GeolCarp_Vol63_No6_481

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GEOLOGICA CARPATHICA

, DECEMBER 2012, 63, 6, 481—489 doi: 10.2478/v10096-012-0037-z

Introduction

Hydrocarbon  reservoirs  can  contain  oil,  condensate  and/or
gas. The conventional approach considers a reservoir as rock
with  reservoir  properties,  namely  enough  highly  effective
porosity and permeability for hydrocarbon recovery with ap-
plication of primary, secondary or tertiary recovery methods.
However, due to wettability, capillary forces and saturation,
only part of the total hydrocarbon reserves (Original Hydro-
carbons In Place, abbr. OHIP) can be recovered from reser-
voirs.  Average  recoveries  are  about  60 %  for  gas  and
20—30 % for oil reservoirs. However, parts of hydrocarbons
remain in reservoir and those remaining quantities are larger
in weakly permeable reservoirs, like tight sandstones. Part is
also kept in the source rocks where hydrocarbons are gener-
ated, and only reached primary migration inside source rocks
initiated by capillary forces and pressures. The range of pri-
mary migration is on a scale of hundreds meters, and of sec-
ondary (from source to reservoir rocks) on a kilometer scale.

The term of unconventional reserves covers hydrocarbons

associated  with  tight  sandstones,  shales  or  marls,  coal  bed
methane,  gas  hydrate  deposits,  heavy  oil,  tar  sands  (i.e.
weakly  permeable  rocks)  and  reservoirs  characterized  by
high pressure and temperature. Unconventional reservoirs of
hydrocarbon  contain  chemically  the  same  hydrocarbons  as
the conventional ones, but trapped in the weakly permeable
rock, which are often also source rocks where it was generated.
The  unconventional  reservoirs  are  also  often  located  geo-
graphically  inside  the  same  borders  that  delineated  the  sur-
face  projection  of  the  hydrocarbon  field  with  conventional
reserves.  In  such  cases  unconventional  reservoirs  are  on

Unconventional hydrocarbon resources of the Bjelovar

Subdepression (Pannonian Basin System) in Croatia:

an overview

TOMISLAV MALVIĆ

and ANA MAJSTOROVIĆ BUŠIĆ

University of Zagreb, Faculty of Mining, Geology and Petroleum Engineering, Pierottijeva 6, 10000 Zagreb, Croatia;

tomislav.malvic@ina.hr

INA-Industry of Oil Plc., Šubićeva 29, 10000 Zagreb, Croatia; ana.majstorovic@ina.hr

(Manuscript received December 8, 2011; accepted in revised form June 13, 2012)

Abstract: The Croatian part of the Pannonian Basin System includes several Miocene chronostratigraphic (sub)stages
mostly characterized by weak permeable clastic sediments. They are often also mature source rocks at depths of more
than 2500 m, from Late Badenian to Early Pannonian ages, represented by marls and calcitic marls, and kerogene
Types II and III. The other types of weakly permeable sediments are tight sandstone mostly of Badenian age. Those two
lithotypes are potential unconventional reservoirs described in the Bjelovar Subdepression, regarding their age, geo-
logical evolution, lithology, porosity and permeability. Domination of kerogene Type III and low total organic carbon
defined marls as gas-bearing source rocks. Both marls and tight sandstones mostly have porosity less than 10 % and
permeability less than 10

—3

. It is about 10—100 times lesser permeability than in conventional reservoirs. Weakly

permeable zones are highly stochastically distributed and fluid flows are relatively short (several meters), which could
be enhanced only by the using hydraulic or other fracturing techniques.

Key words: Miocene, Croatia, Bjelovar Subdepression, tight sandstones, marls, unconventional reservoirs, hydrocarbon gas.

higher  depths,  where  their  rocks,  rich  in  organic  matter,
reached  thermal  maturity,  generated  hydrocarbons  (mostly
during  catagenesis)  and  due  to  expulsion,  that  is  secondary
migration, had been trapped in conventional reservoirs with
high  primary  or  secondary  porosity  (sandstones,  breccia,
fractured carbonates, etc.).

The geological properties of such reservoirs in northern

Croatia are shown here. Some previous preliminary geological
studies and numerical calculations for such reservoir systems
in Croatia defined some petroleum engineering and fractur-
ing properties (e.g. Page & Miskimins 2009) as well as geo-
logical potential (e.g. Miskimins 2006).

