U.S. Department of the Interior

U.S. Geological Survey

Bulletin 2204–E

Transylvanian Composite Total Petroleum System

of the Transylvanian Basin Province, Romania,

Eastern Europe
Transylvanian Composite Total Petroleum System 

of the Transylvanian Basin Province, Romania, 

Eastern Europe

By Mark Pawlewicz

Bulletin 2204–E

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005

Posted online August 2005, version 1.0

This publication is only available online at:

http://pubs.usgs.gov/bul/2204/e

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Pawlewicz, Mark, 2005, Transylvanian Composite Total Petroleum System of the 

Transylvanian Basin Province, Romania, Eastern Europe: U.S. Geological Survey Bulletin 2204–E, 10 p.
Contents

Foreword ................................................................................................... 1

Abstract ................................................................................................... 2

Introduction................................................................................................ 2

Province Geology ....................................................................................... 2

Structural Setting ................................................................................. 2

Stratigraphy.......................................................................................... 2

Hydrocarbons in the Transylvanian Basin ............................................................ 5

Hydrocarbon Source Rocks........................................................................ 5

Seals ................................................................................................ 7

Reservoirs .......................................................................................... 7

Traps ................................................................................................ 7

Burial History and Generation and Migration of Hydrocarbons .............................. 7

Assessment Results and Summary ..................................................................... 8

References Cited.......................................................................................... 9

Figures

1. Map showing location and structural features of the Carpathian Mountains region, 

eastern Europe................................................................................. 3

2. Map showing location of Transylvanian Composite Total Petroleum System in

Romania, eastern Europe..................................................................... 4

3. Events chart for the Transylvanian Basin Neogene Suprasalt Gas Assessment

Unit ............................................................................................. 5

4. Cross sections showing stratigraphic relation of Upper Cretaceous

through Pliocene rocks for the Transylvanian Basin, central Romania,

eastern Europe................................................................................. 6

5. Cross section showing stratigraphic relation and basement configuration of

Jurassic through upper Miocene sediments, central Romania, eastern Europe ...... 6

6. Events chart for Transylvanian Basin Neogene Suprasalt Gas Assessment Unit,

central Romania, eastern Europe............................................................ 7

7. Graph showing undiscovered field size distribution for the Transylvanian

Neogene Suprasalt Gas Assessment Unit of the Transylvanian Composite

Total Petroleum System ..................................................................... 8

Table

1. Transylvanian Composite Total Petroleum System assessment results ............... 9

iii
Multiply By To obtain

kilometer (km) 0.6214 mile

square kilometer (km2) 0.3861 square mile

meter (m) 3.281 foot

cubic feet 0.02832 cubic meters

Vertical coordinate information is referenced to the North American 

Vertical Datum of 1988 (NAVD 88).

Conversion Factors

iv
Transylvanian Composite Total Petroleum System of the 

Transylvanian Basin Province, Romania, Eastern Europe

By Mark Pawlewicz

number and sizes of undiscovered fields is applicable. A TPS 

might equate to a single AU or it may be subdivided into two 

or more AUs 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 TPS is provided in the form of an events chart that shows: (1) the times 

of deposition of essential rock units; (2) the times that processes, such as trap formation, necessary to the accumulation 

of hydrocarbons took place; (3) the critical moment in the total 

petroleum system; and (4) the preservation time, if any. 

A numeric code identifies each region, province, TPS, 

and AU; these codes are uniform throughout the project and 

will identify the same item in any of the publications. The 

code is as follows:

Example

Region, single digit 4

Province, three digits to the right of the

region code 4057

Total Petroleum System, two digits to the

right of province code 405701

Assessment unit, two digits to the right of

petroleum system code 40570101

The codes for the 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 areas reports from Petroconsultants, Inc., unless otherwise noted.

Boundaries of TPSs, AUs, and pods of active source 

rocks were compiled using geographic information system 

(GIS) software. Political boundaries and cartographic representations were taken, with permission, from Environmental 

Systems Research Institute’s ArcWorld 1:3 million digital 

coverage (1992), have no political significance, and are displayed for general reference only. Oil and gas centerpoints are 

reproduced, with permission, from Petroconsultants (1996).

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 have been ranked accordingly to the discovered oil 

and gas volumes 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 regional economic importance) 

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 World Energy Assessment Team, 2000). 

