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Strontium isotope evidence on the history of oilfield brines, Mediterranean Coastal Plain, Israel
00167037/83/04068709503.00/0
Strontium isotope evidence on the history of oilfield brines, Mediterranean Coastal Plain, Israel A. STARINSKY, M. BIELSKI, and B. IXZAR Department of Geology, The Hebrew University, Jerusalem, Israel
and G. STEINITZ and M. FLAAB The Geological Survey of Israel, 30 Malchei Israel St.. Jerusalem (Received April 23, 1982; accepled in revised form December 23, 1982)
Abstract-The isotopic composition of Sr in oil field brines from the Mediterranean Coastal Plain was determined in 18 drillholes ( 13 of which are oil producing wells). The brines are characterized by salinities ranging from 35 to 93 g/l (TDS), Sr from 28 to 350 mg/l, Sr/Ca molar ratios from 0.011 lo 0.053 and “Sr/% ratios from 0.7075 to 0.7090. E & A = 0.708 1 2 0.0004 (2~). The brines are classified into two groups: (a) Mavqi’im groug-brines with relatively high s’Sr/%r ratios (0.7087 and 0.7090), sampled from elastics, dolomites and anhydrites of Upper Miocene age. (b) Helctz group-brines with relatively low “‘Sr/“%r ratios (0.7075 to 0.7081). sampled from sandstones and dolomites of L.ower Cretaceous age. Equations were derived to show the relations hetween “Sr/%r ratio of the brines and the processes through which they evolved (dolomitization and dissolution-repmcipitation). It is suggested that both groups of brines originated from Mediterranean evaporated seawater during the Messinian desiccation. The strontium isotope composition of the seawater is r&cted in that of both groups of brines, the Mavqi’im group containing the original 87Sr/%r ratio (-0.7088). The Heletz group evolved later on, through exchange reactions of those primary brines with a carbonate sequence of Cretaceous age and consequently new “Sr/%r ratios (-0.7078) could have been developed. The effect of the dissoiutionreprecipitation reactions on the carbonate rocks is estimated to be very slight, only around 1%(by weight). INTRODUCI’ION
ALTHOUGH THE application of strontium as a geochemical tracer for processes taking place between subsurface saline water and sedimentary rocks is well known (KINSMAN, 1969; CARPENTER and MILLER, 1969; VEIZER and DEMOVIC, 1974; SASS and STARINSKY, 1979: and many others) very fnv studies concerning the Sr isotopic composition have been published so far. Sr isotopic studies of subsurface oil field brines from oil producing wells from Kansas and Colorado (CHAUDHURI, 1978) and Ohio (SUNWALL and FWSHKAR, 1979) show relatively high 87Sr/86Sr ratios (0.7113 to 0.7341 and 0.7092 to 0.7143, respectively) compared to the present or paleo-seawater values. The high ratios are attributed to Sr isotopic exchange reactions between the brines and the surrounding rocks. The hot brines of the Red Sea, which form a part of the seawater body, show relatively low values of 87Sr/“Sr ratio (around 0.708) compared to that of the present day Red Sea (FAURE and JONES, 1969). The ratios are attributed to volcanic activity. HAWKESWORTHand ELDERFIELD(1978) have studied the relationship between Sr content and Sr isotopic composition of interstitial seawater along two sections from deep sea cores (DSDP, sites 245 and 336). They have found an enrichment of Sr with depth and explained it by the alteration of volcanic material as well as by the introduction of Sr derived from calcium carbonate. 687
The waters studied are oil field brines from the southern Coastal Plain of Israel (Fig. la). They were sampled from two Merent parts of the sequence (Fig. lb): (a) Mavqi’im Fm. (Neogene), composed of clastics, anhydrite and dolomite, representing the phase of the Mediterranean Sea desiccation during the Late Miocene (Messinian) about 6 million years ago (Hsu, 1972). The unit is enclosed by a thick sequence (hundreds of meters) of impermeable clays and marls of the Saqiye group. (b) Heletz Fm. (Lower Cretaceous) composed mostly of sandstones and dolomitic lenses. These are the main oil reservoir rocks of the southern Coastal Plain in Israel. The sandstones are cemented by calcite or dolomite (SHENHAV, 1971). The Heletz Fm., which attains a thickness oi up to 300 m, was deposited in Valanginian-Barremian times. A single sample, from the Upper Jurassic carbonates is also attributed to this group. The major chemistry and H and 0 isotopes of the water were studied earlier by BENTOR (1969), STARINSKY (1974) and FLEISCHERet al. (1977). The studied waters are Cachloride brines with Ca/(HC03 + S04) > 1. Their fairly high Ca/Mg ratios are explained by dolomitization processes in which they were involved (BENTOR, 1969; STARINSKY, 1974). Based on a quantitative treatment of Sr/Ca ratio variations in the brines, and assuming certain values for the Sr content of aragonitic and calcitic sediments and Sr/Ca ratios for lagoons and sabkhas, SASS and STARINSKY (1979) suggested that the brines were
ial
c,
%rPSr 0 707
1
Veoqene
0709
a.704
I
:
UPPW
Miocene i bleoqene
I
Upper
Cretoceeus
Ceoomanton - Turonian
brines
m
Juross~c
Anhydrite
FIG. i. Location map and a generaiized columnar section of the studied area.
filtered through 0.45 pm “Milhpore” filter and stored in a new set of polyethyiene bottles for chemical and isotopic analyses. Chemical components were determined by atomic absorption spectroscopy (Na, K. Mg, Ca and Sr), potentiometrienhy (Cl and HCOr) and gravimetricaily (SO,). Rb was determined by an AAS graphite furnace system. 87Sr/%r ratios were determined on a Varian CH-5 mass spectrometer (21.4 cm, 90” magnet sector) equipped with a Gary 401 vibrating reed electrometer. Digital readout of the results was accomplished via a modified DVM. The ratios were normahzed to “%/@Sr = 8.3752. A value of 0.708 I + 0.0004 (28) for Eimer and Amend SrCO, standard was obtained. Sr was extracted by conventional ion exchange techniques using resin Dowex SOW-8X, 200-400 mesh. Chemical blanks were found to be less than 2 ng( sample.
formed by do~omi~tio~ of aragonitic sediments and to a lesser amount were a&c&xi by substiace processes like dissolution-reprecipitation. The present study is an investigation of the possibility of using the Sr isotopic composition of oil field brines as a genetic indicator for (1) kinds of reactions in which these brines were involved during their evolution, (2) a quantitative estimate of reactions taking place in the subsurface between the brines and the surrounding rocks. METHODS 13 samples were collected fmm the weh head of producing welts (Table 1). The sampies were mixed with oil and a gravitative separation has been carried out in the tield. 5 samples horn dry holes were co&acted during drilling op erations. All samples were stored in I liter polyethylene bottles. Before chemical analyses were carried out, the water was
701 710 300 172 169 SI 703 301 706 726 219 175 lrn
Hidei-2 Hden.3 Halsec.9 ti&a.lo H&lP2%4 tkiea-27 Helecr.31 Kakhw-I KaAr.3 Kddur~ Kokhw7 Kokhw.15 had.1
” L ” ” ” “ ‘a ‘I (’ ‘. ” ‘.
THE
MODEL
Ca-chloride bnnes are believed to be formed mostly from ancient seawater through several pro-
”
1521.23 1511.17 1523.24 154146 1575 i49l.V 1680.92 1561.64 164754 1493”SlO I.51244 186187
UC. Sit. sn. dol. wt. sat. ut. st.;dol.
