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Physical and chemical properties of flysch sediments in the

Ždánice oil deposit

(Outer Western Carpathians, Czech Republic)


















Charles University in Prague, Faculty of Science, Albertov 6, 128 43 Prague 2, Czech Republic


Exploranium CZ, Ltd., Hudcova 56b, 621 00 Brno, Czech Republic; *


Czech Geological Survey, Leitnerova 22, 658 69 Brno, Czech Republic


Slavíčkova 415/10, 638 00 Brno, Czech Republic


Geomin Cooperative, Znojemská 78, 586 56 Jihlava, Czech Republic


Boháčova 866/4, 149 00 Praha, Czech Republic

(Manuscript received October 21, 2005; accepted in revised form June 22, 2006)

Abstract: A petrophysical investigation was carried out on the hydrocarbon deposit, which occurs in East Moravia near
the village Ždánice at the depth of 1 km. The objective of investigations was to find out whether there is a significant
difference in the radioactivity of the rock and in other studied parameters between rocks above the deposit and rocks
from a distance of 2—3 km. Shallow boreholes were drilled above the deposit and in its vicinity to get rock samples
(claystones and siltstones – CLS and sandstones – SNS). Petrophysical parameters – densities, porosity, magnetic
susceptibility, radioactivity, and contents of carbon and sulphur were determined and statistically evaluated. Statistical
tests of difference between petrophysical parameters of rocks over the oil deposit and outside the oil deposit show a low
indicative power of separate petrophysical parameters, while their comprehensive application may enhance their petro-
leum indicative value.

Key words: radioactivity, densities, magnetic susceptibility, oil deposit, prospective indicators, flysh sedimentary
rocks,  contents of sulphur.


In East Moravia between the villages Nevojice and
Ždánice a hydrocarbon deposit occurs at the depth of ap-
proximately 1000 m (Krejčí 1993). The deposit is located
in the surface part of Upper Proterozoic granitoids, with
overlying flysch sediments and overlying Miocene sedi-
ments buried by Carpathian flysch thrust sheets. Above
the deposit and in its vicinity, shallow boreholes (5 of
them 20 m deep and 13 ones 12 m deep) were drilled to
search for indicators of a buried hydrocarbon deposit in
near-surface portions of the flysch sediments (Fig. 1). A to-
tal of 444 borehole samples were collected for determina-
tion of the contents of natural radioactive elements and
specific activity of 


Cs and 300 samples for determina-

tion of densities and porosity and magnetic susceptibility.
Two hundred selected samples were taken for determina-
tion of mineral and organic carbon and content of sulphur.

The objective of the investigations was to find out

whether there is a significant difference in the studied pa-
rameters between the boreholes above the deposit and the
boreholes situated at the distance of 2—3 km. Research
conducted in other countries found various changes in
sediments over hydrocarbon deposits, including radioac-
tivity, for example Shideler & Hinze (1971) or Saunders et
al. (1993), which prompted the assumption that similar

petrophysical changes in overlying beds might be present
at the Ždánice deposit, though the deposit proper occurs
at a depth of approximately 1 km under the surface.

Geology of the deposit and its vicinity

The Ždánice oil-gas bearing deposit can be found on

the NW border of the pre-Devonian elevation in the
Ždánický les Highlands. The reservoir rocks of natural hy-
drocarbons are tectonically deformed and weathered
granitoids (granodiorites, diorites, rarely granites) of
Brunovistulicum and Lower Miocene sediments (sand-
stones and conglomerates) of the Carpathian Foredeep.
The seal of the deposit was formed during Late Miocene
by the tectonically emplaced Ždánice thrust sheet, which
attains the thickness of 750 to 870 m over the hydrocar-
bon field (Krejčí 1993). From the orographic viewpoint
the area of interest is part of  the Ždánický les (Forest) unit
whose highest landmark, elevation 438 m “U slepice”, is
situated in the middle of the area.