The purpose of this effort is to aid in assessing the 

volumes 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 must be at least 1 million barrels 

of oil equivalent) or occur as reserve growth of fields already 

discovered.

The total petroleum system (TPS) constitutes the basic 

geologic unit of the oil and gas assessment. The TPS includes 

all genetically related petroleum that occurs in shows and 

accumulations (discovered and undiscovered) that has been 

generated by a pod or by closely related pods of mature 

source rock and which exists 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 TPS 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 (AU) is a mappable part of a TPS in 

which discovered and undiscovered fields constitute a single 

relatively homogeneous population such that the chosen methodology of resource assessment based on estimation of the 
Abstract

The Transylvanian Composite Total Petroleum System 

and the Transylvanian Neogene Suprasalt Gas Assessment 

Unit were identified in the Transylvanian Basin Province 

(4057), which lies entirely within the country of Romania. The 

assessment unit is composed of middle Miocene (Badenian) 

to Pliocene strata. Gas from the assessment unit is biogenic 

and originated from bacterial activity on dispersed organic 

matter in shales and siltstones. Gas migration is believed to 

be minimal, both vertically and laterally, with gases trapped 

in local stratigraphic traps and in structural traps (most likely 

domes and anticlinal folds created by salt diapirism) in the 

central part of the basin. For this reason, the gas fields also are 

concentrated in the central part of the basin. Reservoirs are 

isolated and composed of vertically stacked sandstones and 

siltstones sealed by shales and possibly by evaporite layers. 

Traps result from stratigraphic and facies changes within the 

entire assessment unit stratigraphic section. Some gas fields 

contain as much or more than a trillion cubic feet, but in most, 

the estimated resources are between 6 billion and 96 billion 

cubic feet. Petroleum exploration in the province is in the 

mature stage, which, combined with the geologic complexity of the region, is considered to limit future discoveries to a 

relatively few small fields. The undiscovered resources for the 

Transylvanian Basin Neogene Suprasalt Composite AU in the 

Transylvanian Hybrid Total Petroleum System (4057) are, at 

the mean, 2.083 trillion cubic feet of gas. No oil is produced in 

the basin. Rocks underlying the salt layers were not assessed 

for hydrocarbon potential.

Introduction

The Transylvanian Basin lies entirely within the country 

of Romania in eastern Europe (fig. 1). The basin is a middle 

Miocene to Pliocene sag basin that covers an area of about 

20,000 km2 in the central part of the Transylvanian intraCarpathian depression (Popescu, 1995). Surrounding the basin 

are the Carpathian Mountains on the north, east, and south and 

the Apuseni Mountains on the west.

The Transylvanian Basin Province (4057) contains the 

Transylvanian Composite Total Petroleum System (TPS 

405701), within which a single assessment unit (AU) (with 

a boundary coincident with that of the TPS), the Transylvanian Neogene Suprasalt Gas AU, is identified (fig. 2). The 

AU includes all rocks above the base of the Neogene stratigraphic sequence, where there is a basinwide evaporite layer. 

Potential petroleum resources in rocks below this layer were 

not assessed due to a lack of exploration and production and 

limited geologic and seismic data. 

The results of USGS World Energy 2000 assessment 

ranked the Transylvanian Basin Province (4057) as the 56th 

largest in the world, exclusive of the United States, with 0.2 

percent of the world’s oil and gas resources. This ranking is 

based on the volume of known reserves plus cumulative production (Klett and others, 1997). The assessment results are 

summarized in the USGS Digital Data Series DDS–60 (U.S. 

Geological Survey World Energy Assessment Team, 2000).

Province Geology

Structural Setting

The Transylvanian Basin is a depression superimposed 

on the Cretaceous nappe system of eastern Europe and the 

Romanian Carpathian Mountains (fig. 1). The basin began to 

form in Late Cretaceous time, and the present morphology 

and structure of the Carpathian (and Pannonian) region is the 

result of Cretaceous to Miocene convergence and collision of 

the European plate with several small continental blocks to the 

south (Royden and Baldi, 1988). Following the Cretaceous, 

the inner Carpathian zone, including the Transylvania area, 

became rigid while the outer zone remained mobile (Visarion 

and Veliciu, 1981).

