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GEOLOGICA CARPATHICA,  48, 6, BRATISLAVA,  DECEMBER 1997

371–386

DIAGENESIS AND POROSITY OF THE UPPER TRIASSIC

CARBONATES OF THE PRE-NEOGENE VIENNA BASIN BASEMENT

 PETER MASARYK

and OTÍLIA LINTNEROVÁ

2

VVNP, Research Oil Company, Votrubova 11/a, 825 05 Bratislava, Slovak Republic

2

Departrment of Mineral Deposits and Geology, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovak Republic

(Manuscript received March 18, 1997; accepted in revised form October 15, 1997)

Abstract

: The Upper Triassic carbonates of the Opponitz Formation and Hauptdolomit Formation represent reservoir

rocks of gas deposits in the basement of the Vienna Basin Neogene fill. During study of the reservoir rock properties of
dolomites in the well Kuklov-3 (K-3) significant variations in porosity as a result of late diagenesis processes in buried
sediment were found. On the basis of petrographical, SEM and mineralogical-geochemical methods, we identified neo-
morphic calcite layers, also found in the Kuklov-4 (K-4) well. Coarse crystallized calcite crystals are idiomorphic (e.g.
ditrigonal scalenohedron). They have a relatively high content of Sr, Fe and Na, and decreased isotopic ratio of O (

δ

18

O:

–6 to –9 ‰) or also C (

δ

13

C: –0.7 up to +1.8 ‰) in comparison with values in dolomites (

δ

18

O: –4.8 up to;1.3 or also 

δ

13

C:

0.1 to +4.1 ‰) or also in limestones (mostly 

δ

18

O: –3.9 to –3.6 ‰). Microstructural analysis indicates that they substi-

tute dolomites as a result of dedolomitization under conditions of deep burial. Diagenesis under conditions of deep burial
results in forming of new minerals such as kaolinite, pyrite and illite. The observed changes (increasing) of reservoir rocks
porosity of dolomites both in the well K-3 and K-4 (at the depth of 3660 to 3830 m) were caused by diagenetic processes
taking place in the deep burial environment and these processes were probably limited to a layer of (originally dolomitic)
breccias.

Key words: 

Upper Triassic, carbonate reservoir, chemical and mineralogical composition, SEM, dedolomitization, stable

isotopes of O and C.

Geological and reservoir setting

Both, in the Austrian and the Slovak part of the Vienna Ba-

sin, three nappe zones (Bajuvaricum, Tirolicum and Juvavi-
cum) were interpreted separated from each other by Upper
Cretaceous and Paleogene sediments of the Gosau type (Wes-
sely 1983, 1988; Jiříček 1984). The most external, thrusted
over the Klippen Belt are the Bajuvaric nappes (Frankenfelds
and Lunz nappes). The Upper Cretaceous and Paleogene sedi-
ments of the Giesshübel Syncline have been deposited on
these nappes in a transgressive position. According to Wessely
(1983, 1988), Jiříček & Tomek (1981), Jiříček (1980, 1984),
Sauer et al. (1992) the Bajuvaric nappes are extended in the
belt Aderklaa–Schönkirchen–Prottes–Borský Jur–Kuklov-
Šaštín–Senica and are submerged under the Upper Cretaceous
and Paleogene sediments of the Myjavská pahorkatina Up-
land. They appear at the surface in form of isolated structures
southwards of the Klippen Belt between Podbranč and Lubina.

Well exploration into pre-Neogene basement of the Vienna

Basin was aimed at elevation structures (Šaštín, Závod, Stu-
dienka, Borský Jur, Kuklov, Senica) with a mean depth of the
Neogene basement in the interval of 3000 to 4000 m. In the
Slovak part of the basin two gas deposits — Borský Jur and
Závod were discovered in Upper Triassic dolomite sequences
of the Opponitz Limestone and Hauptdolomite Formations.
Both wells K-3 and K-4 were drilled into the marginal zone
of the Borský Jur reservoir (Fig. 1). The well K-3 is consid-
ered to have one of the most complete sections through the
Lunz Nappe sequence (the interval of 2700 to 5200 m). Be-

Introduction

The presented paper is aimed at the question of whether the
observed change in Upper Triassic dolomites of wells Kuk-
lov-3 (K-3) and Kuklov-4 (K-4) in the basement of the Neo-
gene fill of the Vienna Basin (Fig. 1) is a result of late-diage-
netic processes. Upper Triassic carbonates (Opponitz
Limestone Formation and Hauptdolomite Formation) repre-
sent reservoir rocks of gas deposits in the Slovak part of the
basin. The present burial depth of the Triassic sequence in the
Vienna Basin is predominantly more than 3000 m (in an in-
terval of 2000 to 3000 m, except near basin rims). The porosi-
ty values of the carbonate rocks are frequently below the lower
limit referred to good carbonate reservoir rocks (< 6 %). The
porosity record of the dolomite sequence in the well K-3
shows that the usual porosity reduction with increasing depth
reversed. We observed abnormal increased porosity values
(above 15 %) in this part of the well. Study of reservoir rocks
properties of dolomites was carried out in the framework of
the state research project „Evaluation of prospectivity of
searching for hydrocarbon in selected areas of the Western
Carpathians“ (Masaryk 1996 in: Janků et al. 1996). Preceding
results (Borza et al. 1985; Masaryk et al. 1988; Ostrolucký &
Jiříček 1986; Ostrolucký 1994) were supplied with other pet-
rographical, mineralogical and geochemical results (Lintner-
ová 1988; Masaryk 1990, 1996). It is becoming evident that
late-diagenetic alteration of a carbonate sequence affected
mostly parts built up by breccias and significantly influenced
their reservoir rock properties.

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372                                                                            MASARYK and LINTNEROVÁ

sides the Upper Triassic dolomites (2700 to 3832 m), the well
also penetrated the Lunz-Reingraben Formation (up to 4758
m) and a part of the Reifling Formation (up to 5200 m). The
lithological nature and changes in reservoir rocks were
checked in logs (Figs. 2–3). Biostratigraphical subdivision of

the carbonate sequences was problematical because of the lack
of fossils.

