U.S. Department of the Interior

U.S. Geological Survey

Bulletin 2204–F

Total Petroleum Systems of the Carpathian–Balkanian Basin 

Province of Romania and Bulgaria
Total Petroleum Systems of the 

Carpathian–Balkanian Basin Province of 

Romania and Bulgaria 

By Mark Pawlewicz 

Bulletin 2204–F

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Pawlewicz, Mark, 2007, Total Petroleum Systems of the Carpathian–Balkanian Basin Province of Romania and 

Bulgaria: U.S. Geological Survey Bulletin 2204–F, 17 p.; available at <http://pubs.usgs.gov/bul/2204/f/
iii

Foreword

This report was prepared as part of the World Energy Project of the U.S. Geological 

Survey. In the project, the world was divided into 8 regions and 937 geological provinces. The 

provinces were first ranked according to the volumes of discovered oil and gas within each 

(Klett and others, 1997). Then, 76 “priority” provinces (exclusive of the United States and 

chosen for their high ranking) and 26 “boutique” provinces (exclusive of the United States and 

chosen for their anticipated petroleum richness or special economic importance to a region) 

were selected for appraisal of oil and gas resources. The petroleum geology of these priority 

and boutique provinces is described in this series of reports. A detailed report containing the 

assessment results is available separately (U.S. Geological Survey, 2000). 

The purpose of this effort is to aid assessment of the quantities of oil, gas, and natural gas 

liquids that have the potential to be added to reserves within the next 30 years. These volumes 

either reside in undiscovered fields whose sizes exceed the stated minimum-field-size cutoff 

value for the assessment unit (variable, but at least 1 million barrels of oil equivalent), or they 

occur as reserve growth in fields already discovered.

The total petroleum system constitutes the basic geologic unit of the oil and gas assessment. The system includes all genetically related petroleum that occurs in shows and accumulations (discovered and undiscovered) that have been generated by a pod or by closely related 

pods of mature source rock, and that exist within a limited mappable geologic space, together 

with the essential mappable geological elements (source, reservoir, seal, and overburden rocks) 

that control the fundamental processes of generation, expulsion, migration, entrapment, and 

preservation of petroleum (Magoon and Dow, 1994). The minimum petroleum system is that 

part of a total petroleum system encompassing discovered shows and accumulations together 

with the geologic space in which the various essential elements have been proved by these 

discoveries.

An assessment unit is a mappable part of a total petroleum system in which discovered and 

undiscovered fields constitute a single relatively homogenous population such that the chosen 

methodology of resource assessment based on estimation of the number and sizes of undiscovered fields is applicable. A total petroleum system might equate to a single assessment unit or 

it may be subdivided into two or more assessment units such that each is sufficiently homogeneous in terms of geology, exploration considerations, and risk to assess individually. Assessment units are considered “established” if they contain more than 13 fields, “frontier” if they 

contain 1–13 fields and “hypothetical” if they contain no fields. 

A graphical depiction of the elements of a total petroleum system is provided in the form 

of an events chart that shows the times of (1) deposition of essential rock units; (2) processes, 

such as trap formation, necessary to the accumulation of hydrocarbons; (3) the critical moment 

in the total petroleum system; and (4) the preservation of hydrocarbons. 

A numeric code identifies each region, province, total petroleum system, and assessment 

unit; these codes are uniform throughout the project and will identify the same item in any of the 

publications. The code with respect to the Carpathian–Balkanian Basin Province is as follows:

Example

Region, single digit 4

Province, three digits to the right of the region code 4061

Total petroleum system, two digits to the right of province code 406101

Assessment unit, two digits to the right of petroleum system code 40610101

The codes for regions and provinces are listed in Klett and others (1997).

Oil and gas reserves quoted in this report are derived from Petroleum Exploration and 

Production database (Petroconsultants, 1996) and other area reports from Petroconsultants, Inc., 

unless otherwise noted.

Boundaries of total petroleum systems, assessment units, and pods of active source rocks 

were compiled using geographic information system software. Political boundaries and cartographic representations were taken, with permission, from Environmental Systems Research 

Institute’s ArcWorld 1:3,000,000 digital coverage (1992), have no political significance, and are 

displayed for general reference purposes only. Oil and gas center points are reproduced, with 

permission, from Petroconsultants (1996). 
iv

Contents 

Foreword ........................................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Geology of the Carpathian–Balkanian Basin Province ...........................................................................1

Structural Setting..................................................................................................................................1

Stratigraphy ...........................................................................................................................................3

Source Rocks.........................................................................................................................................7

Reservoirs ............................................................................................................................................10

Traps and Seals...................................................................................................................................11

Hydrocarbon Generation and Migration..................................................................................................11

Resource Assessment ...............................................................................................................................11

Moesian Platform Composite Total Petroleum System, Moesian Platform Assessment 

Unit...........................................................................................................................................11

Dysodile Schist–Tertiary Total Petroleum System, Romania Ploiesti Zone and Romania 

Flysch Assessment Units ....................................................................................................12

Assessment Results and Summary ..........................................................................................................13

References Cited..........................................................................................................................................16

Figures

1–4. Maps showing:

1. Total petroleum systems and oil and gas fields in the Carpathian–Balkanian Basin 

geologic province....................................................................................................................2

2. Generalized geological and geographic features of the Carpathian–Balkanian Basin 

Province....................................................................................................................................3

3. Location and structural features of the Carpathian Mountains region...................................4

4. Geological features of the eastern Carpathian Mountains .......................................................5

5. Cross section of nappe geology near the Carpathian bend, Romania .............................................6

6. Map of geological features in Romania.................................................................................................7

