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International Geoinformatics Research and Development Journal
Vol. 5, Issue 3, September 2014
Using Remote Sensing Techniques and GIS to Study Hydrocarbon Leakage
from Oilfields in Urban Areas
(A Case Study of Masjed-Soleiman City)
Khedri Gharibvand L. 1 *, Rangzan K.
2
1Department of engineering, Dezful Branch, Islamic Azad University, Dezful, Iran
Email: [email protected]
2Department of Geology, Shahid Chamran University, Ahwaz, Iran
Abstract
To examine gas and hydrocarbon leakage from oilfields of Masjed-Soleiman city, information layers such as
surface and subsurface structures, locations of gas leakage, stratigraphy, status of population density, and
topographic altitude of the region were prepared. Information from field investigations and remotely sensed
data have been analyzed and were entered into geographic information systems (GIS) software for mapping. By
using analytical functions in GIS, the layers were overlaid, and the zoning of gas leakage risk was accomplished
through Boolean logic. The results indicated high fracture density at the intersection of major faults
corresponding to the location of a deep anticline hinge in Masjed-Soleiman. It is a factor of decreasing the
impermeability of the Asmari Reservoir cap rock. Gas leakage occurs through these fractures; as a result, the
highest risk of leakage in Masjed-Soleiman was detected in its urban areas, including Siberenj and Naftun
districts. In addition to the high density of fractures, the gas cap depth is low. These areas also have the highest
amount of gas leakage and the highest population density. In other areas, the leakage risk was lower than
average due to lower population density, increased depth of gas cap, and lower fracture density.
Keywords: Masjed-Soleiman, Gas leakage, Remote sensing, GIS
Introduction
Hydrocarbon leakage has been observed in surface sediments of reservoirs in close proximity to the mountainous
Khuzestan province in southwest Iran. In most of these reservoirs, such leakage occurs in the south wing of
reservoirs with somewhat distance from the crest line of anticlines, which is due to the structural conditions of
these areas. In 1908, after drilling well number Mis1, Masjed-Soleiman reservoir was the first to be explored for
economic potential in the Middle East. The crest of this reservoir is at a depth of 183 m, which makes it one of
the shallowest reservoirs in Khuzestan province. As a result, several leakages have occurred. The first led to the
discovery of oil; however, recent changes in reservoir conditions have altered these leakages to a gaseous state.
H2S gas leakage in various areas of Masjed-Soleiman has created the potential for environmental disaster; any
factor that allows the exit of this gas can put the city at substantial risk [1]. Attempts have been made to
determine the origin of this gas and the cause of these leakages. In a 1994 report, number 73.3-2.438, the
National Iranian Oil Company probed the status of hydrocarbon leakage in Masjed-Soleiman city; the
investigation only identifying these leakages. The next report determined the type and concentration of the
pollutants and examined their environmental effects [2].
Similarly, the third report identified the origin rock and determined an inorganic nature for these gasses [3]. The
fourth attempted to determine the relationship between gas exit points and structural factors such as the depth of
the gas cap and its fault zones [4]. In other parts of the world, the high importance of maintaining the ecosystem
has dictated the implementation of certain conditions to avoid gas leakage from oilfields in urban areas, resulting
in the closure of several oilfields. Researchers have examined the surface and semi-deep migration patterns of
hydrocarbons in California and investigated the rates and mechanisms of the gas leakages [5]. In addition, an
attempt was made to determine the amount of leakage and hydrocarbon migration using remote sensing and
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three-dimensional seismology [6], and hyper spectral remote sensing was applied to recognize hydrocarbon
leakage [7].
General Geology of the Region
Masjed-Soleiman is located between 49°0'26" and 49°34'39" eastern longitude and 31°40'8" and 32°11'43"
northern latitude and is the major population center in the northeast region of Khuzestan province. This city is
bordered by Dezful to the north, Chaharmahal-o-Bakhtiary province and Izeh to the east, Ramhormoz to the
south, and Shushtar to the west (Figure 1).