--.-
l+dipmduc&*stl;-dwdriUhde “Ccxq. = C~omun~a; hf. - limeme; dd. - dolovnte. ut. = anhydme. PPZ = tmdsfo~e
-
Sr isotopes in oilfield brines
w
content-and hence their (Sr/Ca)R--a brine which was evolved through dolomitization processes may show different concentrations of Sr depending on the actually replaced mineral. The dolomitization reaction progmss (CaJCaJ in natural cases is unknown and practically, Eqn, (1) can be solved for the limiting case in which (CaJCarJ - co. In this case
;B
SURFACE
689
j: :7 ‘Z 1
Sr cau=iqjx
(1
1 LAGOON1
2
Sr ( caR)
(2)
In order to estimate quantitatively the reaction progmss, a new independent parameter concerning the brine and the replaced rock is introduced herein. Rearrangement of Eqn. ( 1) yields: TYPICAL
OIL FIELD
BRINE
Sr
= _L.(?&(l
%
1 + x,9
(1
SUBSURFACE
TYPICAL
OIL
FIELD
_ (L%-o+g]
BRINE
FIG. 2. Schematic evolution of oil field brines from seawater.
cesses (Fig. 2). The most important are surface evap oration, dolomitization, SO, reduction, and membrane filtration or mixing (WHITE, 1965; STARINSKY, 1974; COLLINS, 1975; and others). Once a typical brine has been formed, it may interact with the surrounding rocks during its migration. Usually these reactions of dissolution and reprecipitation fall under the general name of exchange reactions. While Sr concentration may change in each of the above mentioned processes, Sr isotopic composition may vary only in two of them, viz. dolomitization and exchange. Dolomitization SASSand STARINSKY ( 1979) presented a model showing that Sr/Ca ratio in Ca-chloride brines varies during the process of dolomitixation according to the equation:
As the instantaneous (Sr/Cah in the brine equals the algebraic sum of (Sr/Ca) ratio contributed by the rock-the first term of Eqn. (3)-plus that of the initial solution-the second term of Eqn. (3)-the fractions of both contributions are: fraction contributed by the rock
1
fraction contributed by the initial solution
1
Since no fractionation of “‘Sr/Sr exists during the process, the “Sr/%r ratio of the brines vaiies as a weighted average of the contributions of both the initial solution and the rock (Eqn. 5).
Sr
(EL1=i&&d, +(43,
+z fraction contributed ()(“hsr L,’ by the initial solution )
(5)
By rearrangement, where X&-distribution coefficient of Sr in dolomite; (Sr/ Ca), and (Sr/Ca)G-instantaneous and initial ratios of the brines, respectively; (Sr/Ca)x-the ratio in the replaced rock: (CaL/Cati)_the ratio between the instantaneous and the initial Ca. This last quantity expresses the reaction progress. As calcite and aragonite differ significantly in their Sr __---__-
[Sr) Fr
2(Z).(3.(1
- $)“+*“’
+ (1 + G&+j&ra)L($)+‘+~E’ (7)
L= 2(E),
+ (-2(g),
Equation (7) shows that at the beginning of the reaction, when CaL/CaL, = 1, (2)
By introducing the value of (Sr/Ca)t from Eqn. (I). Eqn. (6) becomes
= (z$),,
+ (I + xe@))($)“+*“’ while in more advanced stage of the reaction, when CaL/ CaL, - ffi. (2)
= (2).
4. Stannsky rt 61
690 (a) DOLOMlTlZATlON
OF
(b)DOLOMlTlZATlON
ARAGONITE
OF CALCITE
‘Sr/Ca)$,
(Sr /Ca)L, 0709 i
0707. ysr/
8*Sr)R
- - -
--
'0Lii-m co, 1
9706.
07od
0.01
0.02
003
004
37031
003
OQI
a02
003
0.04
00s
(Sr / co JL
(Sf/Ca)L
FIG. 3. The variation of “‘Sr/?3r and Sr/Ca ratios of the brmes during dolomIt~zation. The direction of the process is shown by arrows. X,9 = 0.025: (a) for aragonite (Sr/Ca)s = lo-‘; fb) for calcite (Sr/Ca)R = 5 x IO-’
The variation of (a7Sr/86Sr)~ as a function of (Sri Ca),. and reaction progress (CaL/Cati) is demonstrated in Fig. 3. The curves were constructed for hypothetical cases by choosing the following parameters: (87Sr/*6Sr)R - 0.7065, chosen arbitrarily. X,9 = 0.025, after KATZ and MATTHEWS (1977) from experimental data at 252-295°C; (B7Sr/86Sr)~, = 0.704 and 0.709; (Sr/Ca)Li = 0.01, 0.03 and 0.05. In case (a) (Sr/Ca)a = 1O-’ for replaced aragonite and in case (b) (Sr/Ca)R = 5 X lo-* for repiaced calcite. All ratios are expressed in molar units. Figure 3 and Eqn. (7) show that no matter what were the initial values of (Sr/Ca)Li and (“Sr/“%r)Li, and what was the kind of mineral (calcite or aragon&e), the final values of (*7Sr/~r)ti are the same, depending onIy on the “Sr/@‘Sr ratios of the repfaced minerals (Xe is very small and can be neglected).
+$
(8) ($J,il- exP(-A~~.~)j
where & Gas/Cal
distribution coetlicient of strontium in calcite. the ratio between the amount of Ca which was dissolved from the rock to its amount in the sofurion;i.e.the progress reaction.
L;ke In the case of do~omltl~tion Eqn. (8) can be practically used in natural cases. only when (Q&al) is large enough and then, Eqn. (8) becomes
191 By analogy to the case of dolomiti~tio~, the ~ntr~u~lng of fX’Sr/saSr)L and (“‘Sr/e”Sr)R, into Eqn. (8) yields:
Dissolution-reprecipitation According to SASS and STARINSKY( 1979) the fSr/Ca)L as follows:
varies during ~~s~lii~tion
by substitutmg the value of (SriCa), from Eqn. (8) mto Eqn. ( IO) it becomes:
At the beginnmg of the exchange reaction, CadCaL = 0,
Eqn. ( I 1) becomes
-_.. Eqn.
_ _.. _~
__ ____II___.__
C1I) becomes:
(Eglj,. =(igi, whileataveryadvan&~oftheizaction.Ca&Ca~-~3
Dunng exchange reactions. the final value
of ("'Sr/"SrfL
Sr isotopes in oilheld brines
691
(01 ARAGONITE
(b)
/
CALCITE (SrKo),,
(Sr/Co)L, I
0708-
0707-
____.(“Sr/“SrfR
____ 0706
____(6’Sr/86Sr)L,
____
0704
t
FIG. 4. The variation of @‘Sr/%r and Sr/Ca ratios of the brines during di~lution-~p~ipi~tion ’ - 0.058; (a) for aragonite (Sr/C& = IO-*; reactions (exchange). Arrows show direction of progress. Xsr (b) for &cite (Sr,Qn = 5 X IO-‘.
in the brine is controlled exclusively by the (?Sr/%r)R ratio of the reacting minerals (Fig. 4).