7. Stratigraphic column of the Moesian platform, Romania and Bulgaria...........................................8

8. Stratigraphic column of eastern Carpathians and Moesian platform, Romania ............................9

9. Events chart for the Moesian Platform Composite Total Petroleum System.................................12

10. Events chart for the Dysodile Schist–Tertiary Total Petroleum System .......................................14

11. Diagram showing estimated undiscovered oil fields in the Carpathian–Balkanian Basin 

Province...........................................................................................................................................15

12. Diagram showing estimated undiscovered gas fields in the Carpathian–Balkanian Basin 

Province...........................................................................................................................................15
v

Tables

1. Summary of allocated oil and gas resources, Carpathian–Balkanian Basin Province, as 

evaluated per assessment unit....................................................................................................13

2. Summary of allocated oil and gas resources, Carpathian–Balkanian Basin Province ...............14

Abbreviations

API American Petroleum Institute

BCFG billion cubic feet of gas

HC/g TOC hydrocarbon/gram of total organic carbon

MMBNGL million barrels of natural gas liquids

MMBO million barrels of oil

MMCFG million cubic feet of gas

TOC total organic content (usually, total organic carbon)

°C/km degrees Celsius per kilometer

g/cm3 grams per cubic centimeter

g/m3 grams per cubic meter

km kilometer

m meter

Ma Mega-annum

mD millidarcy

mg milligram

ppt parts per thousand

Ro vitrinite reflectance

wt percent weight percent

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

kilometer (km) 0.6214 mile (mi)

meter (m) 3.281 foot (ft) 

meter (m) 1.094 yard (yd) 
Abstract

The U.S. Geological Survey defined the Moesian Platform Composite Total Petroleum System and the Dysodile 

Schist–Tertiary Total Petroleum System, which contain three 

assessment units, in the Carpathian–Balkanian Basin Province 

of Romania and Bulgaria.

The Moesian Platform Assessment Unit, contained within 

the Moesian Platform Composite Total Petroleum System, 

is composed of Mesozoic and Cenozoic rocks within the 

Moesian platform region of southern Romania and northern 

Bulgaria and also within the Birlad depression in the northeastern platform area. In Romania, hydrocarbon sources are 

identified as carbonate rocks and bituminous claystones within 

the Middle Devonian, Middle Jurassic, Lower Cretaceous, and 

Neogene stratigraphic sequences. In the Birlad depression, 

Neogene pelitic strata have the best potential for generating 

hydrocarbons. In Bulgaria, Middle and Upper Jurassic shales 

are the most probable hydrocarbon sources. 

The Romania Flysch Zone Assessment Unit in the 

Dysodile Schist–Tertiary Total Petroleum System encompasses three structural and paleogeographic subunits within 

the Pre-Carpathian Mountains region: (1) the Getic depression, a segment of the Carpathian foredeep; (2) the flysch 

zone of the eastern Carpathian Mountains (also called the 

Marginal Fold nappe); and (3) the Miocene zone (also called 

the Sub-Carpathian nappe). Source rocks are interpreted to 

be Oligocene dysodile schist and black claystone, along with 

Miocene black claystone and marls.

Also part of the Dysodile Schist–Tertiary Total Petroleum 

System is the Romania Ploiesti Zone Assessment Unit, which 

includes a zone of diapir folds. This zone lies between the 

Rimnicu Sarat and Dinibovita valleys and between the folds 

of the inner Carpathian Mountains and the external flanks of 

the Carpathian foredeep. The Oligocene Dysodile Schist is 

considered the main hydrocarbon source rock and Neogene 

black marls and claystones are likely secondary sources; all 

are thought to be at their maximum thermal maturation. 

Undiscovered resources in the Carpathian–Balkanian 

Basin Province are estimated, at the mean, to be 2,076 billion 

cubic feet of gas, 1, 013 million barrels of oil, and 116 million 

barrels of natural gas liquids. 

Introduction

The Carpathian–Balkanian Basin Province lies in northern Bulgaria and southern and eastern Romania (fig. 1). The 

boundaries are (1) the Romania-Ukraine national border on 

the north, (2) Moldavia (of the former Soviet Union) and the 

Black Sea on the east, (3) the southern extent of the Moesian 

platform in central Bulgaria on the south, and (4) the Transylvania Basin and the southern Carpathian Mountains on the 

west (figs. 2, 3). As defined, the western and northern parts 

of the province are dominated by a series of extensive nappes 

(fig. 4) that form much of the Carpathian Mountains chain, 

whereas the eastern and southern parts are characterized by a 

relatively stable structural platform containing several intraplatform basins. Petroleum is produced mainly in the northern 

and western parts. On the basis of known petroleum volumes 

(amount produced to date plus remaining reserves), the province is ranked 42d in the world (exclusive of the United States 

and Canada), with 5.9 billion barrels of oil, 7.3 trillion cubic 

feet of gas, and 100 million barrels of natural gas liquids, for a 

total of 7.2 barrels of oil equivalent (Klett and others, 1997). 

For purposes of this study, the Carpathian–Balkanian 

Basin Province is divided into two total petroleum systems 

(TPS) and three assessment units (AU): (1) the Moesian 

Platform Composite TPS, containing the Moesian Platform 

AU; and (2) the Dysodile Schist–Tertiary TPS, containing the 

Romanian Ploiesti Zone and Romania Zone AUs (fig. 1).

