GEOL. C ROAT. 49/2 19 1 - 195 7 Figs. 1 Tab. ZAGREB 1996 

Scientific paper 

Organic Geochemistry in the Rationalization of Oil and Gas 
Exploration and Production 

Gerlrud BARIC 

Key words: Source roc ks, Organ ic fac ies, Maturi ty, 
Pe troleum alte rat ion. 

Abstract 
Organic geoc hemistry. a comparatively youn g scientific d isci­

pline integrated into the basic geological model can affect the reduc­
tion of o il and gas exp lo rati on ri sks. T he correct lise of these dala, 
panicularly on a regiona l level, so-called geochemica l mapping, has 
been success fu l in di scovering the hydrocarbon reservo ir. By select­
ing rclcv:lI1! geochemical parameters (source, migration, temperature 
regime) il has been possible 10 obtain p<l ft icli lary useful inform ati on 
abOUI margina l, markedly unfavourable parts of lh e basin. wh ere 
ex pl oration costs arc very high. 

The geoc hemic:l! re sults arc used as co rrelat io n paramete rs, not 
on ly in ex ploration, but al so duri ng hydrocarbon produc ti on. Th ey 
enable determination of reservoir destruction , as well as co mpositi on 
change of reservoi r hydrocarbons . 

Howeve r, the combination o f all geo log ical and geopbysica l 
met hods and techn iq ues in the course of exploration process results in 
the ma ximum effect of geoc hemical prospection. 

1. INTRODUCTION 

Oil and gas exploration is a complex process requir­
ing the application of numerous geological, geophysical 
and geochemi ca l methods. Every mcthod has cert a in 
d isadvantages anci it s proper selec tion is impera tive to 
gain the maximum benefits and reduce exp lora tion ri sks 
(MU RRI S, 1984). 

The old theory that oil is so widespcad in sed imen­
lary basins that it "generates everywhere and mi gra tes 
anyw here" has becn abandoned long ago. The mecha­
nisms of hydrocarbon generation, migrat ion and accll­
mulat ion are well known today and are considered uni­
versa l, but depend on basic fact s, i.e. sources, rese r­
voirs, cap rocks and traps (DEMAISON, 1984). 

Geochemistry, as a relati vely new scientifi c di sci­
pline, is primarily used in the process of explora tion ror 
hydrocarbons, since it enables the determination of 
so urce rocks, eval uati on of their generating potential , 
effi ciency or expulsion and detection of migration phe­
nomena. The genetic characterization of reservoir 
hydrocarbons is achieved by th e applica ti on of geo-

lNA -Nafiaplin, Exploration and Development Di visio n, Lov inCiceva 
I , IIR - lOOOO Z<lg reb, Croatia. 

PROCEEDINGS 

Kljucne rijeci: maticne stijene, organski fac ijes , zre­
lost, izmjenjenost ugljikovocli ka. 

Sazctak 
Organska geokemija, rclaliv no mlada znllllstvcna discipi ina , inle­

gri mml U osnovn i geoloski model moze utjecal i na sm:lnjenje ri zik a 
pri istraiivanju nafte i plina. tspnw na primjena ov ih podataka poseb­
no na reg ionainom planu , tzv . geokemij sko kartiranjc, pokazalo jc 
II spjeh U ol krivanju leiiSta uglj ikovodika. tzbororn re levantn ih 
geokemij skih paramclara ( izvor, mi g racija. Icmpcralurni rdim), 
mog ucc je dobiti posebno kori sne in formacij e II g rani cnim, izrazilo 
nepovo ljnim dijelovima bazena , u kojima su cijcnc istrazivanja vrlo 
visokc. 

Osirn It istrazivHnj u i lijekom proizvodnje uglj ikovodika koriste 
sc geokemijski rezu lta ti kao koreJacijski pararnctri. Nj ima je moguce 
takodc r utvrditi destrukciju Id ist a, kao i izmj enu sas tava lci isnih 
uglj ikovod ika. 

Maksima lan ueinak geo kernijske prospekcije pos ti zc sc ko mbi ­
nacijom sv ih geoloski h i gco fi zic kih metoda i tchnika u is traznOlll 
procesu . 

chemical analyses, as well as by the ir co rrelation, iden­
tifi cation of al terati on and trans formation of hydrocar­
bons wi thin accumu lations. 

An objective evaluati on of the role and application 
of" geochem istry is possible today and its importance in 
the hydrocarbon exp loration process has also been 
defin ed. Greater efficiency in the search ror hydrocar­
bon resources and discovery 0[" hydrocarbon reservoirs 
can be achieved by incorporating geochemical data into 
basic geological models. 

