Copyright © 2020 by Modern Scientific Press Company, Florida, USA
International Journal of Modern Applied Physics, 2020, 10(1): 54-68
International Journal of Modern Applied Physics
Journal homepage:www.ModernScientificPress.com/Journals/ijmep.aspx
ISSN: 2168-1139
Florida, USA
Article
Structural Interpretation of Three-dimensional Seismic Data
from B-field, Located in the Niger Delta Area, Nigeria
Bright C. Abanum.1, Egbo D. Okechukwu.2
1Department of Physics, University of Benin, Benin City, Nigeria
2Department of Physics, Ambrose Alli University, Ekpoma, Nigeria
* Author to whom correspondence should be addressed; E-Mail: [email protected]
Article history: Received 29 May 2020, Revised 5 August 2020, Accepted 15 August 2020, Published
24 August 2020.
Abstract: In this research work, a three-dimensional(3-D) seismic data from B-field, located
in the Niger Delta, Nigeria has been interpreted with the aim of generating a structural model
of the subsurface of the area with a view to reveal special features favorable to the
hydrocarbon prospectivity of the study area. The 3-D seismic volume data was interpreted
using OpendTect 4.3.0 software. Faults were delineated in field but only four was of interest
F1, F2, F3, F4. The fault F3 is the major growth fault in the field. Its setting tips it as a good
reservoir sealing structure. We have the faults F2 and F4 described as the antithetic faults.
Fault F1 can be describe as normal fault. Two seismic reflection horizons H1 and H2 were
mapped based on their reflection patterns. The seismic section reveals the structural
configuration of the field as an anticlinal dip closure. Since anticlinal and fault assisted
closures are regarded as good hydrocarbon prospect areas in the Niger Delta. It can be
therefore suggested that the trapping potential of the field are attributed to faults, acting as
fault assisted closures which have been perceived to be responsible for high retentive
capacity of the reservoirs and the hydrocarbon trapping mechanism in the studied area.
Keywords: 3-D (Three Dimensional), seismic data, Fault, Horizon, OpendTect 4.3.0
software, B-field, Niger Delta
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1. Introduction
The increasing population and standards of living of people globally has caused a growing
demand for energy source and despite the many efforts made to exploit `new' energy sources such as
solar energy and bio-mass in other to containing this increasing demand, oil and gas has continue to be
the primary sources of energy. In Nigeria, oil almost constitutes exclusively the revenue base for national
development and as such, demands greater efforts from both the Government and the research
institutions to ensure that this non-renewable resource is adequately and optimally tapped.
In view of the exploration of the huge deposits of the natural resources, particularly the
hydrocarbon deposits, the study of exploration geophysics has been of great relevance to the oil and gas
industry as this helps the exploration industry in identifying and delineating structural features that could
serve as possible traps for the accumulation of hydrocarbons Owing to the fact that this lucrative natural
resource has become the major source of the Nigerian economy, all efforts have been intensified over
the years to ensure the continuous exploration and production of this hydrocarbon deposits.
The production of oil and gas is from accumulation in the pore spaces of reservoir rocks usually
sandstone, limestone and dolomite. The formation is characterized by alternating sandstone and shale
units varying in thickness from 100ft to 1500ft [8].
There are various geophysical exploration methods applicable in tapping this great natural
resource which includes; the gravity, magnetic, seismic, electrical method and others. However, the
biggest breakthrough in petroleum and natural gas exploration came through the use of seismic method
while magnetic and gravity methods are used for reconnaissance surveys to delineate areas of interest
[11].
The goal of oil and gas exploration is to identify and delineate structural and stratigraphic traps
suitable for economically exploitable accumulations and delineate the extent of discoveries in field
appraisals and development [2].
[4] worked on the Reservoir characterization and structural interpretation of seismic profile: A
case study of Z-field, Niger Delta, Nigeria, showed that detailed study of the petrophysical results of the
field will provide an understanding of the geometric properties of the reservoirs, lateral variation in
thickness and possible hydrocarbon accumulations.
Three-dimensional (3-D) seismic exploration is capable of providing the most complete
subsurface picture of any surface-based geophysical technique. The essence of the method is a real
deployment of sources and receivers on a 2-D grid, followed by processing and interpretation of the
resulting densely sampled volumetric data [1].
Fundamentally, however, the greatest benefit of 3-D resides in its spatial resolving power both in
terms of absolute spatial resolution and relative accuracy in image positioning [20]. Features such as
Int. J. Modern App. Physics. 2020, 10(1): 54-68
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56
fault systems can now be mapped in much more detail than with 2-D seismic data, with its inherent
limitations of spatial aliasing [10].
[12] worked on feature detection algorithms using a Hough transform, and deduced that 3D fault
surfaces can automatically be extracted.
[3] worked on Emi Field of Niger Delta region, they showed that Anticlinal closures and fault
assisted closures which are regarded as good hydrocarbon prospect areas were found in the study area
and apart from the structural traps delineated, other stratigraphic pays including pinch-outs,
unconformities, sand lenses and channels were also suspected.