The lowest structural units in the eastern Carpathian 

Mountains consist of a continuous belt of Upper Cretaceous to 

Miocene flysch that was deformed into north- and east-verging 

thrust and fold nappes in Tertiary time (Royden and Baldi, 

1988). Flysch sedimentation proceeded diachronously from 

west to east, ending in early Miocene time in the western Carpathians, whereas sedimentation of molasse facies continued 

until Pliocene time in the eastern Carpathians (Royden and 

Baldi, 1988). The basin was subjected to intensive Neogene 

subsidence and molasse sedimentation (Visarion and Veliciu, 

1981), resulting in middle Miocene to Pliocene molasse overlying the Upper Cretaceous and Paleogene.

The Apuseni Mountains, forming the western boundary of the Transylvanian Basin (fig. 1), are an agglomeration 

of autochthonous and allochthonous rocks of Precambrian 

to Mesozoic age, with lithologies ranging from medium- to 

high-grade schist, granite, and gneiss to limestone (Burchfiel, 

1976). In Pliocene to Holocene time, these mountains, along 

with the eastern and southern Carpathians, were uplifted, 

which probably resulted in the overall shape of the present-day 

Transylvanian Basin (Huismans and others, 1997). 

Geophysical data show that an estimated 7.5 to 8 km of 

sediments accumulated in the two deepest parts of the basin, 

one near the geographical center and the other near the northeast margin (Burchfiel, 1976). The structure of the sedimentary-rock terrain was strongly influenced by salt tectonics, 

which created salt domes 10–15 km in diameter in the central 

part of the basin and diapiric folds with a north-south orientation along the basin perimeter (Mocanu and others, 1996). 

Stratigraphy 

The Transylvanian basement consists of two distinct 

sequences: (1) a lower sequence of folded and thrusted 

 Transylvanian Composite Total Petroleum System of the Transylvanian Basin Province, Romania, Eastern Europe
Province Geology 

Figure 1. Location and structural features of the Carpathian Mountains region in eastern Europe. After Roca and others (1995). Inset 

map after Ciulavu and others (2000). A,A’ and B, B’: location of cross sections in figure 4; C, C’: location of cross section in figure 5.

Germany

Poland

Czech

Republic

Slovakia

Ukraine

Austria

Romania

Yugoslavia 

Bulgaria

Black

Sea

Moldavia

Hungary

Danube R.

0 300 600

KILOMETERS

Belarus

0 100 KILOMETERS 

16° 18° 20° 22° 24° 26° 28°

44°

45°

46°

47°

48°

49°

50°

Thrust fault edge covered 

Thrust fault teeth in upper unit 

VIENNA

KRAKOW

CLUJ

BELGRADE

ZAGREB

BRATISLAVA 

BUDAPEST

LVIV 

BUCHAREST

AUSTRIA

POLAND

SLOVAKIA UKRAINE

HUNGARY 

MOLDAVIA

ROMANIA

BULGARIA

Alpine inner zones 

Ophiolites

Pieniny klippen belt 

Alpine units Alpine outer units 

Magura and Transcarpathian 

 flysch units 

Outer Carpathian 

 flysch units 

Buried flysch units 

Foredeep

Neogene volcanism 

EXPLANATION 

Central

European

Platform

Eastern

European

 Platform 

Moesian Platform 

Transylvanian 

Basin

Dinarides

Miechow

Depression

Lublin-Lviv

Depression

Inner

Carpathians

Apuseni

Mtns.

Southern Carpathians 

Inner Carpathians

Pannonian

Basin

Stebnik

Stebnik

Stebnik

Borislav-Pokut

Skole

Silesian

Chornagora

Magura

Dukla

A

A'

B

B'

C

C'
crystalline rocks, Triassic to Lower Cretaceous sedimentary 

strata, and pre-Upper Cretaceous volcanic rocks, all of which 

represent an extension of Carpathian flysch stacked in nappes 

emplaced during Late Cretaceous compression; and (2) an 

upper sequence forming the post-tectonic basin fill composed 

of Upper Cretaceous to Tertiary formations, with thicknesses 

as much as 8 km (Ciulavu and others, 2000) (fig. 3). Paleogene to lower Miocene deposits are mostly confined to the 

northwestern part of the Transylvanian Basin and an area just 

to the north-northwest. Middle Miocene–Pliocene strata are 

restricted to the central and southern parts of the basin (Gyorfi 

and others, 1999), and middle to upper Miocene volcanic rock 

overlies the Paleogene to lower Miocene strata in the eastern 

and northern parts.

