Hydrocarbon Generation Modelling in the West Moesian Platform - Pene, 1996

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Hydrocarbon generation modelling in the west of the

Moesian Platform, Romania

Constantin Pene Bucharest University, Faculty of Geology and Geophysics 6, Traian Vuia St., RO-70139 Bucharest 1, Romania

ABSTRACT: Silurian, Midd le Carboni fer ous, Middle Jur assi c and Sarmatian shales as

well as Devonian bituminous limestones are thought to be main the source rocks in

the study area. The the rmal matur ation level of the so urce roc ks was com put ed

using burial history curves for 175 wells. Maps of isoreflectance show that all the

source rocks are mature in the north of the study area. The onset of oil generation

from most of the source rocks took place during Sarmatian and Pliocene times,

after most traps were formed. The calculations suggest that the volumes of

hydrocarbons available to be reservoired are much greater than the volumes

discovered to date.

KEYWORDS: Moesian Platform, Romania, hydrocarbon generation modelling

INTRODUCTION

The Moesian Platform is bordered by the Carpathian, Balkanand North Dobrogea orogenic systems, respectively. It alsocovers to the east the continental platform of the Black Sea(Fig. 1). The Moesian Platform represents one of the mostimportant petroliferous basins of Romania. Exploration startedin the early 1950s. After 1956—when well 2 Ciuresti encountered the first hydrocarbon accumulation reservoired in theSarmatian sandstone—more than 130 oil and gas fields havebeen discovered. Initial reserves are 235 x 1061 and ultimate

resources are at least 237 x 1061 (Popescu 1995). At present,the Moesian Platform yields about 40% of the hydrocarbonproduction of Romania. Over the last 35 years, numerousbooks and articles on the stratigraphy and structural development of the Moesian Platform have been published; for recentreviews see Mutihac & Ionesi (1974), Sandulescu (1984),Paraschiv (1974, 1979a, b, 1983, 1984), Patrut et al. (1983),Mutihac (1990) and Pene (1995). A good compilation on thehydrocarbon-producing areas of the Moesian Platform and fielddescriptions was published by Paraschiv (1979b). Also there isa huge body of literature on geological and geophysicalresearch represented by unpublished reports (PetromArchive).

This paper refers only to the west of the Moesian Platform.

The study area is bordered southwards by the Danube andeastwards by the Olt river. Westwards, the limit is provided bythe Southern Carpathians and, in the north by the CarpathianForedeep, delineated by the Pericarpathian Fault (Fig. 2). Inthe northwest of the Moesian Platform more than 20 oil andgas fields have been discovered in Devonian, Permo-Triassic,Middle and Upper Jurassic, Lower and Upper Cretaceous,Sarmatian and Lower Pliocene reservoirs (Paraschiv 1979a, b,1984; Patrut et al. 1983; Pene 1995). The region contains theonly hydrocarbon accumulation in Palaeozoic (Devonian)bituminous and fissured dolomites and limestones discoveredin Romania. Oil and gas accumulations in Permo-Triassic and

Presented at the EAGE Conference, Glasgow, June 1995 (P538).

Middle Jurassic reservoirs have been discovered only in thenorthwest of the Moesian Platform. The reservoir rocksconsist of dolomites and limestones in the Devonian, MiddleTriassic, Upper Jurassic and Lower Cretaceous and sandstonesin the Lower and Upper Triassic, Middle Jurassic, UpperCretaceous, Sarmatian and Lower Pliocene (Fig. 3). The sealsconsist of marls and evaporitic rocks. Faulted anticlines andmonoclines are the main traps.

No data are available on the resource sizes, number ofwells drilled and seismic line-kilometres shot over this area.

Publications describing source rock type, organic mattercontent, thermal maturity, expulsion potential, migrationroutes, source-to-oil and oil-to-oil correlations and carbonisotopic composition are scarce or absent. Unpublished dataon organic matter content occur in the Petrom Archive. Somedata on the geochemical analysis of the source rocks andgeothermal gradient are reported by Paraschiv (1979b) and

Fig. 1. Study area and adjacent geological units of the Moesian

Platform.