The Ždánice Nappe, which is the subject of our study, be-

longs to the Outer Flysch Belt of the Western Carpathians.
During the orogenetic movements after the Lower Miocene
the nappe shifted far to the south-eastern margin of the Eu-
ropean Platform. Its rock structure is made up of Paleogene

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to Lower Miocene sediments of the Ždánice Unit and Qua-
ternary cover. The Ždánice Unit contains the Němčice,
Menilite and Ždánice-Hustopeče Formations.

The Němčice Formation (Paleocene to Lower Oli-

gocene) is composed of prevalent green-grey, brown-grey
and grey, and scarce red claystones to clays, alternating in
irregular interbeds and schliers, with thin intercalation of
sandstones, and sporadic occurrences of conglomerates.

The Menilite Formation  (Oligocene) is made up of dark

brown bituminous shales with cherts, overlying brown
Dynow Marls and Šitbořice Member with characteristic
rhythmically alternating green-grey and grey claystones
with fine-grained sandstones.

The Ždánice-Hustopeče Formation  (Upper Oligocene to

Aquitanian) is the most extended strata within the study
area. It is characterized by alternating intercalations,  lay-
ers and bodies of light grey, as a rule little consolidated
sandstones, siltstones and grey lime claystones. There are
also scarce beds of conglomerates. A strong facial variabil-
ity is typical of the strata.

The study area is part of the Ždánice synclinorium where

fold structural elements strike in the south-eastern—south-
western direction. Dipping beds, only observed in the
Ždánice-Hustopeče Formation, are prevalently medium.
The Menilite and Němčice Formations crop out in the core
of the anticline in the east, and the Menilite Formation in
the south-west of the area, presumably in the overthrust
line. The sediments of the Ždánice Unit were folded during
neo-Alpine orogeny. According to the study of magnetic

anisotropy (Hrouda & Stráník 1985) the deposits
of the Ždánice Unit underwent at least two
foldings: in the Late Oligocene and after the
Early Miocene. During the post-Early Miocene
orogeny the sedimentary fill of the Ždánice ba-
sin was folded and detached from the home area
and thrust in the form of a shear nappe over the
Lower Miocene cover of the Ždánice elevation
crystalline unit. The fold structures are dis-
turbed with south-western—south-eastern trans-
verse faults. Natural water springs, in places
with structural foamstones affiliated with faults,
testify to their recent activity.

Measuring methods and evaluation

Mineralogical density

 (Dm), which is an

important material parameter, was measured
together with porosity (Por) giving the vol-
ume of pore space and bulk density (Do) by
the ‘triple weighting’ method. Samples were
dried up at a temperature of 100 ºC up to con-
stant mass, weighted and then in vacuum satu-
rated with kerosene, which is a suitable liquid
as it does not penetrate the crystalline struc-
ture of clayey minerals and easily fills all open
pores. The fully saturated samples were weight-
ed again in kerosene and in air. Standard devia-
tion of one determination is  ± 0.001 g · cm



Dm, ± 0.003 g · cm


for Do, and  ± 0.02 % for porosity.

Magnetic susceptibility

 is a dimensionless parameter

expressed in SI units. Although it is a tensor, for observa-
tion of material changes its mean value (SUSC) is used.
Measurements were made on the kappabridge KLY-4 with
sensitivity of 10


 SI in the laboratory of company AGICO

in Brno. As a rule, the rock is composed of diamagnetic
minerals as quartz, pure felspars, calcite, and paramagnetic
minerals such as felspars with admixtures (SUSC in the or-
der of 10


 to 10


), mica, chlorides (SUSC in the order of



to 10


), amphiboles, pyroxenes and ilmenite (SUSC

in the order of 10


). Magnetic susceptibility is signifi-

cantly affected by ferrimagnetic minerals, mainly magne-
tite and maghemite (SUSC in the order of 10


 to SI units)

and monoclinic pyrrhotine (SUSC in the order of 10


), in

sediments also greigite. The antiferromagnetic minerals
hematite, hexagonal pyrrhotine, etc. are major carriers of
remanent magnetization (NMR). However, their SUSC
does not exceed the value of 10