Early Miocene sedimentation in the Transylvanian Basin 

changed to mainly coarse-grained siliciclastics. North-south 

transpressional contraction between tectonic blocks caused 

uplift of the lower Miocene rocks and created a wedge-shaped 

Figure 2. Location of Transylvanian Composite Total Petroleum System (TPS) in Romania, eastern Europe. 

Red dots are gas-field centerpoints. Boundaries for TPS and Neogene Suprasalt Assessment Unit are coincident.

 Transylvanian Composite Total Petroleum System of the Transylvanian Basin Province, Romania, Eastern Europe

Transylvanian Composite Total Petroleum System (405701) 

Transylvanian Basin Province (4057) 

EXPLANATION 

Gas-field centerpoint 

LVIV 

BELGRADE

BUCHAREST

ROMANIA

SERBIA

AND

MONTENEGRO

HUNGARY 

SLOVAKIA 

POLAND

UKRAINE

BULGARIA

0 100 200 KILOMETERS 
Hydrocarbons in the Transylvanian Basin 

configuration—that is, the rocks thin markedly from north to 

south (fig. 4). In late early Miocene (late Burdigalian), basin 

deformation and erosion were followed by deposition of 

volcanics represented by the Dej Tuff, at the base of which is 

a regional unconformity (fig. 3). In turn, the tuff is overlain 

by thick evaporites that form diapirs in the central part of the 

basin (fig. 4) (Huismans and others, 1997).

Badenian strata encompass two distinct facies: a sandy 

marlstone facies in the western and southwestern parts of the 

basin and a pelitic facies the central part. Both facies contain dacitic tuffs and evaporites (Visarion and Veliciu, 1981). 

Basinwide Badenian to Pliocene claystones and sandstones 

overlie the evaporites and form continuous layers as much as 

4 km thick (fig. 5).

Sediments in the northwestern part of the Transylvanian Basin can be separated into three depositional cycles: 

(1) Eocene continental, brackish, evaporitic, and shallowmarine sequences; (2) lower Oligocene continental brackishwater deposits and marine deposits; and (3) lower Miocene 

siliciclastic deposits. During these depositional intervals, the 

direction of transgression shifted clockwise from northwest to 

south-southeast (Gyorfi and others, 1999).

Volcanism occurred during Late Cretaceous–Paleocene 

time on the west side of the basin, whereas within the basin 

proper, volcanic activity started during the Badenian. Volcanic 

activity, represented primarily by calc-alkaline tuffs, reached a 

peak in the Pliocene, and the final episode of volcanism was in 

the form of basaltic eruptions in the late Pliocene and Pleistocene (Visarion and Veliciu, 1981).

Sedimentary strata in the Transylvanian Basin were 

strongly affected by salt tectonics, which created domes 10–15 

km in diameter in the central part of the basin and diapiric 

folds with a north-south orientation in the marginal areas 

(Mocanu and others, 1996).

Hydrocarbons in the

Transylvanian Basin

The Transylvanian Basin is the main gas-producing region 

in Romania, with about two-thirds of the total gas reserves and 

four-fifths of the gas fields. Dinu and others (1996) reported the 

distribution of gas pools in the basin as follows: Badenian reservoirs, 54 pools; Sarmatian, 45 pools; and Pliocene, 2 pools. 

Gas composition is 98 percent methane (CH4), with negligible 

amounts of CO2 and N2 (Dinu and others 1996).

Popescu (1995) reported that most of the gas within the 

Transylvanian Composite TPS and the Neogene Transylvanian Composite Suprasalt AU is biogenic, although there may 

be some early thermogenic gas intermixed with the biogenic 

gas. There is speculation, prompted by the presence of gas 

accumulations with high concentrations of CO2 (as much as 

90 percent) and N2 (as much as 40 percent), that gases in several fields close to Neogene volcanic rocks in the central and 

eastern parts of the basin may have originated from a higher 

heat flow.