2. RESULTS AND DISCUSSION 

The paper presents the results o f geochemical inves­
tiga ti on carried ou t to date in the Croati an part of the 
Pannonian basin, in the Dinaridcs and in the Adriatic 
offshore. The main oil and gas fi e lds in Croatia are 
located in the south-western , marginal part of the Pan­
nonian basin (Fig. 1). Research included the identifica­
tion of source rocks, genelic characteri za tion of hydro­
carbons within accumul ati ons, detennina tion of alter­
ation and destruction of hydrocarbons within reservoirs 
and ex planation of the mechanisms for the appearance 
o f high concentrations of chem ica ll y inorganic con-



192 

KAAlOVAC 

" 

KAAPINA 

ZAGREB 
@ 

CA)(OV~C 

VAAiIZOlN 

% 

SISAK -

" 

" 
'<9l..~ 

KO~IVN1tA ~~, 

" 

VII'!OVITICA 

® 

POtEGA 
® 

LEGEND 

_ OIlFIELD 

.,. GAS FIELD 

SLAVQNSI( I eAOO 

~ ~ 
,,", ~..J ,--, 

Geologi:J. Croatica 49/2 

" __ ... ~r 
rJ 

§ ~--;; :,:: r) 
' L-__ ~~ ________________ ~ ______________________ ~ _______________ ~ __ -= ____ ~ __ ~ 

\ 
I 

} 

6 300 000 6'" 000 

Fig. 1 Index map with oil and gas fields in the Croati an part of the Pannonian basin. 

DEPRESSION MURA DRAVA west DRAVA east SAVA 

Depth (rn) >3000 >3000 >2100 >2300 

Thickness (m) 200-500 200-500 200-300 100-300 

Mid. to Low. Mid. to Low. Mid. Miocene Mid. (0 Low. Age 
Miocene Miocene, Mesowic Miocene 

Lithology mudstone, mudstone, marlstone marly limestone 
siltites silt it es limy marls 

TOC (%) average 0.9 1.5 1.2 1.4 

Kerogen lype III - (I I) 111 -(1 1) II - (Ill ) 11 - (1 11) 

Maturity (% Ro) 0.7 - 1.8 0.5 - 2.2 0.4 - 0.9 0.6 - O.~ 

b 13Cl'D13 (%0) -25.8 to -2~.7 -26.3 [0 -28.S -22.3 to -26.2 -21.6 to -26.6 

Table I Basie Source Rock Qlmlity Data in the Croatian part of the Pannonian Basi n. 

sti tuents in gas and gas-condensate fie lds of the Mura 
and Drava depression. 

In the Croatian pan of the Pannonian basin, source 
rocks were identified in a large number of wells by geo­
chemica l analyses and the regional determinat ion of 
sedimentation facies (Table 1). Source rocks are pre­
dominantly marly limestones and limy marls of Lower 
to Middle Miocene age, while bacteria-degraded algal 
matter and terrigenous lip ids are the precursors of 
petroleum t

• 

Recent investigation established that thick, compact 
dark-gray (0 black carbonaceous fossiliferous mud -

stones, clayey mudstones and si ltstones of Lower 
M iocene and Mesozoic age at depths exceeding 3,000 
m in the Mura depression and in the western part of the 
Drava depression represent source rocks in these areas 
(Figs. 2 and 3). Terrest rial organic matter and longer 
exposure to high temperatures resulted in generation of 
gas-condensate and dry thermogenic gases (BARIC et 
a l. , 1990, 1996). 

Geochemical characterization of hydrocarbons with­
in the reservoirs (Fig. 4) confi rmed that they genetically 
belong to the determined source rocks and at the same 
time established the relatively short source rock - accu-

I Corn ford , C. (1989): Petrol cum Gcochemistry of the Pannonian 13asin.- Unpublished ECL Report, lNA -Naftaplin Archive, Zagreb. 



n ~ric: Org~nic Gcochem istry ill tile R~tionalizatio ll of Oil and Gas Exploration and Production 193 

fOCt .. HI 

I ~I~I "GH 

''''''' 

"'" 
00 

o ~ 

"'" 

~::: ~=~ 
AYOfIIA EX'''' Ca I fOC 

GAS/oIL ~S~ 
:10000.010 IqO tilO 19G 

- I 
I 

- I 
1 

I I 

I I 
1 

I 

1 

~ 
I 

i 

1 I 

I 

JI 
Co 1 I 
~ , 
~I 

I 

=- I I 
I 

~ I I 

~I I 
~I I 

~ I I 
I I 
I 

mulation migration paths. The investigat ion confirmed 
that in som e oi l fields of the East Dra va depression 
source rocks, fi ne-grained pellitic sediments also repre­
sent cap rocks, and in certain locali ties they si multane­
ously act as source and reservoir rocks due to secondary 
porosity (HERNITZ ct aI., 1995). 

