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GEOLOGICA CARPATHICA, JUNE 2007, 58, 3, 229—235


In East Moravia between the villages of Nevojice and
Ždánice a hydrocarbon deposit can be found at a depth of
nearly 1 km. The reservoir is located in a fractured and
weathered surface part of Upper Proterozoic granitoids and
in Lower Miocene sandstones and conglomerates overlying
the crystalline basement. The seal was formed during the
Late Miocene by the tectonically emplaced Ždánice thrust
sheet, which attains the thickness of 750 to 870 m over the
hydrocarbon field (Krejčí 1993). To study prospective indi-
cators of a buried hydrocarbon deposit in near-surface por-
tions of overlying flysch sediments, several shallow
boreholes were drilled above the deposit and in its vicinity.

The objective of investigations was to find out whether

there is a significant difference in the studied parameters
between the boreholes above the deposit and the bore-
holes situated at a distance of 2—3 km. Research conduct-
ed in other countries revealed 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
in the Ždánice deposit, though the deposit proper occurs
at a depth of approximately 1 km under the surface.

Our research results (Matolín et al. 2007) confirm

changes of the physical and chemical parameters in near-
surface flysch sediments due to hydrocarbon contamina-
tion over the Ždánice oil deposit. In this case, cores from
‘shallow’ boreholes were used as a comparatory material.
Interpretation of results yielded information on changes to

Physical and chemical properties of hydrocarbon-bearing

sediments (Outer Western Carpathians, Czech Republic)












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


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


Slavíčkova 415/10, 638 00 Brno, Czech Republic


Exploranium CZ, s.r.o., Hudcova 56b, 621 00 Brno, Czech Republic;

(Manuscript received January 23, 2006; accepted in revised form October 5, 2006)

Abstract: The present study deals with the impact of hydrocarbons on changes in the physical properties of the sediments
of the Ždánice-Hustopeče strata in the flysch sedimentary cover in the south-eastern part of the Czech Republic. The
discussed changes are mainly concerned with contents of natural radioactive elements Th, U and K. The subjects of the
study are rocks from deep boreholes collected down to the depth of 3.3 km. The boreholes are situated in an area of local
hydrocarbon occurrences and are divided into two groups – with and without hydrocarbon indications. Physical
properties of sandstones and claystones from the two groups are compared. It has been concluded that the distribution of
U and K in the Ždánice-Hustopeče Formation indicates shifts in contents of these elements which are undoubtedly due
to the presence of hydrocarbons.

Key words:  petrophysical data, flysch sedimentary cover, statistic evaluation, oil deposit, deep boreholes, prospective

the physical and chemical properties of rocks in the vicin-
ity of the deposit delimited in this way. It can be summed
up that the contents of potassium and uranium are general-
ly lower in the overlying area. Despite a considerable dis-
tance from the deposit, the existence of phenomena
causing changes in contents of radioactive elements was
confirmed. Contents of sulphur in claystones and sand-
stones as well as magnetic susceptibility and mineralogi-
cal density can be regarded as reliable indicators of an oil
deposit. The deposit body can be identified on the basis of
physical-chemical parameters measured on specific rock
samples from surface or shallow boreholes; however, in
most evaluated parameters this is not reliable.

Following this experiment it was decided to evaluate

the available petrophysical data from deep boreholes in
analogical geological areas using the methodology veri-
fied in investigations of so-called shallow boreholes. Se-
lected measurements of physical properties on cores from
deep boreholes drilled in the sediments of the Carpathian
Foredeep and Outer Group of Nappes of the West Car-
pathian Flych Belt were used for this purpose. The objec-
tive was to find links between the presence of
hydrocarbons and changes in the physical-chemical pa-
rameters of rocks. The study material was obtained within
the framework of the geophysical database Grant of the
Czech Geological Survey – Geofond (Dědáček et al.
2003). Over the past decades the geological formations of
the Carpathian Neogene Foredeep and Carpathian Flysch
Belt were subject to intensive reconnaissance and explor-
atory drilling with extensive state investment. That is why
a relatively large set of measurements was obtained and ar-

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chived, and is now being transferred into an electronic da-
tabase administered by the Czech Geological Survey –
Geofond Praha. On the territory of the Czech Republic
there are approximately 30,000 samples from drill cores
including coded petrographic data.