Contents of natural radioactive elements Th, U and K

in the measured samples were determined by gamma spec-
trometry in the laboratory of Exploranium CZ, Ltd. Along
with natural radioactive elements, specific activities of



 were automatically determined from the measured

spectra. The samples crushed to grains under 3 mm were
closed in plastic cases, and when equilibrium between ra-
dium and radon was gained, they were placed in the
shielded NaI(Tl) detector 4

” in diameter and 4” in height,

with energy resolution of 7.9 % (662 keV). Spectra mea-

Fig. 1. Location of boreholes in the area between Nevojice and Ždánice.

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sured with a multichannel spectrometer PCAP (Nucleus
USA) were evaluated by comparison with spectra of the
IAEA standards from Vienna and IIZ standards from
Prague. The measuring time was 20 minutes, sample mass
ranged around 400 g. As U content evaluated on the basis
of low-power gamma radiation of 


Th and 


U isotope

is not very accurate, the content of 


Ra was determined

and recalculated to the content of Ura. If radioactive equi-
librium is retained in the decay chain, then the values of U
and Ura should be equal. However, differences frequently
occur in Quaternary sediments and altered rocks. Standard
deviation of one determination is  ± 0.3 ppm


for Th,

± 0.5 ppm


for U,  ± 0.1 ppm for Ura,  ± 0.04 % for K, and

± 4  Bq · kg





Contents of C and S

 were measured in the Testing Labo-

ratory of the Czech Geological Survey, branch Brno, on the
apparatus METALYT CS 100/1000S (ELTRA, Germany).
The procedure consists of determination of CO



in combustion at 1250 ºC (total C, TC), and determination
of mineral C (TIC) by sample decomposition by acid. The
content of organic carbon (TOC) is given by the difference
TC-TIC or is found by direct analysis of the decarbonated
sample. The content of S is measured analogically to TC ac-
cording to SO

generated in combustion at 1250 ºC.

A theoretical substantiation of observed differences in

contents of radioactive elements in sediments above the
hydrocarbon deposit and outside it was published by
Saunders et al. (1993). They assume that by the action of
organic acids and CO




clay and other minerals are decom-

posed, and consequently U and K are released and re-
moved. As a result, sediments affected by hydrocarbons
are poorer in these radioactive elements than sediments
not affected. It is also presumed that Th remains immobile.
The following derived radioactive parameters were
identified as suitable indicators of radioelement changes:

Kd = [Ka — (Kmo/Thmo) · Tha]/Ka
Ud = [Ua — (Umo/Thmo) · Tha]/Ua
Drad = Ud — Kd

Ka, Ua, Tha are contents of radioactive elements in indi-

vidual samples of the investigated sedimentary unit with
hydrocarbons occurrence (or altered by their products),
Kmo, Umo and Thmo are mean contents of these elements
in the same unit outside the deposit accompanying alter-
ations. The authors are of the opinion that negative values
of Kd and Ud and positive values of Drad indicate hydro-
carbons. Their findings, however, relate to arid climates,
and the theoretical model may not be generally valid and
the distribution of radioactive elements can be more com-
plicated (see Gnojek 1976; Borovec 1985; Fiala 1989; etc.).

Statistical data processing


The measured and derived values of studied variables

representing a random selection were, according to the

geological character of the samples, grouped into two
working sets: Claystones and siltstones (CLS) and Sand-
stones (SNS)


Both these lithological types differ in a number of

physical and chemical properties, and if they were evalu-
ated together, the results might not be objective.