The Uppermost layers of the Upper Triassic dolomites con-

tain foramifers Uppermost Norian to Rhaetian (Borza et al.
1985). It is possible to accept the lithofacial subdivision of
the Hauptdolomite Formation proposed by Scherreiks (1971)
and later made more precise by Fruth & Scherreiks (1982,
1984, 1985) also for sequences of the Vienna Basin base-
ment. On the basis of this subdivision, the well K-3 drilled
through the middle and basal part of the Hauptdolomit For-
mation and Opponitz Limestone Formation (Fig. 3). The Op-
ponitz Limestone Formation was also found in the well K-4,
but the Mesozoic sequences were drilled in an inverted ar-
rangement (Masaryk et al. 1988).

Methods

150 thin-sections were studied from the wells K-3 and

K-4. Some of them were coloured with alizarine red. The
porosity of well core rocks was evaluated by methods of
triple scaling, Hg-porosimetry and optical coloured poro-
simetry. We also utilized the results of processed logs of
the well K-3 (processed by computer). For all methods of
porosity measurements in details see Masaryk (1996). We
studied the fracture surfaces of dolomite and also etched
and polished rocks thin plates (0.5% formic and 1% hydro-
chloric acid, 15 sec. to 1 min.) under a scanning electron
microscope (SEM). The mineral composition of rock sam-
ples was studied by X-ray diffraction analysis (CuK

α

).

The dolomite stechiometry (mole calcite– dolomite ratios)
and the dolomite crystal ordering have been evaluated by
methods of Lumsden (1979). The calcium and magnesium
content of the carbonate parts of rock samples (HCl dis-
solved) were checked by chemical analysis. The calcite and
dolomite contents were corrected on the basis of insoluble resi-
due (IR) gravimetrical determination (Table 3). The elements
listed in Table 2 were determined by the AAS method in the
sample portions soluble in hydrochloric acid. The 67 whole-
rock analyses from both wells were done by the X-ray fluo-

Fig. 1. 

Localization map of the Kuklov 3 (K-3) and Kuklov 4 (K-4)

boreholes.

WELL

DEPTH (m)

P HG

(%)

P HG

>20 nm (%)

TOP

(%)

VP(%)

LITHOLOGY

AGE

Kuklov 3

2708–3013

1.50

0.39

3.05

2.03

Grey-brown loferitic and

brecciated dolomites

Norian

Kuklov 3

3089–3238

1.33

0.45

 2.10

0.64

Grey-brown laminated

dolomite with anhydrite

Norian

Kuklov 3

3390–3665

0.91

0.32

  0.00

1.07

Grey-brown massive

limestone

Carnian

Kuklov 3

3676–3832

2.03

0.37

  8.77

3.35

Grey-brown limy

dolomite and dedolomite

Carnian

Kuklov 4

3329–3573

0.30

0.10

  0.00

0.40

Grey-brown massive

limestone

Carnian

Kuklov 4

3641–3644      10.42

6.72

      13.00

   11.75

Grey-brown porous

dolomite

Carnian

Kuklov 4

3697–3734

 2.95

1.55

  3.50

3.07

Grey-brown carbonate

breccia

Carnian

P HG (%)— average of the total porosity measured by Porosimetro 2000, (Hg-porosimetry)

P HG >20 nm (%)—  average of the effective porosity measured by Porosimetro 2000

TOP(%)—  average of the total optical porosity (colour optical porosimetry)

VP (%)— average of the total volume porosity (triple weight)

Table 1: 

The results of porosity measurement by four methods.

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DIAGENESIS AND POROSITY OF THE UPPER TRIASSIC CARBONATES                                         373

Fig. 2. 

Borehole K-3: Lithological section and the values of log porosity.

rescence method. Some of them are listed in Table 2, but the
complete analyses are unpublished (Borza et al. 1985; Lint-
nerová 1988).

Powder specimens for isotopic analysis from analysed

cores were prepared. Samples were dissolved by the stan-
dard method (McCrea 1950), i.e. in 100% phosphoric acid
in vacuum at 25 

o

C. In samples containing calcites and dolo-

mites, carbon dioxide has been separated step by step, ac-

cording to reaction time. The values of the isotopic ratio
were corrected (decreased by 0.8 ‰) in respect to a different
fractional factor of oxygen in reaction with the acid. The
quoted way of separation is favourable for samples with the
content of one component over 10 %. Results were quoted
as an isotopic ratio 

δ

 in per mile, related to the PDB standard

for both elements. The accuracy of the assurements is better
than 0.1 ‰ for both 

δ

13

C and 

δ

18

O.

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374                                                                            MASARYK and LINTNEROVÁ

Fig. 3.

 Upper Triassic carbonate cores in boreholes Kuklov 3 and Kuklov 4.

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DIAGENESIS AND POROSITY OF THE UPPER TRIASSIC CARBONATES                                         375

Results

Petrographic characteristics of the specimens

Well Kuklov-3 (K-3)

The Upper Triassic carbonate rocks of the well K-3 were

subdivided into three lithological types (Figs. 2–3). The dolo-
mites of cores 6–11 represent the middle part of the Hauptdo-
lomit Formation sequence, while cores 12 to 14 were taken
from the basal dolomitic-anhydritic complex (after Scherreiks
1971; Fruth & Scherreiks 1982, 1984, 1985). The limestones
and dolomites of cores 15 to 22 (Fig. 3) belong to the Opponitz
Limestone Formation.

Cores 6 to 11 are formed by grey-brown tectonized lofer-

ite dolomites (Fig. 4: 1–2). The uppermost part of the dolo-
mites (cores 6,6a) are formed primarily by dolosparite
(grain size 200–250 

µ

m,) and sporadically by microsparite.

In other cores recrystallization is not so intensive, and fine-
grained dolomitic matrix (5–25 

µ

m in size) is partly recrys-

tallized into microsparite (30–100 

µ

m in size). Initial sedi-

mentary textures and structures were almost completely
wiped away and are preserved only in the form of indistinct
ghosts (Fig. 4: 1). Fossil fragments (ostracods, foraminifers,
crinoids) occurred only rarely (Fig. 4: 2). The primary stro-
matolite structure is the most typical phenomenon in this part
of the sequence (Fig. 4: 3–4, 7). The dolomites were relative-
ly intensively fractured and the secondary joints are filled
primarily by calcite, less by dolomite and rarely by anhydrite
crystal aggregates. The open jointed network in tectonized
layers (tectonic breccia) wholly increase communication of
pore spaces. Locally, the rocks have the nature of pseudoru-
dite and are formed by lighter dolomicrosparitic clasts en-
closed by dark dolomitic matrix.