Petrophysical measurement methods

Archived petrophysical measurements  were evaluated.

Petrophysical measurements are laboratory measurements
on specimens of rocks for mineralogical and bulk density,
porosity, specific magnetic susceptibility and contents of
Th, U and K.

Petrophysical measurements archived in the database of

the Czech Geological Survey – Geofond have been con-
ducted since the mid-sixties on cores of most core-drilled
deep boreholes in the territory of the former Czechoslova-
kia. The methodology is governed by analogical princi-
ples, though updated in the course of time. Measurements
conducted at different times are therefore comparable.

Density measurements  –  the parameters measured are

bulk density (Do), mineralogical density, that is the densi-
ty of the mineral component of a rock without pores (Dm),
and open porosity. In tables it is designated as (Por). It is a
proportional volume of communicating pores – a scalar
quantity. Measurements were carried out by the triple
weighing method using kerosene as the saturating medi-
um. It should be noted that porosities of rocks dried at
100 ºC are considered.

The standard deviation of one measurement is

± 0 .001 g·cm


for Dm,  ± 0.003 g·cm


for Do, and  ± 0.02 %

for porosity.

Magnetic susceptibility  was measured on the KLY kap-

pabridges with the sensitivity of 10


. It is a dimensionless

material parameter (tensor), its mean value (SUSC), how-
ever, is used for observations of material changes.

Th, U and K contents  and the total gamma activity were

determined using the scintillation spectrometry. U was de-
termined through its daughter nuclide 


Ra. The mean

measurement errors were: Th ± 0.8 ppm, Ura ± 0.4 ppm and
K ± 0.2 %.

Investigated deep boreholes and drilled tectonic-

stratigraphic units

Data for our investigation were  taken from  numerous

measurements of physical properties on cores from deep
boreholes, drilled in the sediments of the Carpathian Fore-
deep and Carpathian Flysch Belt and reaching the
Ždánice-Hustopeče Formation whose sediments are evalu-
ated.  The relatively extensive area is shown in Fig. 1.
Apart from the hydrocarbon occurrences known as the
Ždánice deposit there were numerous indications in other
localities. Among the most significant ones is the Lubná
area where an oil field occurs in Precambrian (Cadomian)
Brunovistulicum granitoids under the ovethrust sediments
of the Carpathian flysch nappes. Moreover, drilling

reached Paleozoic rocks of the Bohemian Massif, Cam-
brian and Devonian (old-red) clastic rocks (?), Devonian
carbonates, lower Carboniferous (Culm facies) and sporad-
ically coal-bearing lowermost upper Carboniferous (Na-
murian). From the Mesozoic and Tertiary rocks making up
the autochthonous cover of the Bohemian Massif, the
presence of Jurassic carbonate and pelitic-carbonate sedi-
ments, Paleogene limestones and Miocene sediments of
the Carpathian Foredeep was proved by drilling.

The Carpathian Flysch Belt investigated by deep drilling

in south-east Moravia consists by Pouzdřany, Ždánice and
Zdounky Units belonging to the Outer (Menilite-Krosno)
Group of Nappes and by the Magura Group of Nappes.

The Ždánice Unit in the studied area is composed of Pa-

leogene to Lower Miocene sediments (Ždánice-Hus-
topeče, Menilite and Němčice Formations). Older
sediments (Upper Jurassic to Upper Cretaceous) are known
from the surrounding area of this unit (Chlupáč et al.
2002). Some of these boreholes revealed manifestations of
hydrocarbons, some did not. A few boreholes offered only
weak indications.