Hydrocarbon Source Rocks

Nonmarine and brackish-water shales of Miocene (Badenian) to Pliocene age are recognized as the source rocks for 

the hydrocarbon (all gas, no oil) in the Transylvanian Composite TPS (fig. 6); these rocks cover about 12,000 km2

 (Popescu, 

1995) in the Transylvanian Basin. Total organic carbon content 

(TOC) is as much as 1.2 percent, averaging 0.5 percent Corg 

that is reported to consist of Types II and III kerogen (Ciulavu 

and others, 2000; Dinu and others, 1996).

Rock Eval data from 10 samples of drill cuttings collected from one well at depths ranging from 950 to 2,250 m 

show that the rocks are thermally immature and thus have low 

source potential. The TOC values range from 0.21 to 0.85 

Figure 3. Stratigraphic column for the Transylvanian Basin, central Romania, eastern Europe. After Ciulavu and others (2000).

5

10

12.2

16.5

22

24

30

36

40

43.5

50

55

100

140

230

2,000

3,000

1,500

3,000

3,000

1.8

Hadareni Tuff

Salt

Dej Tuff Fm

Hida Beds

Ileanda Beds

Jibou Fm

? ?

S

EXPLANATION

limestone

conglomerate

tuff layers

metamorphics

salt

shale unconformity

sandstone

siltstone

S

Strong uplift

in north part

of basin and 

 salt

 diapirism

Basin begins

to assume 

present-day

morphology

?

W-E foreland

 basin

West-east

subsiding

basins

North-south

piggyback

basins

North-south

subsiding

basins

North-south

subsiding

basins Inner Dacides Transylvanides

Middle Dacides

Basin fill Basement

Eocene Oligocene Miocene Quaternary

Pontian

Pannonian

Sarmatian

Badenian

Burdigalian

Aquitanian

Chattian

Rupelian

Priabonian

Bartonian

Lutetian

Ypresian

Upper

Cretaceous

Lower

Cretaceous

Jurassic

Triassic

Metamorphics

Pliocene

Ma Geologic

age Lithology

Thick

 max.

 (m)

Selected

formation

 names

Regional

volcanism

Basin tectonic

units
Figure 4. Cross sections showing stratigraphic relation of Upper Cretaceous through Pliocene rocks for the Transylvanian Basin, 

central Romania, eastern Europe. Location of sections shown in figure 1. From Huismans and others (1997).

Figure 5. Cross section of Jurassic through Miocene showing stratigraphic relation and basement configuration for Transylvanian 

Basin, central Romania, eastern Europe. Location of section shown in figure 1. From Ciulavu and others (2000).

 Transylvanian Composite Total Petroleum System of the Transylvanian Basin Province, Romania, Eastern Europe

VOLCANICS

0LIOCENE

3ARMATIAN

"ADENIAN

"ADENIANSALT	

0ALEOGENEANDLOWER-IOCENE

5PPER#RETACEOUS

PRE
3ENONIANBASEMENT

- LÀii

%80,!.!4)/.

 ",/7
-/ -"1/
-/

 ",/ -"1/

! !g

" "g

ÎäÊ"
/
,-

Ó8ÊÛiÀÌV>ÊiÝ>}}iÀ>Ì

"V>Ê`iÊ-ÕÃ <iÊÌi>

/À>Û>V>

,ÛiÀ

-V VÕ -V> V>`

-iÃÕV

,ÛiÀ

-iÃ

,ÛiÀ

	iÕ`Õ *}>Vi>Õ>

ÕÀiÃ

,ÛiÀ

/À>Û>>Ài

,ÛiÀ

	À>Ìi }Ì>

"ÌÊ,ÛiÀ

1Vi>

ä

Metamorphic

basement

Metamorphic

basement

Metamorphic

basement

Inner Dacides Mures-Vardar zone Middle Dacides 

Ophiolites

Badu 4301 Besa 1 Rotbav 2 

C C'

Upper

 Miocene 

Salt

Middle

 Miocene 

Burdigalian

Oligocene

Eocene

Upper

 Cretaceous 

Lower

 Cretaceous 

Jurassic

 sedimentary rocks 

EXPLANATION 

Thrust fault 

(?) Strike-slip fault 

borehole

KM 0.5 

−3.5

−1.5

−5.5

−7.5

0.5 KM 

−1.5

−3.5

−5.5

−7.5

0 50

KILOMETERS
Hydrocarbons in the Transylvanian Basin 

percent, S2 ranges from 0.14 to 0.48 mgHC/g (milligrams of 

hydrocarbon per gram of rock), and Tmax (temperature corresponding to the maximum of hydrocarbon generation during 

pyrolysis used mostly for evaluation of the maturation stage of 

the kerogen) ranges from 423° to 436°C (Popescu, 1995).