Overview of geology and petroleum potential of the

Croatian part of the Pannonian Basin System

The Pannonian Basin System (PBS) is a back-arc basin

system superimposed on an earlier, mostly Cretaceous, com-
pressional realm (Tari & Horváth 2006). Royden (1988) pro-
vided a modern understanding of the Neoalpine evolution of
the PBS and relation to the Carpathians. However, the pre-
Neogene evolution of the PBS substrata has been discussed
only in a few studies (e.g. Tari 1994, 1995). During the Mid-
dle Miocene the PBS (Fig. 1) was mostly covered by the
Central Paratethys, and later by younger brackish and fresh-
water basins and lakes formed from the Paratethys.

The PBS was formed mainly due to the continental collision

in the Carpathians to the north and northeast. At the southern
and southwestern margin convergence and subduction re-
sulted from moving of the Apulian Plate under the Dinarides

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during the Styrian orogenic phase. That created, e.g. the Pe-
riadriatic-Vardar lineament and some other regional features.
However,  Pavelić  (2002)  even  located  the  southwestern
boundary of Central Paratethys more to the south from the Pe-
riadriatic-Vardar  lineament  in  area  of  small  and  fresh-water
within Dinarides. Those basins were  never flooded  by  marine
transgression in the Paratethys.

The very first extensions in the PBS were initiated in the Ott-

nangian (Royden 1988; Rögl 1996, 1998), but were docu-
mented only locally, and continued through the Karpatian when
lacustrine and fluvial sediments were locally deposited also in
the different surrounding areas of the PBS like the Apuseni
Mts, Carpathians and Podolian Upland as well as on the mar-
gin of the Alps and the Bohemian Massif (Rögl 1996, 1998).

The Badenian was period of the largest extensional dis-

placements  and  maximal  extensions  of  the  marine  environ-
ment  (e.g.  Rögl  1996,  1998;  Vrbanac  1996),  mostly  because
of the existence of connections (through Trans-Tethyan trench
corridors)  with  the  Mediterranean  in  the  southwest  and  the
Indo-Pacific  in  the  southeast.  These  connections  resulted  in

two  regional  transgressions.  The  first  happened  in  the  Early
Badenian and covered the entire basin system, from Austria to
Romania (Transylvania) and from the Carpathians to the Di-
narides. The second followed in the Middle Badenian, caused
development of the marine environment of normal salinity in
the western part of the Central Paratethys (i.e. parts of Poland,
Hungary,  Slovenia  and  Croatia).  The  evaporate  sediments  in
other parts are documents of regression and increased salinity.
Such  thick  evaporate  sediments  are  described  as  the  Middle
Badenian  salinity  crisis,  and  documented  in  the  Transcar-
pathian and Transylvanian Basins as well as the Carpathian
Foredeep (Kováč et al. 2007). The trench corridors that con-
nected  the  Paratethys  and  Mediterranean,  or  Indo-Pacific
were eventually closed in the Late Badenian (e.g. Steininger
et  al.  1978).  This  caused  another,  this  time  final  deposition
of regional evaporates, as in the Carpathian Foredeep and the
Transylvanian  Basin  (Andreyeva-Grigorovich  et  al.  1997;
Mărunteanu  1999;  Peryt  1999;  Chira  2000).  In  the  Croatian
part of the Pannonian Basin System (CPBS), Vrbanac (1996)
interpreted the Badenian as a stage with a marine environment

Fig. 1. Pannonian Basin System (area of Central
Paratethys) and its Croatian part.

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when present-day mountains were islands or shallow subma-
rine  uplifted  paleoreliefs  and  beginning  of  initial  marine
transgression  (Ćorić  et  al.  2009).  Pavelić  (2001)  described
the Late Badenian as a period when an extensional had been
gradually  transformed  into  a  post-extensional  period,  when
tectonics were caused mostly by thermal subsidence and had a
dominant  compressional  character.  During  the  entire  Bade-
nian, as a result of weathering of uplifted paleorelief and car-
bonate reefs as well as the activities of numerous alluvial fans,
large quantities of coarse-grained sediments were deposited.

In the Sarmatian the marine environment was progressively

reduced. This eventually finished with the formation of more
or less isolated brackish and freshwater lakes during the Late
Miocene  across  the  entire  basin  system  (e.g.  Rögl  1996,
1998).  Dominantly  pelitic  sediments,  mostly  marls  and  cal-
citic  marls,  were  deposited.  Royden  (1988)  pointed  out  the
Early Pannonian as a period when a major extensional phase
finished over most of the PBS. It was followed by a post-ex-
tensional  phase,  generated  by  thermal  subsidence,  and  ac-
companied by local alkali volcanic activity. Turbidites were
a  characteristic  transport  mechanism  in  the  CPBS,  moving
silty and sandy detritus that originated from the Eastern Alps.
They were activated due to gravitational and tectonic insta-
bility mostly on depression margins, or ramps (e.g. Vrbanac et
al. 2010; Malvić & Velić 2011).

