Methane seepage in an urban development area (Bacau, Romania): origin, extent, and hazard

Methane seepage in an urban development area(Bacau, Romania): origin, extent, and hazard

C. BACIU1, G. ETIOPE2, S . CUNA3 AND L. SPULBER1

1Faculty of Environmental Sciences, Babes-Bolyai University, Cluj-Napoca, Romania; 2Istituto Nazionale di Geofisica

e Vulcanologia, Rome, Italy; 3National Research and Development Institute for Isotopic and Molecular Technologies,

Cluj-Napoca, Romania

ABSTRACT

The paper describes a case of a natural emission of methane from soil in an urban development area, generating

a significant risk for the local population and buildings, due to gas explosiveness and asphyxiation potential. The

site is located on the south-western margin of the East-European Platform in eastern Romania, in a hydrocarbon-

prone area crossed by the Pericarpathian lineament and regional faults. Molecular composition of gas and stable

isotopic analyses of methane (CH4>90%, d13C1: )49.4&, dD1: )173.4&) indicate a dominant thermogenic ori-

gin, with significant amounts of C2-C5 alkanes (�5%), likely migrating through faults from a deep reservoir. Pos-

sible candidates are the Saucesti and Secuieni gas fields, located in the same petroleum system. Two surface

geochemical surveys, based on closed-chamber flux measurements, were performed to assess the degassing

intensity and the extent of the affected area. Methane fluxes from soil reach orders of 104 mg m)2 day)1. Gas

seepage mainly occurs in one zone 30 000 m2 wide, and it is likely controlled by channeling along a fault and

gas accumulation in permeable sediments and shallow subsoil. The estimated total CH4 emission is about

40 t year)1 CH4, of which 8–9 t year)1 are naturally released from soil and 30–35 t year)1 are emitted from shal-

low boreholes. These wells have likely channeled the gas accumulated in shallow alluvial sediment but gas flux

from soil is still high and mitigation measures are needed to reduce the risk for humans and buildings.

Key words: gas hazard, methane seepage, soil degassing, thermogenic gas

Received 24 August 2008; accepted: 24 September 2008

Corresponding author: C. Baciu, Faculty of Environmental Sciences, Babes-Bolyai University, Cluj-Napoca, Romania.

Email: [email protected]. Tel: +40 264 307030. Fax: +40 264 599444.

Geofluids (2008) 8, 311–320

INTRODUCTION

Natural seepage of methane produced in the lithosphere is

a widespread phenomenon, especially in sedimentary areas

with petroleum production (Etiope & Klusman 2002;

Abrams 2005; Etiope 2008; Etiope et al. 2008a). Methane

can be released from recent and shallow (Holocene) sedi-

ments (e.g., Judd 2004), produced by recent microbial

activity, or from deeper and older geological formations,

often hosting gas accumulations with economical impor-

tance, where methane is fossil (radiocarbon free), with

microbial, thermogenic, or mixed origin (e.g., Clarke &

Cleverly 1991; Etiope et al. 2007, 2008a,b). More than

13 000 seeps exist on lands (Etiope et al. 2008b). When a

methane seep develops in correspondence with urban,

agricultural, or industrial areas, it can represent an

important hazard for local communities: explosions and

sudden flames may occur especially in confined spaces, if

methane concentrations reach explosive levels between 5%

and 15% in the presence of air. The level of 5% is referred

as the lower explosive limit (LEL). A concentration of 0.5

% CH4, i.e., 10 % of the LEL, should be considered a limit

above which mitigation measures must be undertaken.

Methane is a non-toxic gas, but high contents in the ambi-

ent air may induce asphyxia, causing headaches, nausea,

dizziness, and death; long-term exposures to methane-rich

air may lead to hypoxia in the blood and brain damages

(e.g., Nagao et al. 1997). Methane’s non-specific odor

provides no warning of its presence in potentially danger-

ous concentrations. When gas includes hydrogen sulfide

(H2S, generally associated with CH4 in salt diapirism

zones) it has a typical odor but can also be toxic and

lethal. Gas in subsoil and soil can, then, damage building

and infrastructures by gas-pressure build-up or by

Geofluids (2008) 8, 311–320 doi: 10.1111/j.1468-8123.2008.00228.x

� 2008 The Authors

Journal compilation � 2008 Blackwell Publishing Ltd

degradation of geotechnical properties of soil foundations.