Boreholes indicating the presence of hydrocarbons

Bařice 1 – without hydrocarbon presence, Karlín 1 –

productive, gas,  Krumvíř 1 – no hydrocarbons, Letošov 1 –
weak indications, Lubná 1 –  productive,  Lubná  2 – weak
indications, Lubná  5 –  productive, Lubná 7 – weak indica-
tions, Mouchnice 1, 2 – indications, N. Mlýny 1 – no indi-
cations, Němčičky 1 – contentious indications, Uhřice 1 –
hydrocarbon occurrences, Vranovice 1 – contentious indi-
cations, Snovídky 2 – contentious indications, Ždánice 2 –
strong indications, Ždánice 4 – no indications, Křepice
5 – no indications, Bučovice 1 – no indications, Bul-
hary 1 – no indications, Kobeřice  4 – no indications, Ko-
bylí 1 – productive, Nikolčice 2a – productive, Sedlec 1 –
weak indications, Uhřice 6 – productive, Žarošice 1 – weak
indications,  Žarošice 2 – productive.

The boreholes have been processed in two groups:

 Productive boreholes, with hydrocarbon indications in-

cluding those classified as contentious or bearing only weak
traces (in short ‘productive’).

 Boreholes without hydrocarbon indications (‘non-pro-


Comparison of the physical parameters in psammites and

pelites from the two processed groups indicated potential
changes of physical parameters due to the presence of hy-
drocarbons. Hydrocarbon indications are related to the en-
tire borehole, that is the Ždánice-Hustopeče strata reached
by the borehole which produced the evaluated samples.

Tectonic position and stratigraphic classification of
evaluated samples

Each borehole reached the Ždánice-Hustopeče strata as

had been planned. The cores from these boreholes provid-
ed a sufficient number of samples (mainly of psammites),
which does not apply to the Miocene of the Carpathian
Foredeep or the underlying Menilite and Němčice Forma-

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tions. We assume that if there are hydrocarbon occurrences
or indications in some layers, the other reached stratigraph-
ic levels are likely to be at least partially geochemically
‘contaminated’, including the investigated Ždánice-Hus-
topeče Formation. Our considerations are therefore based on
the assumption that hydrocarbon ‘contamination’ is more
obvious in borehole group 1 as compared with group 2 al-
though some weak contamination effects cannot be exclud-
ed in group 2. Indications of epigenetic metasomatism of
clastics (‘contamination’) due to hydrocarbons can be sta-
tistically evaluated by comparing two sets (pairs of sand-
stone sets and pairs of claystone and siltstone sets).

The Ždánice-Hustopeče Formation is assigned to the Up-

per Oligocene (Chattian) to Lower Miocene (Aquitanian). It
is an identical lithostratigraphic unit investigated in ‘shal-
low’ boreholes between the Ždánice and Nevojice villages.
The thicknesses are considerably higher in deep boreholes,
with a high probability of a close, direct contact with hy-
drocarbons. In deep boreholes the strata was drilled at
depths between 5 and 3300 m under the recent surface.
The greatest depth – over 3 km was reached in boreholes
Sedlec 1 (the stratum is represented by samples from the
interval 1000—3300 m). The real thickness of the
Ždánice-Hustopeče strata is approximately 1200—1300 m.
In individual boreholes the strata is often tectonically
scaled and thus it is encountered in the borehole profile in

more intervals (e.g. borehole Kobylí 1), or double thick-
nesses (Sedlec 1).