total dissolved solids of the samples. The 87Sr/*6Sr ratio of the brines (Fig. 5) shows two distinct groups: (a) brines with high 8’Sr/86Sr ratios, 0.7087 and 0.7090, and (b) brines with low *‘Sr/*%r ratios, from 0.7075 to 0.708 1. The waters with the higher values were sampled from the Neogene Mavqi’im Fm. and those with the lower ones from the Lower Cretaceous Heletz Fm. The average *‘Sr./%k ratio of the Mavqi’im brines is around 0.7088 and that of the Heletz brines is 0.7078 + 0.0002( 1a). Since our anaIyticaI precision is around +0.0002( 1CT) the scatter around the average is referred to analytical rather than geochemical reasons. The Sr/Ca ratios of the waters from both groups fall in the same range, from 0.01 to 0.05. This relatively wide range will be discussed later. 13 samples of the Heletz group were pumped out from oil producing wells (Table l), i.e. from a water
RESULTS The major chemical components of the studied samples are listed in Table 2. The brines have a typical Ca-chioridic composition with salinity (TDS) ranging up to ca. 90 g,/l. Whereas Na/Cl ratio of the brines is simiiar to that of seawater (O&3), the Caf Mg ratio shows considerably higher vaIues, of 0.9 to 2.4. (0.2 for seawater). The content and isotopic composition of Sr are given in Table 3. The Sr content (tens to hundreds mg/l) is significantly higher compared to evaporated seawater with the same salinity. The G/Cl ratio ranges from 0.6 X 10e3 to 2.6 X 10c3 (0.2 X 10V3for seawater). No correlation exists between Sr and the
48s 734 203 245 302 701 710 300 172 169 51 703 301 706 726 219 175 170 *nanttr I-)noi
tidod.2 A3hdod.S Au.1 Beer Sbeba,l Ehur,l Hekcl-2 Hektr.3 Heku.9 Hcku-10 Heku-2SA Hdeu-27 Hdetl.31 K&h%+I Kddw3 KOkhlVd Kokhw.7 Kdrtuv-IS Sad-1
I
dncrrmned
T&k
2.Chemical
29.7 27.0 16.9 16.0
0.27 0.86 0.18 0.24 0.47 0.59 035 0.52 037 038 0.29 0.70 037 0.48 0.25 033 0.4
18.8
22.0 22.1 18.8 223 26.1 19.9 20.0 22.5 102 12.4 296 23.1 24.2 10,s
0.73 1.0 0.5 0.56 0.6 ob4 062 0,s 0.6 0.7 OS1 0.5 0.83 03 0.44 1.1 0.6 0.5 13
data (m&r
2.6 7.2 1.4 1.7 4 1.7 1.4 13 1.5 2.1 13 1a 3.0 05 13 3.L 1.9 2.1 0.4
1
ekments)
53.8 56.8 303 2.2 26.8 33.7 0.24 38.9 0.02 32.8 032 38.8 45.6 36.1
064 0.1 0.18 0.14 0.17 0.17 0.13
88 93 $1 49 55 63 54 64 7s 58 58 71 28 38 91 67 67 3s
23 4.4 1.6 I .8 I .4 1.6 13 1.5 6 1.8 1.5 1.2 2.2 09 1.8 i .8 1.8 2.4 0.2
I
0.85 0.73 0.87 09.7 0.86 0.87 0.89 0.88 0.87 0.85 0.64 0.78 0.92 0.83 0.81 0.87 0.90 0.86
485
uhdod.2 .tidcd.j
‘2
203 245 302 701 710
loo 172 ih9 51
703 301 706 ‘26
body in contact with oil. Only 3 samples were pumped out from dry holes. In spite of the small number of samples from dry hoies it is seen that no correlation exists between the presence of oil and the strontium isotopic composition of the brine. Rb was determined in 11 samples (Table 3) and falls in the range of cff. 100-400 fig,& No correlation exists between the Rb content and the 87Sr/86Srratios. It is therefore obvious that the minor amounts of “St formed by the radioactive decay of *‘Rb can be neglected. DISCUSSION
As both groups of brines-the high 87Sr/a6Sr ratio Mavqi’im group and the low “SrlssSr ratio Heletz group-have the same chemical composition and salinity and were sampled from the same area it is believed that both groups originated from the same ancient sea.
Mavqi ‘im brines On the basis of geologicai and geochemical data. as well as on hydrological considerations, STAR~NSKY (1974) suggested that the brines (including both groups) originated from Neogene seawater and entrapped during the Messinian desiccation ofthe Mediterranean Sea. In our case, the studied brines, like most of the subsurface saline waters in the crust, differ significantly in saiinity and chemical ratios from present-day seawater. SASS and STARINSKY ( 1979) have suggested that these chemical differences result, at least partially, from dolomitization reactions in which the original Neogene seawaters were involved. The replaced CaC03 minerat was aragonite rather than calcite (op. cit., p. 892-3). If this is the case, Sr contributed to the brines should have a Neogene sea isotopic composition (--0.7088), because when (87Sr/ ‘%r)ti = (S7Sr/86Sr)R= 0.7088, the final (87Sr/“Sr)L will be the same. Heletz brines
“sr / “Sr x sw
0.709 orsroga
_-
-
-
1
0
A
I
0
roei_
nuerogc
1
- -
A
:A*:..
-‘-
4
A
AA A
0 7071 001
002
0.03
004
305
0.06
Sr J Co !molari
FIG. 5 y’Srf”6Sr and SrjCa ratios of the oil field brines from the southern Coastal Plain, Israel. sw-seawater; circIes--Mavqi’im brines: triangles-Heletz brines.
As the Heletz brines are a part of a iarger water body. which includes the Mavqi’im brines also, and provided that both groups have a common source, the difference in their strontium isotopic composition should be explained. Moreover. the high Ca/Mg ratios of both groups indicate their evoiution through dolomitization, and thus the source of the CaCO? minerals. which become dolomitized by the original brines-belonging now to the Heletz group-should be shown. /uj Dolomitizutlon of Lower Cretaceous curbonatr rocks. Based on the suggested model. Eqn. (7) has been solved to show the variation of f”Sr/%),_ VS. (Sr/Ca)L during dolomitization of Lower Cretaceous carbonates (Fig. 6a). The parameters used are the following: (Sr/CajR = 9.35 y 10e4. based on the average concentrations of Sr and Ca in carbonate rocks ~TUREKLAN and WEDEPOHL, 196 I1 was chosen herein to represent the wide range of concent~tions existing
Sr isotopes in oilfield brines in the literature (VEIZER,1978). (*‘Sr/*‘%r),= 0.7072 and (r’Sr/%r)Li = 0.7088 seem to be the best values (Fig. 1c) for the Lower Cretaceous marine carbonates (supposed to participate in the process) and the Up per Miocene seawaters, respectively. The values are obtained from the curve showing the variation of strontium isotopic composition during the relevant times (PETERMAN etal., 1970; DASCH and BISCAY, 197 1; VEIZERand COMPSTON,1974 and STAR~NSKY et al., 1980), with slight corrections for E & A standard. (Sr/Cah = 0.04 (the highest ratio found in the studied brines), 0.06,0.08 and 0.10. A: = 0.025 after KATZ and MATTHEWS (1977). Figure 6a shows that the relations existing in the brines, between *‘Sr/%r ratio and SrfCa ratio cannot be explained by an epigenetic dolomitization process of Lower Cretaceous carbonates. By solving Eqn. (2) one can find that the final (Sr/Ca)u ratio must be about 1.9 x 10m3, about one order of magnitude higher. Moreover, a group of brines having the (Sri Ca), ratios of the Heletz brines should have-if evolved through dolomitization of Lower Cretaceous carbonates-*‘Sr/%r ratios above 0.7084 rather than the values of 0.7075 to 0.7081 of the actual Heletz brines. It seems therefore that epigenetic dolomitization of Lower Cretaceous carbonate rocks could not be primarily responsible for the composition of the actual Heletz brines.
(b) Dolomitization of Neogene sediments. SASS and STARINSKY(1979) have suggested that the Heletz brines (their group C, op. cit., p. 890) were formed through dolomitization of aragonitic sediments in a Neogene lagoon, migrating later into the surrounding permeable Cretaceous carbonates and sandstones
(a) DOLOMITIZATION
693
where they further interacted by dissolution-reprecipitation reactions. This process erased the (Sr/Cah and (*7Sr/86Sr)Lratios inherited from dolomitization and new ratios were acquired through the exchange reactions. If this is the case, and the Heletz brines were evolved through dolomitization of marine Neo gene CaC03 minerals, the different *‘Sr/%r ratios of both groups remain still unexplained. (c) Exchange reactions with Cretaceotu rocks. To show the variation of (87Sr/86srh vs. (Sr/Cah during dissolution-reprecipitation reaction between the brines and the Lower Cretaceous rocks, Eqn. (11) has been solved, using the following parameters: A& = 0.058 (KATZ et al., 1972) distribution coefficient of Sr in the precipitating calcite. (Sr/Ca)R, (*‘Sr/%r)R, (*‘Sr/ W&hi as those previously chosen for the dolomitization processes. The results and calculated curves obtained are plotted on Fig. 6b. It c8n be seen that the field of points of the Heletz water group is practically crossed by the calculated set of curves. The “Sr/%r ratio of the Heletz brines can be mgarded as fairly constant, having an average of 0.7078 A 0.0002( 1a) (the ~0.0002( 1u) analytical error may affect the actual results). On the other hand, the wide distribution range of (Sr/CaL from ca. 0.01 to 0.05, cannot result from the analytical procedure, and can be explained on geochemical parameters as follows: (Sr/Ca)R-the ratio depends on (1) relative amounts of the various rocks interacting with the brines; (2) amount of Sr and Ca in each kind of rock, (3) solubilities of Sr and Ca bearing minerals participating in the dissolution process. of these, parameters (1) and (3) are fairly known whereas no relevant data for parameter (2) in the studied area, are available.