Petroleum Geoscience, Vol. 2, 1996, pp.241-248 1354.0793/96/$07.00 © 1996 EAGE/Geological Society, London



242 C. Pene

Fig. 2. Tect onic map at the t op of Palaeozoic formations with th e main fracture systems, uplifts and depressi ons of the Moesian Platform (Romania).

L E G E N D

I WA

| Anhydrite, salt

1 f J f* | Dolomite

1' 1 • 11Limestone

•r S i l Marl

•• N=! Shale

<>|v:-:-| Sandstone

|-...-| Conglomerate

I *x*xx| Basalt

• 1*++++| Quartzitic porphyry

raw Crystalline basement

• • Oil and gas reservoirs

<̂^ Source rock

•

•

•

•

•

• 4

<>

Fig. 3. Lithostratigraphic column of the sedimentary cover from thewest of the Moesian Platform.

Baltes (1983b). The fossil vegetal contents of the coresfrom Bulbuceni area were examined by Baltes (1983a). Thealteration index values suggest that only Neogene and, in part,Jurassic rocks could have generated liquid hydrocarbons andwet gas. The organic matter in rocks which are older thanJurassic have reached an overmature stage and dry gas has

been generated. Baltes (1983a) suggested that oil reservoiredin Palaeozoic and Triassic rocks originated by lateral migrationfrom younger formations, along the important unconformities.Patrut et a I. (1983) suggested that oil and gas generationfrom Middle Jurassic shales started in the Sarmatian at depthsbetween 1900- 4500 m in a kitchen beneath the CarpathianForedeep, nort h of the Pericarpathian Fault. Recent geochemi-cal studies (Lafargue et a I. 1994) suggest that the MoesianPlatform source rocks could have similar characteristics tokerogen type II from source rocks which are stratigraphicallyequivalent in the Toarcian of the Paris Basin. The geochemicalstudies of Lafargue et al. (1994) suggest that two families ofoil exist in the Moesian Platform. Pene (1995) analysed thethermal maturity, expulsion potential, migration routes and

hydrocarbon volumes available to be reservoired. The mostrecent article on Romania's petroleum systems and theirremaining potential (Popescu 1995) reports that the Doggershales yielded 20-30% DOM (dispersed organic matter) andaverage organic carbon (Cor g) content of 0.35%. Popescu(1995) showed that Dogger shales are equivalent to theEutropole Formation from Bulgaria and are the only sourcerocks proven to have good hydrocarbon potential. The purpose of this paper is to evaluate the hydrocarbon volumesgenerated, expelled and migrated from source rocks in thewestern part of the Moesian Platform.

STRUCTURAL SETTING

The structural style of the Moesian Platform - affecting all thestrata - comprises a fault network wit h two dominant trends.East-west faults are the most significant ones in the basin andare the result of regional extension which affected all thestrata. Another fault trend, generally perpendicular to the



Hydrocarbon generation modelling, Romania 243

former, includes faults of smaller size, often with a localdevelopment. In the Romanian part of the Moesian Platformsome uplift and depression zones are developed. The majoruplift zones are: Strehaia-Chilia, Craiova-Bals-Optasi, Slatina-Corabia and Bordei Verde. The Bailesti, Rosiori-Alexandria

and Urziceni-Calarasi are the main depressions (Fig. 2).The main uplifts and depressions were active until the EarlyTriassic. Jurassic, Cretaceous and Neogene strata have a mono-clinal dip towards the north beneath the Carpathian Foredeep.Middle and Upper Triassic strata have a transitional structure.In the study area only the Strehaia-Chilia, Craiova-Bals-Optasiand Slatina-Corabia uplifts, as well as the Bailesti depressionare developed.

STRATIGRAPHY

The wells drilled in the Moesian Platform show that a more orless complete sedimentary cover of Palaeozoic, Mesozoic and

Tertiary formations overlies a heterogeneous basement. Thesedimentary cover was formed during four main cycles ofsedimentation: Cambrian-Middle Carboniferous, Late Permian-Triassic, latest Liassic-Late Cretaceous and Neogene (Fig. 3).Eocene deposits are developed locally in the southeast of thestudy area. In the main depressions, the sedimentary covermay exceed 8000-10 000 m in thickness.