Geology of the Carpathian–Balkanian 

Basin Province

Structural Setting 

The Moesian platform, a broad crustal block formed by 

Hercynian deformation during Late Devonian to Early Triassic 

time, occupies all of northern Bulgaria and extends north and 

west under the Danubian lowlands into Romania where it borders the foredeep of the Carpathian Mountains (figs. 2, 4). The 

structure of the region was shaped by two principal events, 

one that formed large and moderately faulted anticlines during 

Total Petroleum Systems of the Carpathian–Balkanian 

Basin Province of Romania and Bulgaria 

By Mark Pawlewicz 
 Total Petroleum Systems, Carpathian–Balkanian Basin Province

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Figure 1. Carpathian–Balkanian Basin Province (4061), Romania and Bulgaria, showing boundaries 

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Petroleum System and locations of oil and gas fields.
Paleozoic to Triassic time and another that formed a northdipping homoclinal feature in post-Triassic time. The platform 

was uplifted and exposed subaerially during the Early Jurassic, 

and during Late Jurassic and Early Cretaceous time it formed a 

north-facing Tethyan passive margin that accumulated marine 

sediments while continuously subsiding. Another period of 

uplift during the Early Cretaceous (Aptian) was followed by 

subsidence until the end of the Cretaceous, after which the 

platform was again uplifted and remained exposed until the 

middle Miocene (Stefanescu and others, 2000). 

In Bulgaria, the Moesian platform was a relatively stable 

block with several superimposed structural or sedimentary 

basins receiving sediments until the Tertiary (Vuchev and 

others, 1993). Along its south margin, Early Triassic to 

Early Jurassic intercratonic rifting, followed by compression, shortened the overall width of the basin and produced a 

south-vergent fold and thrust belt (figs. 5, 6). Another phase 

of compression began in the Early Cretaceous and continued 

through the middle Eocene. This tectonic activity created normal fault–bounded rotated blocks in an autochthonous section 

below a detachment fault and elongate, asymmetrical anticlines in an overlying allochthonous section (fig. 5) (Emery 

and Georgiev, 1993; Ionescu, 1993).

A geologic feature in the northeastern part of the 

Carpathian–Balkanian Basin Province is the Birlad depression (fig. 2), a graben that plunges to the west beneath the 

Carpathian foredeep. The 

depression has a depth to basement of more than 6 km. 

In Romania, compressional 

forces of the Alpine orogeny 

formed the Paleogene flysch 

and Neogene molasse basin of 

the eastern Carpathian Mountains and the Paleogene and 

Neogene molasse basin of the 

southern Carpathian Mountains 

(fig. 2) and then gave rise to an 

extensive series of nappes that 

were thrust east over the foreland (Dicea, 1993). The eastern 

Carpathian Mountains are 

650–700 km long and as much 

as 100 km wide, and they consist of a stack of sedimentary 

nappes of a Lower Cretaceous 

to Miocene flysch series that 

extends 75 km eastward over 

the foredeep; total crustal shortening is about 250 km (Ziegler 

and Roure, 1996). During the 

middle Miocene (Badenian), 

the Tarcau and Marginal Fold 

nappes of the eastern Carpathian Mountains were emplaced (fig. 4) (Sandelescu, 1988), followed by emplacement of the nappes of the outermost Carpathian orogen during the middle late Miocene (Sarmatian). Also in 

the late Miocene, the Paleogene and Neogene nappe sequence 

was underthrust by the foreland platforms that form the autochthon of the Sub-Carpathian, Marginal Folds, and Tarcau nappes, 

in the eastern as well as the southern Carpathians (Dicea, 1996) 

(fig. 4). Stefanescu and others (2000) estimated post-Oligocene 

shortening to be greater than 100 km. Salt diapirs, created by 

alternating episodes of extension and compression that began in 

early Burdigalian and Badenian (early to middle Miocene) time, 

are located in the transitional zone between the Getic depression and the eastern Carpathian Mountains, where the 

Sub-Carpathian nappe becomes progressively shallower from 

the west toward the east and northeast (Stefanescu and others, 

2000) (fig. ̨4). The Carpathian foredeep is filled with upper 

Pliocene–lower Pleistocene molasse that resulted from erosion 

of the Carpathians; the molasse covers most of the external units 

of both the eastern and the southern Carpathians, along with part 

of the surrounding platforms.

Stratigraphy

Within the Moesian platform region, the Middle Cambrian to upper Carboniferous sequence is divided into three 

lithologic subunits (fig. 7). Upper Cambrian to Middle 

Devonian rocks are mainly arkosic and quartzitic sandstones 

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Figure 2. Generalized geological and geographic features of the Carpathian–Balkanian Basin 

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Figure 5. Cross section showing complex nappe geology near the Carpathian bend in Romania. After Stefanescu, 

2000. Vertical exaggeration is 2. Line of section shown in figure 4.
with some siltstone, shale, limestone, and conglomerate intercalations (some Silurian shales contain graptolites); Middle 

Devonian to Carboniferous rocks are composed primarily 

of massive limestones and dolomites with lesser amounts of 

bituminous limestones and evaporites; and middle to upper 

Carboniferous clastic rocks are coal bearing in part. This 

sequence is overlain by siltstones, marls, and sandstones (Tari 

and others, 1997).

Two sequences of Permian to Triassic strata are mainly of 

terrestrial origin. A Permian through Lower Triassic red clastic 

sequence consists largely of interbedded claystone, siltstone, 

quartzitic sandstone, calcareous sandstone, and conglomerate 

with minor dolomitic limestone, anhydrite, and salt; an Upper 

Triassic sequence is mostly shales, marls, sandstones, and 

conglomerates (Tari and others, 1997). A third sequence in 

the Middle Triassic, in contrast, is predominantly limestones 

and dolomite with interspersed marl and anhydrite or salt. 

Additionally, magmatic activity was common, especially at the 

beginning of the Permian and into the Middle to Late Triassic.

From Jurassic to Cretaceous time, clastic sedimentation 

was uneven across the Moesian platform; it began earlier on 

the south side than on the north side. Subsequently, massive 

carbonate rocks with interbedded siliciclastic rocks were 

deposited, including a thick turbidite sequence south of the 

platform (Vuchev, 1993). Following a period of erosion or 

nondeposition, limestone was again deposited, more abundantly on the east side than on the west side of the platform 

(Tari and others, 1997).