Cores 12 to 14 (Fig. 3) contain grey-brown brecciated do-

lomites with anhydrite. The dominant breccias have clasts
formed by laminated loferite dolomites (Fig. 4: 3–4, 7–8) and
matrix formed by laminated anhydrites with dolomite inter-
calations (Fig. 4: 5). The anhydrite of these layers is coarse
grained and tabular or needle-shaped crystal forms are char-
acteristic. The dolomite laminae are micritic, frequently with
a clay admixture. The dolomite clasts in the breccia are
formed by the same types of dolomites as described from
cores 6 to 11.

Cores 15 to 19 (Fig. 3) are formed by grey-brown laminat-

ed limestones. The matrix of the limestones is micritic with
an inexpressive fine lamination. The limestones (limy mud-
stones) are poor in organic remnants and other allochems.
Sporadic pellets, ostracods, crinoids, globochaetes and fora-
minifers are most frequently concentrated into thin laminae.
The matrix includes a finely dispersed clayey-silty admixture
(approx. 5 to 10 %) concentrated into stylolite surfaces that
represent a result of pressure dissolution. The limestones
were fractured by a relatively dense net of joints or veinlets
which were formed by secondary sparry calcite, seldom by
dolomite. Diagenetic dolomitization affected these lime-
stones only indiscernibly. Besides sporadic small rhombs in
the matrix of these limestones there are more abundant au-
thigenic pyrites and clay minerals. These limestone types
with a relatively high clayey-silty admixture also include
more intensively dolomitized limestones.

The rocks of cores 20 and 21 (Fig. 3) are formed by pale-

brown brecciated dolomite to dolomitic limestone. The ma-
trix is microsparitic to sparitic with relics of sporadically re-
crystalized foraminifers, ostracods, ooid relics and intraclasts
(Fig. 5: 1–2). The detritus content in dolomites reaches up to
10 % (wackestone type). The matrix has the typical granular
nature of dolomites, but locally with an increased calcite
content, fairly visible in thin sections coloured by alizarin
red and also according to the changes in size of crystals. Ir-
regular islands of dolomite (Fig. 5: 3) and relics of dolomite
(dolomitic ash) between and inside large calcite grains indi-
cate processes of a diagenetic alteration — dedolomitization.
The original laminated structure of dolomite is visible in
hand specimens but completely disappears in thin sections.
Dolomites were also strongly fractured and brecciated. A pri-
mary part of joints and pores is filled by neomorphic sparry
calcite, but part of the pores is free. This is very important for
reservoir rock properties (the increasing of the permeability).

Sample

CaO

MgO

CO

2

Fe

Mn

Na

Sr

wt. %

ppm

K3

 6/2708

31.62

20.58

46.69

  250

 36

   250

    81

6a/2757

32.09

20.19

46.90

  380

 61

  330

    81

 7/2704

32.19

19.92

46.93

  300

 44

  460

    75

8/2851

31.80

20.42

46.87

  310

 39

  450

  690

8/2853

32.29

19.83

47.07

  290

 39

  440

    71

10/2951

28.09

20.40

42.70

1550

  74

  650

    96

10/2952

32.33

20.75

45.44

  920

  87

  580

    94

10/2953

24.66

16.58

36.36

2770

  87

1050

    94

11/3011

28.33

20.36

42.18

  55

  575

  116

12/3090

  3.72

  2.51

  5.66

108

  738

  995

13/3177

  9.01

  6.47

14.12

  63

  295

2218

14/3237

32.11

20.82

45.11

  91

  373

  839

15/3391

53.64

  1.31

42.54

  20

  330

  800

15/3392

53.48

  1.21

42.87

  23

  310

  755

16/3421

51.84

  2.70

42.45

910

  195

  701

17/3566

54.56

  0.72

43.82

870

  150

  500

19/3663

42.45

  4.43

37.48

328

  475

  443

20/3676

32.84

19.73

47.16

  28

  375

  103

20/3677

32.41

19.60

47.03

  32

  388

  100

21/3825

34.36

18.07

47.05

  210

  17

  580

    93

21/3826

55.48

  0.59

43.09

  455

  10

  128

  378

21/3827

55.46

  0.68

42.29

1330

  14

  113

  228

22/3828

53.69

  2.07

43.51

  420

  19

  125

  393

22/3829

50.00

  4.71

44.48

  420

  22

  125

  231

22/3830

52.57

  2.81

43.98

  385

  17

  148

  280

34/4902

48.72

  0.95

41.17

  29

  890

K4

11/3699

53.42

  1.21

42.59

  80

  260

  325

    11/cl

54.96

  2.60

  570

  77

  160

  295

    11/ce

53.14

  0.15

  760

  82

  310

  309

  11a/cl

52.57

  1.61

  580

  45

  190

  305

  11a/ce

38.27

  0.71

1880

  64

  700

  250

12/3732

46.83

  1.41

  68

  450

  230

   12/cl

36.95

  0.71

  960

140

  730

  248

   12/ce

38.55

  0.50

  690

  55

  720

  203

Table 2: 

Chemical composition of the rock samples. Sample 6/

2708 it is: 6-core number, 2708 = 2708 m in the borehole.

cl — clast,  ce — cement

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376                                                                            MASARYK and LINTNEROVÁ

We noticed a significant increase of calcite content in the

lower part of core 21 and in core 22, and although specimens
resembled dolomite by their appearance they are actually
formed by neomorphic calcite. The calcite crystals are sparit-
ic (Fig. 5: 3–8) with relics of dolomicritic matrix. Relics of
dolomicritic grains with their rims partially rhombohedrally
boundered as well as in form of large rhombohedral grains
were typical. Some core parts are formed by a carbonate
breccia with angular clasts with sizes of 0.1 mm to 4 cm.
Clasts were of a different nature, resembling the described
compact dolomites or limestones. The cement of these brec-
cias is formed by coarse-grained neomorphic calcite (Fig. 4:
6–7) and micritic parts (matrix), which are also calcitic.
Ghosts, remnants of tests probably from foraminifers and os-
tracods, appear locally.

