Soil gas radon and thoron measurements in some Venezuelan

Soil gas radon and thoron measurements in
some Venezuelan oilfields
Daniel Palacios1, Emidio Fusella2, Yininber Avila1, Jhonny Salas3, Diana Teixeira1, Guillermo
Fernández4, L. Sajo-Bohus1, E.D. Greaves1, H. Barros1, M. Bolívar5, J. Regalado5
1Universidad
Simon Bolivar, P.O. 89000, Caracas, Venezuela
2Instituto de Estudios Avanzados (IDEA), Caracas, Venezuela
3Universidad Central de Venezuela, Caracas, Venezuela
4Universidad del Zulia, Maracaibo, Venezuela
5Servicios Geofísicos-PDVSA, Puerto la Cruz, Venezuela
Motivation
The existence of radiometric anomalies over hydrocarbons reservoirs is known long
ago, nevertheless, despite several claims of success, few scientifically rigorous
research on the many factors that can influence the results have been done.
Probably those circumstances would lead to lose the motivation and confidence in
these studies, reason why the literature on this matter is very scarce after year
2000. At present, although there are companies that provide services for oil
exploration using radiometric techniques, oil community shows no wide acceptance
of the method. Nowadays, the advances in radiometric methods, techniques of
geostatistics and data processing, and a greater understanding of geochemical and
geophysical alterations induced over oil and gas reservoirs, could indicate a new
and successful stage of the application of radiometric techniques for hydrocarbon
exploration.
CONTENT
™Surface Expression of Seepage
™Radiometrics in Oil and Gas Exploration
™Measuring Techniques
™Case Study 1, Amarilis Oilfield - Monagas State
™Case Study 2, Nodosaria 15, Jusepin - Monagas State
™Case Study 3, Freites Municipality - Anzoátegui State
Surface Expression of Seepage
Possible model of hydrocarbon microseepage-related alteration over petroleum
deposits (modified after Saunders et al., 1999)
RADIOMETRICS IN OIL AND GAS EXPLORATION
How radiometric surveys are used to detect potential oil and gas payzones
related to structural uplifts (anticlines), fault traps, and stratigraphic traps?
Surface geochemical and geophysical expression of petroleum seepage
Radiometric responses to oil and gas payzones
Sketch of upward flow
of ground gases from
an oil/gas reserve
Fleischer and Turner
(1983)
MEASURING TECHNIQUES
Georeferenced measurements sites
Portable meteorological
station
IN SITU GAMMA SPECTROMETRIC MEASUREMENTS (1m soildetector distance)
HPGe detector
BGO detector
Na(Tl) detector
Soil sample collection and HR gamma spectrometric analysis (226Ra-222Rn
progeny in secular equilibrium)
Radon and thoron concentrations by an active method (Lucas cells)
Radon measurements by a passive method (LR-115 SSNTD in diffusion
chambers)
CASE STUDY 1, AMARILIS OILFIELD - MONAGAS STATE
‰ Complex reservoir
‰ Several structural highs and an anticline
fold was defined by the AML-1X well
‰ Objective: To evaluate whether the
distribution of radon concentrations in
soil gas along a transect can give
insights on possible causes of
differences in productivity of two drilled
wells.
‰ Method: 40 measuring points were set
each separated 250 m along a transect
passing through the producer well AML1X and the exploratory well AML-2
which turned out dry
Results:
¾AML-1X well located within in the interval of the transect with low radon concentrations
limited at its boundaries by the maximum values
¾Difference in the radon concentration across the dome agrees with the Fleischer and Turner
model if the maximum had coincided with the edges of the dome structure (not the case).
¾Since no more maximum in radon concentration after the AML-2 well (in the NE direction), it
can be assumed the well to be outside the dome structure and/or the lack of active faulting
Radon highs do not lie over the
projection of possible fractures
produced as a result of differential
compaction of overlying sediments
¾ The highest radon concentrations coincide with the location of Lead “Amarilis Oeste” and the
associated structure ⇒ easier gas transport from deeper layers through the fault.
¾ Results may be rather related to a geochemically active fault (GAF), through which gases
leak out with enough pressure to create the radon anomaly ⇒ area can be considered as
potentially hydrocarbon bearing zone at depth.
¾ Productive well seems to be near the boundary of the anticlinal structure.
¾ Minimum 226Ra/232Th ratios around the
producer well contrasting the higher values
at the dry well
¾ The decrease of eU relative to thorium is in
correspondence with the chemical
alterations induced by the ascending
hydrocarbons
¾ AML-1X well is located in an area with welldeveloped
migration
pathways
and
an
active petroleum system.
Conclusions:
9A relatively low radon concentration in soil pores was identified around the productive well
as compared with its surroundings.
9The radon profile did not spatially coincide with the anticlinal trap structure possibly due to
the complexity of the reservoir.
9Apparently, the productive well is located in an area with well-developed migration
pathways and an active petroleum system.