The first sedimentary cycle begins with mainly areniticdeposits belonging to the Cambro-Ordovician and continueswith argillites and intercalations of sandstones and limestonesin the whole Lower Ordovician-Eo-Devonian interval. AnEifelian complex of Old Red Sandstone type, with partlylagoonal dolomites follows, and Givetian-Visean limestonesand Silesian terrigenous deposits were laid down at the end

of the sedimentary cycle.The second sedimentary cycle of Permo-Triassic age begins

with mainly terrigenous-continental deposits associatedwith effusive volcanic rocks in the Upper Permian to LowerTriassic. The Middle Triassic contains limestones, dolomites,anhydrites and salt, while the Upper Triassic sequence comprises terrigenous-continental deposits. Within this sedimentary cycle three phases of effusive volcanic rocks are present:two in the Upper Permian-Lower Triassic (acid and basic,respectively) and the third (basic) in the Ladinian-Carnianinterval.

The third sedimentary cycle started in the latest Liassic andended in the Senonian. The Liassic-Dogger interval consistsof terrigenous formations. Sandstones (Toarcian-Aalenian)

dominate at the base of this interval, whereas shales andsubordinate marls and limestones occur in the upper part(Bajocian-Lower Callovian). Pelagic, neritic-reefal and locallylagoonal carbonate deposits formed from Callovian to the

Table 1. Computation of the generated hydrocarbon volumes

Platform {Romania)

Source interval Area Thickness V TS

(km2) (m) (106m3)

Sarmatian 661 25 16525Middle Jurassic 778 40 31120

Middle Carboniferous 650 40 26000Upper Devonian 2690 60 161400Silurian 3055 60 183 300

Aptian. During Late Cretaceous times a complex of marls andsandstones, which is dominantly pelitic, was laid down.

The fourth sedimentary cycle began in the Sarmatian(locally Badenian) and consists mainly of detrital deposits.

SOURCE ROCKS

The most important source rocks in the study area areOrdovician-Silurian shales, Devonian bituminous limestones

and dolomites and Middle Carboniferous, Middle Jurassicand lowermost Sarmatian shales (Paraschiv 1979b; Baltes1983b; Pene 1995) (Fig. 3). Also, there are interesting rocksfor hydrocarbon generation in the Oligocene located in thenorth of the area, in the Carpathian Foredeep (Baltes 1983b;Lafargue et al. 1994; Popescu 1995). The hydrocarbon potential of the source rocks of the study area has been estimatedusing Total Organic Carbon (TOC) analyses and determinationsof Organic Matter Type. The organic matter contents arerelatively reduced (Table 1). The geochemical data (H/C and

O/C ratios from Baltes 1983b) suggest that the organic matterconsists predominantly of type I kerogen for the Ordovician-Silurian shales. The organic matter from the Upper Devonianbituminous limestones and dolomites consists of mixed kerogen (types I+ 11, but predominantly type I). In the MiddleCarboniferous the organic matter content varies vertically andconsists of mixed kerogen (sapropelic-humic, but predominantly humic). The geochemical data (H/C and O/C ratiosfrom Baltes 1983b) suggest that the organic matter of theMiddle Jurassic and lowermost Sarmatian source rocks consists predominantly of type II kerogen. The geochemical andlithostratigraphic data suggest that the most important sourcerocks are the Silurian shales.

THERMAL MATURITY

The thermal maturity level of the sedimentary deposits hasbeen calculated from burial histories. Burial history curveshave been constructed for 175 petroleum exploration wellsfrom the western part of the Moesian Platform in Romania. Inthis paper only one example of a burial history curve for the5 Stoenita well is presented (Fig. 4). The thickness of thepreserved stratigraphic units has been determined directlyfrom well logs and from lithostratigraphic profiles. Construction of burial curves and calculation of thermal maturityhas been undertaken using original computer software. Thissoftware has been constructed on the basis of the geologicalmodel of the study area and taking into account its erosional

and depositional history. Because the geological data in theburial history plots are insufficient (e.g. no palaeobathymetry,small erosional phases), the deposits have not been decom-pacted to their original depositional thicknesses. Thus actual

of the source rocks from the western part of the Moesian

C mo Pg Rt P p B p Volume of(%) (%) (%) (%) (%) oil gener ated

(10 6m3)

3.07 35 30 40 1.1

5.31 40 40 45 1.1

6.83 32 50 35 1.15.55 40 55 50 1.1

8.97 50 55 50 1.2

Generated oil volume 4047



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£

Q.