(b)
DISSOLUTION
- REPREClPlTATlON
(Sr /Ca)L,
0.707
’
I
1
0.02
004
I
0.06
I
0.06
(Sr /COIL
I
0.10
0.70T 1.l-L (Sr/Co)L
FIG. 6. The variation of “Sr/%r and Sr/Ca ratios during dolomitization and exchange reactionswith Lower Cretaceous marine carbonates.Triangles-Heletz brines. For the construction of the curves, the followingparameters were used: (*‘Sr/%r)Li = 0.7088; (*‘Sr/%r)R = 0.7072; (Sr/Ca)U = 0.04; 0.06; 0.08; 0.10; (Sr/Ca)R = 0.35 X lo-"(a) for dolomitization-A e = 0.025 (b) for dissolution-reprecipitationA:, = 0.058.
%/‘“Sr 0709
I (%/%), ,
r
I
0
708 i ._i
0707~
__L___1L_____. __
-!
001
002
003
004
j (67Sri’ %jR 0 7072
005
sr/ca (molarI FIG. 7. Calculated curves showing the behavior of ?‘Sr/ “bSr)L YS. (Z&a),_ during exchange reactions assuming a single value (0.07) for (Sr/Ca)L, and varying values for (Sr/ Ca)R. The curve in the middle was calculated using an average vaiue of (Sr/Ca)R = 9.35 X lo-‘. The dashed and dotted lines Iimit ranges (of -+50%and ~6590, respectively) around the (Sr/CafR average: triangles-Heietz brines: for other parameters see Fig. 6b.
The sequence of rocks interacting with the brines is composed mainly of limestones, dolostones, marls and chalks (Fig. 1b). The Sr content of such a rock complex varies within a wide range of concentrations, between several tens and few thousands ppm (VEIZER. 1978) and as a result, a wide range of Sr/Ca ratios is dictated. Figure 7 shows that assuming a varying range of values for fSr/Ca)a, on both sides of the average (9.35 x 10W4,after TVREKIAN and WEDEPOHL, 1961) I? samples (75%) and 15 sampies (90%) fail within the range of 250% and 65%. respectively. fSr/Ca)~~--brines which were formed through dolomitization may acquire during the process different
lSr/CabL vaiues, For example. the two sampies from the Mavqi’im group (which are considered to be formed mainly by dolomitization) have Sr/Ca ratios of 0.019 and 0.053. These (Sr/Ca)‘ values become later the initial values for brines starting the reactions of dissolution-reprecipitation. Using the suggested model and parameters (Fig. 6b) it can be seen that the effect of different tSr/CaL values on the ISr: Call--on the abscissa-is conside~bly large at the t&t stages of the reaction progress while the “?Sr/ ‘“Sr-on the ordinate-is affected significantly at very progressive stages only. For the relevant range c)f(X7Sr/X”Srl,_ of 0.7075 to 0.708 1 the curves converge III spite oftheir initial wide range. Therefore it seems that the wide range of (Sr/Ca)L did not result from different values of (Sr/Ca)L,. Thus, the actual relationship between the Sr ISOtopic composition and Sr/Ca ratios of the Heletz brines, is mainly explained by a wide range of (Sr/ Ca)R values rather than of (Sr/Cak values. id] 4 q~a~titat~~~eestimate @the brine-rock inter~cttun. The Heletz brines have migrated into their present-day reservoir along a marine carbonate sequence built of Cretaceous rocks (Fig. lb). affected meanwhile by exchange reactions. The effect of these reactions of dissolution and reprecipitation can be estimated quantitatively by calculating the ratio of Cas/CaL The relations of (“‘Sr/*?ir), vs. (Cas/CaL) were obtained by solving Eqn. (I 1). (87Sr/“6Sr)Rwas chosen as 0.7072 tfor Lower Cretaceous rocks) and 0.7075 (for Upper Cretaceous rocks). All other parameters are those used to construct the curve of Fig. 8. If the value of 0.7078 represents the average <“‘Sri ‘%rfL ratio. then the corresponding average value for tCas/Car) will be around SO. Assuming a Ca content of 3 gril {Table I. samples No. 219 and 301~ the highest value found in the bnnes), a value of I20 gr/
0 7090
0 7080
FlG. 8. The variation of X’Sr/“hSr ratio of the brmes with reactton progress (c‘a.JCaL) during exchange between brines and Cretaceous marine carbonates. For pfottrng. the following parameters were used:
Sr isotopes in oiltield brines
1 (40 X 3) is obtained for the amount of Ca dissolved by the brines from the rock during the dissolution process. As the average porosity of the section is considered to be cu. 5% (A. ARAD, A. BEIN and H. FINK, pet-s. commun.) and the density is around 2 gr/cm3, the amount of rock attacked by one liter of an average brine is 40 Kg. Following TUREKIAN and WEDEFQHL ( 196 1) using the value of 30% for the Ca content in an average carbonate section, the amount of Ca contained in the attacked rocks is 12 Kg. Of this amount, only 0.12 Kg was dissolved into the brine, i.e. about one percent only. Based on the suggested model, most of the dissolved calcium returns into the rock in the form of secondary calcite (in veins or VI&S). While for a carbonate section, this effect of 1% dissolutionreprecipitation Seems to be minor and negligible, it may be most significant in a non carbonate sequence. Thus, the *7Sr/86Sr ratio of the CaCO, cement of Heletz Fm. (SHENHAV, 1971) may be used as an indicator to its origin, which is of interest sands are the oil reservoir rocks.
as the Heletz
Acknowledgments-The authors are grateful to Mrs. T. Braun and Mrs. M. Hassidim for technical assistance. The research was partially supported by the Ministry of Energy and infrastructure through grant No. 8 I-200-74.