A physiographic feature in eastern and south-central 

Romania, termed the Pre-Carpathian depression, is situated 

between the Mesozoic crystalline core of the Carpathians on 

the west and the adjacent forelands to the east. It has been 

divided into several subunits—the Paleogene Flysch zone (also 

known as the Marginal Fold nappe), Miocene (Diapir Folds) 

zone, Miocene-Pliocene zone, and Getic depression (fig. 2)—

that are differentiated on the basis of stratigraphic, structural, 

and paleogeographic features. The associated flysch and 

molasse sequences range in age from Cretaceous to Pliocene 

(Ionescu, 1993).

The Paleogene Flysch zone is primarily detrital; it is 

composed of sandstones, marls, claystones, dysodile schist (a 

plastic, slightly elastic, yellow or greenish-gray hydrocarbonbearing schist), and black shale (fig. 5; see fig. 4 for location 

of line of section A-A ́). Thickness of this zone exceeds 7 km. 

The Miocene zone (also called the Sub-Carpathian nappe; 

fig. 5) is characterized by detrital molasse units that contain 

sandstones, shales, and marls. The Miocene-Pliocene zone 

(also called the Diapir Folds zone; fig. 2), mainly sandstones, 

black shales, and dysodile shales, is distinguished by having 

salt diapirs that play a large part in trap formation. Thick, 

late synorogenic and in part postorogenic molasse deposits 

cover the Carpathian foredeep, the Sub-Carpathian nappes, 

and the more internal nappes (fig. 5; Dicea, 1996). The Getic 

depression is a section of the Carpathian foredeep between the 

Carpathian Mountains and the Moesian platform that contains 

sandstones, black claystones, and shales (Ionescu, 1993) 

(fig. 2). 

Deposition was uneven during Paleogene to Neogene 

time. Deposits of sandstone, marl and, locally, carbonate 

rocks are as much as 1,600 m thick in the southern part of the 

Moesian platform, but they are thin to absent in the northern 

part. In contrast, during the Miocene more than 5.5 km of 

sandstones and carbonate rocks was deposited near the north 

edge of the platform. In late Miocene time, carbonate rocks 

were deposited in a brackish environment, indicating increased 

isolation from the sea during the Messinian, and extensive salt 

and evaporite deposits accumulated (Tari and others, 1997).

Source Rocks

Source rocks within the Moesian region include both 

clastic and carbonate rocks, although two lithologies—dysodile 

schist and black clay-rich shales—predominate in the most 

prolific hydrocarbon-producing regions of Romania. In these 

regions, known as the Getic depression, the Miocene zone, and 

the Paleogene Flysch zone (figs. 1, 2, 3), the main source rocks 

are Oligocene-Miocene dysodile shales. The shales contain 

Type-II and -III organic matter or kerogen, and are at a maturation level ranging from 1.0 to 1.15 percent vitrinite reflectance 

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Figure 7. Generalized stratigraphic column of the Moesian platform, Romania and Bulgaria. After Tari and others (1997). Plio., Pliocene; 

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(Ro

). In these shales, the total organic content (TOC) ranges 

from 0.35 to 2.5 percent, and calcareous shales it is as much as 

7.6 percent in (Popescu, 1995). In the region of the Marginal 

Fold nappe (figs. 4, 5), lower Oligocene (lower Rupelian) rocks 

comprise two shale sequences (fig. 8). Together, these formations range in thickness from 80 to 280 m. An upper interval 

of dysodile shale ranges from 50 to 100 m thick (Dicea, 1996). 

Source rocks for the Paleogene Flysch zone (Marginal Folds 

nappe) are Paleogene dysodile schist and menilites. In the 

Carpathian fold and thrust belt, lower Oligocene–lower Miocene (Rupelian–lower Burdigalian) dysodile shales are the most 

important source rocks, because they contain a reported 3.7 to 

29.8 percent TOC (Dicea, 1996).

Source rocks for the Diapir Folds zone are the Oligocene 

dysodile schist and possibly middle Miocene and Pliocene 

black shale (Dinu and others, 1996). Within the Marginal 

Folds nappes (fig. 6), the strata consist of the lower Menilite 

Shale (fig. 8) and dysodile shales and their equivalents. Hydrocarbons produced from the molasse deposits in the Carpathian foredeep (fig. 3) appear to be from the Oligocene–lower 

Miocene bituminous formations of the Marginal 

Folds and Sub-Carpathian nappes (Miocene 

zone) (Ionescu, 1991). Hydrocarbon sources in 

the Getic depression are thought to be Oligocene 

shales and Miocene black claystones (Dinu and 

others, 1996). Source rocks within the Birlad 

depression (fig. 2) are thought to be pelitic intercalations of Neogene (Badenian and Sarmatian) 

age, although the Middle Jurassic rocks are also 

a possible source (Ionescu, 1993). The Oligocene 

Menilite Shale is generally present in the eastern 

Carpathian Mountains (fig. 2) and contains a 

reported 10 percent TOC (Ziegler and Roure, 

1996). The organic matter of the Cretaceous 

nappes of eastern Romania contain rocks that 

change in percent TOC and degree of maturity 

from one nappe to another; depending on structural position, and location of the organic-bearing 

rocks, maturation level ranges from mature to 

overmature.