CASE STUDY 2, NODOSARIA 15, JUSEPIN - MONAGAS STATE
‰ Oilfield in exploitation for many years, there are active and non-active wells
‰ The oilfield has its northern border in the Pirital shifting fault and in the western border a
transversal NE fault, which pronounced displacement (up to 1000‘)
‰ Objective: To assess whether or not there are radon anomalies associated with oil production
sites and the type of reservoir trap (fault trap).
‰ Method: 124 measurement stations distributed on a grid array of 200 m spacing.
Results:
¾Anomalous high concentrations (higher than 20 and 16 KBq/m3 for radon and thoron,
respectively) overlying an area of the fault system
¾Distributions are not consistent with the hypothesis that the entire fault lines cross at the
points with the maximum values of radon and thoron concentrations
¾In general, radon and thoron data show good correlation.
¾Higher radon and thoron concentrations were found in an area occupied by some oil wells.
Radon and
thoron
contour
maps
obtained by
the active
method
(Lucas cells)
¾ Radon/thoron contour map reveals anomalous high near the intersection of two faults and in
general, high values to the northeast of the studied area ⇒ gases are transported from the
greatest depth in the area where faults intersect
¾ Detectors closest to fault region showed alpha track density well above the background level
¾ Radon maximum value is located in an area traversed by the fault system and near the
intersection of the two faults
¾ Higher radon concentrations were found almost in all the area occupied by oil wells and at the
surface along the all trace of the fault system
Overlap of faults
and locations of
oil wells with the
contour maps of
radon/thoron
ratios (A) and
relative radon
concentrations
given by the
passive method
(B).
¾ The above results indicate that radon emissions in the studied area are structurally controlled,
which is in correspondence with the fault trap structure of the Nodosaria-15 reservoir.
¾ Radon maximum overlie fault zones developed at the edges of the reservoir acting as
pathways for gas migration to the surface.
Conclusions:
9Radon highs overlie the fault system regardless of the measurement method.
9The anomaly is in correspondence with the fault trap structure of reservoir, so the existence of
well-defined radon anomalies above fault-controlled oil-bearing traps was confirmed.
9A good spatial correspondence was obtained between radon anomalies and oil wells.
9Measurements performed with nuclear track detectors better reveal the fault system traces and
the oil well locations for this reservoir type.
9Highs of radon relative concentrations (by the passive method) and radon/thoron ratio (by the
active method) are spatially better correlated with the intersecting fault system.
9As carrier gases seem to come from deep zones, the results suggest the existence of a
geochemically active fault system associated to an oil reservoir with still enough pressure to
create the detected radon anomalies.
CASE STUDY 3, FREITES MUNICIPALITY - ANZOÁTEGUI STATE
‰ Two water wells resulted contaminated by natural gas and gas leaks exist in the nearby river.
‰ There are active and inactive oil wells, must of the latters abandoned for more than 50 years.
‰ Objective: 1) to identify possible structures (faults or fractures system) related to methane
ascension and suggest which is or may be the source responsible for its occurrence in the
river and aquifer, and 2) to assess whether or not there are radon anomalies associated with
oil production areas.
‰ Method: Calculation of
226Ra
activity concentrations in soil samples. Thus, radon
concentration at equilibrium can be derived to better discriminate the radon
component uprising from a deep gas source along faults. Data were collected on a
grid composed of eight sets of rectangular rings centered on the locations of the
two contaminated water wells, six abandoned gas wells suspected of gas leaks and
the river location where gas was observed to emanate. The survey consisted of 210
survey stations, 250 m apart, covering approximately 22 km2.
Results:
Active radon measurements
¾Lowest radon levels in region C indicates the presence of a hydrocarbon reservoir in depth in
correspondence with the laboratory gamma spectrometric results (expected configuration
according to the model based on the alteration of near-surface gas flux gradient)
¾The radon maximum concentration values are very close to the river gas emanations (zone
A). Apparently, this anomaly is related to the hydrocarbon gas migration from some source
through an interconnected fracture system or a fault.
¾ The radon/thoron ratios also show an anomaly well defined only in zone A .
¾ Since both anomalies spatially coincide and as
222Rn
has a much longer half-life than
220Rn,
this result indicates that both isotopes have been brought to the surface by other carrier
gases from a deeper source.
¾ The existence of a deep fault in that location, through which gases are transported and
penetrate the aquifer, could explain the observed river gas emanations due to its hydraulic
connection with the aquifer.
Passive radon measurements
¾The same anomaly of high radon concentrations near the river gas emanation is apparent in zone A.
¾The highest radon concentrations were found in zone B where there are a few abandoned gas wells.
Such behavior can be explained by the relative high
226Ra
concentrations (214Bi). Since
accumulation is not due to lithological variations, it seems to indicate that both radon and
226Ra
226Ra
are
being transported by an upward gas flow along the pipe of one of the abandoned wells
¾Radon levels seem to suggest this area (B) as possibly linked with the source of the aquifer
contamination.
