GEOL. C ROAT. 49/2 265·269 3 Figs. I Tab. ZAGREB 1996 

Review paper 

Geothermal Energy Resources in the Republic of Croatia -
- Overpressured Zone 

Ivan MESIC, Branko KRISTOFEK and Tamara BABIC PROCEEDtNGS 

Key words: Gcolhcnnal resources, Overpressure, Heat 
storage. 

Abstract 
The g reat q uantities of ..; ta red heal , at relati vely shallow depths, 

enable liS \0 perform two fundamental human activiti es. suitable for 
:t n indu stri a ll y clean COUll try. The first one i<; agriculture throughout 
the w ho le year and the second one is touri sm with the conside rabl e 
po\ e nli:ll fo r health resort s . Besides the subsurface and slIrf<lCC 
hydropo(Cllli al. there j , al so good geothermal energy present , espe­
cially in lhe western and central part o rlhe Drava Basin. 

1. INTRODUCTION 

Subsurface reservoirs from where the earth's heal is 
carried upward by Sleam or hal water convective c irc Ll ­
lati o n a rc c lass ifi cd as hyd rotherm al convectio n sys­
tcms. The exi stence of a hydrothermal resource 
depends on two basic component s: a heat source, and 
adequate ve rtical pcrmeability to allow hot fluid s to 
ri se. 'fhi s type of stored hot wate r is recognized as two 
systems. The first one is vapor-dominated or hot water, 
depending on whether steam or liquid water is the dom­
inant press ure-controlling phase in the reservoir. Th e 
second type or this reservoir is hot water. According to 
the temperature range, hot wate r sys tems a rc divided 
il1lo three categories: 

I ) hi gh temperature ran ge ( ISO-360°C), suitabl e for 
e lec tric power generation, 

2) inte rmed iate temperat ure range (90 - 150DC), suitable 
for space and process heating, 

J) low temperature sys tem (below 90°C) with li mited 
usage. 

Although some data do exist in the local literature, 
no d ata all bas ic geothermal classifi cmion could be 
found. Resource geotherma l e lass i rication follows 
WHITE & WJLLlAMS ( 1975): 

tNA-Nalbplin , Subiccva 29 . tIR- IOOOO Zagreb, Croat ia. 

Kljucnc rijcci: geole rmain i izvori , nadt lak, usk ladi ­
slenjc top tine, 

Saietak 
Zel im o Ii bili c ista zemlja, s d \' ijc tcmc lj nc tjudske djcla lllos ti , 

po tjo(ijcistvo rn i turizmo rn . momlllO rabili cistc c l1c rgc(skc izvorc, 
Osim hidropOIc ncijala, povrs inskog iii pod zcrnnog . imamo vclikc z;:t­
libe toplinske energij e II naSClll podzemlju. Naroc ilO jc 10 iz razc no II 

zapadno III i srcdisnjcTll dij e lll dra\'skog bazena. 

I . Hydrothermal convection resource (heat carried 
upward by water or steam convection): 

a) vapor dominated with temperatures around 
240°C. 

b) hot water dominated wi th tempera tures from 
350°C to less than 90°C. 

2. Hot igneous resources (rock intruded in molten form 
from depth): 

a) part still molten wi th tempera tures higher than 
650°C, 

b) not molten or so called "hot dry rock" with 
temperatures from 650 to 90DC. 

3. Conduction dominated resources (heat carried 
upward by conduction through rock): 

a) radiogene tic (heat generat ed by radioac tive 
decay), with ten1peratures between 30- I SO°C, 

b) sedimentary bas ins (hot fluid in sediment a ry 
rocks) with temperatures be tween 30-1 50°C, 

c) geopress urcd (hOI /luid under high pressure) 
with temperatures between ISO-200De. 

Prom the economic point of view, three of these sys­
tem s apparently exi st in ou r coun try. T he first one is 
"ho t d ry rock" wh ich can be spec ified as old rock 
"in truded" ((ike basement) with temperatures around 
170°C, It is assumed that granite, as in the well MoIR32 

(relat ive depth 3100 111 ) is the local source 1'01' this kind 
of heating. The second is a hOI wate r domi na ted sys­
tems. These systems arc doc limen ted in the literature 



266 

Austria 

Hungary 

Osijek 

Geologia Cro:lIiC:1 49/2 

(J ELlC, 1987; CUBR1C, 1993). The locations Ve lika 
Cigiena, Lopatincc and olhers can be classified within 
this sys tem. The third one is the geoprcssllred reservoir 
recogn ized over the whole area, and which will be dis­
cllssed in more detail. 