Seals

Huismans and others (1997) described Badenian through 

Pliocene strata as forming a relatively monotonous succession 

of mainly claystones and sandstones. A thick evaporite layer 

within the Badenian rocks separates the Transylvanian Basin 

Neogene Suprasalt AU from any hydrocarbons that may be 

migrating upward from older strata. 

Reservoirs

Reservoirs for the Transylvanian Basin gas are in sandstone and marly sandstone units of the middle Miocene to 

Pliocene formations (fig. 6). Some fields can have as many as 

22 individual gas-bearing sandstones (Popescu, 1995), either 

in elongate, lenticular, or sheetlike bodies. Reservoir porosities 

are 25 to 30 percent, and permeabilities range from 10 to 100 

millidarcies (Dinu and others, 1996).

Traps

Hydrocarbon traps in the Neogene Suprasalt AU are 

formed by dome and domelike anticlines as well as by 

stratigraphic traps. The domes were created by salt diapirism, which was probably induced by the rapid burial of the 

evaporite layer at the base of the middle Miocene sedimentary sequence. The stratigraphic traps are the result of facies 

changes related to molasse sedimentation that is characterized 

by a thick sequence of ungraded, crossbedded sandstone, 

shale, and marlstone.

Burial History and Generation and Migration

of Hydrocarbons

The burial history for the Transylvanian Neogene Suprasalt AU is one of gradual subsidence during deposition. Figures 4 and 5 show that sedimentation during Late Cretaceous 

time was in north-south-trending grabens (Huismans and 

others, 1997), a pattern that continued generally through early 

Miocene time. Thereafter, deposition was into a continually 

subsiding basin in which as much as 5.5 km of molasse deposits (middle Miocene to Pliocene) was deposited (Cranganu 

and Deming, 1996). This depositional period of approximately 

17 m.y. was followed by the accumulation of upper Miocene 

sediments ranging in thickness from more than 1 km near the 

perimeter of the basin to more than 3 km in the basin interior.

The heat flow within the Transylvanian Basin ranges 

from about 60 mW/m2 in the northern part to about 40 milliwatts per square meter (W/m2

) in the southern part. Cranganu 

and Deming (1996) reported an average surface heat flow of 

38 mW/m2 compared to 88 mW/m2 in the adjacent Pannonian 

Basin (fig. 1). Thus, the Neogene strata in the Transylvanian 

Basin are cool compared to surrounding areas. Reasons suggested by Huismans and others (1997) to explain this difference are (1) the rapid sedimentation rate near the central part 

of the basin, (2) the relatively thick crust, and (3) the high 

topography of the area. Cranganu and Deming (1996)

Figure 6. Events chart for the Transylvanian Basin Neogene 

Suprasalt Gas Assessment Unit (40570101), illustrating the relative 

timing of critical geologic events in the evolution of the assessment unit. Plio, Pliocene; Mio, Miocene; Olig, Oligocene; Eoc, 

Eocene; Pal, Paleocene; L, Late; E, Early; Ma, million years.

02%3%26!4)/.#2)4)#!,-/-%.4

'%.%2!4)/.


2/#+5.)4

3/52#%2/#+

2%3%26/)22/#+

3%!,2/#+

/6%2"52$%.2/#+

42!0&/2-!4)/.

!##5-5,!4)/. -)'2!4)/.










0ALEOGENE

#RETACEOUS





%

,

.EOGENE



0AL

%OC

/LIG

-IO

0LIO

'EOLOGIC

TIME

SCALE

0ETROLEUM

SYSTEM EVENTS

-A	
interpreted the lower-than-expected present-day heat flow 

numbers to be a probable reflection of the rapid sedimentation 

during the Miocene.

