105
Original scientific paper Received: May 8, 2014
Accepted: December 22, 2014
Pore pressure detection and risk 
assessment of obl oil field, offshore 
Niger delta, Nigeria
Ugotavljanje pornega pritiska in ocenitev 
tveganja v naftnem polju obl v predobalnem 
delu Nigrove delte v Nigeriji
Matthew E. Nton*, Mosopefoluwa D. Ayeni
University of Ibadan, Department of Geology, Nigeria
*Corresponding author. E-mail: matthew.nton@mail.ui.edu.ng; ntonme@yahoo.com
Abstract Izvleček
The Niger Delta is a prolific petroleum sedimentary Za vrsto naftnih polj v bogatem naftnem območju delte 
basin in the Gulf of Guinea and is posed with a major reke Nigra v Gvinejskem zalivu je značilen pojav viso-
challenge of overpressure development in many of its kih pritiskov. Na predobalnem delu Nigrove delte so 
fields. Integrated study, utilising 3D seismic data and izvedli kompleksno raziskavo na osnovi tridimenzio-
well logs from four wells, namely; OBL_1, OBL_2, OBL_4 nalne seizmike in karotažnih podatkov iz štirih vrtin, 
and OBL_5 was carried out in the offshore Niger Delta. in sicer OBL_1, OBL_2, OBL_4 in OBL_5. Namen je bil 
The study aims at evaluation of structural influences oceniti vplive geološke zgradbe na razvijanje pritiskov, 
on pressure development, detection of overpressure ugotoviti prostorsko porazdelitev pritiska in določi-
zones and consequently examining the risk involved ti stopnjo tveganja pri globinskem vrtanju. Karotažni 
in drilling. Log signatures show decrease in density log podatki pričajo o manjšanju gostote in s tem poveza-
values and corresponding increase in sonic log read- nim naraščanjem zvočnih karotažnih meritev v vrtinah 
ings in wells OBL_1, OBL_2 and OBL_4. Well OBL_5, re- OBL_1, OBL_2 in OBL_4. Nasprotno so v vrtini OBL_5 
veals increase density log values but decrease sonic log ugotovili naraščanje vrednosti gostotnih in zmanjševa-
readings. Two regional and few minor faults observed nje zvočnih karotažnih meritev. Dva regionalna in ne-
to penetrate the overpressure zones were mapped out kaj manjših prelomov, ki potekajo čez pasove visokih 
and integrated with the surfaces of the tops of the over pritiskov, so kartirali in povezali s podatki iz teh pasov 
pressured zones to generate three stratigraphic zones. ter opredelili tri stratigrafske cone. Ugotovili so, da ute-
Aside from under compaction mechanism, the region- gnejo vplivati na razvijanje visokih pritiskov v vrtinah 
al faults penetrating the surfaces generated across the razen mehanizma kompakcije tudi regionalni prelomi, 
overpressure intervals could have influenced pressure ki potekajo čez intervale pod visokim pritiskom. 
development in the wells.
Ključne besede: porni pritisk, ocena tveganja, Nigrova 
Key words: pore pressure, risk assessment, Niger Delta delta
UNIVERSITY OF IBADAN LIBRARY
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Introduction Study area and regional geology 
setting 
Drilling in an overpressure zone without taking 
precautionary measures can pose serious chal- The study area, OBL field, lies between Lati-
lenges. Hydrocarbon within the pore structures tudes 4˚ N–6˚ N and Longitudes 4˚ E–9˚ E in the 
of sedimentary rocks tend to migrate from high south western offshore region of Niger Delta, 
pressured zones to relatively low pressured Nigeria (Figure 1) and falls within the Chev-
zones of a basin. However, if the formation is ron Nigeria Limited Concession. The base map 
isolated or laterally sealed by impermeable showing location of study wells is shown on 
clay or shale bed, such that pressure cannot be Figure 2.
dissipated through connected pores to regions 
of low pressure, the formation remains under 
abnormally high pressure [1]. 