¾Apparent spatial connection for higher radon concentrations between gas wells (B) and the river gas
emanations (A) ⇒ the existence of a breakdown in the structure of abandoned gas wells in (B) should
not be discarded. This connection was not observed for active measurements.
¾ The radon activity at equilibrium with soil radium content is very low in zone A compared to
the high radon values measured in siu. This confirms the assumption of other radon source
that could originate from deep layers and migrate to the surface along a fault. Otherwise,
radon values measured in A had been in equilibrium with soil radium content.
¾ The lowest Ue (214Bi) levels in C may indicate the existence of a hydrocarbon reservoir in
depth, according to the model of radiometric anomalies over anticlinal-type hydrocarbon
reservoirs. Geological data confirmed the previous statement, and at present several oil wells
are producing very close to that area.
¾ Results obtained with passive detectors are less affected by meteorological conditions. In
addition, passive sensors do not hamper the rising gas column. Thus, results obtained by
the application of SSNTDs are more reliable.
¾ Both radon measurement methods indicate that the occurrence of gas emanations in the
river and gas escape in water wells, are controlled by a single conduit (fault or
interconnected fractures in zone A) for the ascent of natural gas to the surface.
¾ The magneto-telluric method also showed a vertical conduit connecting the river gas
emanations with very deep zones. Additionally, the resistivity distribution indicates an
apparent connection between some of the abandoned gas wells in zone B and this conduit.
Conclusions:
9Surface hydrocarbon seeps were identified based on soil gas radon data.
9The anomalous zone for the radon/thoron ratio seems to indicate a high permeability of the
soil caused by the effect of a fracture system or nearly vertical deep geological fault.
9The high anomalous radon concentration localized near the river gas emanations indicates
the zone of methane penetration into the aquifer. The gas emanation in the river and in the
water wells are controlled by this source.
9It is suggested that the source of the natural gas may be due to leaks at deep sites along
the structure of some of the abandoned wells indicated in the North-East of the studied area.
9The lowest radon values detected in the eastern area suggest the presence of an anticline
hydrocarbon reservoir.
GRACIAS
ESTUDIO RADIOMÉTRICO EN LA LOCALIDAD DE TASCABAÑA, ESTADO ANZOÁTEGUI
Objetivos: Contribuir en la identificación de causa de
ocurrencia de gas natural (metano) en río y pozos de
agua y ubicación del sitio de penetración del gas al
acuífero.
Relacionar zonas de anomalías radiométricas favorables
con áreas productivas.
Conclusiones:
™Buena correlación espacial entre las anomalías radiométricas favorables y ubicación de
algunos pozos activos de petróleo/gas, donde las anomalías radiométricas se corresponden
con la estructura de trampa anticlinal.
™Reevaluación de reservas.
™Identificación y localización de falla que sirve de conducto para la migración de metano
desde zonas profundas hasta el acuífero libre, causando su presencia en pozos de agua y las
emanaciones observadas en el río aledaño.
™Identificación de pluma de contaminación del acuífero.
™Aparentemente la ocurrencia de metano parece estar relacionada a un fenómeno natural,
aunque no se descarta que la fuente se deba a averías profundas en alguno de los pozos de
gas abandonados ubicados al noreste de la zona de estudio.
ESTUDIO RADIOMÉTRICO EN UN ÁREA DEL YACIMIENTO
NODOSARIA 15, JUSEPÍN, ESTADO MONAGAS
Objetivo: Detectar sistema de fallas asociado al sistema petrolífero. Discriminar el área
donde están los pozos de petróleo y en particular, dentro de ella, la de mayor potencial
productivo.
Estudio radiométrico y geoquímico a lo largo de una
transecta en el campo Amarilis
2,50
2,00
1,50
Zn
1,00
0,50
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
0,00
4,00
3,50
3,00
2,50
2,00
Cu
1,50
1,00
0,50
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
0,00
1,00
0,90
0,80
0,70
0,60
0,50
Mn
0,40
0,30
0,20
0,10
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
0,00
4,00
3,50
3,00
2,50
2,00
Pb
1,50
1,00
0,50
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
0,00
9,00
8,00
7,00
6,00
5,00
4,00
Ca
3,00
2,00
1,00
0,00
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Objetivo: Evaluar si el comportamiento
de la radiactividad natural y distribución
de algunos metales refleja las
diferencias en la productividad de pozos
petroleros
FUGA DE GAS EN FINCA REQUENITA, ESTADO GUÁRICO
Micro y macrofiltración de hidrocarburos
Microseeps:
•Slow and continuous loss of CH4 and other light hydrocarbons from 2 to 5 miles in sedimentary
basins where thermal degradation of organic matter occurs.
•General phenomenon driven by the buoyancy of gas phase in relation to the water.
•Vertical migration with lateral displacements by faulting and flow through transport beds to the
margins of the basin.
Macroseeps:
gas migrates through faults or fractures through a bulk flow model
Faults and fractures as channels for the increase in the flow rate of microbubbles. Seeps along faults
is an effusive process
Esbozo de la producción
geológica del metano y su
liberación