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TYPE
Original Research
PAGE NO.
10-24
10.37547/tajas/Volume07Issue08-02
OPEN ACCESS
SUBMITED
19 July 2025
ACCEPTED
24 July 2025
PUBLISHED
01 August 2025
VOLUME
Vol.07 Issue 08 2025
CITATION
Ibrahim Olanrewaju Ibrahim, Ngozi-Chika CS, Ashioba C, & Ibrahim Bilqees
Damilola-Habeeb. (2025). Environmental Impact Assessment and
Geological Evaluation of Non-Associated Gas Wells in Forcados-Yokri
block, Burutu Area, Delta State, South-south Nigeria. The American
Journal of Applied Sciences, 7(8), 10
–
24.
https://doi.org/10.37547/tajas/Volume07Issue08-02
COPYRIGHT
© 2025 Original content from this work may be used under the terms
of the creative commons attributes 4.0 License.
Environmental Impact
Assessment and Geological
Evaluation of Non-
Associated Gas Wells in
Forcados-Yokri block,
Burutu Area, Delta State,
South-south Nigeria.
Ibrahim Olanrewaju Ibrahim.
Design and Hydrogeology units, Lower Niger River Basin
Development Authority, Ilorin, Kwara state, Nigeria.
Ngozi-Chika CS
Department of Geology, Dennis Osadebay University, Asaba, Delta
state, Nigeria
Ashioba C
Department of Geology, Dennis Osadebay University, Asaba, Delta
state, Nigeria
Ibrahim Bilqees Damilola-Habeeb
Kulende study center, Force Education Unit, Ilorin, Kwara State,
Nigeria
Abstract:
Forcados-Yokri field is situated in Burutu area
of Delta State. Its a large oil and gas province holding
large volume of recoverable hydrocarbon. A Geological
redevelopment was done to harness the abundant gas
contained in it. There is urgent need for Nigeria to shift
to cleaner source of alternative energy and reduce its
carbon footprint, thus, the block was re-evaluated and
re-developed for this gas deposit. The field was thus
initiated to supplement gas production for domestic
reserve and enhance the industrialization of the region
and the country. The field has an expected STOIIP and
GIIP (AG + NAG) of 2878 MMstb oil and 2298.7 Bscf of
gas respectively. Cluster Jackets, 18" x 5.5km Oil Pipeline
12" x 6.4km Gas Pipeline, 33KV Power Cable Back-up, 8"
Gas lift line (North Bank Gas Plant-Yokri Flowstation),
new cluster jackets, 33kV Cable from North Bank
Manifold, 700V Power Control Cable Ring were
connected to the main Pipeline Phase 1. More
importantly, 6" South Bank to Forcados Terminal fuel
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gas back-up Line,
8 km and 6“ flow line were also
connected to slug catcher at North Bank Gas Processing
Facilities with the existing Associated Gas facilities at the
CPF. Findings revealed feasible environmental space for
gas production with phytoplankton distribution of
Bacillariophyceae
52%,
Cyanophyceae
33%,
Euglenophyceae 7%, Dinophycea 5%, Chlorophyceae 3%
with the zooplankton belonged to six (6) taxa;
Copepoda, Hydrozoa, Mollusc larvae, Decapod larvae,
Rotifera, and Chaetognatha. The study concluded the
feasibility and viability of producing the hydrocarbon
content of about 291MMstb of oil and 94 bscf of gas in
the block area under the prevailing environmental
condition.
Keywords:
STOIIP= Stock Tank Oil Initially In Place,
Zooplankton, AG is Associated Gas, NAG is Non-
Associated Gas, CPF is Central Processing Facility and
GIIP is Gas Initially In Place.
Introduction
1.
Background
Forcados-Yokri field is situated in Burutu Area of Delta
State. It is one of the largest oil fields in the area. A total
of 152 wells have been drilled in the field till date. A
redevelopment strategy for the Forcados Yokri
Integrated block area was initiated to shore up the
hydrocarbon reserve base. The scope of work included
development of total expectation reserve target of
292MMstb (proved reserves of 201 MMstb) of oil and 92
bscf (proved reserves of 52 bscf) of gas. This is expected
to reduce carbon emission and greenhouse gases,
moreso, with the rolling out of compressed natural gas
infrastructure (CNG) by Nigerian government in
collaboration with Nigerian Midstream, Downstream
Petroleum Regulatory Authority to convert vehicles
from Premium Motor Spirit (PMS) based to gas based,
cleaner, safer, cheaper and more environmental friendly
fossil fuel. It is worthy of note that most oil producing
companies in the area are expected to key into the real
geoscientific dynamics shaping the world energy
landscape. Climate change is real and can be partly
traced to burning of oil and gas of which the gas can be
converted to cleaner energy.
Before now, the associated gas is commonly flared with
impunity in Niger Delta area and globally (Doust and
Omatsola 1989 and 1990) which has serious
consequential harm on the ecosystem with attendant
serious public health implication on well-being of
communities and the people. Gas utilization has
increased tremendously in the day-to-day socio-
economic activities in Nigeria as a cleaner fuel ranging
from domestic cooking gas, alternative source of
powering generators to generate electricity, abundant
gas-powered stations, gas fired industries, compressed
natural gas in our vehicles etc. These are due to new
Technological and. Geoscientific innovations springing
up globally to reduce carbon footprint and make the
planet earth more habitable for man (Ibrahim et al
2023). The feasibility (ESIA) of producing the gas deposit
was thus evaluated.
2.