The hydrocarbon-generation potential of 

the western portion of the Moesian platform was 

estimated for the region by using TOC analyses 

and type of organic material. Ratios of hydrogen 

to carbon (H/C) and of oxygen to carbon (O/C) 

suggest that the organic material is Type-Ia kerogen of marine origin that produces mainly liquid 

hydrocarbons from Ordovician and Silurian 

shales. In Upper Devonian bituminous limestones 

and dolomites, the organic material appears to 

be both Type-I and -II (Type-II also derives from 

a marine source but yields fewer hydrocarbons 

than Type-I), but Type-I is predominant. The 

content of middle Carboniferous rocks varies 

vertically but is predominantly Type-III (humic, 

from a terrestrial source) (Pene, 1996a). Organic matter from 

Middle Jurassic and lowermost Sarmatian rocks is mostly 

Type-II (Pene, 1996a).

In the northwestern Moesian platform of Bulgaria, 

Middle Jurassic black shales and silty claystones as much as 

400 m thick may have the highest potential for generating 

hydrocarbons, although they contain only 0.35 percent TOC; 

Middle to Upper Triassic carbonate rocks also have potential 

(Popescu, 1995). Pene (1996a) believed that Devonian bituminous limestones, in addition to Silurian, middle Carboniferous, 

Middle Jurassic, and Sarmatian shales, are the main hydrocarbon source rocks in western Romania. 

In the most northern part of the Moesian platform in 

Romania (south of the outer Carpathian flysch units shown 

in fig. 3), Paleozoic and Lower Cretaceous (Albian) strata 

contain the most likely source rocks. Silurian-Ordovician 

shales, which have an Ro of 2.75 percent, are near the end of 

the dry-gas-generation window. Carboniferous dolomites have 

a modest TOC content of 0.3 to 1.7 percent (Type-II and -III 

kerogen) and an Ro of 1.3 percent. Carboniferous paralic shales 

have TOC content of 0.3 to 1.0 percent and an Ro of 1.25 perÊÊ,i} >

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Figure 8. Simplified stratigraphic column of eastern Carpathian Mountains and 

Moesian platform, Romania. After Stefanescu (2000). L., Lower, U., Upper, Plio., 

Pliocene. Not to scale.

Geology of the Carpathian–Balkanian Basin Province 
10 Total Petroleum Systems, Carpathian–Balkanian Basin Province

cent (near the onset of gas generation) with Type-III kerogen. 

Although analytical data are lacking, Oligocene shales are also 

thought to be a potential source of hydrocarbons. 

In the Getic depression (fig. 2), source rocks are in the 

Eocene, Oligocene, and middle to upper Miocene; geochemical data are insufficient to determine whether Cretaceous rocks 

also contribute (Dicea, 1996). 

Overall, numerous formations in Bulgaria can be considered as potential source rocks (Vuchev and others, 1993), but 

no major field has been discovered for several decades. Either 

favorable geologic conditions do not exist or any hydrocarbons 

that were generated have been lost; hence, Bulgaria appears to 

have few good future prospects.

Reservoirs

Upper Miocene clastic rocks, largely derived from the 

Carpathian Mountains, are typically the main reservoir on 

the Moesian platform. However, the first post–World War II 

field in Romania, discovered in the late 1940s, produced oil 

from a Lower Cretaceous carbonate reservoir on the Moesian 

platform. Productive horizons are now known to range from 

fractured Paleozoic carbonate rocks through reservoirs of 

Triassic, Jurassic, and Cretaceous age (Benton, 1997). Oil and 

gas fields have been discovered in Devonian, Triassic, Lower 

to Upper Jurassic, Lower to Upper Cretaceous, upper Miocene, and Pliocene rocks (Ionescu, 1993) and in both clastic 

and carbonate rocks. 

Oil and gas in the Moesian platform is produced mainly 

in Romania, where reservoirs in the producing fields are 

distributed throughout much of the stratigraphic column and 

possess a variety of lithologies. Most of the reserves within 

the platform area are also concentrated in Romania, and most 

of the fields produce oil. More than two-thirds of the oil and 

gas or condensate pools are in Mesozoic rocks, and they are 

equally distributed between the Permian-Triassic, Middle 

Jurassic (Dogger), and Cretaceous (Tari and others, 1997). 

Principle reservoir lithologies are (1) carbonate rocks in the 

Middle or Upper Devonian to lower Carboniferous, Middle 

Triassic, Upper Jurassic–Lower Cretaceous, and Upper Cretaceous; (2) sandstones in the Permian-Triassic and Middle 

Jurassic; (3) glauconitic sandstones in the Lower Cretaceous; 

(4) calcarenites in the Upper Cretaceous; (5) limestones in the 

upper Miocene; and (6) sandstones, some of which are poorly 

consolidated, in the middle Pliocene (Stefanescu, 1991; Tari 

and others, 1997). 

In western Romania, reservoir rocks consist of dolomite 

and limestone in Devonian, Middle Triassic, Upper Jurassic, 

and Lower Cretaceous strata, and sandstones in Lower and 

Upper Triassic, Middle Jurassic, Upper Cretaceous, middle 

Miocene (Sarmatian), and lower Pliocene strata (Pene, 1996a). 

Depth to production in the Moesian platform in Romania 

ranges from 350 to 5,000 m; deeper reservoirs are located in 

the northern part. Regionally, oil is produced mainly in the 

western part and gas is produced mainly in the eastern part 

(Ionescu, 1993). Rifting during the Permian-Triassic created 

geologic conditions favorable for the generation, migration, 

and trapping of hydrocarbons within parts of the Moesian 

platform. Reservoir rocks in this sequence have been discovered only in the northwestern part of the platform in Romania 

(Pene, 1996b). 

In the Carpathian “bend” zone (region around Ploiesti, 

Romania; fig. 2) and in the Getic depression, Oligocene, Miocene, and Pliocene formations contain many pairs of reservoir 

and seal. The lower to middle Miocene (Burdigalian and 

Badenian) salts (fig. 8) provide seals for many accumulations 

in the external Carpathians (Dicea, 1996). Within the Getic 

depression, oil and gas fields produce mainly from sandstones 

with porosities ranging from 15 to 25 percent and permeability 

ranging from 10 to 500 millidarcys (mD) (Dinu and others, 

1996). Drilling depths exceed 4,000 m.