Both sets were statistically processed with the objective

of finding out whether the values of the studied variables
measured above the deposit (samples designated by code
A) and outside the deposit (samples designated by code O)
come from one or two sets, or whether they can be used to
separate the deposit from its surroundings or not. As some
tests of good fit are based on the assumption of normal
distribution, normality of distribution of the values of in-
dividual variables was tested at first. For this purpose the
Kolmogorov-Smirnov normality test  was applied based on
the maximal difference between selective cummulative
distribution (its values are given by relative frequency in
the sampling),  and assumed cummulative distribution. As
a variant the Shapiro-Wilks  normality test W was applied.
It was preferred to other tests for its high efficiency
(StatSoft, Inc. 1999). Probability p-p graphs were con-
structed for visual control of data distribution normality.

The mean values of sets were compared by means of the

commonly used t-test  for independent groups of data (nor-
mal distribution of values and only slightly different
variation coefficients are required). In case of other than
normal distribution of values non-parametric tests were
applied – the Kolmogorov-Smirnov test  for comparison
of means in two independent selections expressing agree-
ment in the form of distribution curve and the Mann-
Whitney U test  which can also be used for work with sets
of a different extent of observation (Koshin et al. 1992),
which is this case. The level of significance

   in all statis-

tical evaluations was set at 0.05 (i.e. 95 %).


Claystones and siltstones (CLS)

The numbers of observations are not equal in all studied

variables within individual sets. However, the ratio of
samples collected above the deposit and outside it is ap-
proximately constant – 3 : 1. The Kolmogorov-Smirnov
and Shapiro-Wilks W tests showed that the condition of
normality is satisfied by a large number of variables (see
Table 1), mainly in the Kolmogorov-Smirnov test.

In the case of equal sets the below given results of t-test

can be considered reliable. Roughly a half of the vari-
ables, however, exhibit normal distribution values in one
subset only (Figs. 2, 3).

According to the results of good fit tests the mean values

of sets from above the deposit and outside it do not exhibit
substantial statistic differences. According to the t-test vari-
ables K and Drad are an exception, in the Kolmogorov-
Smirnov test (see Table 2) and the Mann-Whitney test
variables Th, K, and 


Cs. These variables could be statisti-

cally used as indicators of the deposit. In the case of 



we must not forget that its distribution was affected by the

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Chernobyl accident. Nevertheless, it is remarkable that its
specific activities in the two areas significantly differ.

Even though the mean values of the sets of values from

above the deposit and outside it are statistically different,
it is obvious that in practice differentiation of the manifes-
tation of the deposit from its geological surroundings is
not easy. Bar charts constructed for any of the suitable
variables (e.g. Th in Fig. 4) show that the sets nearly over-
lap in their range of values. On the other hand, it is highly
probable that in the Ždánice deposit the contents of Th
and also K are lower above the deposit.

As for sulphur, the two subsets  substantially differ. The

distribution of values, however, is distinctly asymmetric
(L-distribution) in both of them, but extreme values above
the deposit are much higher (up to 0.53 %), while in the
vicinity of the deposit they do not go beyond the limit of
sensitivity of the analytical method 0.20 % (Fig. 5).

There are almost no low contents of potassium under

1.65 % in the vicinity of the deposit, while above the de-
posit there are approximately 30 % of such values (Fig. 6).

Drad values over 0.330 are rare outside the deposit

(Fig. 7).

The susceptibility of claystones above the deposit also

attains relatively high values over 200 10


 (SI), reflect-

ing the presence of a ferromagnetic mineral (see below).

Sandstones (SNS)

The numbers of observations in the subsets of sand-

stones are again very different, while the number of
samples from above the deposit is roughly two to three
times larger than the number of samples from the vicinity
of the deposit. Similarly to the previous set, the normality
condition is satisfied by more variables in the Kolmogorov-
Smirnov test than in the Shapiro-Wilks W test. They are
listed in Table 3.

All three tests of good fit give roughly the same results.

According to the t-test, the mean values of variables Dm,
S, K and 


Cs are statistically substantially different. The

normality condition is not satisfied by any variable in ei-
ther subset. Strictly in terms of statistics, results of the t-test
are referential only. According to the Kolmogorov-Smirnov
test (Table 4), Dm, U and 


Cs display statistically sig-

nificantly different mean values, in the Mann-Whitney
test differences were found in Dm, S, U, K and 


Cs.  In

terms of statistics, these first four variables could be used
to identify the deposit. As in the previous set CLS, their
application is made difficult by an analogical range of U,
K and 


Cs values in both subsets.