Location
Forcados Yokri field is bounded approximately around
the co-ordinates of 319435mE - 335238mE and
158355mN - 141626mN with area coverage of
244.64sqKm (24464.0 hectares). The block area is
composed of meandering creeks and mangrove swamps
with dredge slots leading to well heads distributed over
the area. The land terrain of the block area is covered by
fresh water swamp
forest in the North Bank - Yokri axis and mangrove
swamp forest in South Bank.
Forcados-Yokri field is a
brown field situated in the coastal swamp area
straddling the mouth of the Forcados River in Burutu
Local Government Area of Delta State (Fig. 1).
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Fig. 1: Geological map of Delta study area, Nigeria
3.
Sampling location and Statistical Analysis
The map showing the study area has shown an
expensive area with vast potential to produce
hydrocarbon in commercial quantity (Figs 1 and 2). The
sampling points were geo-referenced by means of
Global Positioning System (GPS). Purposive sampling
was applied in the selection of study stations, taking into
account tidal influence and regime. Control stations
were located outside the spatial boundary. A number of
statistical tools were employed; the student t-Test
Paired Two Samples for means, and the single-factor
(one-way) Analyses of Variance (ANOVA) adopted. The
t-Test Paired Two Samples for Means was employed to
compare two sets of data (study area and control) while
the single factor ANOVA was eventually not used in this
publication but reserved for an ongoing manuscript to
be published soon.
Table 1: Sampling coordinates for Revalidation of Study block area
S/N
Sample codes
Point X
Point Y
1
AQ1
318722.4562
152599.6653
2
AQ2
318691.9888
151633.6733
3
AQ3
317941.3238
152105.6015
4
AQ4
319494.3507
152077.2955
5
AQC1
318799.9353
153637.2337
6
AQC2
318639.1922
150449.414
7
SW1
318722.4562
152599.6653
8
SW2
318691.9888
151633.6733
9
SW3
317941.3238
152105.6015
10
SW4
319494.3507
152077.2955
The objective was thus to determine statistically,
environmental impacts of existing facilities (e.g
flowstations, manifolds, pipelines and other associated
facilities) on the environment, social and health
components of the area. In essence, it can be vividly said
that the block area in question had its environmental
impact and social assessment to give some details of the
area and allow for further mitigation planning. The
sedimentological evaluation of the study area is typical
of the Niger Delta and as described by previous workers
(Ibrahim et al 2000).
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Fig. 2: Sampling points during the field work to the study area
4.
Materials and Methodology
Materials for the work study included Pipeline Lay
Barges, Drilling Rigs, Piling Rigs, Shuttle boat, Tugboats,
Work barges, Welding Machines, Generator Barges,
Power/Fibre optics, Subsea cable, Estuary flow station
facilities, Cluster Jackets, 18" x 5.5km Oil Pipeline (New
Estuary FS
–
South Bank), 12" x6.4km Gas Pipeline (New
Estuary FS -South Bank), 33KV Power Cable Back-up
(Forcados Terminal - CPF Ring), Construct 8" Gaslift line
(North Bank Gas Plant - Yokri Flowstation), new cluster
jackets, 33kV Cable from North Bank Manifold
–
CPF,
700V Power Control Cable Ring Main Pipeline Phase 1.
More importantly, 6" South Bank to Forcados Terminal
fuel gas back-up Line,
8 km and 6 “ flow line to a slug
catcher at North Bank Gas Processing Facilities and
integrating the whole setup to the existing Associated
Gas facilities at the CPF. For the Phase 2 pipeline: New
gas flowline from NAG Well to CPF with
16’’ Export Gas
pipeline-34km
6’’ Bulkline replacement
-105km. Afremo
pipeline repair works 8’’ X 12.2km (Fig. 3).
With
Gas lift pipeline 8” x 20km,
Gas lift spur-
line 4”x 20km were used,
Old Estuary flow station (OEFS)
pig launcher replacement and inter-connecting piping
works with Non-Associated Gas (1 NAG) Well and 4 Oil
well Development. Water based mud (bentonite) and
pseudo oil-based mud (POBM) were used for the upper
and lower sections of the holes respectively for rigidity
to prevent caving. Spent muds and drilling fluids were
generated during drilling and disposed off appropriately
with no harm to the ecosystem. The POBM drill cuttings
were transported to onshore and treated at the Thermal
Desorption Unit (TDU) at Forcados Terminal. The unit
eventually produced solids that are free of Pseudo Oil
Based Mud (POBM). The treated solid is usually about
99.97% pure solid which is within regulatory limits of
NUPRC and were shipped to deep offshore (depth of
water greater than 200ft and distance greater than 12
nautical miles) and dumped. Drilling fluids and chemicals
were continuously recycled/reused.
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Fig. 3: Diagram showing Forcados Non-Associated Gas wells and clusters of hydrocarbon infrastructures.
5.
Description of the Environmental condition of the
area
The purpose of the data acquisition was to establish the
status of the environmental condition of the study area
before the intervention. Data obtained will be useful for
adequate comparative study with previously obtained
data and shall be used for adequate planning. This
comparison provides us information on impacts of
existing facilities such as flow stations, clusters, pipelines
and manifolds on the environment in addition to
determining future impact of proposed block activities
on the environment to further improve on production.
This network of pipelines was similarly advanced by
previous workers, especially around the Sagbara gas
flow station (Ibrahim et al 2023).