Reservoirs in the Birlad depression (fig. 2) are in Jurassic and Paleozoic quartzitic sandstones at an average depth of 

4,200 m (Ionescu, 1993); average porosity is 4–9 percent and 

average permeability is 1–5 mD. 

The main reservoirs in the Paleogene Flysch zone (fig. 2) 

are the Kliwa Sandstone and the Fusaru Sandstone (OligoceneMiocene) (fig. 8), where drilling depths exceed 4,000 m, 

porosities range from 10 to14 percent, and permeabilities 

range from 2 to 100 mD (Dinu and others, 1996). In the 

eastern Carpathian Mountains and adjoining foreland basins, 

the Kliwa Sandstone is also a reservoir unit whose thickness 

ranges from 20 to 170 m. Secondary reservoirs consist of 

unnamed Oligocene and Miocene sandstones and conglomeratic beds ranging in thickness from 60 to120 m (Dicea, 1996).

In the Miocene zone (Sub-Carpathian nappe, fig. 4), 

hydrocarbon has accumulated in Oligocene and in lower and 

upper Miocene sandstones. The porosity of reservoirs ranges 

from 12 to14 percent and permeability from 5 to 70 mD 

(Dinu and others, 1996). In the Diapir Folds zone farther 

south (fig. 2), the reservoirs are mainly sandstones with high 

porosities, 14 to 25 percent, and permeabilities ranging from 

10 to 500 mD (Dinu and others, 1996). Reservoir characteristics, such as porosity, hydrocarbon content, and net pay 

thickness, change within short lateral distances. In eastern 

Romania, the various Cretaceous nappes also contain reservoir 

rocks—mainly sandstones, conglomerates, and different flysch 

facies—but lithology changes from one nappe to another 

(Stefanescu and Baltes, 1996).

In northern Bulgaria, clastic reservoirs are quartz-rich 

sandstones that contain reduced intergranular and dissolution 

porosity (Emery and Georgiev, 1993). Lower Triassic rocks 

are predominantly fluvial, braided, sandstone sequences with 

interbedded eolian clastic rocks (Ajdanlijsky, 1997). Triassic 

carbonate rocks and Lower and Middle Jurassic clastic rocks 

are the reservoirs for all known oil and gas accumulations in 

the Bulgarian portion of the Moesian platform. They are at 

depths of about 3,000 m (Vuchev and others, 1993).
Traps and Seals

Trap types within the Carpathian–Balkanian Basin 

Province differ throughout this geologically complex region. 

The traps are primarily structural in areas deformed by thrusting and folding or by salt diapirism, but in less structurally 

deformed areas, such as the Moesian platform, stratigraphic 

traps produced by facies changes or by unconformable relations are more common.

In Romania, east-west trending Neogene normal faults 

are the primary control on the distribution of oil fields, 

whereas in Bulgaria most fields combine structural and 

stratigraphic traps. Structural traps are associated with all 

nappe units, and unconformities related to different compressional phases form additional traps (Dicea, 1996). Established 

oil fields are contained within relatively shallow structures 

that attained their present configuration during late Pliocene 

deformation (Dicea, 1996). The Birlad depression is characterized by structural and lithostratigraphic traps, and the 

seals are middle and upper Miocene (Badenian, Sarmatian) 

and Pliocene marls and claystone. Reservoir traps are mainly 

faulted anticlines and monoclines on the west end of the Moesian platform, and the associated reservoir seals are marls and 

evaporites (Pene, 1996a).

In the Getic depression (southern Carpathians), hydrocarbons originated from Oligocene and Miocene source rocks and 

accumulated in both structural and stratigraphic traps. In the 

Paleogene Flysch zone, traps are formed by faulted anticlines, 

folds, duplexes, and imbricate structures (Dinu and others, 1996). 

In the eastern Carpathian Mountains, traps are mainly structural 

and may be anticlines, partly cut by thrust faults, or structures 

modified by salt diapirs; stratigraphic pinchouts and unconformity 

traps are secondary (Dicea, 1996). In the Tarcau nappe region, 

seals are provided by Pliocene rocks (Dicea, 1996).

Hydrocarbon Generation and Migration

Popescu (1995) believed that most of the hydrocarbons in 

Romania were expelled or remigrated into reservoirs after the 

Styrian orogeny about 12 to 14 Ma (middle Miocene); hydrocarbons presumably migrated updip along north-south- and 

east-west-trending normal faults and along unconformities. 

In the northern part of the Getic depression, Oligocene source 

rocks are thought to be at the beginning of the oil window at 

depths of 3,500–4,000 m. Generated hydrocarbons migrated 

updip to the south into structural and stratigraphic traps 

(Dicea, 1996). Oil from Silurian shales may have migrated 

before traps formed and was lost. Migration pathways for 

hydrocarbons and migration distances were directly controlled 

by the location and timing of thrusting (Larfargue and others, 

1995). 

The time during which oil and gas accumulated in the 

Moesian platform is not well known. One interpretation is that 

the hydrocarbons formed in a succession of stages beginning 

in the Paleozoic and were frequently remobilized and redistributed. A second interpretation is that hydrocarbons formed 

in only one period of generation, the Neogene, when all source 

rock units reached maturity owing to subsidence during Carpathian thrusting (Tari and others, 1997). Evidence supports 

the latter interpretation (Dicea, 1996), although a question 

remains as to the possibility of early generation during the 

Hercynian orogeny, about 370 to 220 Ma (Late Devonian–Late 

Triassic) (Tari and others, 1997). One more approximation 

for the onset of generation in the Paleogene Flysch zone is in 

the early Miocene (Burdigalian); however, this episode may 

have been only local owing to the geologic complexity of the 

flysch zone (Popescu, 1995). In the Moesian platform, onset 

of hydrocarbon generation is believed to have been during the 

middle Miocene (Popescu, 1995). In the Birlad depression, 

source rocks are buried to depths greater than 4,000 m and 

generation of hydrocarbons is thought to have begun in the 

late Miocene (Sarmatian) (Pene and others, 1997). 