Other related contribution to knowledge in the context of 3-D structural interpretation of seismic
data can be seen in [1-20]
2. Theory
GEOPHYSICAL TECHNIQUES
Natural resources which are buried underneath the subsurface of the earth requires the application
of certain geophysical methods which reveals the possibilities of hydrocarbon presence and other
essential minerals at the subsurface of the earth and these methods have been of great relevance to the
oil and gas industries.
There are various methods which are available for geophysical survey which provides us with a
higher resolution picture of the subsurface geological features. The geophysical methods are basically
categorized into two which includes:
(1). The natural methods. This comprises of the following:
Magnetic
Gravity
Radiometric
Electrical method comprising of the resistivity, electromagnetic, self and induced potential
method and the telluric methods.
(2). Artificial methods:
Seismic
Electrical method comprising of the resistivity, electromagnetic, self and induced potential
method and the telluric methods.
Seismic survey method consists of creating a mechanical disturbance somewhere at the surface
of the earth or deep in a well and observing its effects at a number of locations along the surface or inside
the well. The objective of seismic exploration is to deduce information about the rock especially about
Int. J. Modern App. Physics. 2020, 10(1): 54-68
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57
the attitude of the beds, from the observed arrival time and from variation in amplitude, frequency and
waveform [16]. This survey method encompasses two methods which includes;
Reflection method
Refraction method.
There are two groups of seismic waves, body waves and surface waves [13]. Body waves are
waves that can propagate through the body of an elastic solid. They are of two types P-waves and S-
waves. S-waves are slower than P-waves and can only move through solid rock, not through any liquid
medium. [13] showed that the velocity of propagation of a body wave in any material is given by
𝑉 = [𝑎𝑝𝑝𝑟𝑜𝑝𝑟𝑖𝑎𝑡𝑒 𝑒𝑙𝑎𝑠𝑡𝑖𝑐 𝑚𝑜𝑑𝑢𝑙𝑒𝑠 𝑜𝑓 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙
𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 ]
12
The general purpose of seismic survey is to give exploration knowledge about the various strata
beneath the earth’s surface. The three major components of the survey are data acquisition, data
processing and interpretation.
Seismic data acquisition refers to field procedures as well as the operational principles of
instruments used in obtaining information about the subsurface structure. The acquisition of seismic data
can either be done by reflection method or refraction method which is either carried out on land (land
survey), transition zones or at sea (marine survey).
The purpose of seismic data processing is to manipulate the acquired data into an image that can
be used to infer the sub-surface structure. Only minimal processing would be required if we had a perfect
acquisition system. Processing consists of the application of series of computer routines to the acquired
data guided by the preference of the processing geophysicist. The interpreter should be involved at all
stages to check that processing decisions do not radically alter the interpretability of the results in a
detrimental manner. From good quality data it is possible to estimate lithology in terms of velocity
information and pore content from amplitudes of reflections. Processing is carried out in time domain,
common midpoint (C.M.P) domain and stack domain [14].
The aim of seismic reflection surveying is to reveal as clearly as possible the structure of the
earth. Seismic interpretation which is part of seismic surveying is the process of determining information
about the subsurface of the earth as well as the geologic significance of seismic reflection data. Seismic
interpretation also involves the construction of a geological model of the subsurface using all available
data such as seismic section, check short data and well log. The interpreter’s job is to extract geological
meaning, both structural and stratigraphic, from these geophysical data.
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3. Methodology
The data for this paper consist of a 3-D volume of seismic reflection data having a large number
of inlines and crosslines. The interpretation of the 3-D seismic volume was carried out using
interpretation software called ‘Opendtect 4.3.0 version’. This software is a C++ programme which is
designed as an open source seismic interpretation programmed software used in interpreting structural
features (faults and horizons) of the earth subsurface from the 3-D seismic volume.
4. Results and Discussion
Results are listed in tables and figures below. Seismic interpretation of Bright-field (B-field)
revealed that the structural style that characterizes the field contains some interesting features. The
seismic section reveals the structural configuration of the field as an anticlinal dip closure as shown in
Figure 7, which is a fault assisted dip closure. This anticlinal configuration has a time structure increasing
in depth from N-E to S-W with colour increasing from dark red to dark blue. That is, these colors on the
time structure map indicate the travel times of depth. It can also be deduced that the blue portion of the
interpreted data have a greater depth than the red portion, that is the red part is shallower than the blue,
thereby making an anticlinal structure.
Seismic attribute analysis (RMS amplitude slices) performed on the interpreted section, shows
that the structural configuration observed (anticline and faults combination) in the field have
hydrocarbon trapping capacities as illustrated by Figures (2-8) and Figures (10-13).