High pore pressure fluids are encountered 
worldwide in formations ranging in ages from 
Paleozoic to Cenozoic era and may be encoun-
tered in shale-sand sequences and/ or carbon-
ate-evaporite sections at depths ranging from a 
few 100 m below the earth’s surface to depths 
exceeding 6 100 m [2]. Therefore as explora-
tion and exploitation of hydrocarbon move 
into deeper water environment, pore pressure 
analysis has become an important asset in the Figure 1: Location map of the study area. [Modified from 33, 34]
team’s planning process [3].
Pore pressure analysis serves as a useful tool 
in many areas. In exploration, it is useful for 
detecting presence of hydrocarbon seals, map-
ping of hydrocarbon pathways, analysing trap 
configuration and for basin modelling. It also 
serves as a great tool in drilling as it helps in 
understanding mechanisms and influences of 
overpressure development on hydrocarbon ac-
cumulation [4]. For example; almost half of the 
gas production in the South Louisiana (Tertia-
ry) in USA has been from a 600 m thick interval 
around the top of abnormal pressure zone [5]. 
Authors [6–8] established that the best zone to 
look for gas accumulation is in the over pres-
sure region and in the formations above such 
area. In line with this assertion, it becomes nec-
essary to critically examine pore pressure anal-
ysis as an integral procedure in exploration.
Like most petroliferous sedimentary basins in Figure 2: Base map of the study area produced using Petrel 
the world, the Niger Delta basin is posed with software.
the challenge of overpressure development in 
most of its oil fields [9]. This study therefore at- In the Niger Delta, clastic wedges were de-
tempts to evaluate the influence of structures posited along the failed arm of a triple junc-
on pore pressure development, detection of tion system. Originally, the Delta was formed 
over pressure zones as well as examine drilling during the break-up of the South American and 
risks in the Niger Delta Basin, Nigeria. African plates during the Late Jurassic [10, 11]. 
Nton, M. E., Ayeni, M. D. 
RMZ – M&G | 2015 | Vol. 62 | pp. 105–115
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The two rift arms that followed the southwest- Six regional depobelts were deposited during 
ern and southeastern coasts of Nigeria and the 25 million years from early Miocene to pres-
Cameroon developed into the passive continen- ent. Depobelts tend to become finer-grained, 
tal margin of West Africa; whereas, the third laterally away from areas of most rapid delta 
failed arm formed the Benue Trough which is progradation and basin ward away from areas 
located under the Gulf of Guinea, offshore Ni- of most rapid growth fault development [13]. 
geria. After an early history of rift filling in the 
Mesozoic, the clastic wedge steadily prograded 
into the Gulf of Guinea during the Tertiary as 
drainage expanded into the African craton with 
consequent subsidence of the passive margin. 
These upward- coarsening strata, offlapping 
the continental margin (Niger Delta), have 
been divided into three diachronous litho-
stratigraphic units, namely from oldest to 
youngest; the Akata, Agbada and Benin Forma-
tions (Figure 3; [12, 13]). The Akata Formation is 
the oldest of the units and comprised mainly 
marine shales which range in age from Eocene 
to Recent. The Agbada Formation overlies the 
Akata Formation and made up of alternating 
deltaic sandstones with shale. Its age ranges 
from Eocene to Recent. The Benin Formation 
is the youngest in the lithostratigraphic succes-
sion, and comprises sandstone, grits, claystone 
and streaks of lignite. Its age ranges from Oligo-
cene to Recent. 
The Niger Delta is subtly disturbed at the sur-
face but the subsurface is affected by large scale Figure 3: Stratigraphic columns of the three formations in the 
synsedimentary features such as growth faults, Niger Delta [13].
rollover anticlines and diapirs [13, 14]. The struc-
tural style, both on regional and on field scale, 
can be explained on the basis of influence of the 
ratio of sedimentation to subsidence rates. The 
different types of structures are namely, sim-
ple non-faulted anticline rollover structures, 
faulted rollover anticline with multiple growth 
faults or anticline faults and complicated col-
lapse crest structures [15]. Others are sub-paral-
lel growth fault (k-block structures) and struc-
tural closures along the back of major growth 
faults (Figure 4)
Continental-margin collapse structures ex-
ert control on depositional and stratigraphic 
patterns within the Niger Delta clastic wedge 
(Figure 4). At the largest scale, these structures 
extend laterally along depositional strike near-
ly across the entire Niger Delta (hundreds of 
kilometers), defining ‘‘mega structures’’ [15] and 
associated ‘‘depobelts’’ that are tens of kilome- Figure 4: Examples of Niger Delta oil field structures and 
ters wide perpendicular to the shoreline [13, 16]. associated trap types[13, 14].