Resource Assessment

Moesian Platform Composite Total Petroleum 

System, Moesian Platform Assessment Unit

The Moesian Platform Composite Total Petroleum 

System (TPS) (fig. 1), which includes the Moesian Platform 

AU, is composed of Mesozoic and Cenozoic rocks within the 

Moesian platform region of northern Bulgaria and southern 

Romania and also of the Birlad depression in the northeastern 

platform area. Jurassic, Cretaceous, and Tertiary formations 

within the platform are in a monocline that dips north and west 

beneath the Carpathian foredeep (fig. 5). Faulting is prevalent; 

some faults are oriented east-west parallel to the Carpathian 

orogen, and others are oriented north-south or northwestsoutheast. The main lithofacies is molasse derived from the 

Carpathian Mountains. 

The greatest volume of discovered oil in Romania is in 

the Moesian platform, where upper Miocene clastic rocks are 

the main reservoir. Triassic, Jurassic, and Cretaceous strata 

also contain several productive horizons. In the Birlad depression, middle to upper Miocene strata are thought to have the 

best potential for hydrocarbon source rocks; the Middle Jurassic is considered to be a possible secondary source rock. Most 

oil and gas discoveries in Bulgaria have been in a structural 

depression in the northwest, one of several superimposed 

structural and sedimentary basins in the Moesian platform 

region.

In Romania, source rocks are interpreted as carbonate 

rocks and bituminous claystones of Middle Jurassic, Early 

Cretaceous, and Oligocene ages; Ordovician-Silurian shales, 

as well as Carboniferous dolomite and shale, may have generated limited amounts of dry gas. Middle and Upper Jurassic 

shales in Bulgaria are considered the sources most likely to 

Resource Assessment 11
12  Total Petroleum Systems, Carpathian–Balkanian Basin Province

have generated oil and gas but to have had less capacity than 

the sources in Romania. 

An events chart (fig. 9) summarizes the ages of and relations among source, reservoir, seal, and overburden rocks and 

the timing of trap formation and the generation, migration, 

accumulation, and preservation of hydrocarbons in the Moesian Platform AU. Hydrocarbon generation is believed to have 

occurred in Permian-Triassic time, but only to a limited extent; 

any hydrocarbons generated then would probably have been 

lost owing to the lack of good reservoir seals. Late Neogene 

time is proposed for the principle generation period, because 

burial of source rocks in Paleogene and lower Neogene strata 

had become great enough for maturation of the organic matter. 

Gas generated from molasse deposits located within the Moesian platform may have followed a path similar to that of gas 

generated in the northern Carpathian Mountains of Poland and 

Ukraine, where the generation of biogenic gas is nearly contemporaneous with deposition of the source rocks (Kotarba, 

1998). The proximity of source rocks and reservoirs assured 

a relatively short migration distance into stratigraphic and 

structural traps, probably less than 5 km in most cases, and 

into reservoirs ranging in age from Paleozoic to mid-Pliocene. 

Carbonate and sandstone are the main reservoir lithologies in 

Romania. In the Bulgarian portion of the Moesian platform, 

reservoirs of all known oil and gas production are contained 

within Triassic carbonate rocks and Lower and Middle Jurassic clastic rocks.

Estimated undiscovered resources (mean values) for 

the Moesian Platform Zone AU are 171 million barrels of oil 

(MMBO), 561 billion cubic feet of gas (BCFG), and 20 million barrels of natural gas liquids (MMBNGL (table 1).

Dysodile Schist–Tertiary Total Petroleum 

System, Romania Ploiesti Zone and Romania 

Flysch Assessment Units 

The Dysodile Schist–Tertiary TPS (fig. 1) is composed 

of the Romania Flysch Zone AU and the Romania Ploiesti 

Zone AU, which differ in their structural configuration but 

share a number of closely related geological features. The 

Romania Flysch Zone AU includes the Getic depression, the 

Marginal Folds zone (part of the Paleogene Flysch zone of 

the eastern Carpathian Mountains), and the Miocene zone (or 

Sub-Carpathian nappe) (fig. 2). Oil with subordinate gas is 

produced from each assessment unit.

An events chart (fig. 10) summarizes, for the Dysodile 

Schist–Tertiary TPS, the ages of and relations among source, 

reservoir, seal, and overburden rocks and the timing of trap 

formation and the generation, migration, accumulation, and 

preservation of hydrocarbons. The sources of the oil for the 

AUs are two particularly rich source rocks: Oligocene dysodile schist and black claystones, and Miocene shales, black 

claystones, and marls. The dysodile schists are divided into 

lower and upper parts, whose thickness ranges from 50 to 100 

m and 80 to 200 m, respectively. Reported percent TOC ranges 

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from 3.7 to greater than 29; thus, the schists are exceptionally 

good sources of hydrocarbons. The main productive intervals 

in the Romania Ploiesti zone are in Oligocene, lower and 

upper Miocene, and Pliocene rocks. In the Getic depression, 

oil accumulations are closely related to the distribution of 

Paleogene formations; gas-prone Miocene-Pliocene rocks are 

the source for upper Miocene and Pliocene reservoirs.