Significant cases of methane seepage in urban or industrial-

ized areas are documented in western Greece (Katakolo

harbor; Etiope et al. 2005), several locations in the United

States (e.g., Los Angeles outskirts; Jones 2000; Chilingar

& Endres 2005), and Italy (Serra de’ Conti, Corporeno

and many sites in the Po Valley; Martinelli & Judd 2004;

Etiope, unpublished data).

When the direct social and economic impact is relevant,

the seep can be the object of forensic studies (e.g., Lunde-

gard et al. 2000). This type of seep is thus investigated in

detail to understand its origin and to evaluate possible

remedial actions. In these cases, to understand the nature

of the seep, its causes and possible evolution, it is necessary

to know, beyond gas chemistry, the geologic stratigraphy,

the geotechnical properties of soil and subsoil, the local

tectonics and hydrogeology, the gas flux, and its spatial

distribution. Such detailed data are generally not acquired

for seeps located far from urban areas, where only knowl-

edge of the gas origin may be of interest, e.g., for hydro-

carbon exploration purposes. Thus, ‘urban seeps’ have not

only a local interest. Related investigations have both

applied and theoretical implications, as they may represent

fundamental sources of information for the protection of

people and property, as well as for more general studies on

geofluid migration and seepage.

With this viewpoint, we report a new severe case of

‘urban seep’ recently investigated at Gheraiesti village,

north of the city of Bacau, eastern Romania. A program

for a new residential area with 248 detached houses started

to be developed in 2002. In autumn 2003, when 102

houses were built and some owners already moved in, a

methane leak was accidentally discovered in a cellar. The

works were stopped and preliminary investigations for the

identification of the gas source and the extent of the seep-

age area were carried out. In the present study the origin

of gas is clarified with the support of isotopic analyses, and

the seepage extent and distribution are more accurately

evaluated by gas flux measurements from the ground. The

conditions controlling methane seepage are then assessed,

as a prerequisite for designing appropriate mitigation mea-

sures.

Geologic setting

The study area (Gheraiesti, Bacau) is located in the fore-

land of the Eastern Carpathians, corresponding to the

southwestern extremity of the East-European Platform

(EEP). A few kilometers westward from Bacau, the

Lower ⁄ Middle Miocene sediments of the Subcarpathian

Nappe have overthrust the Upper Miocene (Sarmatian)

platform deposits along the Pericarpathian lineament –

PCL (Figs 1 & 2). Southward, the NW-SE trending Bistri-

ta Fault delimitates the EEP from the Scythian Platform

(SP). Bacau area is thus in correspondence with the cross

of two fault lineaments of regional importance. The two

crustal blocks, EEP and SP, had a different evolution in

the Palaeozoic and lower part of the Mesozoic, and a simi-

lar one in Cretaceous and Tertiary times. Trotus Fault,

approximately parallel to Bistrita Fault, represents the limit

between the SP and the North Dobrogean Orogen. Beside

the major tectonic features separating the platform units,

two further types of faults, with a lesser amplitude, are

present (Matenco et al. 2003). The first type, NW-SE

trending normal faults, is responsible for the progressive

deepening of the platform basement beneath the Outer

Fig. 1. Main tectonic elements and location of

hydrocarbon fields in the Bacau region.

312 C. BACIU et al.


Journal compilation � 2008 Blackwell Publishing Ltd, Geofluids, 8, 311–320

Carpathians units. The second type, NE-SW to E-W-ori-

ented faults, imaged on seismic surveys, facilitates the base-

ment deepening toward the south. These tectonic

elements, together with some minor sub-vertical faults, are

important for petroleum trapping. Oil and gas accumula-

tions occur in all of the above mentioned tectonic units

(Fig. 1). The Palaeogene and Miocene Carpathian units

contain numerous oil, and to a lesser extent, gas accumula-

tions west of Bacau, in flysch and molasse deposits. On the

western side of the SP, oil and gas deposits occur in the

Sarmatian sediments.

The western side of the EEP hosts mainly gas deposits,

at the level of the Middle and Upper Miocene (Badenian

and Sarmatian regional stages). The petroleum is accumu-

lated in structural traps of the faulted monocline type, or

in mixed (lithological and structural) traps (Paraschiv

1979). The biggest gas-bearing structure is Roman-Secuie-

ni, about 15 km north of Bacau. The gas accumulates in

detrital, more or less extended lens-shaped bodies, grouped

into 12 complexes. The deposit is divided into tectonic

blocks by sub-vertical faults.