Well Kuklov-4 (K-4)

The cores 11, 11a and 12 (Fig. 3) are formed by grey-

brown carbonate limestone breccias. Dolomites are present
only in core 10 (Fig. 3). The appearance and texture proper-
ties of them are very similar to those from well K-3, core 20.

The matrix of the limestone breccias is formed by mi-

crosparitic to sparitic calcite with a clayey admixture. The
matrix is relatively homogeneous without allochems. Be-
sides the matrix, a syntaxial calcitic cement is found at the
rims of some clasts. The clasts are dominantly angular to
subrounded with signs of corrosion. The contacts between
clasts and matrix are frequently obscured and rather resemble
to gradual transition. The breccia is distinctively polymodal,
clast sizes vary between 0.5 to 10 cm. Sporadically the clasts
show sedimentary structures, e.g. lamination, fenestral po-
rosity, loferite structure which are typical for (the described)
dolomites. They are formed by neomorphic calcite sparite
with small inclusions (relics) of dolomite. Micritic aggregate
grains with ghosts of a pseudo-oolitic texture can be ob-
served locally. In the sparite mosaic some grains thicken to-
wards rims, which are formed by completely pellucid rhom-
bohedral calcite grains. Generally recrystallization is
manifested by the presence of numerous inclusions and au-
thigenic minerals, e.g. pyrite. The breccias studied in the
well K-4 were gradually formed by leaching/dissolution and
neomorphic calcite grains substituted the original dolomitic
material.

Properties of the dolomite porosity

The comparison of measurement results gained by a few

methods (Table 1, Fig. 2) confirms a significant variability in
the dolomite porosity in the wells K-3 and K-4 or confirms
some increase of porosity towards depth. This increase at the
depth below 3600 m (Fig. 2) is connected with dolomites or
dedolomitized breccias, which is documented by porosity
based on logs processing of the well K-3 (Fig. 2). On this ba-
sis, it is impossible to make any statement as to the absolute
values of porosity, but the relative changes of reservoir rock
properties provide valuable information which shows that seg-
ments with an increased porosity coincide with intervals of

(late) diagenetic alteration. Apart from significantly (fracture)
porous and permeable dolomites in the beds directly underly-
ing the Neogene, which represent the old erosional surface of
the Triassic dolomites, and are associated with two known de-
posits in the Slovak part of the Vienna Basin, it is possible to
identify further important reservoir horizons. In both wells, K-
3 and K-4, reservoir rocks did not contain gas, but were filled
with salt water. For comparison Ostrolucký (1994) gives mean
porosities for the Opponitz Limestone Formation (K-3) of up
to 7.33 % and a permeability of 3.56 mD, simultaneously he
gives a mean value for the porosity of all the dolomites (reser-
voir rocks) of this well of 1.3 % and a permeability of 0.2 mD.

From the viewpoint of reservoir rock properties we can

classify the petrographic types as follows:

1. The coarse-grained dolosparites of a sugary appearance

are characterized by a predominantly planar polymodal coarse
crystalline mosaic with sizes of crystals above 100 

µ

m (mean

250 

µ

m in size). The extensive dolomitization resulted in a to-

tal destruction of the original rock and therefore we cannot ob-
serve any primary sedimentary textures and structures. Porosi-
ty values (Table 1) reach an average of 2.5 % (the interval
0–7.5 %), scanty communication — permeability of intercrys-
talline porosity represents a certain insufficiency.

2. The laminated muddy dolomites and dolomitic lime-

stones of mudstone type with anhydrite are characterized by
predominance of micritic, microsparitic types of the matrix
with sizes of dolomite crystals of up to 50 

µ

m generally with-

out clasts or other allochems. The anhydrite formed synsedi-
mentary laminae, but was secondarily mobilized and repre-
sents fill of veinlets, stylolites and fractures. The original
sedimentary structural-textural elements are well preserved.
The values of the total porosity (Table 1) are low — 1 to 2 % ±
2 %, the rocks are, with exception of breccia layers, nearly im-
permeable.

The calcarenites of dolograinstone type with irregular clast

recrystallization and dolosparitic cement are characterized
by various stages of original sedimentary texture and struc-
ture preservation. Clasts are predominantly formed by pel-
lets, bio and lithoclasts. Dedolomitization results in a brec-
ciated to microsparitic nature with ghosts of clasts. The
porosity values of the original dolomites were low — 0–5 %,
but dedolomitized layers (Table 1) have higher porosity val-
ues — 5–12 % with a relatively good microfracture porosity.
These layers are some of the best reservoir rocks within the
whole carbonate complex.

Fig. 4. 

Thin section photos of dolomites from well K-3 (cores 8 to

14). 1 — Coarse- to fine-grained dolosparite with relics of the dolo-
micrite in the pseudoclastic structure (2910 m, magnification 7

×

).

— Dolosparite with ghosts of bioclasts (ostracods, foraminifers)

and dolomicrite relics (2849 m, magnification 9

×

). 34 — Dolomite

with stromatolitic (loferite) structure (3091.8 m & 3235.5 m, magni-
fication 7

×

). 5 — Laminated finecrystalline anhydrite with thin lam-

inae of dolomicrite (3178.3 m, magnification 7

×

). 6 — Coarse crys-

talline anhydrite with dolomite relics (3089 m, magnification 7

×

).

7

8 — Dolomitic breccia with a stromatolitic structure in the clast

(3237.4 m & 3012 m, magnification 7

×

).

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DIAGENESIS AND POROSITY OF THE UPPER TRIASSIC CARBONATES                                         377

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378                                                                            MASARYK and LINTNEROVÁ

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DIAGENESIS AND POROSITY OF THE UPPER TRIASSIC CARBONATES                                         379

SEM

A total of 15 samples from the dolomite intervals (Fig. 3)

separated by limestone layers were selected from the well K-3

for SEM study. The first layer was caught by cores 6 to 14
(the interval 2708 to 3240 m), and the second one by cores
20–21 (the depth 3676 to 3830 m).