From the seismic interpretation, a number of faults were identified and mapped which are
displayed on Figures (2-3) and table 1. The fault F3 (Royal blue) is a major growth fault which have a
main trend from the North-East direction to South-West direction and main dip towards southward
direction, which can however be inferred to as a good trap and we have the faults F2 (Dark Sea green)
and F4 (Dark green) described as the antithetic faults which have a main trend from the South-East
direction to north-East direction and a main dip towards Northward direction. Fault F1 (Orchide) dips in
the south direction and it can be described as the normal fault.
From the interpretation of the seismic volume with several horizons, two seismic reflecting
horizons were mapped based on their reflection patterns on every ten in-line and every five cross-line
where data quality was adequate to confidently follow reflectors, thereby generating a 10x5 grid. These
horizons were identified as H1 (Dark Magenta), and H2 (Dark Turquoise) as shown in Figure (6).
Horizon 1(H1) has a good continuity from N-E until truncated by a major fault and later
continued towards S-W and was truncated again by another fault as shown in Figure (7). An Amplitude
attribute was generated for horizon 1 (H1) with a time window of 25ms and 50ms time window and from
Int. J. Modern App. Physics. 2020, 10(1): 54-68
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59
this attribute, it shows from the color key that H1 has an high amplitude at the north direction and south-
west which represent a very large sand body and an intercalated shale body of low amplitude as shown
in Figure (10-11).
Above the H1, is Horizon 2 (H2), which is also continuous from N-E until truncated by a major
fault and later continued towards S-W and was truncated again by another fault as shown in Figure 8.
An Amplitude attribute was generated for horizon 2 (H2) with a time window 25ms and 50ms. From the
color key, H2 has a low amplitude towards North-East direction which shows that the portion is made
up shale body and high amplitude at South-West direction which represent a sand body as displayed by
Figure (12- 13).
Figure 1: Perspective view of the 3-D seismic volume used for this study
Table 1: Different interpreted faults, their dip direction and the fault types
FAULTS FAULT DIRECTION FAULT TYPE
F1 SOUTH Minor growth fault
F2 NORTH Antithetic fault
F3 SOUTH Major growth fault
F4 NORTH Antithetic fault
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Figure 2: The four faults displayed on 11600 inline on the seismic section
Figure 3: A 3-D view of the faults with planes displayed on the seismic section
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Figure 4: A 3-D view of the horizons display (with Z-values) showing the depth characteristics
Figure 5: A 3-D display of two Horizons on inline 11600 (H1 represented by the Dark Magenta
colour that passes through the horizon and H2 represented by the Dark Turquoise colour
that passes through the horizon)
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Figure 6: A 3-D view of the horizons display (without Z-values)
Figure 7: A 3-D view of the horizon one (H1) display and faults
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Figure 8: A 3-D view of the horizon two (H2) display and fault planes
Figure 9: shows that the interpreted seismic section which reveals Anticlinal structure
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Figure 10: A display of amplitude attribute of Horizon 1 (H1) on the seismic section with
time window of 25ms
Figure 11: A display of amplitude attribute of Horizon 1 (H1) on the seismic section with a
window of 50ms
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Figure 12: A display of amplitude attribute of Horizon 2 (H2) on the seismic section with
time window 25ms
Figure 13: A display of amplitude attribute of Horizon 2 (H2) on the seismic section with a
window of 50ms
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5. Conclusion
The seismic data volume resulted in better understanding of the structural styles. The seismic
volume used for this study was interpreted using the software opendtect 4.3.0. From the interpretation,
four faults, were identified which has a major growth fault trending south, two antithetic fault trending
north and a normal fault having a dip direction in the southward.
The seismic section reveals the structural configuration of the field as an Anticlinal dip closure
as shown in Figure 9, which is a dip assisted fault closure where apparent fault dependent element of
closure is less than 50ft, dissected anticlinal dip closure: a pure dip closure dissected by non-sealing
synthetic and antithetic faults. About 50% of hydrocarbon bearing reserves in Nigeria appears to be dip
closed [20]. Furthermore, they also stated that the trapping styles in the Niger Delta are mostly structural.
Therefore, it can be deduced that the growth fault F3 and the normal fault F1 may have acted as migratory
paths for hydrocarbon flow from the underlying Akata Formation. Also, since Anticlinal and fault
assisted closures are regarded as good hydrocarbon prospect areas in the Niger Delta. It can be therefore
suggested that the trapping potential of the field are attributed to faults, acting as fault assisted closures
which have been perceived to be responsible for high retentive capacity of the reservoirs and the
hydrocarbon trapping mechanism in the studied area. It is therefore inferred that large areas covered by
the growth fault, antithetic faults and the normal fault are suggested to be the controlling factors
responsible for economic hydrocarbon accumulation in this particular study area of the Niger Delta.
The horizons H1 and H2 are associated with strong reflections and high amplitudes toward the
S-W direction. These results indicate that the hydrocarbon boundary is close to the strong or high
amplitudes representing a sand body as illustrated by the seismic attributes.
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