Figure 1. Geographical position of Masjed-Soleiman region
The Masjed-Soleiman region is located in Zagros Mountains. The Zagros orogenic belt, which was folded in the
final stages of the Alpine orogenic phase to receive its current shape, is composed of three main zones of thrust
Zagros, folded Zagros, and Khuzestan plain. The region is located in the western part of the folded Zagros
region, west of the Kazeroun fault. The folding pattern of this oilfield is a disharmonic type. Reverse functioning
of faults with a N120–130 trend has resulted in the formation of a pressure zone and the creation of fault-related
folding (Masjed-Soleiman anticline) in hard layers. In addition, the existence of a formable layer of the
Gachsaran Formation is a factor for folding differences at several surfaces, which ultimately converted the
Masjed-Soleiman anticline into a substantially wide syncline at higher horizons (Figure 2) [8].
Figure 2. Structural section of Masjed-Soleiman anticline
The trend of the Masjed-Soleiman anticline is N120–130. On the basis of controlled stratification status, the
anticline is asymmetric in the upper side of the Asmari Formation, and its axial plane slopes approximately 70°
toward the northeast. Oil and gas hydrocarbons are aggregated in the upper region of the Asmari Formation. The
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stratigraphy of outcrop formations, including the Asmari Formation as the oil reservoir, is described in the
following subsections.
Asmari Formation
This formation with Oligocene–Miocene age (Oligomiocene) composition consists of cream-colored limes
among marly limestone layers. Due to its rather high resistance, an in-depth folding pattern has been determined.
Lower and upper borders of this formation are isoclinic with Pabdeh marls and halite and Gachsaran anhydrites,
respectively. This formation has no surface outcrop and has been solely determined by core drilling.
Gachsaran Formation
This formation with lower Miocene age is composed of sequence of anhydrite, halite, and green and red marls.
Due to its low resistance, this formation has become a detached layer as a result of non-harmonic folds in the
region. The upper border of this formation is isoclinic with marls and sandstones of the Aghajari Formation.
Aghajari Formation
This formation was built from the sequence of marl and red sandstones; it has altered to siltstone and marl in the
upper part and contains a Lahbari member. This formation has the age of middle to upper Miocene, and special
angular discontinuities can be observed between the formation and upper side (Lahbari member). This formation
comprises the major part of the surface outcrops in the studied region. Its upper border can be observed as an
unconformity due to conglomerates and sandstones of Bakhtiari Formation.
Bakhtiari Formation
This formation, composed of sequences of conglomerate, sandstone, siltstone, and a low quantity of marl layers,
has a Pliocene–Pleistocene age and is the youngest surface outcrop in the southwestern region [8].
Methodology
The research materials included the following elements: a) Geological map of the studied area: 1:100000 scale,
National Iranian Oil Company; b) Digital topographic maps at a scale of 1:25000 with digital graphic number
(DGN) format related to blocks 58531 NW, 58534 NE, 58542 SW, 58543 SE obtained from Iran Mapping
Organization; c) Underground Contour Map (UGC) related to the upper side of Asmari Formation with a scale of
1:50000 obtained from National Iranian South Oil Company; d) Maps of the city, roads, and countryside with a
scale of 1:15000 prepared by Sahab Geography and Cartography Institute, e) Statistics of the regional population
obtained from the last population census in 1991 by the management and planning organization of the province;
f) Annual statistics of weather stations adjacent to the studied area by Khuzestan; g) water and power
organization; h) Landsat 7-ETM satellite data, 166.038 transmission in 2002 prepared by Iran Remote Sensing
Center with the reference datum “WGS84” and Imaging System “UTM” Zone: north 39; i) ArcGIS 9.0,
Microstation, ArcView 3.2, Rivertools2.4, ENVI 4.0, and ER Mapper 6.1 software; j) Global Positioning System
(GPS) device, model ETREX, with geographical and altitudinal accuracy of ± 1m and ± 50 m, respectively.
Preparing the Required Layers
To prepare the zoning map of gas leakage risk, the following layers were required: fractures, lithology, condition
of underground structures, gas leakage points, demographic data, and altitude and land use. After preparing each
layer, a grid with dimensions of 1 km x 1 km was prepared; on this basis, isopotential surfaces were made for
each layer. Collection, entrance, and preparation methods of these layers are described in the following
subsections.
Structural Factors
To prepare the layers of fractures and faults, we were able to use proper filters for determining structural factors.
Therefore, in the ENVI software, directional filters with various angles of 45, 90, 135, and 190° were used in
different directions to determine faults and fractures. The final purpose of preparing such a layer was for
development of the isofracture potential map. To accomplish this task, a grid was made on the fractures and For
each cell of this grid, the total length of the fracture was calculated, and isofracture potential surfaces were
obtained for the region (Figure 3).