In the region of Bacau, Sarmatian sediments, more than

2000 m thick, include marls intercalated with permeable

sands and sandstones in the lower part, while the upper

1000 m contains fine-grained, impermeable sediments.

The clastic deep levels may host natural gas accumulations.

This entire suite is covered by Quaternary alluvial deposits.

The Miocene sedimentary formations are sub- horizontal,

slightly dipping to the southeast. The seismic activity in

the area is important, with intensity grade up to VIII on

the Medvedev-Sponheuer-Karnik (MSK) scale, and a recur-

rence time of a few decades. The earthquakes seem to be

able to re-activate the deep faults.

The closest known petroleum-bearing structure is Sauc-

esti, located at a distance of 3–5 km eastward from the area

affected by gas seepage. The structure contains eight gas-

bearing horizons, located at 1400–2200 m depth

(Fig. 3A). Gas is accumulated in lens-shaped permeable

bodies, crosscut by faults. Ten exploratory and producing

wells were drilled, with depths in the range of 2300–

3200 m. Paraschiv (1979) reported production rates of

43 000–100 000 m3 day)1. Other significant gas pools are

in the Secuieni field, about 10 km north of Gheraiesti, in

correspondence with the Pericarpathian lineament (Fig. 1).

The closest well (no. 100) to the investigated seepage

site is about 2.5 km SE from Gheraiesti. No exploitable

gas accumulations were intercepted by this well, and it was

abandoned at a final depth of more than 3000 m.

Previous investigations and site characterization

After the discovery of gas leaks in the Gheraiesti residential

village, preliminary investigations were carried out by Ploi-

esti University in 2004 (Vasiliu & Batistatu 2005). Eight

wells, up to 6 m deep, were drilled in the proximity of the

house where the first methane leak was noticed. Succes-

sively, 24 shallow exploratory borings, with an average

depth of 15 m, were drilled by Laforservice (2005). The

wells are spread over an area of about 100 000 m2. Gas

samples were collected and analyzed by gas chromatogra-

phy. A deep provenance of gas was assumed based only on

its chemical composition (e.g., wells FC9, FD4, FD5 in

Table 1). Additionally, a 96-m deep well was drilled to

clarify the geological structure of the area. The lithological

composition was found to be relatively similar in the shal-

low wells, with alternating permeable and impermeable

deposits: topsoil layer from 0 to 1.5 m; gravels and sands

from 1.5 to 7.5 m; marls from 7.5 to 8.5 m; sands from

8.5 to 13.5 m; marls from 13.5 m downward (Fig. 3). In

most of the wells, the groundwater table is about 4.5 m

Fig. 2. Geological features of the study area

(after Murgeanu et al. 1968).

Methane seepage in Bacau, Romania 313



below surface. In the deeper well, the lower marl level con-

tinues down to 63.6 m. Between 63.6 and 80.0 m deep,

sandy marls and sands were identified. From 80 m to the

bottom of the well, at 96 m, another marl layer was found

(Laforservice, 2005).

At the request of the National Dwelling Agency (ANL)

building developer, flux meters were installed by the natu-

ral gas supply company Distrigaz Nord, on some of the

highly degassing shallow wells. A significant flux of meth-

ane, on the order of 0.5–2.5 m3 h)1 was recorded in some

of the wells (ANL, private communication), the gas burn-

ing freely at the end of the exhausting pipe (Fig. 4). As an

example, the average fluxes recorded between August 21,

2006 and September 23, 2006 are presented in Table 2.

The fluxes were found to be variable, slightly increasing or

decreasing in the different wells, with a general tendency

of slow decline. Some remote wells do not show any gas

emission. No gas was found in the lower permeable levels

intersected by the deeper well.

Methodology

Gas flux from soil was measured by the closed chamber

method (e.g., Livingston & Hutchinson 1995; Etiope

et al. 2004) in two different phases (spring and summer

2007) throughout the Gheraiesti residential area. The aim

of this approach was to determine the degassing pattern

and the extent of the area affected by gas emissions.

In the first phase, a closed-chamber equipped with por-

table CH4 solid-state detector (Fig. 5A; Metrex 2, Huberg;

lower detection limit 5 ppmv, accuracy 10%) was used for

rapid detection of large gas exhalations.