MURA -2 
GAS PAONE 

VITRINITE IlERK'lN(t 
"'_'.~ .fU ........ 
OUoi.CU.ATED I~ 
.""..&.5i..REO ..... "'110 

•• .~ 

Tm • x 
·C 

~ 
.~ 

I 

I 

. .., .~ 

MATURITY 
ZONES 
lIIElO5T 

ii, ~I~I! :I 0 ~ ; 

Fig. 2 Organic facies 
and maturity log of 
Mllra-2 wel l (afte r 
BARICetal., [996). 

Current results indi cate that duri ng furt her explo­
ra tion in the C roatian part of the Pannonian basin the 
type of hydrocarbons can be predicted. The generating 
poten tial and volume of the de termined source rocks 
are mostly in agreement with th e size of the ex ist in g 
hydrocarbon accumulat ion. Geological and gcoehcmi-

lN~ Nil/tapl H' 

80 Ie) \-\--7''''''''-----\-------'''<-'+-1 L())jEA PONTI AN 

UPPER PANNOl-I U N 

LOMEA P~NNON IAN 

D Early "'a tur~ ~(o 1ilil" 

0.5 to 0.7 

• 
Mid Matur e (0\ 1) 
0.7 to \ 

IR+l Late Mature (0\ 1) 
!tm, t o 1.3 EGGENBUtlGIAN 

1I!00. HlTA$S!C 

, • 0 Fig. 3 Burial history curves for 
Mllra-2 wel l (after BARI C 
Ct aI., 1996). 



194 

LEGEND' 

-10 + F 

B 0 "" 
-SO • K,I 

XM [::, 51G 

I .... KB 

• -'0 

Ii () L1 
T U [J Mic P ." 

<D ·'0 
- 2.0 <D Lsn 

" 4 ,0 0 01 
-30 .. Hlml 

~ Ok, 

THhl 0 G, 
-20 0 • L'9 

- 250 -200 - '50 - 10() -50 X Go 

50 CH • i""'i 

Fi g. 4 Isotope ratios of natural gases in the western part of the Drava 
depression (gene tic zonation aner SCHOELL, 1983). 

cal reinterpretation or certain areas has to be undertaken 
to evaluate further prospectivity of exploration areas 
and greater attention has to bc paid to those localities in 
which source-rocks have been determined, but in whose 
vicinity hydrocarbon reservoirs havc not been discov­
ered. 

Source rock seq uences have been determined in 
deep exploration well s of the Adriatic offshore, 

Sample 1 
(2 144.2153m) 

Sample 2 
(2300-2311 m) 

• Samp le 4 

_~'.LJ..LU-L. ,,1: 111~~h 
Fig. 6 Gas chromatograms of condensates and bitume n in the Mum 

depression. 

900 

000 

><- 700 

: ~6001 w _ 

°u 
~ :~O 
> E 
x 

1.00 [[ 

)00 

200 

100 

III 

Geologia Croatiea 49/2 

o 

oL-____ I __ ~~~=-____ __ 
1.30 1.65 

IMMATURE lOlL ZONE I GAS ZONE 
, mO' ( ' C) 

Fig . 5 HI - T",u diagram showing types and malurity of kerogen in 
lhe Dugi Olok Basin (Adrialic offshore). 

although their regional dis tribution has not been fully 
defined. The organic matter in those sediments has not 
yet reached the stage of maturity, i.e. the oil window 
(Fig. 5). Poor physical characteristics of cap rocks, 
which permit migration of hydrocarbons, arc a special 
problcm, as bi tuminous impregnations and bi tu men­
filled stylolites confirm (BARIC et aI., 1988). 

Geochemical analyses were used for determination 
of the alteration of reservoir hydrocarbons. The most 
frequent eases are caused by processes of evaporative 
fract ion ation (THOMPSON, 1987; SILVERMAN, 
1965). Fractionation phenomenon caused by structural 
deformation or a low degree of regional seal elleetivc­
ness (DEMAISON & HUIZINGA, 1991) arc manifest­
ed in the separation of the saturated gas phase from gas­
saturated liquid. Separation resu lt s in dispersion and 
dismigration of light hydrocarbons, whi le residual 
hydrocarbons remain in the reservoir and, depending on 
the quantity of waxes or resin-asphaltelle components, 
increase thc viscosity and cause difficulties in produc­
tion. The reservoirs in (he Mum depression are charac­
terized by different degrees of alteration through cvapo­
rative fractionation and in some cases by the total 
destruction of hydrocarbons (Fig. 6). 