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Figure 3. Map of isofractures in Masjed-Soleiman region
Position of Subsurface Structures
To prepare the deep layer of the reservoir gas cap, isodepth maps of the upper region of Asmari Formation were
arranged. The map was then corrected from a georeference geometric and included coordinates and pixel size.
After the digitizing process, a layer was prepared by a vector format from the curves related to deep alignments.
On the basis of depth, they were then classified in five ranks as follows: 100–200 m, 201–350 m, 351–500 m,
501–700 m, and above 700 m. For each cell of this grid, depth expansion was determined, and isodepth potential
surfaces of gas caps were specified (Figure 4).
Figure 4. Isopotential map of gas cap depth in Masjed-Soleiman region
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Gas Leakage Locations
To prepare this layer, points of gas exit were identified after field investigations and questioning the local people.
Afterwards, coordinates of the place were measured and recorded by GPS device based on UTM coordinates
system. A data bank was prepared from the mentioned coordinates and was turned to a point layer in the GIS;
accordingly, isodensity potential surfaces of gas exit points was prepared (Figure 5).
Figure 5. Map of density of gas leakage points in Masjed-Soleiman region
Stratigraphy
Through field sampling and analysis of satellite images in the region, surface outcropping of the formations was
determined. While digitizing layer borders, a map of surface outcrops was prepared in GIS. Stone layers of the
region were divided into five categories of very soft, soft, medium, hard, and very hard, and development of each
lithology was determined. On this basis, isodensity potential surfaces were prepared for the hardness
development of stones (Figure 6).
Figure 6.Map of expansion of rock hardness in Masjed-Soleiman region
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Digital Elevation Model (DEM) of the Region
To prepare this layer, 1:25000 digital topographic maps of Mapping Organization were used. After correcting the
errors and performing required edits, the maps were used for basic altitudinal data. A digital elevation model
(DEM) of the region was developed by using the reverse internal distance weigh (IDW) method and cell size of
30 x 30 m. This layer contained five altitudinal classes of 100–201, 200–300, 301–500, 501–700 and 701–1200.
To prepare the map of altitude density, new values were determined for each cell (Figure 7).
Figure 7. Map of elevation density in Masjed-Soleiman region
Land use
To arrange this layer, urban digital maps of Mapping Organization with a scale of 1:25000 were used.
Accordingly, available functions of the region such as schools, hospitals, and health centers were determined,
and each was digitized as a separate layer and entered into GIS.
Population
To prepare this layer, urban digital maps of Mapping Organization were used with a scale of 1:25000 related to
blocks 58534 NE, 58542 SW, 58543 SE, and 58531 NW, and a vector layer was formed in GIS. By determining
residential area, the unit of population per area was calculated, and a population layer was prepared on the basis
of land use. Thus, the populated regions with such important features as schools were determined in the available
network, and population weight was determined in each district. Isopotential layers of population density were
prepared from this region with consideration of the new values (Figure 8).
Figure 8. Map of population density in Masjed-Soleiman region
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Weighing the Layers
In this stage, each map was divided into the following five levels in terms of available density and effects of gas
leakage in the region: very high density, high density, medium density, low density, and very low density.
Considering the point that most layers have vector quantities and are given equal importance in zoning, an equal
weight system (Boolean logic) was applied to these layers [9]. The following three main placement techniques
were developed: Boolean logic, binary comparison method, and fuzzy logic. The effective factors in method
selection involve identification of priority and importance of elements and parameters that determine placement.
For zoning gas leakage risk, which is the purpose of this research, the involved parameters and factors are
considered equally important; whenever effective parameters and factors have equal importance or are
considered to be equivalent, the proper method would be Boolean logic. Finally, the following levels were
assigned: Very high density had the highest weight and risk (weight number: 5), high density had high weight
and high risk (weight number: 4), medium density had medium weight and medium risk (weight number: 3), low
density had low weight and low risk (weight number: 2), and very low density had very low weight and low risk
(weight number: 1).
Overlaying of Layers and Preparing Zoning Map
After selecting Boolean logic for placing the indexes, the prepared vector layers were overlaid according to risk
classification: very high risk (5), high risk (4), medium (3), low (2) and very low (1). After overlaying the layers,
the algebraic sum of weights of different layers of each grid cell was calculated, this was obviously in the range
of 6–30 for each cell. This range was then divided to the following five risk rankings: very high risk (26–30),
high risk (21–25), medium risk (16–20), low risk (11–15), and very low risk (6–10). On the basis of the
algebraic sum of layers of each cell, isopotential surfaces were finally prepared to develop the zoning map of gas
leakage risk (Figure 9).