In the SEM micrographs it is possible to distinguish two

grain size classes in fine-grained dolomites (Fig. 6A–B). The
dolomite matrix is formed by grains below 10 

µ

m in size and

a coarser grained crystalline phase, above 10 to 100 

µ

m in

size. The grains reach in average around 50 

µ

m in size and

are of typical rhombohedral morphology. These rhombs
grow partly into opened pores, partly replace matrix
(Fig. 6A–B). Their presence generally increases intergranu-
lar pore spaces most expressed in specimens of cores 6, 6a
(Fig. 6B). On rhombohedrons’ surfaces we can see intracrys-
talline pores and fractures, which were a result of proper re-
crystallization of the dolomite. Diagenetic dissolution by

Fig. 5.

 Thin section photos of dedolomites from well K-3 (cores 20

to 22). 1–2 — Oolite dolograinstone with preserved interparticle
and intercrystalline porosity (3676 m, magnification 86

×

, 7

×

). 3–4

— Neomorphic sparry calcite replacing dolomicrite matrix with
vuggy macroporosity ( 3829 m, magnification 86

×

, 7

×

). 5  —

Pseudomorphic dolomitic texture of sparry calcite (3829.5 m,
magnification 86

×

). 6–7 — Neomorhic sparry calcite with relics

of dolomite (3827 m, 3831.5 m, magnification 86

×

). 8 — Pseudo-

morphose after dolosparite (the lower part) and neomorphic cal-
cite with relics of dolomicrite (the upper part), (3829.5 m, magni-
fication 45

×

).

Fig. 6.

 Microphotographs (SEM) of the dolomites in K-3, cores 6 to 13. A — Microsparitic dolomite rhombs in the matrix replaced by

dolosparite rhombs. B — Large dolomite grains were dissolved and the new intragranular porosity was formed. C — Framboidal pyrite
aggregates were grown among dolosparite crystals. Dolomite grains were also partly dissolved. D — Sparry calcite grains were formed in
the pore space and also calcite filled fine veins. E — Dolomite matrix with siliclastic mineral coating (mica, illite), often on the stylolites.
F

 — Large crystal aggregates of anhydrite in dolomite.

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380                                                                            MASARYK and LINTNEROVÁ

pore solutions was indicated by disturbed rhombohedral
grains (Fig. 6B–C) and in places even by irregular residual
grains that were paler. The new phases in dolomites are rep-
resented by calcites (Fig. 6D) in joints as well as framboidal
pyrites (Fig. 6C) studied in specimens of cores 6 to 8. Anhy-
drites reduced by organic substances were probably the
source of the sulphur (Fig. 6F). The dolomite grains near the
framboidal pyrites were also (slightly) corroded (Fig. 6C).
The calcite crystals closely fill joints and are massive with-
out surface disturbance (Fig. 6D). Clastic minerals can be
seen at grain boundaries in typical laminae, deformed after
formation of carbonate grain. They were represented by mica
or chlorite grains pressed out by the dolomitization, but also
by (successive) pressure dissolution. Tiny dispersed particles
at grain surfaces (Fig. 6E) are probably authigenic illites.

The dolomite matrix of the core 20 is microsparitic and it

forms idiomorphic rhombic grains larger than 10 

µ

m, in av-

erage 20 to 40 

µ

m in size (Fig. 7A–C). SEM study showed

that specimens of the lower part of core 21 and in core 22
were formed by predominantly well crystallized, perfectly
limited crystals. We can recognize typical crystalline forms
of calcite, e.g. ditrigonal scalenohedral (Fig. 7C–F). We did
not notice rhombohedral neomorphic crystals. Neomorphic
crystals did not bear any signs of surface disturbance but
were strewn with residual (pale-dissolved) irregularly limit-
ed grains or dust (Fig. 7E–G). Grains frequently bear growth
defects or inclusions of smaller crystals. These crystals do
not fill only cavities but seem to substitute the essential
(large) volume of specimens. Locally grains were closely
crammed (Fig. 7D) but as a whole specimens seem to be rel-
atively loose, porous (Fig. 7E–G) for example in comparison
with calcitic cement in joints (Fig. 6B). We found authigen-
ic kaolinite in etched surfaces (Fig. 7H–I) and from these
parts of cores fluorite was also described (Mišík 1986). Ka-
olinite forms platy pseudohexagonally limited crystals. We
also sporadically observed grains of grown feldspar or
quartz, again in etched surfaces.

From the well K-4 we took 5 specimens from cores 10 to

12, depth interval of 3641 to 3734 m (Fig. 3). Dolomites
were found only in core 10 and they had increased calcite
content (Table 2). Calcite forms predominantly the fill of
cavities, pores and joints (Fig. 8A). Dolomitic micritic to mi-
crosparitic grains (up to 10 

µ

m in size) bear quite apparent

traces of dissolution (Fig. 8B). Cores 11, 11A and 12 have
the appearance of a dolomitic breccia but they were proven
by chemical analysis to be actually limestones (Table 2).
These limestones displayed higher variability of grain size,
intercrystal porosity as well as of the surfaces disturbance
and presence of relic grains of the preceding phases. The
most important for us seemed to be the presence of „fresh di-
agenetic“ calcite grains (Fig. 8C–E) and crystals of clay min-
erals (Fig. 8E–F). These calcite grains were similar after
their morphology to grains studied in the core 22 of the well
K-3 (Fig. 7D–G). Crystals are without a surface dissolution
disturbance but they contain growth’s defects — pores, fre-
quently with crystalline negative morphology. Small relic
grains of the original phase are also present on the surface of
grains and in intercrystal spaces. Some of them have partly
rhombohedral limitation, similar to dolomite grains from the

core 10. Authigenic clay minerals, probably illites in their
typical fine grained form (Fig. 8F) cover the surfaces of cal-
cite grains and fill pore spaces between crystals. In speci-
mens from cores 11A and 12 distinctive signs of recrystalli-
zation and traces of dissolution were also observed
(Fig. 8H–I). On fracture surfaces we could observe preserved
isles of the original phase — fine-grained dolomite which
seems to be displaced towards crystal rims or into the inter-
crystal spaces of neomorphic calcites (Fig. 8H–I). According
to relations between grains, we can see that neomorphic clay
minerals were probably crystallized simultaneously with the
calcite grains formation (Fig. 8H) or later and grow into free
intercrystal spaces (Fig. 8F).