Pore pressure detection and risk assessment of obl oil field, offshore Niger delta, Nigeria
UNIVERSITY OF IBADAN LIBRARY
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Smaller-scale faults and associated structural Methods of Study
deformation accommodating collapse of depo- The methods of study entailed analysis of the 
belts tend to be more complex near the progra- data set by integration of petrel software with 
dational axis of the delta than at its margins. well logs and 3D seismic volume for strati-
This pattern of deposition continues still today, graphic and seismic interpretations. Lithology 
with extensional development of growth faults delineation and well correlation were done 
on the modern shelf and slope and compres- by observing gamma ray log signatures across 
sional uplift near the toe of the slope [17, 18]. the four wells. Similarly, readings on density 
and sonic logs were observed for overburden 
assessment and delineation of overpressure 
Materials and methods of study intervals respectively. Major and minor faults 
were picked at intervals of 10 on in lines and 
Data Acquisition reflected on the cross lines across the planes 
The data set utilized in this study were ac- on the seismic section (Figure 5). Synthetic 
quired from Chevron Nigeria Limited, Lagos, seismogram was generated across the study 
Nigeria and comprised 3-D seismic volume in wells and was utilized in seismic-well tie; thus, 
SEG-Y format and well logs in LAS format. Four enhancing picking of seismic horizons across 
wells namely, OBL_1, OBL_2, OBL_4 and OBL_5 seismic section for generation of time structur-
with relative positions across 2D seismic sur- al contour maps.
vey as shown in Figure 2 were used. The com-
posite logs utilized include among others; gam-
ma ray, sonic, density and resistivity logs. The Results and discussion
3-D seismic volume on the other hand covers 
in-lines and cross lines ranges of 5 800 to 6200 Lithology Identification and Well 
and 1480 to 1700 respectively. Correlation
In lithology identification across wells OBL 1, 
OBL 2, OBL 4 and OBL 5; increase and decrease 
from the baseline on gamma ray log were in-
terpreted as shale and sand intervals respec-
tively. The four wells located within the study 
area penetrated two distinct lithological zones; 
as depicted by the litholog on the second track 
of the wells (Figure 6). Zones with yellow and 
dotted patterns indicate sand unit, while zones 
with black and dash-line patterns indicate 
shale units. 
Zone one (1) lies between 0 m and 1 676 m 
( 0–5 500 ft) and is composed predominantly of 
sand unit. Based on the blocky signature of the 
gamma ray log and the predominantly sandy 
lithology, the zone could be inferred to be the 
Benin Formation [19]. Zone two (2) extends from 
1 676 m (5 500 ft) to about 3 505 m (11 500 ft) 
which can be further delineated into upper and 
Figure 5: Seismic and fault interpretations of the study area. lower sections as reported by Doust and Omat-
sola [13]. The upper section (1 676 m (5 500 ft) 
to 2 286 m (7 500 ft)) shows thicker sand inter-
vals compared to the alternating shale laminae 
which may be indicative of the upper Agbada 
paralic sequence based on the concept of [13]. 
The lower section on the other hand (2 286 m 
Nton, M. E., Ayeni, M. D. 