2. GEOLOGICAL SETTING 

fig. 1 Area o f investigation: shaded areas arc hOI spots which refer 10 
the lower part orlhe LaIc Miocene. 

Sed imentary constitue nt s of the west ern and south ­
e rn parts of Drava Basin (Fi g. I ), are sand stones ane! 
marl s from the Miocene period. The Middle and Earl y 
Miocene arc represented in the western part by sand ­
stones or highstand energy sequences, in the midd le 
part by carbonates sequences (Molve and Gola fie lds), 
while in the eastern part they are represented by sand ­
stones (OrcSac) but lowerstand energy seq uences 
(MES1C, 1995). Sedimentary sequences [rom the Early 

f 
~ 
~ 

TEMPERATURE • 'C TEMPERATURE· 'c 
o 40 60 80 100 120 140 160 20 40 60 80 100 120 140 160 180 200 

~ 

,. 

-
•• 

'~ I 

,~ 

". 

'- I 

'". 
,~ 

". 

"'. 

-I ,-
,~ 

,-

-
'- I 

". 

-
,~ 

= c= = 
.. 
... 't. 

... .. 
:, . 

... ~ 
~ " , . . . 
:. ~ 
': ~ 

~ 't. 
~ " 

TRANSITION 
ZONE 

... ~ 3 't1l00m 
... ~ , , .. 

' .. 
", " 
\\ 

.... :~, ------ ., , . 
"", 

... ~ 
':~ -------- :,~ 
'.' 
\\ \ 
:~ :. • , 
t 
:". 

. . 
" . , 

'. 
" . 

TRANSITION 
lONE 

o 

OVERffiESSURED 
ZONE 

~ :. 
~ ... 

'. 

'SKIN' U>lDICATKlNS 
FROM THE TEM'ERATtIlE 

LOGS ON FOtJR II1:~S 
IMOtVE FIELDI 

~.:-
. . 
'. 
\ -. • 
\ .... 
· , • 
\ · \ . 

o 

o 

00 

120 ° 

o 

EXPECTED 
BED 

OVERPRESSURED 
ZONE loboul 00%1 
II11H GEOTHERMAl 
GRADIENT OF 
ABOlfT 6 QCIl OOm 

000 
o 0 0 

o 0 
o o~ 

\ 
PROVEN 

BED 

rig. 2 Temperature versus depth 
data are ext rapolatcd form 
logs alld DST. Notc high 
geothermal g radi en t OJ! the 
top of overpressured zone 
(over 30°C/lOOI1l). 



Mcsi(;. Kriiitofek & Babic: Gcothennal Energy Rc~oll rce.~ in the Republic of Croatia ... 267 

Temperature Use 

200 0
- 1500 

180 refr igeration by ammonia absorption, d igestion in paper pulp 

170 heavy water via hydrogeo logy sulfide process , drying of d iatomaceous ea rth 

160 dry ing of fis h meal , dryi ng of timber 

150"- 90" 
140 food canning 

130 extract ion of salts by evaporation and crys tallization 

120 fresh water by disti ll ati on 

110 drying and c uring of li ght aggregates cement slabs 

100 drying of o rganic materi als, seaweed, grass, vegetables, washing and d ry ing of wool 

90 - 20" 
90 dry ing of stock fish, intensive de-ic ing o perations 

80 space hea ting , greenhouse by space heat ing, anima l husbandry 

60 greenhouse by combined space and hotbed hea ting 

50 mushroom growing, balneo log ica l baths 

40 so il warming 

30 swimming pools, biodegradation, fe rmenta ti on 

20 fi sh fa rming 

Table I Characteristic geothermal systems - wide lise (WHITE, 1975). 