The Late Pontian, and especially the Pliocene and Quater-

nary  were  characterized  by  fluvial,  lacustrine  and  marshy
sedimentation. Aeolian sediments, namely loess, are very of-
ten found in some parts of the CPBS and dated to the latest
Pleistocene.  This  final  phase  of  formation  of  the  PBS  was
characterized  by  dominant  compressional  tectonic  styles,
where are often documented reactivation of fault planes with
inversion displacements (e.g. Velić 2007). During the entire
Neogene—Quaternary  period  Malvić  &  Velić  (2011)  de-
scribed for the CPBS two transtensional (Badenian and Pan-
nonian—Early  Pontian)  and  two  transpressional  (Sarmatian
and Late Pontian—Quaternary) regional tectonic phases.

Unconventional hydrocarbon resources of the

Bjelovar Subdepression, Croatian part of the

Pannonian Basin System

The area of the Bjelovar Subdepression covers about

2900 km

and has been explored with more than 500 deep

wells, dozens of seismic sections and gravimetric surveys on
the west and northwest (Malvić 2003a; Malvić & Đureković
2004). It is a large southwestern branch of the Drava Depres-
sion (Fig. 1), separated from the central depression zone dur-
ing the Pliocene and Quaternary. The most data is available

Fig. 2. Example of typical hydrocarbon system in the
Bjelovar Subdepression (modified from Malvić et al.
2005; Malvić & Rusan 2007, 2009).

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from  the  12  hydrocarbon  fields  of  different  sizes,  located
mostly on the subdepression margins (Fig. 2). The remaining
(about 4 %) wells are classified as regional. There are several
lithostratigraphic formations defined inside the Neogene and
Quaternary sediments. The oldest one, Moslavačka gora For-
mation  (Fig. 2),  comprises  the  rocks  that  define  the  entire
hydrocarbon system. The reservoirs (mostly breccia and con-
glomerates)  and  traps  are  of  possible  Early  (16.4—15.0 Ma)
and  certainly  Middle  (15.0—13.5 Ma)  Badenian  age.  The
source  rocks  are  of  Late  Badenian  (13.5—13.0 Ma),  Sarma-
tian  (13.0—11.5 Ma)  and  Early  Pannonian  (11.5—9.3 Ma)
ages. The younger regional reservoirs are medium- and fine-
grained  sandstones,  which  belong  to  the  Late  Pannonian
(9.3—7.1 Ma) and Early Pontian (7.1—6.3 Ma) substages, that
is lithostratigraphically to the Ivanić grad and Kloštar-Ivanić
Formations (Croatian lithostratigraphic nomenclature is given

on  Figs. 3  and  4).  Chronostratigraphic  time  spans  listed  for
particular (sub)stages are given according to the values pub-
lished  in  Haq  &  Eysinga  (1988),  Ldi  (2006)  and  Malvić
(2011). A palinspastic reconstruction on the transversal sec-
tion through the Bjelovar Subdepression is given in Fig. 5 to
observe  evolution  of  thicknesses  and  main  faulting  through
the Neogene and Quaternary.

The conventional reservoirs of Badenian age are often con-

nected  in  single  hydrodynamic  unit  with  the  shallowest  part
(about 10—15 meters) of the Paleozoic or Mesozoic basement
rocks.  It  is  result  of  a  long  period  of  continental  weathering
during the Paleogene and locally even longer. Badenian trans-
gression and deposition of coarse-grained sediments with high
primary  porosity  overlaid  such  eroded,  weathered  and  even
cataclized  basement  rocks  where  secondary  porosity  previ-
ously formed, making common space for fluid migration.

Fig. 3. Thickness maps of Badenian and Sarmatian sediments (Moslavačka gora Formation), the Bjelovar Subdepression (modified from
Malvić 2011).

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Table 1: Geochemical values C

org

, S

, HI, OI of core samples from

Badenian to Lower Pannonian in the Bjelovar Subdepression (Malvić
2003).