5.1 Climate and meteorology
The block area environment lies within tropical swamp
belt of southern Nigeria characterized by heavy rain,
frequent thunder storms, high relative humidity and
relatively moderate temperatures. The rain occur mostly
during the wet season from March to November of each
year, with a monthly average of 199.58 mm reported for
the area. Wind in the area is dominated by SW winds
with speeds of 3.1-4.6 m/s measured during the study
(Table 2). The climate of the study area is tropical and
marked by the rainy and the dry seasons. The rainy
season begins around March and lasts till November,
while the dry season commences in December and lasts
till February of each year. The annual rainfall is in the
order of 2395 mm at an average of 199.58 mm per
month. The relative humidity is typically lowest in
January (69%) and highest in July and August (92% and
93% respectively). The mean relative humidity was
84.1%. The mean monthly air temperatures were similar
and almost the same in some months as in February,
May, July, August, September and October. The average
monthly temperature is 27.6°C (Table 2). The lowest
temperature was in July (26.3°C) and August and
September and the highest was in February (29.3°C) and
December. There are winds blowing from different
directions during the year, namely; Northeast (NE), East
(E), Southeast (SE), South (S), Southwest (SW), West (W),
Northwest (NW) and North (N). The block area
environment is dominated by South-west and North-
east trade winds during the wet and the dry seasons
respectively.
5.1.1 Micro-climatic Data
The average annual rainfall ranges from 3,000-3900 mm
with a monthly mean of 270 mm. The mean annual
maximum temperature is between 26.3-29.3°C, while
the mean minimum temperature is between 22-23°C.
Ambient temperature during the study ranged from
26.3°C- 29.3°C with an average value of 28.8°C. Wind
speed in the study area ranged from 3.1 - 4.6 m/s with a
mean value of 3.9 m/s. The South westerly (SW) wind
was the dominant wind direction in the study area
during the study.
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Table 2: Micro-climate Measurement for the Area
Parameters
AQ1
AQ1
AQ1
AQ1
AQ1
AQ1
Temperature
29
29.3
28.3
28.6
27.2
26.3
Wind Speed
(m/s)
4.2
4.6
3.2
3.6
3.5
3.1
Wind
Direction
SW SW/SS
SW SW/SS
SW/SS
SW
SW
SW/SS
5.2 Air quality and Noise
There were no significant differences between the block
area and control locations in noise and air quality
conditions. Maximum noise levels at the project area
was 69.6dB (A) which is within national regulatory limits
set by NUPRC. However, an increasing trend in noise was
observed when compared with previous data from the
field. The increasing trend is attributed to increasing oil
and gas operations as well as ship traffic in the area.
Other air quality indicators including sulphur dioxide,
nitrogen dioxide, volatile organic compounds and
suspended particulate matter were low and within
national regulatory limits of NUPRC. The particulate
fractions SPM, 10.29
μg/m
3
and 17.49
μg/m
3
(Table 3)
were also within stipulated limits of US Environmental
Protection Agency limit. The increasing trend observed
in SPM was attributed to the current particulate matter
(black smoke) pollution in the Niger Delta area. In
contrast to noise and SPM, VOC showed an apparent
decreasing trend which was attributed to improved
management of associated gas, in line with
government’s
flare down policy.
Table 3: Air quality measurement result
Parameters
AQ1
AQ2
AQ3
AQ4
AQC1
AQC2
P-value
NUPRC Limit
Noise (dB(A)
66.9
68.2
62.4
69.6
69.6
62.1
0.765
80-100
NH
4
(ppm)
0.84
1.22
1.42
1.32
1.14
0.25
0.691
NS
SO
2
(ppm)
<0.01
0.01
<0.01
0.13
0.02
0.01
0.5
0.01
NO
2
(ppm)
0.02
0.02
0.01
0.01
0.12
0.01
0.563
0.08
VOC (ppm)
0.86
1.86
1.77
1.6
1.28
0.56
0.699
NS
SPM (μg/m
3
)
10.29
17.49
14.63
14.23
13.99
10.59
0.813
60-90
H
2
S (ppm)
0.01
<0.01
<0.01
<0.01
<0.01
<0.01
-
NS
Air quality parameters monitored included sulphur
dioxide (SO
2
), Nitrogen dioxide (NO
2
), Ammonia (NH
4
),
Volatile Organic Compounds (VOC), Hydrogen Sulphide
(H
2
S) and Suspended Particulate Matter (SPM). Sulphur
dioxide levels ranged from <0.01-0.13 ppm and showed
no significant difference between levels at study area
and those of the control stations. All evaluated samples
showed SO
2
levels lower than NUPRC standard of 0.01
ppm except AQC1 and AQC2 of the control stations.
Nitrogen dioxide concentrations ranged from 0.01-
0.12ppm. There was no significant difference between
values at the study area and the control stations of AQC1
and AQC2. All values were within NUPRC recommended
limits. Volatile Organic Carbon (VOC) values ranged from
0.56
–
1.86 ppm (Table 3). There was no significant
difference between values at the study area and control.
All stations showed VOC levels were high both within the
tolerant limit of 0,09 ppm. Ammonia (NH
4
) ranged from
0.25-1.42 ppm with no significant difference between
block area and control station, while H
2
S were not
detected in the study area, except for sample AQ1 with
0.01 ppm (Table 3).