Figure 9. Map of risk zoning of gas leakage in Masjed-Soleiman region.
Results and Discussion
A separate examination of data the layers revealed that the map of fracture density (Figure 3) indicated that the
highest density was related to the collision point of two structural trends of the Niayesh fault and the fault zone
of Masjed-Soleiman. A structural wedge is located in the central and southern regions of Masjed-Soleiman; as a
result, fracture density exhibited its highest value in this district and was assigned a high risk weight. An
additional center of fracture aggregation was discovered at the collision point of the Lahbari fault and a fault
trend of N 70°, which corresponds to the location of Batvand Village.
The map of expansion of stratigraphic units implied that major surface coverage of the region consists of the
Gachsaran, Aghajari (including the Lahbari member), and Bakhtiari formations. Controlling surface outcrops in
the urban area of Masjed-Soleiman indicated that the northern region of the city is located on the Aghajari
Formation, and the central and southern regions are situated on the Gachsaran Formation. By classifying these
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outcrops to five rankings on the basis of rock hardness and preparing a map accordingly (Figure 6), it was
determined that the most expansion of hard rocks, which are easily broken under tension, occurs in the northwest
districts of the city, from its northwest border to Lali, and in Tembi valley. Regarding the layer of lithology data,
the city proper is located in the zones of very low to low density, which is regarded to carry a low risk for
environmental hazards.
On the map provided, which was based on the depth of the gas cap and introduced as a map of gas-cap density
(Figure 4), it was determined that the shallowest part of the gas cap is located in the southeastern region of
Masjed-Soleiman city. The city proper was placed in zones with low to medium depth; thus, the southeast region
was assigned a medium to high risk weight, and the center of the city was assigned a ranking of high density and
high risk probability.
An examination of gas leakage points revealed that most leakage occurred in the central region of Masjed-
Soleiman city in districts of Siberenj, Dare-Khersan, and Naftun. On the basis of this information, isodensity
potential surfaces were prepared (Figure 5). This map attributed the highest density and risk to the central part of
the city; thus, it carries high weight in terms of risk probability.
A study of population distribution in Masjed-Soleiman city revealed that most of its population resides in the
urban area. A population density map was developed that considered the population number relative to the
residential area (Figure 8). On the basis of this map, Masjed-Soleiman city has a linear shape from northwest to
southeast, and the highest population density is located in the northwest and southeast regions of the city; the
central region has a lower density. Nevertheless, the entire city is located in very high to high density zones,
which indicates very high to high risk probability.
An examination of altitude conditions of the region (Figure7) revealed that Masjed-Soleiman city is in a
relatively low altitude area (200–350 m) and that the central and eastern regions of the city are approximately
150 m higher in altitude than its western region situated in the valley of the Tembi River. Therefore, the western
region carries the highest weight with a very high risk probability, and the central and eastern regions have lower
risk probability and high risk. Altitudes adjacent to Masjed-Soleiman are 500–1000 m and carry medium to very
low weight.
Conclusion
After an examination of the zoning map of gas leakage risk, the following results were obtained: Considering the
point that most urban regions of Masjed-Soleiman are located on outcrops of the Gachsaran Formation, it carries
low weight in terms of lithology data. Thus, no zone of very high risk was assigned to Masjed-Soleiman city.
Due to high density of fractures, gas exit points, and population, the highest risk probability was assigned to
Naftun and Siberenj districts; the high weight of these two data layers demonstrated its effect on zoning patterns.
In addition, Other urban points of Masjed-Soleiman were assigned zones of medium to high risk probability,
which was affected by their relatively high population density, gas leakage points, and shallow gas cap depth. It
is also concluded that in other regions of Masjed-Soleiman, gas leakage risk probability ranged from very low no
risk according to low population density, high UGC depth, low fracture density and lower density of gas leakage
points. After examining altitude conditions of the region (Figure 7) and determining its relationship with gas exit
points, it was clear that regions with lower altitudes located in the gas cap area showed the highest pollution rates
due to the leakage of heavier gases. Therefore, these regions were assigned the highest weight and risk
probability; regions at higher altitudes carried lower risk probability.
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