Gas flux in mg m)2 day)1 was calculated assuming

steady emission and linear increase in methane concentra-

tion in the chamber by the equation (e.g., Livingston &

Hutchinson 1995; Thielemann et al. 2000)

F ¼ Vc

Ac

c2 � c1

t2 � t1½mg m�2day�1�

where VC (m3) is the volume of the chamber, AC (m2) its

area, c1 and c2 (mg m)3) are methane concentrations at

times t1 and t2 (days).

(A) (B)

Fig. 3. General lithological column in Bacau –

Saucesti area (A), and detailed lithological

column in the Gheraiesti perimeter (B).

Table 1 Molecular and isotopic composition of Gheraiesti gas samples,

compared with data from gas fields located in the same petroleum system

(gas composition in %v ⁄ v; data corrected for air contamination).

Gheraiesti

well FD7

(2007)

Gheraiesti

well FC9

(2005)*

Gheraiesti

well FD4

(2005)*

Gheraiesti

well FD5

(2005)*

Secuieni

gas field�

He 0.0033 – – – bdl

Ar 0.014 – – – 0.0044

N2 1.06 6.64 1.38 14.20 0.20

CO2 0.08 0.06 0.07 0.31 bdl

CH4 93.81 88.45 94.47 81.30 94.20

C2H6 2.98 2.75 2.12 2.51 1.75

C3H8 1.13 0.99 1.06 0.87 2.20

i-C4H10 0.26 0.19 0.23 0.17 0.45

n-C4H10 0.31 0.19 0.15 0.15 0.32

i-C5H12 0.13 0.41 0.20 0.19 0.22

n-C5H12 0.089 0.04 0.06 0.06 0.16

C6+ 0.13 0.27 0.26 0.24 0.03

d13C1& (PDB) )49.42

dD1& (SMOW) )173.4

bdl, below detection limit (see text); PDB, PeeDee Belemnite; SMOW, stan-dard mean ocean water.*From Laforservice (2005).�Gas pool, 2170 m deep (Filipescu & Huma 1979).

314 C. BACIU et al.



The first reconnaissance gas flux survey (spring 2007)

was performed at 16 points randomly distributed through-

out a 27 000 m2 area (circles in Fig. 6). The chamber was

positioned for a maximum of 3 min, equivalent to a mini-

mum flux detection of about 100 mg m)2 day)1. The

detector was also used as a sniffer to trace methane anoma-

lies in the atmosphere above soil surface (concentrations

above 5 ppmv) and for measurements in eight sewer man-

holes (triangles in Fig. 6).

Additionally, a gas sample was collected in duplicate in

FD7 shallow borehole (15 m deep), equipped with exhaust

pipe, the gas burning freely when ignited (Fig. 4). Gas was

analyzed for C1–C6 hydrocarbons, d13C1, dD1, He, H2,

Ar, O2, CO2, and N2, by gas chromatography (Carle AGC

100-400 TCD-FID GC; detection limit: CO2, N2, Ar, O2:

40 ppmv; He and HC: 10 ppmv; accuracy 2%; 10% at the

detection limit) and mass spectrometry (Finnigan Delta

Plus XL; accuracy ± 0.1& on 13C and ± 2& on 2H) at

Isotech Labs Inc. (IL, USA). No isotopic data for methane

have been previously published in this region.

In the second survey (autumn 2007), an area of about

250 000 m2 was covered by 58 sampling points, distrib-

uted in an approximately rectangular grid, with 50 m dis-

tance between nodes (Fig. 7). The sampling point

coordinates were determined by using a Garmin eTrex

GPS. Gas accumulated in a 7-l chamber was collected

6 min after chamber deployment, by two 20 ml syringes

(Fig. 5B). The samples were transported to the laboratory

in an isothermal container. Methane concentration

was measured within 3 days by gas chromatography

Fig. 4. Gas burning from the FD7 shallow well (dashed circle) in the main

degassing area of Gheraiesti.

Table 2 Methane concentration and average flux in some high degassing

shallow wells.

Well

Methane

concentration

(%v ⁄ v)*

Flux

(m3 h)1)�

FC 9 82.14 �

FC 13 92.5 0.80

FD 2 93.16 �

FD 4 93.75 1.21

FD 5 69.74 �

FD 6 62.33 0.44

FD 7 87.58 2.13

FD 9 64.33 1.42

*Measured between 27 January and 27 April 2005 (Laforservice, 2005).�Measured between 21 August and 23 September 2005; flux metersinstalled by Distrigaz Nord (ANL, private communication).�Not measured.