5.3 Sediments
Sediments varied from sandy to sand-clay texture
indicating a low affinity for micropollutants including
heavy metals and petroleum hydrocarbons. Sediment
pH was mildly acidic which is related to that of overlying
surface waters. Total organic carbon level was measured
to be <1% signifying absence of organic pollution. Redox
conditions were generally positive, a sign of conducive
environment for microbial degradation of organic
matter. Nitrate levels were low particularly around the
proposed project area, indicating active microbial
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assimilation
associated
with
organic
matter
degradation. Levels of heavy metals in sediments were
low and within recommended sediment quality
guidelines. Cadmium concentration (0.97-1.85 mg/kg)
was, however, higher than sediment guideline of 0.99
mg/kg but within historical levels of BDL to 8 mg/kg for
the Niger Delta. The highest cadmium level of 1.85
mg/kg was obtained at the control indicating the
widespread problem of cadmium pollution in the area.
BTEX were not detected in the sediments indicating
absence of recent petroleum pollution. PAH was also not
detected in the sediments. TPH (5.03-44.03 mg/kg) was
significantly higher around the proposed project area
than control indicating low level contamination around
the area, but all levels were below sediment guideline of
50 mg/kg indicating absence of petroleum pollution.
Microbial counts were generally low compared with
normal counts in unpolluted sediments. This conform to
earlier published figures (McDonald et al 2000).
Total fungi was marginally higher around the project
area while hydrocarbon utilizing fungi was only found in
the project area indicating greater microbial activity
around the project area capable of degrading organic
matter including oil. Trending of sediment parameters
showed decrease in pH possibly linked to discharge of
acidic sediments from mangrove swamp forests into the
coastal waters. Similarly, TOC showed decreasing trend
possibly linked to erosion of organic matter deficient
sediments from coastal areas into the water.
Exchangeable cations showed increasing trends which
may be attributed to variations in tidal conditions during
sampling. Heavy metals showed increasing trends.
Although the levels of heavy metals were within
recommended sediment guidelines (except cadmium),
increasing trends in heavy metal is a course for concern
because of their persistence and toxicity and adequate
attention should be paid to pollution control measures
particularly during the proposed NAG well drilling. Total
petroleum hydrocarbons showed increasing trend with
significantly higher levels around the project area
compared to control indicating local sources of
contamination.
5.4 Phytoplankton:
Phytoplankton diversity and density were generally high
(Fig. 4). The phytoplankton followed the order of
dominance: Bacillariophyceae > Cyanophyceae >
Euglenophyceae > Dynophyceae > Chlorophyceae in the
project study area. The dominance of the phytoplankton
by bacillariophytes (diatoms) is usually considered an
indicator of unpolluted environment. There were no
significant differences between the project location and
control in phytoplankton diversity indices including the
Shannon index. Shannon index was generally above 3
indicating unpolluted water shows the distribution of
major phytoplankton taxa in the study project area and
control. The phytoplankton was represented by five
major families namely: Bacillariophyta, Cyanophyta,
Chlorophyta, Dinophyta and Euglenophyta which
conforms with other reports in Nigerian water (Akoma
and Opute, 2010). Similar results was obtained (DeLaune
2009) in a study of the phytoplankton.
Baccillariophyta was the most dominant taxon
constituting 52% of the total phytoplankton density in
the study area and 55% in the control. Diatom
dominance in phytoplankton of Nigerian waters is
widely reported. The major species of Bacillariophyta
include Pleurosigma angulatum (3.28%), Pinnularia
interrupta (2.25%), Thalassiosira sp. (1.23%), Cymbella
ovals (1.94%) and Cyclotella meneghianina (1.86%). The
dominance of diatoms is usually considered an
indication of unpolluted waters. Cyanophyta was the
next dominant family after Bacillariophyta, with 28
species making up 33% of phytoplankton in the project
area and 28% in the control. The Cyanophyta was
dominated by Oscillatoria pseudominma. (2.05%),
Oscillatoria limosa (2.02%), Oscillatoria princeps
(2.00%), Phormidium forseolarum (1.92%), isocystis
planktonica (1.92%), Oscillatoria terebriformis (1.61%)
and Merismopedia elegans (1.59%). The spatial variation
in total phytoplankton density and species count are
presented in Fig 4.25. Phytoplankton density varied
between 1932 cells/l in station SW 2 to 2347 cells/ml in
SW 4 at the project area and from 2373-3224 cells/l at
the control. A total of eighty-seven (87) species of
phytoplankton were recorded in the study area ranging
from 56 species in station SW3 to 64 species in SW4 with
no significant difference between project study location
and control.
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Fig. 4: Phytoplankton population distribution enumerated in the study Forcados area
5.5 Zooplankton:
The zooplankton belonged to six taxa including
copepods, hyrozoa, molluscs, decapods, rotifers and
chaetognths. Zooplankton of the area (Fig. 5) was
dominated by rotifers followed by copepods in the and
control with no significant difference between study
area and control in the diversity indices. Dominance of
rotifers and copepods is usually associated with
unpolluted environments. Shannon index ranged from
2.7 to 3.1 in the project study area and above 3 in the
control indicating moderately polluted conditions in the
project study area (Fig 5). The percentage composition
of zooplankton taxa has revealed Rotifera were the most
dominant with 12 species accounting for 33% of the
zooplankton in the project area and 29% in the control.
Most dominant members of the rotifers included Lecane
climacois (6.14%), Kelicottia longispina (4.78%),
Monomaratta longiseta (4.02%), Euchlanis dilatata (3.
56%) and Keratella testudo (3.11%). Copepods were the
second in dominance with 14 species representing 33%
in this study and 32% as control. They include Cycopina
longicornis (5.91%) Scaphacalanus magus (5.08%),
Calanus finmarchicus (3.49%), Copila mirabilis (2.96%)
and Euchaeta marina (2.89%). Hydrozoa was measured
to be 13% in this study and 20% in control. More
importantly Mollusca L, Decapod L and Chaetognatha all
recorded 7%, 7% and 4% respectively in this assessment
(Fig. 5).