REFERENCES BENTORY. K. ( 1969) On the evolution of subsurface brines in Israel. Chem. Geo& 4, 80-I IO. CARPENTERA. B. ( 1978) Origin and chemical evolution of brines in sedimentary basins. Oklahoma Geological Survey Circular 79, 60-76. CARPENTERA. B. and MILLER J. C. (1969) Geochemistry of saline subsurface water, Saline County (Missouri). Chem. Geol. 4, 135-167. CHAUDHURIS. (I 978) Strontium isotopic composition of several oilfield brines from Kansas and Colorado. Geechins. Cosmochim. Acta 42,329-33 I. COLLINS A. G. (1975) Geochemistry of Oilfreid Waters. Elsevier. DASCHE. J. and BISCAYEP. E. (197 1) Isotopic composition of Sr in Cretaceous-to-Recent pelagic foraminifera. Earth Planet. Sci. Lett. 11, 201-204. FAURE G. and JONES L. M. (1969) Anomalous strontium in the Red Sea brines. In Hot Brines and Recent Heavy Metal Deposits in the Rea’ Sea (ads. E. T. Degens and D. A. Ross) pp. 243-250. Springer. FLEISCHERE., GOLDBERGM., GAT J. R. and MAGARITZ M. ( 1977) Isotopic composition of formation water from
695
deep drill&s in southern Israel. Geochim. Cosmochim. Acta 41, 51 l-525. GOLDBERGE. D. (I 963) The oceans as a chemical system. In The Sea, Ideas and Observations on Progress in the Studv of the Sea (eds. M. N. Hill, E. D. Goldberg, C. O’D. lselin and W. H. Munk) 2, 3-25, Wiley. HAWKESWORTHC. J. and ELDERF~ELDH. (1978) The strontium isotopic composition of interstitial waters from sites 245 and 336 of the Deep Sea Drilling Project. Earth Planet. Sci. Lett. 40, 423-432. HSU K. J. (1972) When the Mediterranean dried up. Sci. Amer. 227, 27-36. KATZ A. and MATHEWS A. (1977) The dolomitization of CaC03: an experimental study at 252-295’C. Geochim. Cosmochim. Acta 41,297-308. KATZ A., %ss E., STARINSKYA. and HOLLAND H. D. ( 1972) Strontium behaviour in the aragonite-calcite transformation: an experimental study at 40-98’C. Geochim. Cosmochim. Acta 36,48 l-496. KINSMAN D. J. J. (1969) Interpretation of Sr concentrations in carbonate minerals and rocks. J. Sediment. Petrol. 39, 486-508. PETERMAN Z., HEDGE C. E. and TOURTEL~TH. A. (1970) Isotopic composition of Sr in seawater throtqhout Phaneroxoic time. Geochim. Cosmochim. Acta 34, 105-120. SASS E. and STARINSKYA. (1979) Behaviour of strontium in subsurface calcium chloride brines: southern Israel and Dead Sea rift valley. Geochim. Cosmochim. Acta43,885895. SHENHAVH. ( 197 1) Lower Cretaceous sandstone reservoirs, Israel: Petrography, porosity, permeability. Amer. Assoc. Petrol. Geoi. 83, 12, 2 194-2224. STARINSKYA. (I 974) Relationship between &chloride brines and sedimentary rocks in Israel. Ph.D. thesis, The Hebrew Univ. (in Hebrew, with English summary). STAR~NSKYA., BIEISKIM., LAZARB., WAKSHAL E. and ST~INITZG. (1980) Marine *‘Sr/% ratios from the Jurassic to Pleistocene: Evidence from groundwaters in Israel. Earth Planet. Sci. Lett. 47. 75-80. SUNWALLM. T. and PUSHKAR P. i 1979) The isotopic composition of strontium in brines from petroleum fields of southeastern Ohio. Chem. Geol. 24, 189-197. TUREKIANK. K. and WEDEPOHLK. H. ( 196 1) Distribution of the elements in some major units of the earth’s crust. Bull. Geol. Sot. Amer. 72, 175-192. VEIZER J. (1978) Strontium-abundance in common sediments and sedimentary rock types. In Handbook ojGeo_ chemistry (ccl. K. H. Wedepohl), Vol. B/4, Ch. 38, 13 pp., Springer. VEIZERJ. and COMPSTONW. (1974) *‘Wr’%r composition of seawater during the phaneroxoic. Geochim. Cosmochim. Acto 38, 1461-1484. VEIZER J. and DEMOVICR. (I 974) Strontium as a tool in facies analysis. J. Sediment. Petrol. 44,93-l 15. WHITE D. E. (I 965) Saline waters of sedimentarv rocks. In Fluids in Subs&ace Environment (ads. A. Young and J. E. Galley). Amer. Assoc. Petrol. Geol. Mem. 4, 342365.
Strontium isotope evidence on the history of oilfield brines, Mediterranean Coastal Plain, Israel A. STARINSKY, M. BIELSKI, and B. IXZAR Department of Geology, The Hebrew University, Jerusalem, Israel
and G. STEINITZ and M. FLAAB The Geological Survey of Israel, 30 Malchei Israel St.. Jerusalem (Received April 23, 1982; accepled in revised form December 23, 1982)
Abstract-The isotopic composition of Sr in oil field brines from the Mediterranean Coastal Plain was determined in 18 drillholes ( 13 of which are oil producing wells). The brines are characterized by salinities ranging from 35 to 93 g/l (TDS), Sr from 28 to 350 mg/l, Sr/Ca molar ratios from 0.011 lo 0.053 and “Sr/% ratios from 0.7075 to 0.7090. E & A = 0.708 1 2 0.0004 (2~). The brines are classified into two groups: (a) Mavqi’im groug-brines with relatively high s’Sr/%r ratios (0.7087 and 0.7090), sampled from elastics, dolomites and anhydrites of Upper Miocene age. (b) Helctz group-brines with relatively low “‘Sr/“%r ratios (0.7075 to 0.7081). sampled from sandstones and dolomites of L.ower Cretaceous age. Equations were derived to show the relations hetween “Sr/%r ratio of the brines and the processes through which they evolved (dolomitization and dissolution-repmcipitation). It is suggested that both groups of brines originated from Mediterranean evaporated seawater during the Messinian desiccation. The strontium isotope composition of the seawater is r&cted in that of both groups of brines, the Mavqi’im group containing the original 87Sr/%r ratio (-0.7088). The Heletz group evolved later on, through exchange reactions of those primary brines with a carbonate sequence of Cretaceous age and consequently new “Sr/%r ratios (-0.7078) could have been developed. The effect of the dissoiutionreprecipitation reactions on the carbonate rocks is estimated to be very slight, only around 1%(by weight). INTRODUCI’ION
ALTHOUGH THE application of strontium as a geochemical tracer for processes taking place between subsurface saline water and sedimentary rocks is well known (KINSMAN, 1969; CARPENTER and MILLER, 1969; VEIZER and DEMOVIC, 1974; SASS and STARINSKY, 1979: and many others) very fnv studies concerning the Sr isotopic composition have been published so far. Sr isotopic studies of subsurface oil field brines from oil producing wells from Kansas and Colorado (CHAUDHURI, 1978) and Ohio (SUNWALL and FWSHKAR, 1979) show relatively high 87Sr/86Sr ratios (0.7113 to 0.7341 and 0.7092 to 0.7143, respectively) compared to the present or paleo-seawater values. The high ratios are attributed to Sr isotopic exchange reactions between the brines and the surrounding rocks. The hot brines of the Red Sea, which form a part of the seawater body, show relatively low values of 87Sr/“Sr ratio (around 0.708) compared to that of the present day Red Sea (FAURE and JONES, 1969). The ratios are attributed to volcanic activity. HAWKESWORTHand ELDERFIELD(1978) have studied the relationship between Sr content and Sr isotopic composition of interstitial seawater along two sections from deep sea cores (DSDP, sites 245 and 336). They have found an enrichment of Sr with depth and explained it by the alteration of volcanic material as well as by the introduction of Sr derived from calcium carbonate. 687
The waters studied are oil field brines from the southern Coastal Plain of Israel (Fig. la). They were sampled from two Merent parts of the sequence (Fig. lb): (a) Mavqi’im Fm. (Neogene), composed of clastics, anhydrite and dolomite, representing the phase of the Mediterranean Sea desiccation during the Late Miocene (Messinian) about 6 million years ago (Hsu, 1972). The unit is enclosed by a thick sequence (hundreds of meters) of impermeable clays and marls of the Saqiye group. (b) Heletz Fm. (Lower Cretaceous) composed mostly of sandstones and dolomitic lenses. These are the main oil reservoir rocks of the southern Coastal Plain in Israel. The sandstones are cemented by calcite or dolomite (SHENHAV, 1971). The Heletz Fm., which attains a thickness oi up to 300 m, was deposited in Valanginian-Barremian times. A single sample, from the Upper Jurassic carbonates is also attributed to this group. The major chemistry and H and 0 isotopes of the water were studied earlier by BENTOR (1969), STARINSKY (1974) and FLEISCHERet al. (1977). The studied waters are Cachloride brines with Ca/(HC03 + S04) > 1. Their fairly high Ca/Mg ratios are explained by dolomitization processes in which they were involved (BENTOR, 1969; STARINSKY, 1974). Based on a quantitative treatment of Sr/Ca ratio variations in the brines, and assuming certain values for the Sr content of aragonitic and calcitic sediments and Sr/Ca ratios for lagoons and sabkhas, SASS and STARINSKY (1979) suggested that the brines were
ial
c,
%rPSr 0 707
1
Veoqene
0709
a.704
I
:
UPPW
Miocene i bleoqene
I
Upper
Cretoceeus
Ceoomanton - Turonian
brines
m
Juross~c
Anhydrite
FIG. i. Location map and a generaiized columnar section of the studied area.
filtered through 0.45 pm “Milhpore” filter and stored in a new set of polyethyiene bottles for chemical and isotopic analyses. Chemical components were determined by atomic absorption spectroscopy (Na, K. Mg, Ca and Sr), potentiometrienhy (Cl and HCOr) and gravimetricaily (SO,). Rb was determined by an AAS graphite furnace system. 87Sr/%r ratios were determined on a Varian CH-5 mass spectrometer (21.4 cm, 90” magnet sector) equipped with a Gary 401 vibrating reed electrometer. Digital readout of the results was accomplished via a modified DVM. The ratios were normahzed to “%/@Sr = 8.3752. A value of 0.708 I + 0.0004 (28) for Eimer and Amend SrCO, standard was obtained. Sr was extracted by conventional ion exchange techniques using resin Dowex SOW-8X, 200-400 mesh. Chemical blanks were found to be less than 2 ng( sample.