(A)

(B)

Fig. 5. Accumulation chambers for the measurement of methane flux from

soil. (A) Reconnaissance and rapid survey with online portable CH4 detec-

tor. (B) Chamber with collection of gas samples for laboratory analyses.




(Chrompack 438A equipped with FID, 0.3 ppm lower

detection limit). Gas flux in mg m)2 day)1 was then calcu-

lated as above. Additional investigations were performed at

the Calea Romanului location (see Fig. 2), around the gas

exploration well no. 100, at about 2.5 km SE from Gherai-

esti degassing zone. An area of about 50 000 m2 was

covered by 18 sampling points, by using the procedure

adopted in the second survey.

RESULTS AND DISCUSSION

Gas origin

Gas seeping from well FD7 is CH4-dominated (�93%

v ⁄ v), with large concentrations of light alkanes, about 3%

of ethane (C2H6), >1% of propane (C3H8), 0.57% of

butane (C4H10), >0.2% of pentane (C5H12), and 0.13% of

C6+ compounds (Table 1). There are no traces of toxic

gases (H2S). These data are consistent with the analyses

previously performed in other wells distributed over a

60 000 m2 area (e.g., FC9, FD4 and FD5 in Table 1); all

data show remarkable C2–C6 concentrations, typical of

thermogenic natural gas. There are no indications of

microbial gas from any well (Laforservice, 2005). The

thermogenic origin is confirmed by the stable isotopic

CH4 composition analyzed in well FD7. Figure 8 shows

the ‘Schoell’ and ‘Bernard’ diagrams, with the Gheraiesti

sample positioned in the thermogenic fields; both diagrams

suggest a minor component of microbial gas.

The thermogenic character indicates that the gas is prob-

ably related to a deep source, similar to the reservoirs

occurring in the area (e.g., Saucesti, Roman-Secuieni fields,

1400–2200 m deep). Given the lack of isotopic data for

these fields it is not possible to know whether the reservoir

gas is already mixed, or whether mixing occurred during

gas migration to the surface. The high concentration of

ethane and propane at Gheraiesti seep, i.e., the ‘Bernard’

ratio C1 ⁄ (C2+C3) = 23, is exactly the same as the data

reported by Filipescu & Huma (1979) for the close

Roman-Secuieni field, at depths of 2160–2180 m (see

Table 1). Independently of the actual reservoir which is

feeding the seep, the low ‘Bernard’ ratio indicates absence

of molecular fractionation with loss of heavier alkanes dur-

ing gas ascent, a process that can be observed in gas seeps

with relatively low flux (Etiope et al. 2008a). The low Ber-

nard ratio of Gheraiesti seep is therefore consistent with a

high and rapid gas flux.

Seepage extent

In the first survey, flux values above 100 mg m)2 day)1

were detected in 11 sites (out of 16). Up to

400 000 mg m)2 day)1 were measured on a fissure in the

road pavement, close to the house that was first endan-

Fig. 6. Methane fluxes measured in the first survey. Some sampling points

from the second survey are marked as reference. Built houses marked as

gray geometric shapes. House endangered by the gas emission indicated by

block arrow. FC9, FD4, FC5, and FD7 are the boreholes with compositional

analysis reported in Table 1.

Fig. 7. General distribution of fluxes in Gheraies-

ti area (second survey). Area investigated in the

first survey in inset (round dots).

316 C. BACIU et al.



gered by the gas seep. The area where the highest fluxes

were measured coincides with the area covered by the

highly degassing shallow wells (Fig. 6).

The highest flux from soil (21 000 mg m)2 day)1) was

detected near point B1. Despite the fact that sewer man-

holes are not sealed and atmospheric air can enter through

large holes and fissures, CH4 anomalies in indoor air were

detected in four manholes (out of eight), up to 150 ppmv.

The CH4-richest manhole is about 10 m away from the

pavement fissure where the highest methane flux was mea-

sured. In correspondence with the high-degassing sites, the

portable sensor, used as a sniffer, detected CH4 concentra-

tions up to 50 ppmv in atmospheric air, at about 1 m from

the ground.