Geochemical-mineralogical analysis of specimens

We want to document by chemical analyses of specimens

that in cores 21 and 22 (Fig. 3) alteration of the chemical and
mineral composition of the rock arose. In one core (21) the
dolomite or dolomite breccia, has been altered into limestone
(limestone breccia) although the rock is of a dolomitic ap-
pearance. The neomorphic limestones are „pure“ and contain
hardly any aluminosilicate minerals. The limestone layers
continue into core 22 which also has a brecciated appear-
ance. Dolomite content slightly increased in specimens of
core 22 (Table 3). By analogy the breccia from K-4 was
formed by calcites (Table 3), but had an increased or more
variable content of clayey material, as well as of authigenic
material as it was documented also by SEM microphotogra-
phy. Their content in original clasts changed. Mineral com-
position was studied by X-ray diffraction. X-ray records
were assessed by a semi-quantitative method and the ob-
tained assessments correspond well to the mineral composi-
tion calculated from chemical analyses of rock specimens or
from the calcium and magnesium content determined by
classical chemical analysis (Tables 2–3).

Finer-grained, less crystallized dolomites from the well K-3

were close to stechiometric composition (50.5 to 52 mol. % of
CaCO

3

), coarse-grained (sparitic) reveal a slight content of

lime (53 to 56 mol. % of CaCO

3

). However, referred variabili-

ty could reflect alteration processes either in the topmost part
(disturbed rhombohedrons — core 6,6A and relatively little of
calcite cement) or in the lower part in cores 20 and 21. The ste-
chiometry differences were not clearly manifested in differen-
tiation of dolomite crystal ordering. The ratios of reflection in-
tensities 221 and 101 (Lumsden 1979; Hardy & Tucker 1988)
were relatively high in both dolomite grain size groups and

Fig. 7.

 Microphotographs (SEM) of the dolomites and calcites

(dedolomites) in the K-3, cores 20 to 22. A–B — Dolomite mi-
crosparitic to sparitic matrix formed by idiomorphic rhombs. C —
Leaching or partly dissolved dolomite rhombs on both grain gen-
erations. D — Large neomorphic idiomorphic calcite crystals cov-
ered with dolomite ash. E — Neomorphic calcite in some part of
sample filling the space completely. F–G — Calcite crystals (ded-
olomite) with „open“ intercrystalline pores with residual dolomite
grains. H–I — Neomorphic platy kaolinite crystal on the calcite
surface (etched by acid).

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382                                                                            MASARYK and LINTNEROVÁ

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DIAGENESIS AND POROSITY OF THE UPPER TRIASSIC CARBONATES                                         383

were in the range 0.6 to 1.0. Dolomites with evaporates and
specimens with quartz (cores 11 to 13) were not assessed by
this method. Contents of microelements in rock specimens
(Table 2) documented relative differences in composition of
dolomite/limestone layers. Analyses of separated parts of
breccias (Table 2), clasts and matrix/cements show partly
comparably high quantities of observed elements (Sr, Fe, Mg
or Na) in the observed parts, and also comparably higher con-
centrations than in the limestones. The neomorphic calcites
have a higher content of observed elements than the overlying
dolomites. Thus, if they originated by substituting dolomites,
then these alterations indicate buried diagenesis in a buried
sediment and the operation of diagenetic solutions. On the oth-
er hand fluids with a sufficient concentration of calcium or
brines could be obtained by dissolution of evaporite layers or
cements (Fig. 3; the well K-3, cores 12 to 14), although the
fluids could be originally of a meteoric origin (e.g. buried to-
gether with sediment below the Neogene fill, or descending to
depth along faults during different time periods). High con-

tents of Na could indicate the presence of sea water pore solu-
tions or rock brines in the recrystallization process.

Isotopic analysis

We can interpret two basic trends in the distribution of data

in both fields of isotopic data (Fig. 9) which reflect on the
one side preserved sedimentary or early diagenetic dolomiti-
zation distribution of values of isotopic ratios and on the sec-
ond later or deep burial diagenetic influences on isotopic ra-
tios of O and C. The specimens from the Hauptdolomite
Formation (cores 6 to 14, Fig. 9, field 1) have more positive
values of 

δ

18

O than dolomites or calcites from the Opponitz

Limestone Formation and indicate the evaporitic type of do-
lomitizing solutions, probably the sabkha type of dolomitiza-
tion in strata overlying an evaporite sequence. The values of

δ

13

C for these specimens were in the range of normal sea val-

ues (0 to 4 ‰), however, they were slightly decreased (Ta-
ble 3). The specimen from core 6 is fairly shifted to lower
values both for 

δ

13

C (+0.1 ‰) and 

δ

18

O (–2.0 ‰). The dolo-

mites of this core were most tectonically disturbed, but cal-
cite content was low here. The values of couples calcite-do-
lomite were close though they were lower in younger
calcites. The calcites had to be formed in joints from pore so-
lutions which did not significantly differ in the origin of C
and O, but they could be formed at higher temperatures.
However, it could even be a case of mixing of (meteoric) so-
lutions/water dissoluing dolomites in surface (pre-Neogene)
conditions and calcites were formed from them. Limestones
and dolomites of the Opponitz Limestone Formation form
the second field (Fig. 9, field 2) and indicate a different sedi-
mentation or also a dolomitization environment. The values
of 

δ

18

O were predominantly lower and 

δ

13

C were predomi-

nantly higher than for the Hauptdolomite Formation speci-
mens. In dolomites (only 3 analyses) values of 

δ

18

O were

considerably variable in comparison to limestones. The lime-
stones have preserved balanced original (sedimentary) high
values 

δ

13

C which do not follow smaller variations in the

values for 

δ

18

O in the set. In comparison of the couple cal-

cite-dolomite from core 20 we can see that dolomite has
higher ratios of C and also O than the neomorphic calcite
(Table 3) which indicates a (late) diagenetic origin of the cal-
cite (Fig. 9, field 2´). In the value set for the Opponitz Lime-
stone Formation specimens we can follow a more distinctive
diagenetic trend towards the basement (Table 3). The most
significant is the decrease of the 

δ

18

O ratio and also a slight

decrease of the 

δ

13

C ratio in the calcites. Changes of the

same nature were manifested in both ratios, but 

δ

13

C re-

mained relatively small. An exception is represented by a
calcite specimen from the well K-4 (core 11) which has a
more significantly decreased carbon ratio (Fig. 9). The two
specimens were partly enriched in C

12

, and in both cases this

could be also the influence of carbon of an organic origin, for
example as a result of the decay of organic matter (reduction
of sulphates) as well as the influence of mixing of different
solutions. Our set also includes a specimen of the Reifling
Limestone Formation from a depth of 4902 m (i.e. approx.
100 m deeper). This limestone has no apparent signs of di-

Fig. 8. 