Cranganu and Deming (1996) considered numerous 

burial-history scenarios. Their work showed a possible correlation between gas-field location and higher heat-flow values, 

and they concluded that the basin entered the oil window at 

13 to 5 Ma, at depths ranging from 4.4 to 8.8 km. Popescu 

(1995) believed that thermogenic gas generation began in early 

Pliocene time in the central and eastern parts of the basin, possibly as a result of a higher geothermal gradient induced by the 

relatively high thermal conductivity of salt.

Thermogenic gas may be widespread, but biogenic gases 

are commonly present at shallower depths than those required 

for generation of thermogenic gas. Biogenic gases, on the other 

hand, can also persist to a considerable depth; Kotarba and others (1998), for example, showed that generation of biogenic gas 

in Miocene strata of the Carpathian foredeep in Poland could 

occur to a depth of about 3.3 km. Even at that depth, however, 

the organic matter is still immature and has a vitrinite reflectance of only 0.6 percent (Kotarba and others, 1998).

Gas in the Transylvanian Basin Neogene Suprasalt AU, 

primarily of a biogenic origin as previously indicated, is 

generated from organic matter that is dispersed throughout 

the Miocene sandstones and siltstones and not from a single 

rich source-rock unit. Popescu (1995) suggested that the gas 

migrated only a short distance vertically and laterally into 

domelike structures and (or) stratigraphic traps. Timing of 

gas movement is estimated to be late middle Miocene (early 

Sarmatian).

Figure 6, the events chart for the Transylvanian Neogene 

Suprasalt AU, illustrates the relative timing of critical geologic 

events in the evolution of the assessment unit.

Assessment Results and Summary

The gas for the Transylvanian Neogene Suprasalt AU 

is wholly contained within the upper Badenian to Pliocene 

sandstones and siltstones. Field-size distribution ranges 

from 6 billion cubic feet (BCF) to as much or more than a 

trillion cubic feet (TCF) (fig. 7), with the great majority of 

the fields between 6 BCF and 96 BCF. Petroleum exploration in the Transylvanian Basin is in a mature stage, and this 

circumstance, combined with the geologic complexity of the 

Figure 7. Undiscovered field size distribution for the Transylvanian Neogene Suprasalt Gas Assessment Unit of the Transylvanian 

Composite Total Petroleum System. (BCFG, billion cubic feet of gas.)

 Transylvanian Composite Total Petroleum System of the Transylvanian Basin Province, Romania, Eastern Europe
References Cited 

region, is considered to limit the number of fields yet to be 

discovered.

Estimated resources for the Transylvanian Neogene 

Suprasalt AU of the Transylvanian Composite TPS are as follows: at the mean, 2,083 BCFG; at the 95-percent confidence 

level, 460 BCFG; at the 50-percent confidence level, 1,846 

BCFG; and at the 5-percent confidence level, 4,490 BCFG 

(table 1).

References Cited

Burchfiel, B.C., 1976, Geology of Romania: Geological Society of America Special Paper 158, 82 p.

Ciulavu, D., Dinu, C., Szakacs, A., and Dordea, D., 2000, 

Neogene kinematics of the Transylvanian Basin (Romania): 

American Association of Petroleum Geologists Bulletin, 

v. 84, no. 10, p. 1589–1615.

Cranganu, C., and Deming, D., 1996, Heat flow and hydrocarbon generation in the Transylvanian Basin, Romania: 

American Association of Petroleum Geologists Bulletin, 

v. 80, no. 10, p. 1641–1653.

Dinu, C., Morariu, D. C., and Mocanu, V. I., 1996, Hydrocarbon 

resources of Romania—An overview, in Wessely, G., and 

Liebl, W., eds., Oil and gas in alpidic thrustbelts and basins of 

central and eastern Europe: European Association of Geoscentists and Engineers Special Publication no. 5, 456 p.

Environmental Systems Research Institute Inc., 1992, ArcWorld 1:3M digital database: Environmental Systems 

Research Institute, Inc., (ESRI), available from ESRI, Redlands, CA, scale: 1:3,000,000.

Gyorfi, I., Csontos, L., and Nagymarosy, A., 1999, Early 

Tertiary structural evolution of the border zone between the 

Pannonian and Transylvanian Basins, in Durand, D., Horvath, F., and Seranne, M., eds., The Mediterranean Basins—

Tertiary extension within the Alpine Orogen: Geological 

Society, London, Special Publication 156, 570 p.