Mi ocene, i. e. J6-8 .7 Illy, are overpressured. All fl uids 
in th is region, particulary water, incl uded in sed iments 
of Mesozoic and Palaeozoic age, are also overpressured 
(BARIC el 'II. , 1991 ). Walel in o lder sed imenl s is 
mixed wit h hydrothermal sol uti ons and comains other 
com ponents like H2S, CO2 e tc. 'fh is type of water is 
useless on the surface and has to be used in closed cir­
culation o r heaL a lte rat ion. However, this water is a lso 
interes tin g, for use in sma ll areas of hu man activi ty 
(e.g. e lectrical power plant). Thi s means there is a great 
volullle or restricted waleI' in which a great mass of heat 
is accumulated , which is shown by the great va lue o f 
the geothermal gradient (Fig. 2). 

3. PHYSI CAL PR O PERTIES or POT ENTIA L 
RESER VOIRS 

Overpress ure zones afe re lated to deposition of the 
regressive sedi ment ary sequences [hal began in the ear­
ly Late Miocene . T hcse sequences are recogn ized, at 
the begi nnin g, by the sedi mentary marke r Rs7• T his 
mudstone bed is bot h a sed im entary and stra tig raphic 
marker ( 11.7 my - BARIC ct aI. , 1992). This sedimen­
tati on event marks the presence o/" regressive sequences 
in all these areas, with di ffe rent phys ical properti es due 
to the energy in which they were deposited. Examina­
tion of th is bed shows temperatures around 154°C at the 
top of the overpressure zone, at approx imately 2580 III 

depth in the centra l area (Molve), at 3500 m in the 
western area (Koprivnicki Breg i), and at 3800 m in the 
eastern area (OreSac), as shown in Fig. 3. The thickness 
of these sedimentary sequences, ealled "Okoli and Iva 

sandstones" or sediment s between markcrs R S7 (Upper 
Badenian, i.e. I J my) and Z ' (Sarmatian, i.e. 8.7 my), 
are d ifferent throughout the bas in. In Ihe Koprivn ick i 
Bregi a rea the thickness is approx imately 1000 tn , 

whi le in the Molve area it decreases to 600 m and in the 
easte rn part (the Oresac area), on ly 300 m is recorded 
(MESIC, 1995). These sandslones sequences are of low 
poros ity, dependi ng a lso on the ene rgy level a t the 
beg inn ing of sed iment at ion. The wes tern part , repre­
senting a highstanding energe tic level or de lta plain, i.e. 
the upper part o f a delt a front , has sedi men ts w ith an 
average po rosity of around 18%, while the porosity o f 
the central pan or lower part of the delta front is around 
J 2%, and the poros ity o f eastern pa rt (o r trans ition 
bet wcen the lower pa n or the de lta front and to prodelta 
sed imentat ion), is c lose to 8%. 

The gas eomposit ion, dissol ved in the water of over­
pressured zone, is in the upper part qui te d ifferent fr0111 
that in the lower pan. This can ind icate: 1) influence of 
the known reservoir and imperfection o/" the cap rock, 
or 2) the same carrier fl uid in the mai n reservoir and the 
lower geopress ured zone. T he upper di sso lved gas is 
dom inanl ly methane (89%) with low CO, (0. 5%) . 
Water mineraliza tion is low, around 2.6 gil eg. Nne!. 
The composition o f the lower dissolved gas is simil ar to 
tha t of the main reservo ir , i. c. methane (7 1 %), CO

2 
(16%), Ii ,S (64 PPM), Il g (0.5 mg/m' ), b UI wil h higher 
water mineral izat ion (4 gil eg. NaCI). 

4. ENER G Y POINT OF VIEW 

The amou nt of energy in this volume of sedi mentary 
sequences can be roughly calculated. Although a ll sedi-



26< Geologia Cro:l1ica 49/2 

PRESSURE GRADIENTS· bar 110 m 
u u u u u u u u u 

TEMPERATURE · ·C 
40 BO f20 160 200 

It--.. 
l\ f'-r-.. 

1\ 1\ 1\ 
\ 1\ 

i~ V 

I / 
II 

\ - - - ---- --

\ --I~ - - - -

1\ 1\ 
J 

/ I ( 

mcntary masses contain accumulated water, the average 
cfTcc ti vc thickness of 100111, w ith an average porosity 
o r 8% can result in a imosl 16 klll 3 o r storage capacity. 
The average temperature we used was 154°C. 