formed by do~omi~tio~ of aragonitic sediments and to a lesser amount were a&c&xi by substiace processes like dissolution-reprecipitation. The present study is an investigation of the possibility of using the Sr isotopic composition of oil field brines as a genetic indicator for (1) kinds of reactions in which these brines were involved during their evolution, (2) a quantitative estimate of reactions taking place in the subsurface between the brines and the surrounding rocks. METHODS 13 samples were collected fmm the weh head of producing welts (Table 1). The sampies were mixed with oil and a gravitative separation has been carried out in the tield. 5 samples horn dry holes were co&acted during drilling op erations. All samples were stored in I liter polyethylene bottles. Before chemical analyses were carried out, the water was
701 710 300 172 169 SI 703 301 706 726 219 175 lrn
Hidei-2 Hden.3 Halsec.9 ti&a.lo H&lP2%4 tkiea-27 Helecr.31 Kakhw-I KaAr.3 Kddur~ Kokhw7 Kokhw.15 had.1
” L ” ” ” “ ‘a ‘I (’ ‘. ” ‘.
THE
MODEL
Ca-chloride bnnes are believed to be formed mostly from ancient seawater through several pro-
”
1521.23 1511.17 1523.24 154146 1575 i49l.V 1680.92 1561.64 164754 1493”SlO I.51244 186187
UC. Sit. sn. dol. wt. sat. ut. st.;dol.
--.-
l+dipmduc&*stl;-dwdriUhde “Ccxq. = C~omun~a; hf. - limeme; dd. - dolovnte. ut. = anhydme. PPZ = tmdsfo~e
-
Sr isotopes in oilfield brines
w
content-and hence their (Sr/Ca)R--a brine which was evolved through dolomitization processes may show different concentrations of Sr depending on the actually replaced mineral. The dolomitization reaction progmss (CaJCaJ in natural cases is unknown and practically, Eqn, (1) can be solved for the limiting case in which (CaJCarJ - co. In this case
;B
SURFACE
689
j: :7 ‘Z 1
Sr cau=iqjx
(1
1 LAGOON1
2
Sr ( caR)
(2)
In order to estimate quantitatively the reaction progmss, a new independent parameter concerning the brine and the replaced rock is introduced herein. Rearrangement of Eqn. ( 1) yields: TYPICAL
OIL FIELD
BRINE
Sr
= _L.(?&(l
%
1 + x,9
(1
SUBSURFACE
TYPICAL
OIL
FIELD
_ (L%-o+g]
BRINE
FIG. 2. Schematic evolution of oil field brines from seawater.
cesses (Fig. 2). The most important are surface evap oration, dolomitization, SO, reduction, and membrane filtration or mixing (WHITE, 1965; STARINSKY, 1974; COLLINS, 1975; and others). Once a typical brine has been formed, it may interact with the surrounding rocks during its migration. Usually these reactions of dissolution and reprecipitation fall under the general name of exchange reactions. While Sr concentration may change in each of the above mentioned processes, Sr isotopic composition may vary only in two of them, viz. dolomitization and exchange. Dolomitization SASSand STARINSKY ( 1979) presented a model showing that Sr/Ca ratio in Ca-chloride brines varies during the process of dolomitixation according to the equation:
As the instantaneous (Sr/Cah in the brine equals the algebraic sum of (Sr/Ca) ratio contributed by the rock-the first term of Eqn. (3)-plus that of the initial solution-the second term of Eqn. (3)-the fractions of both contributions are: fraction contributed by the rock
1
fraction contributed by the initial solution
1
Since no fractionation of “‘Sr/Sr exists during the process, the “Sr/%r ratio of the brines vaiies as a weighted average of the contributions of both the initial solution and the rock (Eqn. 5).
Sr
(EL1=i&&d, +(43,
+z fraction contributed ()(“hsr L,’ by the initial solution )
(5)
By rearrangement, where X&-distribution coefficient of Sr in dolomite; (Sr/ Ca), and (Sr/Ca)G-instantaneous and initial ratios of the brines, respectively; (Sr/Ca)x-the ratio in the replaced rock: (CaL/Cati)_the ratio between the instantaneous and the initial Ca. This last quantity expresses the reaction progress. As calcite and aragonite differ significantly in their Sr __---__-
[Sr) Fr
2(Z).(3.(1
- $)“+*“’
+ (1 + G&+j&ra)L($)+‘+~E’ (7)
L= 2(E),
+ (-2(g),
Equation (7) shows that at the beginning of the reaction, when CaL/CaL, = 1, (2)
By introducing the value of (Sr/Ca)t from Eqn. (I). Eqn. (6) becomes
= (z$),,
+ (I + xe@))($)“+*“’ while in more advanced stage of the reaction, when CaL/ CaL, - ffi. (2)
= (2).
4. Stannsky rt 61
690 (a) DOLOMlTlZATlON
OF
(b)DOLOMlTlZATlON
ARAGONITE
OF CALCITE
‘Sr/Ca)$,
(Sr /Ca)L, 0709 i
0707. ysr/
8*Sr)R
- - -
--
'0Lii-m co, 1
9706.
07od
0.01
0.02
003
004
37031
003
OQI
a02
003
0.04
00s
(Sr / co JL
(Sf/Ca)L
FIG. 3. The variation of “‘Sr/?3r and Sr/Ca ratios of the brmes during dolomIt~zation. The direction of the process is shown by arrows. X,9 = 0.025: (a) for aragonite (Sr/Ca)s = lo-‘; fb) for calcite (Sr/Ca)R = 5 x IO-’
The variation of (a7Sr/86Sr)~ as a function of (Sri Ca),. and reaction progress (CaL/Cati) is demonstrated in Fig. 3. The curves were constructed for hypothetical cases by choosing the following parameters: (87Sr/*6Sr)R - 0.7065, chosen arbitrarily. X,9 = 0.025, after KATZ and MATTHEWS (1977) from experimental data at 252-295°C; (B7Sr/86Sr)~, = 0.704 and 0.709; (Sr/Ca)Li = 0.01, 0.03 and 0.05. In case (a) (Sr/Ca)a = 1O-’ for replaced aragonite and in case (b) (Sr/Ca)R = 5 X lo-* for repiaced calcite. All ratios are expressed in molar units. Figure 3 and Eqn. (7) show that no matter what were the initial values of (Sr/Ca)Li and (“Sr/“%r)Li, and what was the kind of mineral (calcite or aragon&e), the final values of (*7Sr/~r)ti are the same, depending onIy on the “Sr/@‘Sr ratios of the repfaced minerals (Xe is very small and can be neglected).
+$
(8) ($J,il- exP(-A~~.~)j
where & Gas/Cal
distribution coetlicient of strontium in calcite. the ratio between the amount of Ca which was dissolved from the rock to its amount in the sofurion;i.e.the progress reaction.
L;ke In the case of do~omltl~tion Eqn. (8) can be practically used in natural cases. only when (Q&al) is large enough and then, Eqn. (8) becomes
191 By analogy to the case of dolomiti~tio~, the ~ntr~u~lng of fX’Sr/saSr)L and (“‘Sr/e”Sr)R, into Eqn. (8) yields:
Dissolution-reprecipitation According to SASS and STARINSKY( 1979) the fSr/Ca)L as follows:
varies during ~~s~lii~tion
by substitutmg the value of (SriCa), from Eqn. (8) mto Eqn. ( IO) it becomes:
At the beginnmg of the exchange reaction, CadCaL = 0,
Eqn. ( I 1) becomes
-_.. Eqn.