In the second survey, methane flux was in the range of

0–5852 mg m)2 day)1. Figure 7 shows the spatial distribu-

tion of the fluxes in the investigated area. Most of the

points were in the range of 10–20 mg m)2 day)1. The

highest value was measured at point B1, approximately

coinciding with the highest emission from soil measured

during the first survey. The main degassing area, character-

ized by fluxes higher than 40 mg m)2 day)1 is 30 000 m2,

around points B1 and B2. Lower fluxes, of 20–

40 mg m)2 day)1, were identified in an approximately

north-south strip in the central part of the investigated

area. Very low, but still positive, fluxes, below

10 mg m)2 day)1, were identified in the southeastern part

of the map.

Table 3 summarizes the gas flux data acquired in the two

surveys. At first sight, the results of the two surveys appear

to be strikingly different, especially the average flux value.

The difference in the two averages can be explained by the

fact that the first survey was performed only within the main

degassing area and that the average flux of the first survey

considers only values above 100 mg m)2 day)1, which is

the detection limit of the system used (accumulation box

Fig. 8. ‘Schoell’ (A) and ‘Bernard’ (B) diagrams

for the genetic characterization of methane

(Schoell 1983; Etiope et al. 2008a). In (A): To,

thermogenic with oil; Tc, thermogenic with con-

densate; TD, dry thermogenic. In (B), a mixing

line is plotted by assuming a microbial end-mem-

ber with d13C = )70&, C1 ⁄ (C2 + C3) = 10 000,

and thermogenic end-member with

d13C = )40&, C1 ⁄ (C2 + C3) = 10. Both dia-

grams show that Gheraiesti-Bacau sample (black

square) is thermogenic with a slight mixing char-

acter. The Secuieni reservoir (dashed horizontal

line) has the same ‘Bernard’ ratio (C1 ⁄ (C2 + C3))

as Gheraiesti seep (isotopic data of Secuieni are

lacking). Other gas seeps from Romania (from

Etiope et al. 2008a) are reported for comparison

(1: Sarmasel, Transylvania; 2: Andreiasu, Eastern

Carpathians; 3 Praid, Transylvania-Carpathian

margin).




with portable methane sensor for quick and semi-quantita-

tive measurements). The second survey covered an area

almost ten times greater than in the first case, including a

wide area with low fluxes, measured by means of gas chro-

matography analysis.

Around the abandoned exploration well at Calea

Romanului, methane fluxes are much lower, not exceeding

10 mg m)2 day)1, with an average of 4 mg m)2 day )1.

Such low positive fluxes are common in areas with petro-

leum systems, where methane microseepage can be

pervasive (e.g., Etiope et al. 2004; Etiope 2008).

Gas channeling

The gas seepage basically occurs within one confined area

and not in multiple sites. Gas is coming from depth and

the seepage pattern suggests a process of gas channeling, a

typical result of gas migration along the most permeable

sector of a fault (e.g., Guerra & Etiope 1999; Ciotoli et al.

2005). Gas migrates upward following channels of

enhanced permeability, i.e., of minor resistance to gas

motion, providing a minimum dissipation of gas energy

and conservation of pressure and flow rate. As experi-

mented in gas injection tests (Ciotoli et al. 2005), once a

preferential pathway (the ‘channel’) has been activated, gas

flow occurs just along the channel and is insensitive to

other, even if contiguous, volumes of rock. This means

that seepage does not occur along an entire fault surface,

but it develops only in one or more spots. Natural gas

seeps are, in fact, typically circular or egg-shaped spots or

clusters of aligned spots, not continuous leaking strips. At

Gheraiesti the gas-bearing fault could be one related to the

tectonic dislocations suggested by Matenco et al. (2003),

previously described, or a local fault affecting the Neogene

sediments. Further investigations should be however per-

formed to confirm the existence of such a fault.

The relatively large seepage area is likely due to gas

spreading in shallower and permeable strata, such as the

coarse sediments (gravels and sands) occurring down to

13–14 m below the surface (Fig. 3). Due to the less per-

meable topsoil layer, such sediments could act as shallow

gas accumulation pool where gas, coming from the fault

‘channel’, can spread laterally and eventually be carried by

groundwater. Lower methane fluxes from soil at large dis-

tances from the seep ‘epicenter’ could then result from

groundwater degassing. A significant amount of gas is

released by some of the degassing wells intercepting the

permeable sediments. Based on flux-meter recordings we

have calculated a total emission from boreholes of about

30–35 t CH4 year)1. It can be assumed, then, that the

boreholes represent a preferential shallow pathway for the

gas, further channeling the flux, and as a result, lowering

the soil degassing intensity in the surrounding area. A pos-

sible seepage model, including both fault and borehole-

induced gas channeling is illustrated in Fig. 9.

