concentrations from the Middle Triassic evaporitic formation, although difficult to determine due to dissolution of anhydrite and probably glauberite, are estimated to be close to that of evaporated seawater at (or close to) the state of halite precipitation;

(6) the Lower Triassic sandstone and Upper Car- boniferous siltstone appear to have drained intense hydrothermal circulations of, probably (Ca or Mg)C12 and KCI brines.

These results are in good agreement with data from mineralogic, petrographic, rock-chemistry, fluid- inclusion, organic-matter and isotopic studies carried out within the fourth theme of the GPF Programme (DRoMAaT et al. , 1992; VINCrtON et al. , 1992; PAGEL, 1992; DmNAR et al., 1992). They are of major importance since they help determine the location of movement of fluids along the passive margin of Southeast Basin of France. They indicate the origin of the circulations and clearly distinguish zones of basinal and basement-derived fluid flow. They can also provide an indication of the processes that may have

AG 9°6°E


opera ted dur ing the hydro the rmal activity and metal- logenesis of ore deposits such as the LargentiEre district of France.

Acknowledgements--The authors wish to thank 1. Fournier-Ruet, F. Vidal, Ph Degranges, S. Polizzi, D. Breeze, D. Defoix, J.L. CEcile and T. Bariac team for their considerable contribution through field sampling and analytical work. They would also like to thank P. Skipwith for text editing. Particular thanks are due to J. Dudley, A. Pekdeger and C. Knutson for very useful reviews. Financial support for this research was provided by INSU-CNRS, french MinistEre de l'Education, MinistEre de la Recherche and BRGM.

All the data basis (chemical analyses of drilling fluids, concentration of interstitial waters deduced from model and concentration of interstitial waters deduced from core leaching) representing 10 pages are available upon request to the authors.

Editorial handling: Mike Edmunds.

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