Samples from Badenian–
Sarmatian (age 16.4–11.5 Ma)

Mean Minimum Maximum

org

1.14 0.01

5.30

6.91 0.01

22.91

237.81 120.00

397.78

447.83 107.30

1100.00

Samples from Lower Pannonian
(age 11.5–9.3 Ma)

Mean Minimum Maximum

org

0.61 0.19

1.41

1.64 0.92

2.89

271.74 184.00

433.00

76.70 31.53

121.87

The Badenian sediments are characterized by average total

porosity of about 10 % (or locally more in non-cemented
breccia) and permeability of few hundreds 10

—15

(Malvić

2003b). Upper Miocene conventional reservoirs (sandstones)
have good petrophysical properties only in the northern and
(partially) northeastern Bjelovar Subdepression and elsewhere
they are often completely replaced by impermeable sediments,
especially on the south and southwest. The average total po-
rosity ranges very wide (10—30 %) reflecting a wide range of
permeabilities (Malvić 2003b).

Sediments of the Upper Badenian, Sarmatian and Lower

Pannonian, which belong to the Moslavačka gora Formation
are considered as potential source rocks, regarding total or-
ganic  carbon  (TOC)  and  thermal  maturation.  The  thickness
of  the  entire  formation  can  reach  more  than  1000  meters
(Figs. 3 and 4), where those sediments reached the relevant
depth for maturity in two synclines, Rovišće on the west and
Velika  Ciglena  on  the  east  (Fig. 2,  points A  and  C).  These
two structures cover about 18.5 % of the total subdepression
area (Malvić 2003a).

The complete set of paleostructural maps (structural and

thickness maps) has been interpolated (Malvić 2003a, 2011)
using  hand-drawing  of  isochors.  Thicknesses  calculated  be-
tween  electro-log  markers  (determined  on  the  curves  of
spontaneous potential and apparent resistivity) that separate
particular members in the Moslavačka gora Formation (Mosti
Member – Badenian and Sarmatian at Fig. 3, and Križevci
Member – Early Pannonian at Fig. 4) were used as control
points.  The  control  points  were  located  along  seismic  sec-
tions  and  regional  wells  inside  the  Bjelovar  Subdepression.
Their  locations  are  given  on  A0  maps  published  in  Malvić
(2003a).

The thickness maps related to rocks of the Moslavačka gora

Formation have been statistically analysed (Majstorović Bušić
2011) with the goal of calculating the volumes of particular
lithostratigraphic members as well as determining the end of
the 1

transpressional phase in this part of the CPBS. Despite

the general opinion that the 1

transpression phase in the

CPBS finished in the Lower Pannonian, in the Bjelovar Sub-
depression its end is observed at the end of the Sarmatian.

The methodology of volume calculation has been based on

estimation  of  the  grid  point’s  thickness  value  extrapolated
from the closest isochore. Using this approach, the sum of all
grid node thicknesses has been divided by the number of used
nodes. The result is the average thickness. That value for the
Badenian—Sarmatian  interval  (Moslavačka  gora  Formation,
Mosti  Member)  is  192 m,  and  for  the  Lower  Pannonian
(Moslavačka  gora  Formation,  Križevci  Member)  is  465 m.
The  average  thickness  (m)  was  multiplied  by  the  subdepres-
sion area (m

) shown with borders on Figs. 3 and 4. The vol-

ume of Badenian-Sarmatian rocks is 35,040 10

. The

volume of the Lower Pannonian interval is 127,875 10

The pelitic sediments that could reach the stage of mature
source rocks approximately belong to the younger half part of
the Mosti Member and to the entire Križevci Member.

The surface outcrops of the Sarmatian sediments, as typical

pelitic rocks with increased total organic carbon content in the
CPBS, are found on the margins of the most present-day
mountains in Northern Croatia. Here are given photos of two

characteristic sequences located (locations are given on
Fig. 1b) at Medvednica Mt (Fig. 6) and Samoborsko gorje Mt
(Fig. 7), that is on the very southwestern margin of the CPBS.
Lithologically these are marls and calcitic marls, with more or
less sandy component. The sand in such rocks is the result of
deposition in a near-shore and so shallow environment, in the
range of local alluvial fans.