Alteration of hydrocarbons in a reservoir can be 
influenced by water action, too (TISSOT & \VELTE, 
1984). Water washing processes usually characterize 
shallow accumulations with active hydrodynamic 
flows. Those processes only sligh tly affect the compo­
sition and physical properties of oil. A higher stage of 
degradation is the result of the combined ac ti on of 
water and microorganisms, known as biodegradation. 
Depending on their in tensi ty, partial or full alteration of 



l3:l ric: Organic Geoche mistry in the Rationalization of Oi l and Gas Exploration and Production 195 

p, Ph 

Fig. 7 Gas chromatograms of oil s (Sava depression) altered by water 
action. 

certain types of hydrocarbons is possible. Differenllev­
els or biodegradat ion have been established at the Bu­
njani field, while tota l alteration from paraffinic-aro­
matic to naphtcnic-aromatic type characterizes the Kriz 

field oil (Fig 7). 
Stud ies rescarc hing the origin of chemically inor­

ganic const ituent s (H 2S and CO2) in the Molve-Kalino­
vac-Stari Gradac field gases, as well as in the exp lo­
ration loca liti es of Vuckovec and Vukanovec, were 
conducted in our geochemical laboratory. It was deter­
mined th at H2S was the result of the thennocatalitic 
reduction or dissolved sulphates with the simultaneous 
partial su lphurization of reservoir hydrocarbons. The 
processes arc thermodynamically controlled, and cyclic, 
but higher em ission s and degasification can be pred ict­
ed by continuous monitoring of lluid during production 
(BARI C & JUNGWIRTH, J 995). Carbon dioxide in 
gases is of inorganic origin, and occurs as a result of the 
thermal decomposition of reservoir carbonates or car­
bonates from the deeper parts of the basin. Changes of 
equ ilibrium in reservoir sys tems are the res ult of 
decreased formation pressure, primarily caused by 
explo itat ion, but can also resuiL from thc breakthrough 
o f formation water with large quantities of di ssolved 
CO2, which causes an increase in the CO2 levels in gas­
es of some wells. 

3. CONCLUSION 

A short rev iew of thc results indicates that in th e 
forecasting and es timation of the further prospectivity 
or exploration areas, a re latively large amount of ex ist-

ing geochemical data has to be used in addition to other 
geological and geophysical methods, to lower the risk 
involved in findin g new hydrocarbon reservoirs . Th e 
results also indicate that geochemistry can be success­
fully appli ed in solving the problems arising in the 
process of hydrocarbon production. 

4. REFERENC ES 

BARIC, G., BRTTVIC, V. & DRAGAS , M. (1996): 
Source rocks and hydrocarbon accumulations in the 
Mura depression, Republic of Croatia.- Nana, 47/1., 
25 -34, Zagreb. 

BARIC, G. & JUNGWIRTH, M. (1995) : Pocirij cllo 
H2S i organsko sumpornih spojeva u plin sko-kon­
dcnzatnolll polju Simi Gradec.- 1. C roatian Geologi­
ca l Congress, Opatija, Proceedin gs, 1, 47-51, 
Zagreb. 

BARIC, G. , MARICIC, M. & RADlC, J. (1988 ): Geo­
chemical characterization of organic faci es in the 
Dugi Otok Basin, Adria tic Sea. - Organic Geochem­
istry , 13,343-349. 

BARIC, G. , MESIC, I. , JUNGWIRTH, M. & SPANIC, 
D. (1990): Plinsko i plinsko-kondcnzatna polja 
Molve, Kalinovac i Stari Gradac.- Nana , 41/2, 71-
89, Zagreb. 

DEMAISON, G. (1984): The Generati ve Bas in Con­
cept.- In: DEMAISON, G. & MURRIS, R.J. (cds.): 
Pe troleulll Geochemistry and Basin Evaluation. 
AAPG Memoir, 35, I-IS. 

DEMAISON, G. & HUIZINGA, B.J. (J991): Genetic 
Class ification of Petroleum Systel11s. - AAPG Bull. , 
75/10,1626- 1243. 

HERNITZ, Z., VELlC, J. & BARIC, G. (1995): Origin 
o f Hydrocarbon s in th e Eas tern Part of the Drava 
Depression (Eastern Croatia).- Geologia Croa ti ca, 
48/ J,87-95. 

MURRIS, R.J. (1984): Introduction. - In: DEMAISON, 
G. & MURRIS , R.J. (ed s.) : Petro leum Geochem­
istry and Basin Evaluation. AAPG Memoir, 35. 

SILVERMAN, S.R. (1 965): Migration and segregation 
of oil and gas.- AAPG Memoir, 4, 53 -65. 

SCHOELL, M. (t983): Genetic characterization of nat­
ura l gases. - Am. Assoc. Pe trol. Gcol. Bull., 67, 
2225 -2238. 

THOMPSON, K.F.M. (1987): Fractionated aromatic 
pet roleum and generation of gas-condensates .­
Organic Geochemistry, 11/6, 573-590. 

TISSOT, B.P. & WELTE, D.M. (J984): Petroleum For­
mation and Occurrence.- 2nd cd ., Springer-Verlag, 
459-469, New York. 



196 Geologia Croalica 49/2