Sulphur exhibits values analogical to the CLS set.

Above the deposit, increased values up to 0.37 % occur,
while no contents over 0.02 % were observed outside the
deposit (Fig. 5).

Although data distribution in K is nearly identical in

both statistical populations, outside the deposit right-
hand asymmetry is apparent and indicates decrease in po-
tassium over the deposit (Fig. 6).

In the subset of mineralogical density (Dm) values over

2.735 g· cm


 are rare and it is obvious that the mineral-

Table 1: Variables in subsets of claystones (CLS) with normal asymp-
totic data distribution.

Table 2: Results of good agreement test for claystones (CLS).

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ogical density of sandstones above this value is a signifi-
cant indicator of the area above the deposit (Fig. 8).

As far as uranium itself is concerned, the bar chart

(Fig. 9) shows an obvious left-hand asymmetry of the dis-
tribution of values measured above the deposit. This fact
also indicates that the difference in the content of uranium
above the deposit and in the vicinity of the Ždánice de-
posit is  much more significant in sandstones than in
claystones, which is presumably due to the better migra-
tion abilities of uranium in porous environments.

An attempt was made to identify the defined bodies

(above the deposit, outside the deposit) by means of hierar-
chical clustering. The Ward method was applied where the
block distance (Manhattan) was used as the rate of distance.
This attempt resulted in separation of the two initial data

sets (CLS, SNS) into two main sets.
However, they contain a mixture of
samples of both types.

In conclusion it can be said that dif-

ferentiation of the area above the oil de-
posit from its vicinity is not easy or
even impossible in a majority of the
evaluated parameters. To a certain ex-
tent this can be due to the different num-
bers of observations in sets and their
small number (samples from outside the
deposit), which significantly affects the
shape of the distribution curve (Žáček &
Křivánek 1991). Another cause may be
the sampling itself (large distance be-
tween sampled objects, approx. 1 km
from the deposit) and in the studied
variables. The content of sulphur seems
to be a good identification parameter,
which in both sets (CLS and SNS) ex-
hibits higher values in samples col-
lected above the deposit only (Fig. 5). In
claystones and siltstones these samples
represent 18 % and in sandstones nearly
27 % of values. Regardless of statistics,
content of sulphur can be regarded as a
good indicator of a potential oil deposit.
Analogically, magnetic susceptibility
maxima and minima of K content in

Fig. 2. Example of a variable with normal distribution of values in subsets.

Fig. 3. Example of a variable with different distribution of values in subsets.

Fig. 4. Bar chart for thorium.

claystones and siltstones or maxima of mineralogical density
or the ratio Ura/U in sandstones can be indicators of oil.
However, these indication samples represent only a low per-
centage of the whole set, and therefore should be combined
in the search for oil.


Statistical evaluation proved that the sets of measured

values exhibit normal distribution in rare cases only. In
smaller portions of sets data distribution is right-hand
asymmetric. For this reason the Kolmogorov-Smirnov and
the Shapiro-Wilks W tests were chosen for testing of
agreement. According to these tests the distribution of ra-
dioactive elements can be evaluated as follows.

Contents of thorium in claystones outside the deposit

are substantially higher than above the deposit, contents
of Th in sandstones are equal in both areas. It is apparent
that claystones above the deposit are presumably slightly

Table 3: Variables in subsets of sandstones (SNS) with normal asymp-
totic data distribution.

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depleted in Th, and it can be assumed that its small, but
noticeable migration occurred. This is in contrast with the
assumption of Saunders et al. (1993) of geochemical sta-
bility of Th.

The Kolmogorov-Smirnov test shows the same distribu-

tion of uranium in claystones, but the bar chart (Fig. 9) in-

Fig. 5. Bar charts for sulphur.