Fig. 5: Zooplankton population distribution enumerated in the study Forcados area
5.6 The Benthics
The benthics of the study area was evaluated to be a
total of fifty-four (54) organisms in the project study
area and 27 organisms in the control. Paleontologic
benthic fauna were largely larval forms including,
polychaetes, crustaceans, gastropod and bivalve
molluscs. The polychaetes with a relative abundance of
52% were the most dominant in the area and while
crustaceans with an abundance of 41% were dominant
in the control (Fig 6). Dean (2009) has reviewed the use
of polychaetes as indicators of pollution in aquatic
environment and has shown that they are very
important indicators of organic enrichment, heavy
metals and hydrocarbons. Their dominance in the
project study area is an indication of a relatively more
polluted sediment conditions compared to the control.
Polycheata, Bivalves, Gastropods and Crustaceans all
recorded values of 52%, 9%, 11% and 28% respectively
Baccillariophyta 52%
Cyanophyta 33%
Euglenophyta 7%
Dinophyta 5%
Chlorophyta 3%
Phytoplankton
Study area
Baccillariophyta 55%
Cyanophyta28%
Euglenophyta 5%
Dinophyta 6%
Chlorophyta 6%
Phytoplankton
Control
Zooplankton
Rotifer 33%
Copepoda 33%
Hydrozoa 13%
Mollusca L 7%
Decapod L 7%
Chaetognatha
4%
Study area
Zooplankton
Rotifer 29%
Copepoda 32%
Hydrozoa 20%
Mollusca L 7%
Decapod L 4%
Chaetognatha
8%
Control
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in this study compared to the control station that
recorded 28%, 1%, 30% and 41% respectively (Fig. 6).
Fig. 6: Recorded Benthics in the study area of Forcados, Delta area
5.7 Fisheries:
Fishes of the study area included both fin and shell
fishes. The shellfishes include the crustaceans namely,
blue swimming crab (Callinectes spp.), the mangrove
swamp crabs (Cardiosoma sp and Sesarma sp), the
shrimps (Penaeus notialis, Parapaeneopsis atlantica and
Palaemonetes africanus) and prawns (Macrobrachium
spp. and Nematopalaemon hastatus). The molluscs
include cockles (Senilia senilis), whelks (Thais sp.),
oysters (Crassostrea gasar) and the periwinkles
(Tympanotonus fuscatus and Pachymelania aurita). The
fin fishes include bonga fish, croakers, gobies, groupers,
grunts, snappers, sole, shad, mullets and tilapias. Fishes
such as the bonga-Ethmalosa fimbriata migrate along
the nearshore-inshore axis in relation to changes in
salinity, food availability and age. Fishing gears of the
area are many and varied, but commonly include
gillnets, tow nets, cast nets, beach seines, lift nets, traps,
hooks and lines, fences and stakes. Fishes are processed
for preservation by gutting or merely washed in water
for smoke-drying (DeLaune 1999
)
.
5.8 Vegetation
Vegetation characteristics reflect a typical fresh water
and mangrove swamp forests that extends from Yakri to
Sikebolon, Ogbotobo and Forcados. The physiognomy of
the fresh water swamp is at different stages of re-
growth with vegetation cover of 75
–
85% with an
average ranged between 20
–
26m. The mean tree
density was 900 trees/ha. A total of 33 plant species
belonging to 23 families was recorded in the study area,
common tree species in the swamp forest include Elaeis
guineensis, Alstonia boonei, Ceiba pentandra, lophira
alata,
Pycnanthus
angolensis
among
others.
Microphanerophyes
and
Nanophanerophytes
constituted 70 and 80% of the vegetation respectively.
Common plant diseases in the study area include leaf
spot and mosaic virus caused mostly by Aspergillus,
Fusarium spp and Curalaria spp. The plant tissues
analysis indicated that
the mean concentration of Fe, Zn, Cu and Mn, Cd, Ni,
and Pb were 110.34, 44.82, 4.4 and 90.19, <0.01, 0.06
and 0.14 mg/kg respectively. The results indicate that
there was no toxicity in the plant tissues in the study
area.
5.9 Mangrove Vegetation
This kind of vegetation cover is observed mostly west of
the Forcados tank farm. Mangroves are naturally near
homogenous vegetation systems. They are adapted to
the hostile environmental conditions characterized by
variable
salinity,
hypoxia
(oxygen
deficient),
waterlogged soil strata, tidal pressures, strong winds
and sea waves. Such adaptations include possession of
Stilt roots for
anchorage; prop roots, and root pneumatophores for
dealing with anaerobic mud conditions, and lenticels on
the bark to aid aeration; high water retention in
succulent leaves to keep salt levels diluted;
rhizofilteration to exclude salt during uptake of water;
possession of salt glands for secretion/excretion of salt
onto their leaf surfaces, or bark, or accumulating salt in
older leaves which are discarded as leaf abscission takes
place (Dean, 2009). The combination of these
mechanisms differs from one species to another.
Mangroves (Chinda et al 2007) are among the most
productive terrestrial ecosystems, sustain a huge
Benthics
Polycheata 52%
Bivalves 9%
Gastropod 11%
Crustacea 28%
Study area
Benthics
Polycheata
28%
Bivalves 1%
Gastropod
30%
Crustacea
41%
Control
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hydrobiological system and play a very important role in
stemming coastal erosion and land formation; as well as
providing the quiet back waters for spawning of many
marine species.