RMZ – M&G | 2015 | Vol. 62 | pp. 105–115
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(7 500 ft) to 3 505 m (11 500 ft)) has thicker correlations, based on the lithologies and simi-
shale units which increases with depth com- lar log motifs, which may be indicative of simi-
pared with the alternating sands. This zone lar depositional processes and environment [22].
characterised the Lower Agbada paralic se-
quence and is an indication of gradation into Detection of Overpressure Zones
deeper portion of the basin [14]. It is assumed Overpressure detection is based on the premise 
that the Akata Formation was not penetrated that pore pressure affects compaction-depen-
by the wells, probably due to its overpressured dent geophysical properties like bulk density, 
nature [20]; however, seismic section of the study velocity and porosity. An overpressured interval 
area covers this zone (Figure 5). displays geophysical characters different from a 
From Figure 5, seismic reflection patterns be- hydrostatic reservoir such as decrease in densi-
tween 0 ms and 1 350 ms two-way travel time; ty log values and increase in sonic log values [3]. 
shows low amplitude, parallel and discon- Burst [6] established that the principal reason 
tinuous types. Based on regional studies and for this abnormality in velocity and density logs 
seismic reflection patterns observed within readings in shale units is attributed to the pres-
this interval; the zone could be inferred to be ence of trapped interstitial water in pore spac-
the Benin Formation [19, 21]. Similarly, reflection es of shale layers which could not escape before 
patterns at interval between 1 350 ms and being sealed. Consequently, the trapped fluids 
2 800 ms two-way travel time shows high am- within the pore spaces of the shale unit inev-
plitude, parallel continuous reflection and fault itably begin to support overburden pressure 
zones which are diagnostic of Agbada Forma- which slows down the mechanical process of 
tion (Figure 5). Below this depth, chaotic reflec- compaction by increase in pore pressure which 
tions are observable which is inferred to be the consequently resist further compaction [23]. Log 
Akata Formation (Figure 5). signatures show decrease in density log values 
Based on the relative positions of the wells on and corresponding increase in sonic log read-
the base map on Figure 2, the wells were cor- ings at delineated overpressure intervals of 
related in the order, OBL-2, OBL-5, OBL-1 and (42.57 m, 84.43 m, 34.75 m, 30.02 m, 58.81 m, 
OBL-4 as shown on the well section in Figure 6. 63.95 m, 75.50 m, 119.86 m, 93.56 m) in well 
These wells seem to have some units with good OBL_1, (152.4 m, 31.12 m, 25.14 m, 25.67 m, 
146.74 m) in OBL_2 and (35.84 m, 61.08 m, 
21.28 m) in OBL_4 (Figures 7–9). Based on the 
findings of authors [6, 23], incomplete dewater-
ing and compaction could have occurred in the 
sediments within the delineated overpressure 
intervals, resulting in slight increases in sonic 
log values and consequent reduction in density 
log readings at these intervals. However, densi-
ty and sonic log readings in well OBL_5 reveal 
hydrostatic pressure condition as log readings 
show no deviation from normal, thus indicat-
ing a normal compaction trend (Figure 10). 
This view corroborates the findings of [24]. The 
intervals, depth to top and base of these over-
pressure zones in the study wells are shown on 
Table 1.
Arising from regional study, predominantly san-
dy lithology and depth of delineated overpres-
sure zone in well OBL 4 (1 597.82 m (5 239 ft) 
to 1 619.10 m (5 311.68 ft), Figure 11)); the 
mechanisms or causes of overpressure devel-
Figure 6: Lithology identification and well correlation. opment at such a relatively shallow depth may 
Pore pressure detection and risk assessment of obl oil field, offshore Niger delta, Nigeria
UNIVERSITY OF IBADAN LIBRARY
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not necessarily be due to under compaction. The observed faults penetrate all the overpres-
Tissot and Welte [25] established that the gen- sure zones delineated. The major faults F1 and 
eration of biogenic gas by anaerobic bacteria F2, show sub-parallel relationship and extend 
which occurs during diagenesis at depths rang- up to 70 % across the breadth of the region 
ing from a few hundredths of metres to about as shown on the structural time maps and 3D 
1 500 m or more in sedimentary rocks could be grids on Figures 11–16. Fault F3, an antithet-
a possible cause of overpressure. However, the ic fault, extends to about 25 % across the map 
pressure posed by these sources may be short- and lie discordantly against faults F1 and F2 
lived. Gas generation at shallow depth could be (Figures 13–16). 