The Ždánice-Hustopeče Formation is developed in

psammitic (Ždánice Sandstone), pelitic (Hustopeče Marl)
and psammitic-pelitic rocks  (alternating sandstones and

Methodology of processing

Routine Excel programs were used for processing, en-

abling selection from database and calculations of statisti-
cal parameters and creation of histograms. Parameters Kd,
Ud and Drad were used for evaluation of hydrocarbon in-
dications as defined by Saunders et al. (1993). The authors
published a theoretical explanation of the differences in
radioactive element contents observed in sediments over
the hydrocarbon deposit and outside it. They assume that
by action of organic acids and CO

clayey and other min-

erals are destructed in the vicinity of the deposit, and in
consequence U and K are released and drifted away. The
contents of radioactive elements in affected sediments are
therefore lower than in sediments not affected by this pro-
cess. The authors also assume that Th remains immobile in
these processes. In their opinion the following parameters
can be regarded as indicators:

Fig. 1. The study area with deep boreholes.

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

Ka, Ua, Tha are contents of radioactive elements

in samples of the investigated sedimentary unit
containing hydrocarbons or altered by their prod-
ucts, Kmo, Umo a Thmo are contents of these ele-
ments within the same unit outside the deposit
accompanying the alteration. According to the
authors negative Kd and Ud values indicate the
presence of hydrocarbons. However, their find-
ings related to arid climate, and therefore the the-
oretical model need not be of a general value and
the distribution of radioactive elements may not
be so simple (see Gnojek 1976; Borovec 1985;
Fiala 1989).

Processing of archived and derived data is based

on statistical comparison of two corresponding sets
(pairs of sandstone sets and pairs of claystone and
siltstone sets) from the so-called productive and
non-productive area, as explained above. In corre-
lation matrices for individual parameters we give
the correlation coefficient R and the critical value
for 95 % probability for N number of samples (for
the minimal value N at different numbers of mea-
surements of individual parameters).

The crucial problem in evaluating the changes

of physical parameters due to hydrocarbon ‘con-
tamination’ is the assessment of the significance of
differences between averages of productive and
non-productive boreholes. Statistical distribution
of parameters is often asymmetric, very different
from the Gaussian distribution, as the distribution
of values from ‘shallow’ boreholes was analysed
by tests. Using standard significance tests of dif-
ferences in arithmetic means is not without prob-
lems. It appears that to solve this problem it is
best to evaluate the character of the distribution
of individual parameter values by means of histo-
grams,  as was found out in processing of ‘shallow’

Presentation of results

The results of the evaluation are presented in ta-

bles and histograms appended to this paper. The
parameters from ‘productive’ (code A) and ‘non-
productive’ (code O) boreholes are compared.

Tables 1 and 2 contain statistical data on the

physical parameters including dimensionless pa-
rameters Kd, Ud and Drad in the sense of Saunders
et al. (1993). Each table contains data from pro-
ductive and non-productive boreholes. In the ma-
trix for dimensionless parameters average values
characteristic for the investigated lithotypes must
be used. An average of all available samples of a
particular lithotype is taken.

Table 1:

 Physical parameters of sandstones.

Table 2:

 Physical parameters of calcareous claystones.

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Tables 3 and 4  contain correlation matrices for individ-

ual parameters with the correlation coefficient R and criti-
cal value for 95 % probability given for N samples (always
for the minimal value N at different numbers of measure-
ments of individual parameters). The distribution of pa-
rameters which exhibited differences between productive
and non-productive boreholes in preliminary evaluation
are documented by histograms – Figs. 2, 3, 4, 5 and 6.

Discussion and conclusions

Parameters dependent on sediment texture and struc-

Bulk density and porosity undergo compaction and

lithification in dependence on the depth. The differences
between productive and non-productive boreholes (in
both sandstones and claystones) (Tables 1 and 2) are due
to different depths of occurrences of processed samples
under the recent surface. Compaction and lithification, as
shown earlier, are mainly dependent on the depth of ‘buri-
al’ in the sedimentary basin (Ondra & Hanák 1988). It
took place predominantly in the period before tectonic de-
formation. It is closely related to the basin (collector)
properties of sediments. The discussed impact of hydrocar-
bons on these parameters cannot be evaluated here,
though it cannot be excluded.