6.0 Physicochemical properties of surface water in the
area
The results of physicochemical measurements in surface
waters of the Field area are presented.
6.1 Temperature
Temperature ranged from 27.8 to 29.7oC with no
significant difference between the project location and
control. The measured temperatures are normal for
tropical coastal waters.
Table 4: Summary of results of Physicochemical measurements in surface water of the study area
Parameters
Proposed project area
Control
P values
Minimum
Maximum
Mean
Minimum
Maximum
Mean
Temperature ('C)
27.8
28.8
28.35
29.2
29.7
29.4
0.156
pH
7.5
7.9
7.75
12040
23250
17645
0
Electrical
Conductivity,
μS/cm
13650
42157
26752
12040
23250
17645
0.37
Salinity, ‰
9,9
32.1
20.03
8.2
16.6
12.4
0.18
Total
Dissolved
Solids (TDS), mg/l
7234
22260
14156
6301
12332
9312
0.37
Turbidity, NTU
13.4
81.7
37.1
16.5
19.1
17.8
0.01
Total
Suspended
solids (TSS), mg/l
19.8
34.6
27.9
6301
12322
9312
0.655
Colour, Pt.Co.
25
50
36
40
40
40
0.317
Redox
potential
(±mV)
135.5
184.7
172.3
184.8
186.1
185.45
0.18
Dissolved Oxygen,
mg/l
5.4
5.43
5.42
5.38
5.38
5.38
0.18
Biological Oxygen
Demand
(BOD),mg/l
4.11
8.31
6.01
6.27
6.27
6.27
0.18
6.2 pH, Conductivity, Salinity, Total Dissolved Solids
(TDS)
Hydrogen ion concentration (pH) varied between 7.5
and 7.9 with no significant difference between proposed
project location and control. The pH is normal for
tropical marine waters. According to CWT (2004), the pH
of seawater is usually between 7.5 and 8.4. A study
(Emere 2007) reported that the pH of marine waters is
similar to that of estuarine waters and is usually stable
between 7.5 and 8.5 worldwide. NNPC (1985) reported
a range of 3.1-8.6 for surface waters of the Niger Delta.
Lethal effects of pH on aquatic life occur below pH 4.5
and above pH 9.5. Electrical conductivity, Salinity and
Total Dissolved Solids are interrelated parameters of salt
concentration. Electrical conductivity ranged from
12040 to 42157 μS/cm with significantly higher (P<0.05)
values around the project location compared to control.
The fact that conductivity was higher around the
proposed project area than further offshore at SWC2
may indicate local influence of saline conditions due to
tides or effluent discharges. The observed conductivity
values are typical of brackish waters. Estuaries usually
have electrical conductivity typically from >1500 to
51,500 μS/cm wi
th values increasing as salinity increases
(NSW, 2010).
The levels of salinity (8,2-32.1 ppt) and TDS (6301-22260
mg/l) are also characteristic of brackish waters.
Dissolved oxygen (DO), Oxidation-Reduction Potential
(EH), Chemical Oxygen Demand (COD) and Biochemical
Oxygen Demand (BOD) The levels of DO, EH, COD and
BOD are all indicators of the redox conditions in the
environment. Dissolved oxygen concentrations ranged
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from 5.38-5.43 mg/l with no significant difference
between study locations and control. According to
Chapman (1996) DO concentrations in unpolluted
freshwaters are usually close to, but less than, 10 mg/l.
Concentrations below 5 mg/l may adversely affect the
functioning and survival of biological Community while
levels below 2 mg/l may lead to the death of most fishes.
The observed DO levels are normal in brackish tropical
waters. The moderately lower values may be attributed
to the effect of salinity because the amount of oxygen
that can dissolve in water, decreases as salinity increases
(NOAA, 2017). NNPC/RPI previously reported a range of
2 to 9 mg/l for the Niger Delta area. The present
concentrations are normal for the study area. The redox
potential (EH) ranged from 135.5 to 186.1 mV with no
significant difference between project location and
control. According to previous workers, (Chapman 1996)
surface water containing dissolved oxygen are usually
characterized by a range of EH values between +100 mV
and +500 mV. The observed EH values are in tandem
with the levels of DO in the water.
Chemical Oxygen Demand varied between 9.58 and 18.2
mg/l while BOD ranged from 4.11-8.31 mg/l with no
significant difference between facility location and
control.
According
to
Chapman
(1996)
the
concentrations of COD observed in surface waters can
reach up to 20 mg/l in unpolluted waters indicating that
the waters were unpolluted by organic matter. Also,
NNPC/RPI (1985) reported a range of 1.9 to 2460 mg/l in
the Niger Delta waters indicating that the observed
values are usual for the study area. The BOD values were
also within levels that do not indicate organic pollution.
Typical natural water has a BOD from 0.8 to 5 mg/l.
6.3 Alkalinity
Surface water alkalinity ranged from 220-460 mg/l.
Alkalinity is the measurement of the water's ability to
neutralize acids. It represents the buffering capacity of
water and its ability to resist a change in pH. According
to previous workers, alkalinity of seawater averages 116
mg/l to 127 mg/l with lower values in brackish water.