Fig. 6. Bar charts for potassium.

Fig. 7. Bar chart for Drad.

dicates a certain depletion in the deposit area as compared
with the area outside the deposit. The difference is signifi-
cant in sandstones poorer in uranium above the deposit.
Contrary to thorium, uranium exhibits undoubtable mo-
bility in sandstones. In radium (Ura) no difference was
found. There is no significant disequilibrium between ura-
nium and radium (slightly more radium then uranium) in
the area above the deposit.

The distribution of potassium in compared areas is dif-

ferent in claystones (Fig. 6), in sandstones the difference is
statistically negligible. In contrast to thorium the content
of potassium is substantially higher outside the deposit,
and it seems that the sediments from the deposit area are
depleted in potassium.

To summarize, the area above the deposit is generally

poorer in potassium and uranium, which confirms the find-
ings of Saunders et al. (1993). Moreover, it is likely that the
content of thorium is also not stable. Despite the large dis-
tance from the deposit, existence of phenomena causing
changes in contents of radioactive elements was confirmed.

No significant differences were found in coefficients of

Ud, Kd and Drad. Nevertheless, it can be said that the very
low values of Kd, in claystones under —0.28 and in sand-

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Table 4: Results of good agreement test for sandstones (SNS).

Fig. 8. Bar charts for mineralogical density.

Fig. 9. Bar charts for uranium.

stones under —0.3, mainly occur  above the deposit (Fig. 10).
Drad values in claystones (bar chart in Fig. 7) exhibit a
slight increase over the deposit (see Table 2) in accor-
dance with the assumption of Saunders et al. (1993).

The contents of sulphur in claystones and sandstones

can be regarded as suitable prospective indicators of an oil
deposits. Extreme values, substantially different from me-
dians, increase in sandstones (25 % of samples above the

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Fig. 10. Bar chart for Kd.

Fig. 11. Bar chart for magnetic susceptibility.

deposit) up to 0.25 %, while outside the deposit they are
below the limit of detection. In claystones 18 % of
samples are above 0.20 %, while outside the deposit there
is no such sample (Fig. 5).

Our results – higher contents of sulphur in rocks above

the Ždánice oil deposit support the theory of Saunders et
al. (1993) who suppose the reaction of ascendent
hydrocarbonates with sulphates in subterranean waters
producing hydrogen sulphide and carbon dioxide. This
causes secondary carbonate mineralization. Sulphate re-
ducing bacteria can also take part in these processes. As-
cendant diffusion and penetration of small bubbles of
light hydrocarbonates with proportions of colloidal par-
ticles through the system of microcracks and intergranual
spaces filled with subterranean water enable distribution
of sulphur ionts above the oil deposit. High concentra-
tions of carbon dioxide generate carbonic acid reacting
with clay minerals starting secondary carbonate mineral-
ization and silicification in intergranual spaces. This min-
eralization could have caused the higher mineralogical
densities found above the Ždánice oil deposit.

Magnetic susceptibility can also be an indicator. Its val-

ues go up significantly in some beds of claystones (Fig. 11).
A ferrimagnetic mineral is the carrier, not magnetite. On the
curves of SUSC dependence on temperature in the range of
0 down to —192 ºC no Verwey transition (corresponding
with multidomain magnetite) can be observed.


Evaluation of the results of  petrophysical investiga-

tions on the hydrocarbon deposit near the village of
Ždánice based on statistical tests of differences between
petrophysical parameters of rocks over the oil deposit and
outside it shows a low indicative power of separate
petrophysical parameters, although their comprehensive
application may enhance their petroleum indicative value
(Tables 2 and 4).


We thank the Grant Agency of the

Czech Republic for the possibility to carry out the investi-
gation under Grant No. 205/03/1256 “The migration of flu-
ids, element redistribution, geochemical and petrophysical
changes over the Ždánice hydrocarbon deposit”.


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