QSm = SarmatianK1 = Lower CretaceousJ3 = Upper JurassicJ2 = Middle JurassicT3 = Upper TriassicT2 = Middle TriassicT1 = Lower TriassicD1 = Lower Devonian

Fig. 4. Burial curves of the sedimentary deposi ts of the Moesian Platform at the Stoenita 5 well loca tion.

Fig. 5. Vitrinite reflectance map of the Upper Devonian interval from the wes tern part of the Moesian Platform (Romania). 1, Vitrinite reflectanceisoline (%); 2, mature source rock for oil generation; 3, depositional and erosional limit of the Upper Devonian; 4, well location used to compute thevalues of vitrinite reflectance.




stratigraphic thicknesses have been used. Also, because oflack of data, the present day thermal regime has been used.Time-temperature indices (TTI) (Lopatin 1971; Waples 1979,1980) were calculated.

Corresponding vitrinite reflectance values have been deter

mined using the following equation (Kalkreuth & McMechan1988):

logVRmax(%) = - 0.4769 + 0.2801 (log TTI)

-0.007472(logTTI)2(1)

where: VRmax = calculated vitrini te reflectance; TTI = timetemperature index. At present, in the Romanian area thereare no measured vitrinite reflectance values available to becompared with the calculated data. The calculated valueshave been used to make vitrinite reflectance maps for UpperDevonian (Fig. 5), Middle Carboniferous (Fig. 6), Middle Jurassic(Fig. 7) and Sarmatian (Fig. 8) stratigraphic levels. The timing

of generation within the study area as a whole was estimatedfrom its overall thermal and burial history. This suggests that inthe southwest of the study area (Girla-Cetate zone) the onsetof oil generation for the Silurian shales took place 278 Ma ago;in the Morunglav area these source rocks began to generateoil 80 Ma ago. The onset of the oil generation from the otheranalysed source rocks took place during Sarmatian and Pliocene times. Sarmatian source rocks began to generate oil0.2-0.5 Ma ago and the process of generation continues to

the pre sen t day (vitrinite reflectance < 1.3%) and these rockshave not passed through the oil generation window.

MODELLING OIL GENERATION

The volume of oil generated from a mature source rock isrelated to the amount, type and maturity of its kerogen. Theamount of the oil generated within the study area may becalculated using a geochemical mass balance method (White& Gehman 1979). The basic equation may be expressed asfollows:

V = V rs x C mo xP„xRt xF p xB p (2)

where: V is volume of oil generated (m3); V rs is volume ofsource rock (m3); C mo is organic matter content by volume(%); Pg is genetic potent ial (%); Rt is transformat ion rat io (%);F p is fraction of oil in the hydrocarbon yield; B p is volumeincrease on oil yield. The organic matter content by volume(Cmo) has been com puted using an original volumetric model

ling based on a few geochemical analyses. These values weregiven in weight percentiles. The type of the kerogen hasdetermined the evaluation of the genetic potential (Pg) andtransformation ratio (Rt ) values (Tissot & Welte 1978; Goff1983). The fraction of oil in the hydrocarbon yield and thevolume increase on oil yield have been chosen based onproduction data from oil and gas fields. The values of theparameters and the mode of computation are presented inTable 1. The area of the mature source rocks was computed

Morunglav

Fig. 6. Vitrinite reflectance map of the Middle Carboniferous deposits from the western part of the Moesian Platform (Romania). 1, Vitrinitereflectance isoline (%); 2, mature source rock for oil genera tion; 3, depositional and erosional limit of the Middle Carboniferous; 4, well location usedto compute the values of vitrinite reflectance.