Microphotographs (SEM) of the dolomite and the calcite in

the K-4, cores 10 to 12. A — Micrite to microsparite dolomite ma-
trix with large calcite grains in the join. B — Microsparite dolo-
mite rhombs were dissolved/leached (detail from A). C–D — New
generation of the idiomorphic calcites, with the characteristic cal-
cite crystal-morphology. E–F — Neomorphic illite particles cov-
ering calcite surface and also filling the open pore spaces between
grains. G — Siliclastic minerals mainly accumulated in the stylo-
lite were altered and form new mineral phases. H–I — Dissolution
and substitution of dolomite crystals by calcites. Small residual
dolomite rhombs were accumulated among large calcite grain.

Table 3:

 Mineralogical composition and isotope analyses of the

rock samples.

cl — clast,  ce — cement

Sample

calcite

dolomite

IR

calcite

dolomite

wt. %

per. mil PDB

δ

13

O

δ

18

O

δ

13

C

δ

18

O

K3

6/2708

  6.67

91.67

1.66

0.1

-2.1

7/2704

  8.15

90.80

1.05

3.0

1.3

8/2851

  8.19

91.06

0.85

2.1

0.2

2.4

0.5

10/2952

12.05

84.08

3.78

1.2

0.6

1.4

0.9

11/3011

  5.18

83.58

11.24

2.3

0.2

12/3090

  0.39

11.50

88.11

0.5

-0.1

13/3177

29.58

70.11

1.8

-0.3

14/3237

14.30

79.22

6.48

2.9

-2.0

15/3391

94.72

  1.88

3.23

3.8

-3.7

15/3392

93.43

  3.73

2.67

3.7

-3.6

19/3663

66.32

17.41

16.27

2.5

-4.3

2.7

-4.8

20/3676

  9.94

89.63

0.56

3.3

-2.1

4.1

-1.6

20/3677

  8.71

90.50

0.46

3.9

-1.4

21/3825.5

81.20

17.75

1.05

1.5

-7.6

21/3825.8

90.32

  8.86

0.92

1.4

-8.6

21/3826

95.11

  4.68

0.21

1.5

-9.0

22/3827

90.11

  9.47

0.42

3.5

-4.6

22/3827.5

85.92

12.90

2.18

2.0

-8.6

22/3828

92.73

  5.71

1.56

1.8

-7.7

34/4902

86.96

  3.42

9.16

1.7

-3.9

K4

11/3697

92.36

  5.51

2.18

-0.7

-7.4

12/3730cl

90.26

  5.00

4.74

1.1

-6.4

12/3730ce

90.26

21.68

1.4

-8.2

12/3732

80.08

  6.43

10.87

12/3732cl

60.10

29.90

1.2

-8.0

12/3732ce

63.40

26.60

1.2

-8.2

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384                                                                            MASARYK and LINTNEROVÁ

agenetic alteration, (Masaryk et al. 1993) as with limestones
from core 15. It differed rather by isotopic ratio of C than O
and the decreased ratio could be a reflection of an originally
deeper sedimentary environment.

Discussion

Taking into account microstructural properties as well as

changes of the chemical composition of dolomites or cal-
cites, from wells K-3 and K-4, we judged that dolomites
were also diagenetically altered under conditions of deep
(present) burial (Fig. 3). We described a diagenetic stratum
characterized by the presence of a new generation of coarse
grained, well crystallized calcites (Figs. 7–8) in the Opponitz
Limestone Formation at a depth of approximately 3660 to
3830 m with an observed significant increase of porosity.
These calcites substituted partially or totally the original sed-
imentary dolomite breccia in wells K-3 and K-4. In both
wells this layer is situated at an equivalent depth which itself
indicates that it was created under conditions of the deep
(present) burial, especially since we know that in spite of the
close position of these wells (Fig. 1), they did not penetrate
the same lithological succession. In well K-4 the Opponitz
Limestone Formation is in a reversed position indicating the
tectonic complications in the basin basement. Diagenetically
altered beds are joined rather to layers of original dolomite,
and or dolomitized breccias. As a matter of course, there was
a disadvantage in this comparison caused by the fact that we
could study only separated segments of sequence from the
well cores and the missing segments were much longer than
the available cores.

Based on our results we assume that the neomorphic cal-

cites were the result of the dedolomitization process in a
deeply buried sediment. Dedolomitization was originally de-
scribed as a close subsurface process, but later on also for
different conditions and depths (DeGroot 1967; Back et al.
1983; Land & Prezbindowski 1981; Stoessel et al. 1987;
Kastner 1982; Budai et al. 1984; Loucks & Elmore 1986 and
others). We noticed the disturbed, mostly dolosparitic grains
in the dolomites of the highest part of the basement (e.g.
cores 6, 6A, 8, Fig. 6). However calcite fills have mainly tec-
tonic joints (Figs. 4, 6D). The disturbance/dissolution of do-
lomites has a similar nature in a deeper part of the well sec-
tions (cores 20 to 21), but the neomorphic calcites are
entirely different. According to our observation, calcites
crystallized not only in joints, but they substituted a greater
volume of the dolomite or dolomitic breccia. A style of dolo-
mite rhombohedrons substitution is characteristic for the
dedolomitization (Rao 1969; Mišík 1988; Holail et al. 1988)
and can virtually be considered as a primary proof. We used
a SEM for the study of microstructures as well as an optical
microscope. The SEM microphotographs illustrated relations
between the new and preceding grains well. The neomorphic
calcites are different from calcites in joints and their mor-
phology is quite similar to laboratory evolved dedolomite
crystals (Stoessel et al. 1987). It is also possible to see the
presence of other mineral phases and judge the relative (tem-
poral) succession of the origin of phases. We could see the

presence of post-dedolomitization phases — illite, kaolinite,
pyrite or fluorite. Fluorite was described together with celes-
tine by Mišík (1986) in well K-3. The coexistance of these
phases documents the efficiency of diagenesis and the extent
of deep burial conditions. Neomorphic illites spreading to the
free spaces between calcites (Fig. 8) could be a good indica-
tor of such a process. Kaolinite with a platy-morphology
(Osborne et al. 1994) is also characteristic product of diagen-
esis in deep burial basin conditions (at least 2000 m).