6.4 Turbidity and Total Suspended Solids (TSS)
Turbidity and TSS are related parameters indicating
particulates load in water. Turbidity ranged from 13.4 to
81.7 NTU while TSS ranged from 24.2 to 34.6 mg/l (Table
4). The USEPA guidelines on suspended solids for the
protection of fisheries resources prescribes values
below 25 mg/l as indication of no harmful effects (Emere
and Nasiru, 2007). In Estuaries, turbidity less than 10
NTU is considered healthy while poor water quality is
indicated by levels above 20 NTU. For most surface
waters, turbidity is usually between 1 NTU and 50 NTU
with possibility of higher values after heavy rains when
the water levels are high while lower values can be
expected in still water where suspended particles have
settled. The observed levels of turbidity and TSS are
indicative of poor water quality but such levels are
commonly encountered in natural tidal waters due to
tide-induced re-suspension of sediments (Chapman,
(1996).
6.5 Nitrates
Nitrate ranged from 0.5-0.6 mg/l with no significant
difference between project location and control (Table
5). Nitrate levels above 22 mg/l in natural water
normally indicates man made pollution (Chapman,
1996). In marine environments, levels of 0.44 to 0.89
mg/l are considered ideal (Alken Murray, 2006)
indicating that the water was unpolluted with regards to
nitrates. Nitrites ranged from 0.1-0.3 mg/l with no
significant difference between project location
compared to control. Nitrites occur in water as an
intermediate product in the biological breakdown of
organic nitrogen, their presence may infer recent input
of organic wastes. According to notable researchers
(Hem, (1985) the level of nitrite considered ideal for
marine fish is between 0.04 and 0.15 mg/l. The observed
levels should be expected because of multiple sources of
organic matter inputs including tidal export from
associated mangrove swamps. Phosphate ranged from
0.013 to 0.09 mg/l with no significant difference
between project area and control. According to
Chapman (1996), phosphorus ranges from 0.02 to 0.06
mg/l (Table 5) in most natural waters. A study by
previous researchers (NNPC research group, 1985) gave
a range of 0.049 to 0.584 mg/l for phosphate in rivers of
southern Nigeria. Present values are therefore normal
for the Niger Delta (Omiema and Ideriah, 2012).
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Table 5: Major Cation and Anion result with control data of the study area
Parameters
Proposed project area
Control
P
values
Minimum
Maximum
Mean
Minimum
Maximum
Mean
Chemical Oxygen Demand
(COD),
mg/l
9.58
18.2
14.35
12.55
12.55
12.92
0.18
Alkalinity, mg/l
400
460
430
220
420
320
0
Bicarbonate (CO
3
2-
) mg/l
488 5
561.2
524.6
268.4
512.4
390.4
0.05
Nitrate, mg/l
0.5
0.6
0.56
0.53
0.57
0.55
0.317
Nitrite (NO
2-
), mg/l
0.01
0.03
0.02
0.02
0.02
0.02
1
Phosphorus, mg/l
0.02
0.09
0.05
0.013
0.029
0.021
0.655
Phenol, mg/l
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
-
Chloride (Cl
-
), mg/l
4.33
9235
3478.6
4.254
53.18
28.72
0
Sulphate (SO
4
2-
), mg/l
25.08
49.71
34.94
15.97
24.63
20.3
0.043
Na
2+
, mg/l
3546
8461
5929
1953
4333
3143
0
K
+
, mg/l
188.58
443.7
321.6
118.82
223.89
171.36
0
Ca
2+,
mg/l
70.74
109.37
90.95
64.78
87.43
76.11
0.213
Mg
2+,
mg/l
359.11
1280
822.5
287.34
355.39
321.37
0
7.0 Major Anions and Cations
Chloride ranged from 4.25 to 9235 mg/l (Table 5), while
sulphate ranged from 15.97 to 49.71 mg/l with
significantly higher levels at the project location
compared to control. Chloride and sulphate are major
anions which contribute to the salinity of marine waters.
For typical ocean waters, average concentration of
chloride is 19,345 mg/l and that of sulphate is 2,701 mg/l
(Anderson, 2008). These typical ocean levels are usually
diluted in estuaries and in the vicinity of large rivers
discharging into the sea as is typical of the present study
area (Mouth of Forcados River) (Garrison, 2005). It is
worthy of note that (RPI/NNPC 1985) previous workers
reported values of 3 to 18,648 mg/l for chloride and BDL
to 2,796 mg/l for sulphate in waters of southern Nigeria.
The values of chloride and sulphate obtained are normal
for the study area. However, the observation of higher
values of chloride and sulphate within the proposed
project study area compared to control (SWC2) which is
further offshore may indicate local saline intrusion due
to tides or saline discharges. Sodium, potassium, calcium
and magnesium are major cations that contribute to the
salinity of marine waters.
Sodium ranged from 1953 mg/l to 8461 mg/l, potassium
ranged from 118.82 to 443.7 mg/l, calcium ranged from
64.78 to 109.37 mg/l and magnesium ranged from 2.72
to 2.83 mg/l (Table 5)with significantly higher values at
project location compared to control. Levels of major
cations in sea water (Hem, 1985) average 105 mg/l for
sodium, 380 mg/l for potassium, 410 mg/l for calcium
and 1350 mg/l for magnesium. Concentrations of major
cations were within normal levels for nearshore waters.
Lower values compared to average sea water is
associated with riverine dilution. Although SWC2 is the
farthest offshore location and should naturally show the
highest levels of major cations, concentrations at SW2
and SW3 within the study area were the highest. A
similar trend was found in conductivity, chloride and
sulphate levels. This may be attributed to variations in
tidal conditions during sampling or saline effluents from
variable marine operations in the area.