_ _.. _~
__ ____II___.__
C1I) becomes:
(Eglj,. =(igi, whileataveryadvan&~oftheizaction.Ca&Ca~-~3
Dunng exchange reactions. the final value
of ("'Sr/"SrfL
Sr isotopes in oilheld brines
691
(01 ARAGONITE
(b)
/
CALCITE (SrKo),,
(Sr/Co)L, I
0708-
0707-
____.(“Sr/“SrfR
____ 0706
____(6’Sr/86Sr)L,
____
0704
t
FIG. 4. The variation of @‘Sr/%r and Sr/Ca ratios of the brines during di~lution-~p~ipi~tion ’ - 0.058; (a) for aragonite (Sr/C& = IO-*; reactions (exchange). Arrows show direction of progress. Xsr (b) for &cite (Sr,Qn = 5 X IO-‘.
in the brine is controlled exclusively by the (?Sr/%r)R ratio of the reacting minerals (Fig. 4).
total dissolved solids of the samples. The 87Sr/*6Sr ratio of the brines (Fig. 5) shows two distinct groups: (a) brines with high 8’Sr/86Sr ratios, 0.7087 and 0.7090, and (b) brines with low *‘Sr/*%r ratios, from 0.7075 to 0.708 1. The waters with the higher values were sampled from the Neogene Mavqi’im Fm. and those with the lower ones from the Lower Cretaceous Heletz Fm. The average *‘Sr./%k ratio of the Mavqi’im brines is around 0.7088 and that of the Heletz brines is 0.7078 + 0.0002( 1a). Since our anaIyticaI precision is around +0.0002( 1CT) the scatter around the average is referred to analytical rather than geochemical reasons. The Sr/Ca ratios of the waters from both groups fall in the same range, from 0.01 to 0.05. This relatively wide range will be discussed later. 13 samples of the Heletz group were pumped out from oil producing wells (Table l), i.e. from a water
RESULTS The major chemical components of the studied samples are listed in Table 2. The brines have a typical Ca-chioridic composition with salinity (TDS) ranging up to ca. 90 g,/l. Whereas Na/Cl ratio of the brines is simiiar to that of seawater (O&3), the Caf Mg ratio shows considerably higher vaIues, of 0.9 to 2.4. (0.2 for seawater). The content and isotopic composition of Sr are given in Table 3. The Sr content (tens to hundreds mg/l) is significantly higher compared to evaporated seawater with the same salinity. The G/Cl ratio ranges from 0.6 X 10e3 to 2.6 X 10c3 (0.2 X 10V3for seawater). No correlation exists between Sr and the
48s 734 203 245 302 701 710 300 172 169 51 703 301 706 726 219 175 170 *nanttr I-)noi
tidod.2 A3hdod.S Au.1 Beer Sbeba,l Ehur,l Hekcl-2 Hektr.3 Heku.9 Hcku-10 Heku-2SA Hdeu-27 Hdetl.31 K&h%+I Kddw3 KOkhlVd Kokhw.7 Kdrtuv-IS Sad-1
I
dncrrmned
T&k
2.Chemical
29.7 27.0 16.9 16.0
0.27 0.86 0.18 0.24 0.47 0.59 035 0.52 037 038 0.29 0.70 037 0.48 0.25 033 0.4
18.8
22.0 22.1 18.8 223 26.1 19.9 20.0 22.5 102 12.4 296 23.1 24.2 10,s
0.73 1.0 0.5 0.56 0.6 ob4 062 0,s 0.6 0.7 OS1 0.5 0.83 03 0.44 1.1 0.6 0.5 13
data (m&r
2.6 7.2 1.4 1.7 4 1.7 1.4 13 1.5 2.1 13 1a 3.0 05 13 3.L 1.9 2.1 0.4
1
ekments)
53.8 56.8 303 2.2 26.8 33.7 0.24 38.9 0.02 32.8 032 38.8 45.6 36.1
064 0.1 0.18 0.14 0.17 0.17 0.13
88 93 $1 49 55 63 54 64 7s 58 58 71 28 38 91 67 67 3s
23 4.4 1.6 I .8 I .4 1.6 13 1.5 6 1.8 1.5 1.2 2.2 09 1.8 i .8 1.8 2.4 0.2
I
0.85 0.73 0.87 09.7 0.86 0.87 0.89 0.88 0.87 0.85 0.64 0.78 0.92 0.83 0.81 0.87 0.90 0.86
485
uhdod.2 .tidcd.j
‘2
203 245 302 701 710
loo 172 ih9 51
703 301 706 ‘26
body in contact with oil. Only 3 samples were pumped out from dry holes. In spite of the small number of samples from dry hoies it is seen that no correlation exists between the presence of oil and the strontium isotopic composition of the brine. Rb was determined in 11 samples (Table 3) and falls in the range of cff. 100-400 fig,& No correlation exists between the Rb content and the 87Sr/86Srratios. It is therefore obvious that the minor amounts of “St formed by the radioactive decay of *‘Rb can be neglected. DISCUSSION
As both groups of brines-the high 87Sr/a6Sr ratio Mavqi’im group and the low “SrlssSr ratio Heletz group-have the same chemical composition and salinity and were sampled from the same area it is believed that both groups originated from the same ancient sea.
Mavqi ‘im brines On the basis of geologicai and geochemical data. as well as on hydrological considerations, STAR~NSKY (1974) suggested that the brines (including both groups) originated from Neogene seawater and entrapped during the Messinian desiccation ofthe Mediterranean Sea. In our case, the studied brines, like most of the subsurface saline waters in the crust, differ significantly in saiinity and chemical ratios from present-day seawater. SASS and STARINSKY ( 1979) have suggested that these chemical differences result, at least partially, from dolomitization reactions in which the original Neogene seawaters were involved. The replaced CaC03 minerat was aragonite rather than calcite (op. cit., p. 892-3). If this is the case, Sr contributed to the brines should have a Neogene sea isotopic composition (--0.7088), because when (87Sr/ ‘%r)ti = (S7Sr/86Sr)R= 0.7088, the final (87Sr/“Sr)L will be the same. Heletz brines
“sr / “Sr x sw
0.709 orsroga
_-
-
-
1
0
A
I
0
roei_
nuerogc
1
- -
A
:A*:..
-‘-
4
A
AA A
0 7071 001
002
0.03
004
305
0.06
Sr J Co !molari
FIG. 5 y’Srf”6Sr and SrjCa ratios of the oil field brines from the southern Coastal Plain, Israel. sw-seawater; circIes--Mavqi’im brines: triangles-Heletz brines.
As the Heletz brines are a part of a iarger water body. which includes the Mavqi’im brines also, and provided that both groups have a common source, the difference in their strontium isotopic composition should be explained. Moreover. the high Ca/Mg ratios of both groups indicate their evoiution through dolomitization, and thus the source of the CaCO? minerals. which become dolomitized by the original brines-belonging now to the Heletz group-should be shown. /uj Dolomitizutlon of Lower Cretaceous curbonatr rocks. Based on the suggested model. Eqn. (7) has been solved to show the variation of f”Sr/%),_ VS. (Sr/Ca)L during dolomitization of Lower Cretaceous carbonates (Fig. 6a). The parameters used are the following: (Sr/CajR = 9.35 y 10e4. based on the average concentrations of Sr and Ca in carbonate rocks ~TUREKLAN and WEDEPOHL, 196 I1 was chosen herein to represent the wide range of concent~tions existing
Sr isotopes in oilfield brines in the literature (VEIZER,1978). (*‘Sr/*‘%r),= 0.7072 and (r’Sr/%r)Li = 0.7088 seem to be the best values (Fig. 1c) for the Lower Cretaceous marine carbonates (supposed to participate in the process) and the Up per Miocene seawaters, respectively. The values are obtained from the curve showing the variation of strontium isotopic composition during the relevant times (PETERMAN etal., 1970; DASCH and BISCAY, 197 1; VEIZERand COMPSTON,1974 and STAR~NSKY et al., 1980), with slight corrections for E & A standard. (Sr/Cah = 0.04 (the highest ratio found in the studied brines), 0.06,0.08 and 0.10. A: = 0.025 after KATZ and MATTHEWS (1977). Figure 6a shows that the relations existing in the brines, between *‘Sr/%r ratio and SrfCa ratio cannot be explained by an epigenetic dolomitization process of Lower Cretaceous carbonates. By solving Eqn. (2) one can find that the final (Sr/Ca)u ratio must be about 1.9 x 10m3, about one order of magnitude higher. Moreover, a group of brines having the (Sri Ca), ratios of the Heletz brines should have-if evolved through dolomitization of Lower Cretaceous carbonates-*‘Sr/%r ratios above 0.7084 rather than the values of 0.7075 to 0.7081 of the actual Heletz brines. It seems therefore that epigenetic dolomitization of Lower Cretaceous carbonate rocks could not be primarily responsible for the composition of the actual Heletz brines.