Parameters independent of texture and structure – con-
tents of radioactive elements

a) The differences in the distribution of Ura in sand-

stones are manifested by a shift to lower contents in pro-

Table 3: Correlation of the physical parameters of sandstones.

Table 4: Correlation of the physical parameters of calcareous claystones.

Fig. 2. Uranium contents – deep boreholes.

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ductive boreholes as opposed to non-productive bore-
holes (Fig. 2).

b) The same trend can be observed in claystones and

siltstones. It must be noted, however, that particularly in
pelites Ura can be related to the primary organic element,
which need not coincide with migrating oil and gas. It is
confirmed by the highly significant negative correlation
(Table 4) between Dm and Ura in claystones from produc-
tive and non-productive boreholes. The organic element
in claystones binding Ura reduces the Dm value. In sand-
stones, however, this phenomenon is not of importance, or
is not present at all, as the correlation relationships Dm
and Ura (Table 3)  are contrasting. The contents of the or-
ganic element binding Ura in sandstones are substantially
lower. In claystones with siltstones there are extreme con-
tents of Ura attaining 25 ppm, most probably due to the
content of the organic element. The claystone-siltstone set
exhibits extreme Kd and Drad values observed in average
values (Table 2) due to samples from the borehole Vranov-

Fig. 3. Bar chart for Drad values – deep boreholes.

Fig. 4. Bar chart for potassium – deep boreholes.

Fig. 5. Bar chart for thorium – deep boreholes.

Fig. 6. Bar chart for magnetic susceptibility – deep boreholes.

ice with very low contents of potassium and extreme con-
tents of thorium.

c) Positive Drad values – Saunders et al. (1993) are in-

dicative of hydrocarbons. Fig. 3  shows that 12.6 % of
sandstone samples from productive boreholes exhibit
Drad values higher than 1, in non-productive boreholes
only 1.8 %. This difference in the distribution of Drad is
considered to be due to the presence of hydrocarbons in
‘productive’ boreholes. These manifestations are indicated
by random choice of samples more numerous in produc-
tive boreholes.

d) The histogram for the distribution of K in sandstones

(Fig. 4) shows that 23 % of samples from productive bore-
holes exhibit contents of K lower than 1 %. In the second
group (from non-productive boreholes) there are no sam-
ples with contents of K lower than 1 %.

e) On the other hand, it is difficult to interpret the differ-

ences between the two groups in the histogram for Th con-
tents in sandstones (Fig. 5).

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In conclusion it can be summed up that the analysis of

the distribution of Ura and K in the Ždánice-Hustopeče
Formation reached by deep boreholes indicates changes in
contents of these elements due to the presence of hydro-
carbons. Both observed elements are in the process of epi-
genesis forced out  due to the presence of hydrocarbons. It
is obvious in psammites, and in accordance with the as-
sumptions of Saunders et al. (1993). Indications of this
phenomenon are much stronger in deep boreholes as com-
pared with near-surface portions as deep boreholes reach
the contaminated portions nearer the source of hydrocar-
bons, or even the samples come directly from deposit ac-
cumulations. It is obviously necessary for recording the
geochemical contamination by means of the used method-
ology. In other studied parameters the effect of the contact
with hydrocarbons has not been observed in the studied
set of samples.

On the basis of measurements of the physical-chemical

parameters of rock samples from shallow and deep bore-
holes in the Ždánice Unit a methodology can be recom-
mended for employment in oil prospecting:

 Make use of ‘shallow’ boreholes in seismic shooting

for collection of rock samples for determination of radio-
active elements and contents of sulphur.

 Adapt the gammaspectrometric probe for shallow log-

ging in these boreholes.

 Specific petrophysical measurements in selected hori-

zons and lithotypes.

Acknowledgment:  We thank the Grant Agency of the
Czech Republic for  the possibility to carry out this study
under Grant No. 205/03/1256 “The migration of fluids, el-
ement redistribution, geochemical and petrophysical
changes over the Ždánice hydrocarbon deposit”.


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