8.0 Heavy metals:
Trends in sediment heavy metals generally followed
those of zinc (Table 5) as observed in the trends of
copper and chromium (Table 5). Heavy metal levels in
sediments increased markedly in a previous study
following an initial decrease from baseline to 2012. Since
there were no significant differences in heavy metal
levels in sediment between the study location and
control, the observed increasing trend cannot be linked
to operations in the study area. Although the levels of
heavy metals were within recommended sediment
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guidelines (except cadmium), increasing trends in heavy
metal is a cause for concern because of their persistence
and toxicity and adequate attention was paid to
pollution control during well drilling.
Table 6: Heavy metal and control result of the study area
Parameters
Proposed Project area
Control
P- values
Minimum
Maximum
Mean
Minimum
Maximum
Mean
Cadmium, (mg/l)
<0.005
<0.005
<0.005
<0.005
<0.005
<0.005
1
Zinc, (mg/l)
0.55
0.62
0.6
0.544
0.57
0.544
0.655
Iron, (mg/l)
0.45
3.68
1.431
0.5
0.64
0.5
0.655
Copper, (mg/l)
0.02
0.04
0.03
0.029
0.032
0.031
0.18
Chromium, (mg/l)
<0.005
<0.005
<0.005
<0.001
<0.001
<0.001
1
Nickel, (mg/l)
<0.002
<0.002
<0.002
<0.002
<0.002
<0.002
1
Lead, (mg/l)
<0.002
<0.002
<0.002
<0.002
<0.002
<0.002
1
Vanadium, (mg/l)
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
1
Arsenic, (mg/l)
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
1
Mercury, (mg/l)
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
-
Barium, (mg/l)
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
-
Magnesium, (mg/l)
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
-
Silver, (mg/l)
<0.002
<0.002
<0.002
<0.002
<0.002
<0.002
-
9.0 Organics:
Total Petroleum Hydrocarbon was characterized and
measured to have a mean of 0.109 mg/l at the study
area (Table 7). This was computed from the maximum
and minimum values of 0.006 mg/l and 0.212 mg/l
respectively. A slightly higher value was obtained as
control (0.361mg/l). THC ie Total Hydrocarbon Content
was averagely measured to be 0.193mg/l. Oil and grease
measurement was moderate (Table 7). BTEX was
evaluated to be <0.001 throughout the study area. The
present EIA study has showed significantly higher levels
in the Non Associated gas content kin the study area
compared to control indicating possible impact from
ongoing operations around the well area. Such
operations may include discharge of deck/ballast
effluents from vessels as well as operational effluents
from existing oil and gas facilities in the area. Although
the present levels of TPH were within sediment quality
guidelines, the increasing trend calls for close attention
to pollution control measures in the area particularly
during the proposed Non associated gas well drilling.
Aliphatic Hydrocarbon measurement recorded 0.241
maximum values and <0.031mg/l.. This is in variance
with what is abtainable with previous workers (Massoud
et al 1999) in the Kuwait oil slik.
Table 7: Organic Petroleum and control results obtained from the area
Parameters
Project study area
Control
P-
values
Minimum
Maximum
Mean
Minimum
Maximum
Mean
Total
Petroleum
Hydrocarbon
(TPH),
(mg/l)
0.006
0.212
0.109
0.361
0.361
0.361
0.18
Total
Hydrocarbon
Content (THC), (mg/l)l
0.118
0.241
0.193
0.29
0.61
0.45±
0.18
Oil and grease (O&G),
(mg/l)
0.112
0.24
0.187
0.261
0.27
0.266
0.18
BTEX, (mg/l)
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
-
PAH, (mg/l)
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
-
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Aliphatic
Hydrocarbon,
(mg/l)
<0.031
0.241
0.124
<0.031
0.305
0.305
0.18
10.0 Mitigation measures:
The following mitigation measures will need be adopted
for negative and adverse impacts on the ecosystem.
• use of wet scrubbers for all emission sources
• use of mufflers for vehicle exhaust.
• ensure that
the cluster is located outside the border of
the navigation fairway in
keeping with government regulations requiring the
fairway to be kept free and open
to water transport traffic
• ensure that Vessel/boat operators observe
recommended speed limits
• ensu
re effective consultation with stakeholders
• ensure commitment and transparent adherence to P
-
GMoU programmes.
• identify and address legacy issues promptly
• support skills acquisition and empowerment schemes
to facilitate occupational
proficiency, productivity and sustainability
11.0 Conclusions
The study concludes on a favorable climatic condition in
the area with adequate technological tools and greater
scientific skills capable to produce the gas deposit for
deeper and better utilization as alternative fuel in
Nigeria and diaspora market to reduce global carbon
footprint. The Environmental Impact Assessment has
thus also provided great opportunities for skilled and
unskilled job creation in the area. All identified adverse
effects most especially the severe pollution of the air are
short-term and will need be mitigated as outlined and
can be reduced or controlled.
More importantly, this assessment will guide the
operators and inhabitants on the need to protect the
environment as much as possible during such
intervention to add value to the community and lastly,
restoration of all sites from oil spill during production
phase will need be attended to in the area to prevent
contamination of the soil, water, air and other vital
ecosystem components that will strengthen the public
health of inhabitants.
Acknowledgements
The authors sincerely thank friends and well wishers for
the encouragement during the most tedious part of this
work. Special thanks to support staffs and Industrial
Training Geoscience students of 2024 and 2025 sets at
Lower Niger River Basin development Authority, Ilorin,
Kwara State for the support in compiling articles and
useful data.
Funding
No external funding has been received during the
preparation, typesetting, compilation and review of
manuscripts or publication of these findings.
Conflict of Interest
No conflict of Interest has been established in the work.
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