In the entire CPBS and PBS in general from Late Badenian

to Early Pannonian sedimentation was characterized by pelitic
rocks like marlstone, limestone, calcitic marlstone and marly
limestone (Royden 1988; Rögl 1996, 1998; Vrbanac 1996).
It  was  a  result  of  a  shallow  and  calm  brackish  environment
favourable  for  preservation  of  organic  matter,  transformed
into kerogene Types II and III. Pelitic sediments in the CPBS,
today  at  depths  of  more  than  2500 m  are  mostly  in  the  cat-
agenesis phase. The Bjelovar Subdepression source rocks are
dominantly  characterized  by  kerogene  Type III,  which  is
mostly  a  precursor  for  gas.  The  generation  potential  of  sam-
ples  collected  in  the  subdepression  is  described  with  several
geochemical variables (given in Table 1). These were organic
matter content (C

org

) representing total carbon in sediments (%).

represents hydrocarbon volume generated during pyrolysis

(420—460 °C) and is expressed as g HC/kg rock. Hydrogen in-
dex (HI) represents the S

org

ratio, which indicates the rock

maturation level. Oxygen index (OI) is expressed as the ratio
S

org

, where S

is carbon dioxide volume created in pyroly-

sis (g CO

/kg rock).

All the values given in Table 1 are indicated on kerogene

Type III, sometimes even oxidized, which mostly generates
gas with expelling efficiency usually less then 20 %. It is in-
teresting to compare with kerogene Type II (average
C

org

> 1.5 %), where expelling efficiency can reach 60—90 %.

The expelling process is less efficient from lean source rocks
(i.e. thin and/or source rocks with low generative potential).
In such cases the generated volumes are small, but also most
of hydrocarbons remain trapped in the source rocks (espe-
cially oil because of viscosity) (Cooles et al. 1986).

Figure 8 presents such a system of thin and interlaying con-

ventional  and  unconventional  reservoirs  in  Badenian  sedi-
ments,  connected  in  a  single  hydrodynamic  unit  with
reservoirs  in  fractured  and  weathered  Paleozoic  rocks  in  the
basement.  Frequent  alteration  of  numerous  Badenian  thin
layers, characterized by different clastic lithologies (breccias,

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Fig. 6. Calcitic marl blocks outcrop (vertical scale about 10 m,
Zagreb, Trzinove pećine locality, Medvednica Mt, N 45° 50. 321

’,

E 015° 54. 178

’, altitude 264.4 m) (photo Majstorović Bušić, 4. 11.

2011). Location is shown onto Figure 1.

Fig. 7. Calcitic marl blocks outcrop (scale is hammer, Sveta Nedjelja
hill locality, Samoborsko gorje Mt, N 45° 46.779

’, E 015° 44. 868’,

altitude 249.4 m) (photo Majstorović Bušić, 5. 11. 2011). Location
is shown on Figure 1.

Fig. 8. Alternation of breccia reservoirs and tight
sandstones in Badenian sediments, the Bjelovar
Subdepression, Croatia (modified from Malvić
2003a).

sandstones, tight sandstones, sandy marls, marls) consequently
resulted  in  different  permeability  zones,  where  even  weakly
permeable sediments had been saturated during secondary mi-
gration of gas. Trapping in such thin and tight reservoirs is ad-
ditionally  supported  by  frequent  alternation  of  isolator  rocks
(sandy  marls  and  marls)  which  formed  hydrodynamic  sub-
units  that  very  slowly  communicate  among  themselves  or
temporarily even reached naturally stable state of saturation.

The example shown in Fig. 8 belongs to the field discovered

on the east of the Bjelovar Subdepression (right-most location
on Figs. 3 and 4). The described tight gas sandstones in alter-
nation with breccias and marls are one of two expected types
of unconventional reservoirs in the Middle Miocene sedi-
ments in the analysed subdepression, but also in the entire
CPBS. The second are marls and calcitic marls of Late Bade-
nian to Early Pannonian age, which are regionally proven as
the source rocks with mixture of kerogene Types II and III (in

the  Bjelovar  Subdepression  kerogene  Type III  is  dominant).
Exploration of these two types of unconventional reservoir is
different,  although  major  data  for  both  cases  come  from  re-
flection  seismic  sections  and  regional  wells.  Production  and
exploration wells on existing conventional fields can be useful
only  if  weakly  permeable  and  source  sediments  have  been
drilled  and  examined  together  with  conventional  reservoirs.
However, such analyses are rare because these sediments were
not exploration and production targets in the past.

Furthermore, tight sandstones can also be expected in Up-

per  Miocene  sandstone  sequences,  where  the  main  conven-
tional  reservoirs  in  Croatia  have  been  proven,  namely  in
medium-grained  sandstones,  with  thicknesses  of  a  few  to
dozens  of  meters  and  porosity  15—20 %  and  saturated  both
with oil and gas. Due to depositional conditions in the Upper
Miocene  (e.g.  Malvić  &  Velić  2011)  such  channel  sand-
stones  laterally  are  gradually  replaced  with  weakly  perme-

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Table 2: Porosities and variogram ranges of core samples from
Badenian to Lower Pannonian in the Bjelovar Subdepression
(Malvić 2003a).