Large, idiomorphic calcite crystals evidently needed a suf-

ficiently long time for their formation. The presence of im-
permeable beds (e.g. the Lunz Formation, Fig. 3) as well as
the actual depth (below 3 km) favours the lateral flow of the
solutions. However, it is impossible to exclude the role of
tectonic joints as ways for dedolomitizing solutions. In the
highest part of dolomites almost exclusively tectonic joints
are filled by calcite. It is evident that also in a dedolomitized
layer some calcites crystallized into free space (tectonic frac-
tures?). However, proper calcitized parts were frequently
packed, which was indicated by measured porosity data. The
first (principal) increase in porosity was attached to partly
substituted dolomites where there are many partially dis-
solved grains. The porosity was also decreased by a subse-
quent illite formation (mostly K-4, Fig. 8). The affect of so-
lutions rich in calcium and sulphates/chlorides was
considered in the interpretation of dolomite alteration. We
assume that, for example, dissolved anhydrite layers could
serve as the source of calcium. A sufficient amount of Ca

2+

 is

a critical factor for the process of dedolomitization (Kastner
1982; Back et al. 1983; Stoessell et al. 1987), and not a high
content of sulphates (Katz 1968; Land & Prezbindowski
1981; and others). However, the composition of the solutions
is still not clear (Land & Prezbindowski 1981; Stoessell et
Moore 1985).

Dedolomites created under conditions of deep burial (Bu-

dai et al. 1984) differ from close subsurface ones by having a
relatively high content of elements such as Sr, Fe, but also
Na. Na indicates non-meteoric origin of solutions (of water)
leading to the formation of the neomorphic calcites. It is typ-
ical for Fe that it is joined to the carbonate and oxide or hy-
droxide minerals. Comparing the content of observed ele-
ments in dolomites, limestones and diagenetic calcites, we
can see that neomorphic, coarse grained calcites have rela-
tively high content of the above mentioned elements, higher
than dolomites.

The isotopic ratios of O and C in the neomorphic calcites

significantly decreased, especially that of 

δ

18

O. The 

δ

13

C is

shifted only relatively, and still represents values of marine
carbonates and such values Budai et al. (1984) mentioned for
a joint type of dedolomites. The isotope values of the studied
rock specimens are in general comparable with the previous-
ly studied Triassic carbonates (Lintnerová & Hladíková
1992; Soták & Lintnerová 1994), and with some ancient
sabkha facies (Tucker 1990). Calcites forming fill of joints in
tectonized dolomites have also relatively high isotopic ratios,
although relatively lower then adjacent dolomites. It is im-
possible simply to identify these conditions only as a reflec-
tion of the thermal differentiation of isotopes in respect to an
evaporite environment and enrichment of oxygen ratio in a

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DIAGENESIS AND POROSITY OF THE UPPER TRIASSIC CARBONATES                                         385

heavier isotope connected with that. This process can also be
considered for late diagenetic (dedolomitization) fluids, al-
though in a longer transport (higher amount of water) this ef-
fect could become less important. The Vienna Basin is re-
garded as a „cold basin“ and the interpolated temperatures at
the depth of 3000 to 4000 m are 107 to 127 

o

C (max. 150 

o

C:

Franko et al. 1995). From diagenetic alteration of the organic
matter of the Lunz Formation (3940 to 4550 m) a tempera-
ture range of 100 to 120 

o

C was deduced (Borza et al. 1985).

The process of dedolomitization could quite well operate at
such temperatures (Stoessell et al. 1987).

Conclusions

1. Diagenetic post-sedimentation processes influenced the

properties of the dolomite rocks to a different extent. Most ex-
tensively the breccia layers in the depth of 3660 to 3830 m
were altered, so that significant values of secondary porosity
(locally up to 15 %) appeared.

2. The top parts of the dolomite sequences forming the ero-

sive surface of the Mesozoic basement to a certain extent pre-
served their properties since they were formed by dolomitiza-
tion in a sabkha environment. This is indicated by their
relatively high (evaporitic) isotopic ratio of O and C. Values of
isotopic ratios of calcitic and dolomitic cements (tectonic
breccia) were at least lowered, which could have been affected
precisely by the evaporite participation in the sequence.

The porosity changes are found in the calcitic dolomite to

limestone layers where sparitic, idiomorphic calcites with re-
mains of preceding dolomite can be observed. The microstruc-
tural and also geochemical properties of these calcites indicate
their late-diagenetic origin in the buried sediment. The 

δ

18

O

was significantly decreased, but the content of microelements
as well as values of 

δ

13

C remained relatively high.

Acknowledgements

: The analyses of the carbonate rocks

were performed in the chemical laboratories of the Geological
Institute of the Slovak Academy of Sciences (Bratislava). We
are grateful to Dr. B. Toman, Dr. A. Čelková and Dr. E. Mar-
tiny, and also to Dr. I. Holický for helpful assistance during

SEM study. The porosity measurements were done in the labo-
ratory of VVNP, (Research Oil Company, Bratislava) and our
thanks are given to Dr. S. Jakubov, Mr. J. Valček. The optical
colour porosimetry analyses were performed by Dr. J. Bebej
from the Geological Institute of the Slovak Academy of Sci-
ence, Banská Bystrica. The isotopic analyses were performed
in the Laboratories of the Czech Geological Institute in Pra-
gue. We are grateful to Dr. J. Hladíkova for precise analyses of
calcites and dolomites. We would like to thank the head of
VVNP, (Research Oil Company), Dr. J. Kováč for financially
supporting of the porosity measurements and isotopic analy-
ses. This work was partially supported by the Scientific Grant
Agency (VEGA) of the Ministry of Education of Slovak Re-
public and the Slovak Academy of Science (GJA 1/4090/97).
We are very obliged to Dr. J. Zelman (VVNP, Research Oil
Company) for the translation of this paper and Mrs. V. Mak-
kyová for typing. We want to give thanks to all anonymous
reviewers for their critical comments on the manuscript and
their suggestions to improve the paper.

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