The complex structure of the Ploiesti region is characterized by numerous overthrusts and folds and by deformation 

Figure 9. Events chart for the Moesian Platform Composite Total 

Petroleum System, illustrating relative timing of critical geologic 

events in the evolution of the total petroleum system. E., Early; 

M., Middle; L., Late; Eoc., Eocene; Mio., Miocene; Olig., Oligocene; 

Pal., Paleocene; Plio., Pliocene.
caused by salt diapirs, all of which placed potential reservoirs 

and source rocks in proximity (maximum distance 1–3 km). 

This proximity facilitated hydrocarbon migration along faults, 

both vertically and horizontally, into clastic rocks reservoirs 

with porosity as much as 25 percent and permeability as much 

as 500 mD and into fractured carbonate reservoirs. One sandstone reservoir in the external Carpathians and the foreland 

basin, the Kliwa Sandstone (fig. 8) is 20 to 170 m thick, and 

other unnamed sandstones and conglomeratic beds range in 

thickness from 60 to120 m. Reservoir characteristics, such 

as porosity, hydrocarbon content, and net pay thickness, can 

change laterally over short distances throughout the Dysodile 

Schist–Tertiary TPS. In the Tarcau nappe of the Romania 

Flysch Zone AU (fig. 4), hydrocarbons are produced from 

the Eocene Tarcau Sandstone, Oligocene Kliwa Sandstone, 

and Oligocene–lower Miocene Fusaru Sandstone (fig. 8). In 

the Getic depression, reservoirs are in Paleogene, Miocene, 

and Pliocene sandstones with porosity ranging from 15 to 

25 percent, and permeability from 10 to 500 mD. Generally 

throughout the total petroleum system, hydrocarbon accumulations are contained within a variety of reservoir lithologies 

and are sealed primarily by claystones, evaporites, and marls; 

however, fault planes seal many of the structural traps.

Generation of hydrocarbons is proposed to be late Neogene (fig. 10), at which time source rocks were buried sufficiently deep by various nappes and molasse deposits to mature. 

Estimated undiscovered resources (mean values) in the Romania Flysch Zone AU are 564 MMBO, 2,177 BCFG, and 81 

MMBNGL; in the Romania Ploiesti Zone AU these mean values 

are 278 MMBO, 441 BCFG, and 14 MMBNGL (table 1).

Assessment Results and Summary

Table 1 summarizes the allocated oil and gas resources 

in the Carpathian–Balkanian Basin Province of Romania and 

Bulgaria. Assessment units are listed for each total petroleum 

system. The resources are grouped into fractiles, which provide a means of assigning a risk factor to the actual discovery 

of these resources; that is, the higher the “F” value, the higher 

the confidence that new reserves will be discovered in the next 

30 years. For example, at the F95 level, a 95 percent chance 

exists that at least the estimated amount of oil or gas will be 

discovered. Total undiscovered resources for the province, 

estimated at the mean (F50 values), are 1,013 MMBO, 3,179 

BCFG, and 116 MMBNGL (table 2). Although the province 

presents a difficult challenge for future petroleum exploration 

and production, owing to its complex geologic structure and 

stratigraphic variability, the potential exists for the discovery 

of large amounts of additional oil and gas, most probably in 

small fields (figs. 11, 12). 

Table 1. Summary of allocated oil and gas resources, Carpathian–Balkanian Basin Province (4061), as evaluated per assessment unit.

[MMBO, million barrels of oil. BCFG, billion cubic feet of gas. MMBNGL, million barrels of natural gas liquids. MFS, minimum field size assessed 

(MMBO or BCFG). Prob., probability (both geologic and accessibility probabilities) of at least one field equal to or greater than the MFS. Results shown are 

fully risked estimates. For gas fields, all liquids are included under the NGL (natural gas liquids) category. F95 represents a 95 percent chance of at least the 

amount tabulated. Other fractiles are defined similarly. Fractiles are additive under the assumption of perfect correlation. Shading indicates not applicable]

Assessment Results and Summary 13
14  Total Petroleum Systems, Carpathian–Balkanian Basin Province *,
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Figure 10. Events chart for the Dysodile Schist–Tertiary Total 

Petroleum System, illustrating relative timing of critical geologic events 

in the evolution of the total petroleum system. E., Early; L., Late; Eoc., 

Eocene; Mio., Miocene; Olig., Oligocene; Pal., Paleocene; Plio., Pliocene.

Table 2. Summary of allocated oil and gas resources, Carpathian–Balkanian Basin Province (4061).

[BCFG, billion cubic feet of gas; MFS, minimum field size assessed (1 MMBO or 6 BFCG); MMBNGL, million barrels of natural gas liquids; MMBO, million 

barrels of oil. Prob., probability (including both geologic and accessibility probabilities) of at least one field equal to or greater than the minimum field size. 

Results shown are fully risked estimates. For gas fields, all liquids are included under the natural gas liquids (NGL) category. F95 represents a 95 percent chance 

of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation. Shading indicates 

not applicable]
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Figure 11. Estimated number and size of undiscovered oil fields in Carpathian–Balkanian Basin Province (4061) of Eastern Europe. 

MMBO, million barrels of oil.

Figure 12. Estimated number and size of undiscovered gas fields in Carpathian–Balkanian Basin Province (4061) of Eastern Europe. 

BCFG, billion cubic feet of gas.

Assessment Results and Summary 15
16  Total Petroleum Systems, Carpathian–Balkanian Basin Province

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References Cited 17

Published in the Central Region, Denver, Colo.

Manuscript approved for publication February 23, 2007

Graphics by author

Photocomposition by Margo L. Johnson

Edited by Mary-Margaret Coates (Contractor, ATA Services)
Pawlewicz—Total Petroleum Systems, Carpathian–Balkanian Basin Province, Romania and Bulgaria—Bulletin 2204–F

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