Average porosity (%)

12.86 12.03

6.14

6.01

Vertical variogram range (m)

0.70 4.28

0.63

1.25

able and thin lithofacies of marlitic sandstones or sandy
marlstone, but such unconventional play needs to be more
explored before it can be outlined as potential.

However, the above mentioned pelitic sediments of source

rocks (marls, calcitic marls) are deposited over a longer geo-
logical period in a relatively calm environment and the thick-
nesses  are  significantly  larger  (dozen  or  hundred  meters).
Consequently  they  will  be  much  easier  to  follow  on  seismic
sections,  even  if  only  a  few  regional  wells  with  interpreted
e.log markers are available.

Observing the example given in Fig. 8, the unconventional

Badenian play includes both marls and (proven gas saturated)
tight  sandstones.  The  measured  petrophysical  values  are  sig-
nificantly  lower  than  in  the  “conventional”  part  of  Badenian
reservoirs (coarse-grained sandstones and breccias). In “tight
sandstones”  porosity  is  slightly  higher  than  10 %  and  in  the
naturally fractured marls and carbonates about 6 % (Table 2).

Consequently, their permeability is less than 10

—3

µm

(or

1  millidarcy)  which  is  about  10—100  times  less  than  in  con-
ventional ones in the analysed field. Moreover, weakly perme-
able zones are highly stochastically distributed. Consequently,
zones favourable for fluid flow are relatively short, as is proven
by variogram analyses (Malvić 2003b) calculated for porosity
data from the Neogene reservoir’s sediments collected in the
Bjelovar  Subdepression  (Table 2).  Vertical  variogram  ranges
calculated for sandy marls were not longer than 4.3 meters,
although such entire depositional sequence can reach several
tens of meters. The presented variogram ranges in Table 2 can
be  considered  as  limits  of  approximately  isotropic  zones  for
fluid  flow  in  sediments  belonging  to  the  Moslavačka  gora
Formation.  It  means  that  natural  flow  rates  in  such  uncon-
ventional reservoirs would be low, with drainage on the scale
of several meters, which could be enhanced only by the using
hydraulic or other fracturing techniques.

Conclusions

The presented geological analysis of one larger hydrocarbon

province in the Croatian part of the Pannonian Basin System
(CPBS),  named  the  Bjelovar  Subdepression,  outlined  the
lithological and lithostratigraphical units that included poten-
tial  unconventional  hydrocarbon  reservoirs.  These  are  tight
sandstones  of  Middle  and  Late  Miocene  ages,  and  source
marls  of  Late  Badenian  to  Early  Pontian  ages.  Those  sedi-
ments  are  proven  in  all  CPBS  depressions  (Sava,  Drava,
Mura, Slavonija-Srijem), often distinguished only by their dif-
ferent depths (marls also in maturity level).

Here are outlined the main conclusions valid for such res-

ervoirs analysed in the Bjelovar Subdepression:

a) Unconventional reserves in source rocks are located in

the sediments of Upper Badenian, Sarmatian and Lower Pan-
nonian stages;

b) Such rocks are dominantly gas-bearing due to dominant

kerogene Type III;

c) Their organic carbon content in source rocks of that age

is about 1 %, and the depth for reaching thermal maturity
would need to be more than 2500 m;

d) Those unconventional reservoirs could be tight sand-

stones of Badenian age also assumed deeper than 2500 m;

e) The sandy marls, as weakly permeable sediments, are

proven only on sites located along paleo-shore, but with very
low or absent TOC due to re-working of sediments;

f) The permeability of potential unconventional reservoirs

is lower than 10

—3

µm

which means that any production

such reservoirs could be depending on artificial fracturing.

The future production from such reservoirs in Croatia de-

pends on several factors (like fracture density, drainage radius,
production period, miscibility of possible injected fluids).
Such values could be calculated only after the first pilot-
project, including early production stage.

Acknowledgments:  The  results  presented  here  are  derived
from the work developed during 2011 and 2012 as part of the
Project  “Stratigraphical  and  geomathematical  research  of
petroleum geological systems in Croatia” (No. 195-1951293-
0237),  financed  by  the  Croatian  Ministry  of  Science,  Educa-
tion  and  Sport.  The  authors  would  like  to  thank  reviewers
Prof. Piotr Krzywiec and Dr. Jozef Vozár.

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