(b) Dolomitization of Neogene sediments. SASS and STARINSKY(1979) have suggested that the Heletz brines (their group C, op. cit., p. 890) were formed through dolomitization of aragonitic sediments in a Neogene lagoon, migrating later into the surrounding permeable Cretaceous carbonates and sandstones
(a) DOLOMITIZATION
693
where they further interacted by dissolution-reprecipitation reactions. This process erased the (Sr/Cah and (*7Sr/86Sr)Lratios inherited from dolomitization and new ratios were acquired through the exchange reactions. If this is the case, and the Heletz brines were evolved through dolomitization of marine Neo gene CaC03 minerals, the different *‘Sr/%r ratios of both groups remain still unexplained. (c) Exchange reactions with Cretaceotu rocks. To show the variation of (87Sr/86srh vs. (Sr/Cah during dissolution-reprecipitation reaction between the brines and the Lower Cretaceous rocks, Eqn. (11) has been solved, using the following parameters: A& = 0.058 (KATZ et al., 1972) distribution coefficient of Sr in the precipitating calcite. (Sr/Ca)R, (*‘Sr/%r)R, (*‘Sr/ W&hi as those previously chosen for the dolomitization processes. The results and calculated curves obtained are plotted on Fig. 6b. It c8n be seen that the field of points of the Heletz water group is practically crossed by the calculated set of curves. The “Sr/%r ratio of the Heletz brines can be mgarded as fairly constant, having an average of 0.7078 A 0.0002( 1a) (the ~0.0002( 1u) analytical error may affect the actual results). On the other hand, the wide distribution range of (Sr/CaL from ca. 0.01 to 0.05, cannot result from the analytical procedure, and can be explained on geochemical parameters as follows: (Sr/Ca)R-the ratio depends on (1) relative amounts of the various rocks interacting with the brines; (2) amount of Sr and Ca in each kind of rock, (3) solubilities of Sr and Ca bearing minerals participating in the dissolution process. of these, parameters (1) and (3) are fairly known whereas no relevant data for parameter (2) in the studied area, are available.
(b)
DISSOLUTION
- REPREClPlTATlON
(Sr /Ca)L,
0.707
’
I
1
0.02
004
I
0.06
I
0.06
(Sr /COIL
I
0.10
0.70T 1.l-L (Sr/Co)L
FIG. 6. The variation of “Sr/%r and Sr/Ca ratios during dolomitization and exchange reactionswith Lower Cretaceous marine carbonates.Triangles-Heletz brines. For the construction of the curves, the followingparameters were used: (*‘Sr/%r)Li = 0.7088; (*‘Sr/%r)R = 0.7072; (Sr/Ca)U = 0.04; 0.06; 0.08; 0.10; (Sr/Ca)R = 0.35 X lo-"(a) for dolomitization-A e = 0.025 (b) for dissolution-reprecipitationA:, = 0.058.
%/‘“Sr 0709
I (%/%), ,
r
I
0
708 i ._i
0707~
__L___1L_____. __
-!
001
002
003
004
j (67Sri’ %jR 0 7072
005
sr/ca (molarI FIG. 7. Calculated curves showing the behavior of ?‘Sr/ “bSr)L YS. (Z&a),_ during exchange reactions assuming a single value (0.07) for (Sr/Ca)L, and varying values for (Sr/ Ca)R. The curve in the middle was calculated using an average vaiue of (Sr/Ca)R = 9.35 X lo-‘. The dashed and dotted lines Iimit ranges (of -+50%and ~6590, respectively) around the (Sr/CafR average: triangles-Heietz brines: for other parameters see Fig. 6b.
The sequence of rocks interacting with the brines is composed mainly of limestones, dolostones, marls and chalks (Fig. 1b). The Sr content of such a rock complex varies within a wide range of concentrations, between several tens and few thousands ppm (VEIZER. 1978) and as a result, a wide range of Sr/Ca ratios is dictated. Figure 7 shows that assuming a varying range of values for fSr/Ca)a, on both sides of the average (9.35 x 10W4,after TVREKIAN and WEDEPOHL, 1961) I? samples (75%) and 15 sampies (90%) fail within the range of 250% and 65%. respectively. fSr/Ca)~~--brines which were formed through dolomitization may acquire during the process different
lSr/CabL vaiues, For example. the two sampies from the Mavqi’im group (which are considered to be formed mainly by dolomitization) have Sr/Ca ratios of 0.019 and 0.053. These (Sr/Ca)‘ values become later the initial values for brines starting the reactions of dissolution-reprecipitation. Using the suggested model and parameters (Fig. 6b) it can be seen that the effect of different tSr/CaL values on the ISr: Call--on the abscissa-is conside~bly large at the t&t stages of the reaction progress while the “?Sr/ ‘“Sr-on the ordinate-is affected significantly at very progressive stages only. For the relevant range c)f(X7Sr/X”Srl,_ of 0.7075 to 0.708 1 the curves converge III spite oftheir initial wide range. Therefore it seems that the wide range of (Sr/Ca)L did not result from different values of (Sr/Ca)L,. Thus, the actual relationship between the Sr ISOtopic composition and Sr/Ca ratios of the Heletz brines, is mainly explained by a wide range of (Sr/ Ca)R values rather than of (Sr/Cak values. id] 4 q~a~titat~~~eestimate @the brine-rock inter~cttun. The Heletz brines have migrated into their present-day reservoir along a marine carbonate sequence built of Cretaceous rocks (Fig. lb). affected meanwhile by exchange reactions. The effect of these reactions of dissolution and reprecipitation can be estimated quantitatively by calculating the ratio of Cas/CaL The relations of (“‘Sr/*?ir), vs. (Cas/CaL) were obtained by solving Eqn. (I 1). (87Sr/“6Sr)Rwas chosen as 0.7072 tfor Lower Cretaceous rocks) and 0.7075 (for Upper Cretaceous rocks). All other parameters are those used to construct the curve of Fig. 8. If the value of 0.7078 represents the average <“‘Sri ‘%rfL ratio. then the corresponding average value for tCas/Car) will be around SO. Assuming a Ca content of 3 gril {Table I. samples No. 219 and 301~ the highest value found in the bnnes), a value of I20 gr/
0 7090
0 7080
FlG. 8. The variation of X’Sr/“hSr ratio of the brmes with reactton progress (c‘a.JCaL) during exchange between brines and Cretaceous marine carbonates. For pfottrng. the following parameters were used:
Sr isotopes in oiltield brines
1 (40 X 3) is obtained for the amount of Ca dissolved by the brines from the rock during the dissolution process. As the average porosity of the section is considered to be cu. 5% (A. ARAD, A. BEIN and H. FINK, pet-s. commun.) and the density is around 2 gr/cm3, the amount of rock attacked by one liter of an average brine is 40 Kg. Following TUREKIAN and WEDEFQHL ( 196 1) using the value of 30% for the Ca content in an average carbonate section, the amount of Ca contained in the attacked rocks is 12 Kg. Of this amount, only 0.12 Kg was dissolved into the brine, i.e. about one percent only. Based on the suggested model, most of the dissolved calcium returns into the rock in the form of secondary calcite (in veins or VI&S). While for a carbonate section, this effect of 1% dissolutionreprecipitation Seems to be minor and negligible, it may be most significant in a non carbonate sequence. Thus, the *7Sr/86Sr ratio of the CaCO, cement of Heletz Fm. (SHENHAV, 1971) may be used as an indicator to its origin, which is of interest sands are the oil reservoir rocks.
as the Heletz
Acknowledgments-The authors are grateful to Mrs. T. Braun and Mrs. M. Hassidim for technical assistance. The research was partially supported by the Ministry of Energy and infrastructure through grant No. 8 I-200-74.
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