246 C. Pene

^ u ^ 10 10 Km ' ^ ^ - J ,

^ u ^ 1

^ < * ~ 21 Strehoio

f t ^ t f 30

80

oA

55

Morunjlov

60

' t / t r t f *•

Segorcea

Bailestl

40

Fig. 7. Vitrinite reflectance map of the Middle Jurassic deposits from the western part of the Moesian Platform (Romania). 1, Vitrinite reflectanceisoline (%); 2, mature source rock for oil generation; 3, depositional and erosional limit of the Middle Jurassic; 4, well location used to compute thevalues of vitrinite reflectance.

from the vitrinite reflectance maps. The average thickness ofthe active source rocks was determined from lithostrati-graphic and geochemical analyses. If, in this modelling of oilgeneration, the potential of other source rocks from the studyarea (e.g. Middle Triassic limestones and dolomites, Oligoceneof the Carpathian Foredeep) had been included, the volume ofthe petroleum generated would be even higher.

available to be reservoired (Table 2). These volumes are muchgreater than the actual oil amounts which have been discovered in the study area. This shows the high petroliferouspotential of the western part of the Moesian Platform. Thestudy encourages new investment in geological and geophysical exploration, because of the favourable conditions for oilgeneration and accumulation.

MODELLING EXPULSION AND MIGRATION

The mode of calculation of the expelled and migrated oil is

presented in Table 2. The parameters used in the computation(expulsion and migration coefficients) are assumed by similarity with other petroliferous basins (North Sea Basin, Goff1983; Mackenzie etal. 1987; Forbes etal 1991). The quantitiesof oil expelled and migrated represent the volumes of the oil

CONCLUSIONS

(1) The most important source rocks in the study area are the

Silurian, Middle Carboniferous, Middle Jurassic and LowerSarmatian shales as well as Upper Devonian bituminouslimestones and dolomites. The Silurian shales are therichest source rocks in this part of the Moesian Platform(Romanian area).

Table 2. Computation of the expelled and migrated hydrocarbon volumes

Source interval Volume of Expulsion Volume of Migration Volume ofgenerat ed oil coefficient expelled oil coefficient migrated oil

(106m3) (%) (1 06m3) (%) (106m3)

Sarmatian 23 50 11 70 8Middle Jurassic 130 50 65 60 39

Middle Carboniferous 109 50 54 40 27Upper Devonian 1083 50 541 30 162Silurian 2712 60 1657 20 325

Oil volumes 4057 2328 561




-—10 ^"1lOKm

^or

80

o5

Fig. 8. Vitrinite reflectance map of the Lower Sarmatian deposits from the western part of the Moesian Platform (Romania). 1, Vitrinite reflectance

isoline (%); 2, mature source rock for oil generation; 3, depositional and erosional limit of the Lower Sarmatian; 4, well location used to compute thevalues of vitrinite reflectance.

(2) The calculated thermal maturity indicates that the onsetof oil generation (VR = 0.6%) took place at more than2000 m dep th for the Silurian source rocks and morethan 2900 m for the other source rocks.

(3) In the southwest (Girla-Cetate) and east (Morunglav) ofthe study area, the onset of oil generation for the Silurianshales took place 278 Ma ago and 80 Ma ago, respectively.The oil could not have accumulated in younger reservoirsbecause the majority of traps were formed after oilgeneration started. Thus only a small fraction has beenreservoired from the 325 x 106m3 of oil calculated to

have been expelled from Silurian shales.(4) The onset of oil generation for the other analysed source

rocks in the study area took place during Sarmatian andPliocene time. Thus hydrocarbon generation started aftermost traps were already formed.

(5) The calculations suggest that the amounts of generated,expelled and migrated hydrocarbon available to be reservoired are much greater than the actual oil and gasvolumes which have been discovered in the westernpart of the Moesian Platform.

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248 C. Pene

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Received 18 September, 1995; revised typescript accepted 23 April, 1996

Hydrocarbon Generation Modelling in the West Moesian Platform - Pene, 1996

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