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 reduc­tion 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 select­ing 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 requir­ing 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 imen­lary basins that it "generates everywhere and mi gra tes anyw here" has becn abandoned long ago. The mecha­nisms of hydrocarbon generation, migrat ion and accll­mulat ion are well known today and are considered uni­versa l, but depend on basic fact s, i.e. sources, rese r­voirs, cap rocks and traps (DEMAISON, 1984).

Geochemistry, as a relati vely new scientifi c di sci­pline, 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 phe­nomena. 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 , zre­lost, 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 poseb­no 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, iden­tifi cation of al terati on and trans formation of hydrocar­bons 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 hydrocar­bon 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 inves­tiga 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 Pan­nonian basin (Fig. 1). Research included the identifica­tion of source rocks, genelic characteri za tion of hydro­carbons within accumul ati ons, detennina tion of alter­ation 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-

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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 geo­chemica l analyses and the regional determinat ion of sedimentation facies (Table 1). Source rocks are pre­dominantly 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

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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 with­in 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.

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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 repre­sent cap rocks, and in certain locali ties they si multane­ously act as source and reservoir rocks due to secondary porosity (HERNITZ ct aI., 1995).

MURA -2 GAS PAONE

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Current results indi cate that duri ng furt her explo­ra 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-

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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 discov­ered.

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

Sample 1 (2 144.2153m)

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Geologia Croatiea 49/2

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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 men­filled 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 elleetivc­ness (DEMAISON & HUIZINGA, 1991) arc manifest­ed in the separation of the saturated gas phase from gas­saturated 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 produc­tion. The reservoirs in (he Mum depression are charac­terized by different degrees of alteration through cvapo­rative 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 compo­sition 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. Differenllev­els or biodegradat ion have been established at the Bu­njani field, while tota l alteration from paraffinic-aro­matic 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-Kalino­vac-Stari Gradac field gases, as well as in the exp lo­ration loca liti es of Vuckovec and Vukanovec, were conducted in our geochemical laboratory. It was deter­mined 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 ict­ed 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 car­bonates 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 gas­es 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 success­fully 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-kon­dcnzatnolll polju Simi Gradec.- 1. C roatian Geologi­ca l Congress, Opatija, Proceedin gs, 1, 47-51, Zagreb.

BARIC, G. , MARICIC, M. & RADlC, J. (1988 ): Geo­chemical characterization of organic faci es in the Dugi Otok Basin, Adria tic Sea. - Organic Geochem­istry , 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 Con­cept.- 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 Geochem­istry 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 nat­ura 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 For­mation and Occurrence.- 2nd cd ., Springer-Verlag, 459-469, New York.

196 Geologia Croalica 49/2