Vw = 16kmJ = 16* I09 m3 = 16"' IOlldmJ 

Cw = 4. 1 R7* I 03 J/oC"'dm J 

t l = 154°C 

12 = 1 J .6°e (average surracc temperature in north 

Croatia) 

Qw = 16* 10 12"'4 .1 87* 10:1*( 154_ 11.6) = 9539"' 10 15 J 
= 9.54 1'.1 

1-

--

> 6"/1 00m 

1\ 

I 3 10 m 
--

"/10 m 

--- ---- ---

---- -- + 

l-

e /10 ' m 

I 

Fig. 3 Progn osi~ of pres­
sure [I nc! geothermal 
grad ient for lil e a ve r­
age we ll 0 11 lile ta rget 
area. 

The complete sto red geothe rmal e ne rgy in these 
scd imcnlary sequences is close to 

9.54 ' 10" J = 9540 I'J = (2.65 • 10" Wh = 2.65 Mwh). 

where Vw:::: volume of water (dm J), Cw:::: caloric value 
o f 1 elm "' water (Jr e .. dm.!), I I :::: temperatu re o f water in 
reservo ir (OC ), 12 :::: average surface temperature (OC) , 

and Qw = storage of hot water (P J). 

Taking in consideration di sso lv ed gas, the to ta l 
stored current is even greate r. 

This stored energy can bc useful on the surface only 

if great quantities or wate r a rc be in g produced. The 



iv1csic, Kri~lOrek & l3allic: Geothenlla l Energy Resources in the Republic of Croatia . 269 

water production depends on the reservoir permeability. 
The low penneability zone can be avoided by using 
various attributes of the seismic traces, interval veloci­
ty, gradient and AVO (BARIC et aI., 199 1) permitting 
us to make several maps and a geological model. This 
model reduces the chances of incorrect est imation of 
sedimentary environment, and the position of the zone 
with sufficient permeability for an economically justifi­
able production rate. 

'fhe temperat ure range of conventional power pro­
duction is 180·140"C. 

5. CONCLUSION 

According to world experience, geothermal reso­
urces arc suitable for a wide variety of lIses in human 
act ivities. Especially from the ecological point of view, 
i.1 geopressured reservoir with clean water, the water 
being clean due to the sedimentary environment, is or 
great importance to our country. It is shown how 
important it is to recognize temperature anomalies' vs. 
depth, not only as the average geothermal gradient. 
These hot spots, which exist in some parts of the Drava 
Basin, especially in the area between Koprivnicki Bregi 
and Ordae, make thi s area suitable for agriculture, 
tourism, and other activities that arc uscl'ul , clean, and 
economically viablc. What is required in thc fu ture are 
legislative, institutional and environmcntal actions. 

6. REFERENCES 

BARIC, G .. MESIC. I., JUNGWIRTH, M. & SPAN IC, 
D. (1991): Gas·condensate fields in the N·W of the 
Drava dcpression. - Tn: Generation, accumulation & 
production of Europes hydrocarbons. EAPG, I , 

Oxford Uni versity Press, 323-339. 

BARIC, G., MESIC, 1. & JUNGWIRTH, M. (1992): 
Sedimentary and geochemical characteristics or 
rocks and fluids of the western part of the Drava 
depression.· Nana, 43, 225-238, Zagreb. 

CUBRIC, S. (1993): Power and energy of geothermal 
reservoir in the Republic of Croatia. - Nafta, 44/7-8, 
397-403, Zagreb. 

JELIC, K. (1987): Stacionarna gcotennijska energija u 
Savskoj i Dravskoj potal ini Panonskog bazena SR 
Hrvatskc. · Nana, 38/6, 333-340, Zagreb. 

MESIC, 1. (1995): Naftnoplinonosnost kon tinentalnog 
prcgiba Go la-Molve-Kalinovac.- Unpublished M.Sc. 
Thesis, University of Zagreb, 80 p. 

WHITE, D.E. & WILLIAMS, D.L. (1975): Assesment 
of geothermal resourccs of United Slales.- USGS, 
Cire.,726,147 -1 55. 

WHITE, J.E. (1975): Computed se ismic speeds and 
attenuat ion in rocks with partial gas saturation.­
Geophysics, 40, 224-232 . 



270 Geologia Cromiea 49n