CONCLUSIONS

A significant seepage of thermogenic methane occurs

within the urban development area of Gheraiesti, north of

Bacau. Gas is also enriched in C2-C5 alkanes (�5% v ⁄ v in

total) and it most likely originated in deep sedimentary

strata, probably related to local gas fields (e.g., Secuieni,

Saucesti, or other unknown). The predominantly thermo-

genic gas has a minor microbial gas component, whose

location (in shallow sediments or in deep reservoirs) cannot

be defined, however, due to the lack of isotopic data from

reservoirs. There is no evidence of molecular fractionation

of the seepage gas compared to the deep reservoir gases.

Taking into account the deep provenance of gas and its

possible link with major petroleum reservoirs, it is unlikely

that the seepage will become exhausted in the immediate

future. Seepage is likely due to locally enhanced permeabil-

ity related to the intersection of the N-S Pericarpathian

Table 3 Main statistics of methane flux from soil in Gheraiesti area.

First survey

(closed chamber

and portable sensor)

Second survey

(closed chamber

and laboratory GC)

No. measurements 17 58

Detection limit 100 mg m)2 day)1 2 mg m)2 day)1

Mean* 2183 20

Minimum <100 <2

Maximum 20 900 5852

*Calculated for values greater than the detection limit and excluding themaximum value.

Fig. 9. A proposed seepage model with gas channeling.

318 C. BACIU et al.



lineament with E-W faults belonging to the Bistrita fault

system.

Positive fluxes of methane to the atmosphere, up to

orders of 103–104 mg m)2 day)1 were detected through-

out an area of about 250 000 m2. The main seepage zone,

with fluxes above 40 mg m)2 day)1, is however confined

in an area of about 30 000 m2. The seepage distribution is

probably controlled by a local fault, channeling the gas

flow from a deep reservoir, and shallow (<14 m deep) sub-

soil permeable sediments in which gas is transiently accu-

mulated and then spreads laterally. The gas flux from soil

in the Gheraiesti investigated area (250 000 m2) conserva-

tively accounts for about 8–9 t CH4 year)1. The degassing

wells are releasing at least 30 t CH4 year)1. The total

amount of gas released into the atmosphere in the Gherai-

esti area may be thus on the order of 40 t year)1. Gas

fluxes on the order of units to tens of tons per year are

typical and were measured in many other thermogenic gas

seeps in Europe (Etiope 2008).

The Gheraiesti methane seepage represents a serious

hazard for the population and the built environment. It is

possible that boreholes channel flow of gas and effectively

help degas the shallow accumulations, probably also reduc-

ing the natural leakage from soil. However, gas flux from

soil is still high and an uncontrolled artificial channeling

could lead to further accumulations in the subsoil, under

asphalt or cement paves, manholes and foundations, reach-

ing explosive levels. More detailed research is needed to

detect local faults, define their geometries and depth, and

to design technical solutions able to reduce gas accumula-

tions and related risk for humans and buildings.

In contrast, the measurements performed at Calea

Romanului did not show significant methane fluxes. Gen-

erally the risk induced by such microseepage is very low;

however, in some cases even a small microseepage could

correspond to gas concentrations in soil or subsoil able to

degrade the geotechnical properties of the sediments, espe-

cially the foundation-bearing capacity.

As general note, we can outline that knowing this

geofluid origin, seepage distribution and link with geo-

structural (faults) and fluid (reservoirs, gas pools) systems

is essential for the assessment of gas hazards. To this

end, detailed chemical and isotopic analysis of the gas

and spatial surveys of exhalation flux measurements are

key tasks.

ACKNOWLEDGEMENTS

The present work was financially supported by the Roma-

nian National Authority for Research, under Project 31-

094 FLUX. Constructive suggestions and comments from

three anonymous reviewers were very useful in improving

the manuscript. Special thanks are due to R. Worden for

his editorial work.

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Methane seepage in an urban development area (Bacau, Romania): origin, extent, and hazard

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