Huismans, R.S., Bertotti, D., Ciulavu, D., Sanders, C.A.E., 

Cloetingh, S., and Dinu, C., 1997, Structural evolution of 

the Transylvanian Basin (Romania)—A sedimentary basin 

in the bend of the Carpathians: Tectonophysics, no. 272, 

p. 249–268.

Klett, T.R., Ahlbrandt, T.S., Schmoker, J.W., and Dolton, 

G.L., 1997, Ranking of the world’s oil and gas provinces by 

known petroleum volumes: U.S. Geological Survey OpenFile Report 97–463, 1 CD–ROM.

Kotarba, M. J., Wilczek, T., Kosakowski, P., Kowalski, A., and 

Wieclaw, D., 1998, A study of organic matter and habitat 

of gaseous hydrocarbons in the Miocene strata of the Polish 

part of the Carpathian Foredeep: Przeglad Geologiczny, 

v. 46, p. 742–750.

Magoon, L.B., and Dow, W.G., 1994, The Petroleum System, 

in Magoon, L.B., and Dow, W.G., eds., The petroleum 

system—From source to trap: American Association of 

Petroleum Geologists Memoir 60, p. 3–24.

Mocanu, V.I., Dinu, C., Radulescu, F., Diaconescu, M., Diaconescu, C., and Pompilian, A., 1996, Seismogeological 

features of the crust in Romania, in Wessely, G., and Liebl, 

W., eds., Oil and gas in alpidic thrustbelts and basins of central and eastern Europe: European Association of Geoscientists and Engineers Special Publication no. 5, p. 289–299.

Code and 

field type MFS Prob. (0-1)

Undiscovered resources

Oil (MMBO) Gas (BCFG) NGL (MMBNGL)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Oil fields 1

1.00

0 0 0 0 0 0 0 0 0 0 0 0

Gas fields 6 460 1,846 4,490 2,083 1 6 15 6

Total 1.00 0 0 0 0 460 1,846 4,490 2,083 1 6 15 6

40571 Transylvanian Hybrid Total Petroleum System

40570101 Transylvanian Neogene Suprasalt Gas Assessment Unit (100 percent of undiscovered oil fields and 100 percent of undiscovered

 gas fields allocated to ONSHORE province 4057)

Table 1. Transylvanian Composite Total Petroleum System assessment results.

[MFS, minimum field size assessed; Prob., probability of at least one field equal to or greater than the MFS; MMBO, million barrels of oil. BCFG, billion cubic 

feet of gas. MMBNGL, million barrels of natural gas liquids. Results shown are fully risked estimates. For gas fields all liquids are included under the NGL 

(natural gas liquids) category. F95 denotes a 95-percent chance of at least the amount tabulated. Other fractiles defined similarly. Fractiles are additive under the 

assumption of perfect positive correlation. Gray shade indicates not applicable]
Petroconsultants, 1996, Petroleum exploration and production 

database: Houston, Texas, Petroconsultants, Inc. [database 

available from Petroconsultants, Inc.,P.O. Box 740619, 

Houston, TX 77274–0619 USA].

Popescu, B.M., 1995, Romania’s petroleum systems and their 

remaining potential: Petroleum Geoscience, v. 1, p. 337–350.

Roca, E., Bessereau, G., Jawor, E., Kotarba, M., and Roure, F., 

1995, Pre-Neogene evolution of the Western Carpathians—

Constraints from the Bochnia-Tatra Mountains section (Polish Western Carpathians): Tectonics, v. 14, no. 4, p. 855–873.

Royden, L.H., and Baldi, T., 1988, Early Cenozoic tectonics and paleogeography of the Pannonian and surrounding 

regions: American Association of Petroleum Geologists 

Memoir 45, p. 1–16.

U.S. Geological Survey World Energy Assessment Team, 

2000, Geological Survey World Petroleum Assessment 

2000—Description and results: U.S. Geological Survey 

Digital Data Series DDS–60, 4 CD–ROMs.

Visarion, M., and Veliciu, S., 1981, Some geological and geophysical characteristics of the Transylvanian Basin: Earth 

Evolution Sciences, v. 1, no. 3–4, p. 212–217. 

10 Transylvanian Composite Total Petroleum System of the Transylvanian Basin Province, Romania, Eastern Europe