the possible cause of overpressure observed at Four surfaces, representing the tops of over-
such a shallow depth in OBL_4. Similar obser- pressure zones across wells OBL 1, OBL 2 and 
vations have been made in the Gulf of Mexico, OBL 4 were mapped out (Figures 11, 13, 15 and 
where a major cause of overpressure was for- 16) and observed to be penetrated by the re-
merly attributed to under compaction. A de- gional, antithetic and synthetic faults. The co-
tailed study by Hunt [26] has shown convincing lour legend indicates increasing depth from the 
evidence that generation of gas by decomposi- top (red) to the base (purple).
tion of organic matter from freshly deposited Surface map 1 represents the top of over-
mud could be a major cause of overpressure pressure zone1, delineated at depth of about 
development. 1 676 m (5498.7 ft) (Figure 11). The influence 
of the antithetic and synthetic faults at this 
Risk Assessment depth seems to be unnoticed, compared with 
Influence of Faults on Overpressure zones at greater depth. This could be interpret-
Development ed to be the base and top of Benin and Agbada 
As aids to structural studies, two major faults Formations. Such structures have been report-
and some minor faults picked at intervals of ed by Doust and Omatsola [13].
10 across the seismic planes were mapped. 
Table 1: Depth to Top and Base of Overpressure interval in OBL_1, OBL_2 & OBL_4
Well overpressure
Name Zone (OVZ) Top (m) Base (m) Thickness (m)
OVZ 1 884.49 927.06 42.57
OVZ 2 1 379.66 1 464.09 84.43
OVZ 3 1 674.70 1 709.45 34.75
OVZ 4 2 256.04 2 286.06 30.02
obl_1 OVZ 5 2 464.64 2 523.44 58.81
OVZ 6 2 644.48 2 708.43 63.95
OVZ 7 2 775.95 2 851.44 75.50
OVZ 8 2 911.83 3 031.69 119.86
OVZ 9 3 212.33 3 305.89 93.56
OVZ 1 762.00 914.40 152.4
OVZ 2 1 372.44 11 403.56 31.12
obl_2 OVZ 3 1 911.21 1 936.39 25.14
OVZ 4 2 260.26 2 285.93 25.67
OVZ 5 2 398.26 2 539.00 140.74
OVZ 1 1 356.91 1 392.75 35.84
obl_4 OVZ 2 1 460.39 1 521.47 61.08
OVZ 3 1 597.82 1 619.10 21.28
Nton, M. E., Ayeni, M. D. 
RMZ – M&G | 2015 | Vol. 62 | pp. 105–115
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Figure 7: Depth to top and base of overpressure interval in Figure 9: Depth to top and base of overpressure interval in 
OBL_1 at depth 2 256.04 m to 2 286.06 m. OBL_4 at depth 1 597.82 m to 1 619.10 m.
Figure 8: Depth to top and base of overpressure intervals in 
OBL_2 at depths of 2 260.22 m to 2 285.93 m and 2 398.26 m 
to 2 539.00 m. Figure 10: Hydrostatic pressure condition in OBL-5.
Pore pressure detection and risk assessment of obl oil field, offshore Niger delta, Nigeria
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Figure 11: Time structure map 1 - Top of overpressure zone1 Figure 15: Time structure map 3-Top of overpressure zone 3 
above the faults at depth of about 1 676 m. across the fault at depth of about 2 400.95 m.
Figure 12: 3D grid showing geometry of faults and position 
of surface 1 across the faults. Figure 16: Time structural map 4 - Top of over pressure zone 4 
across the fault at depth of about 2 744.73 m.
Surface maps 2 and 3 represent tops of over-
pressure zones 2 and 3, delineated at depths of 
about 2 257.54 m (7 406.63 ft) and 2 400.95 
m (7 877.13 ft) respectively (Figures 13 and 
15). These zones are penetrated more deeply 
by the regional growth faults, antithetic and 
synthetic faults; indicating that they have pen-
etrated the Agbada Formation. The antithetic 
fault F3, plays a significant role in trapping of 
hydrocarbon within the reservoir especially, 
Figure 13: Time structural map 2 - Top of over pressure zone 2 around the fault assisted closure as depicted 
across the fault at depth of about 2 257.54 m. by contour values of 2 010 to 1 990 on surface 
map 2 (Figure 13); as most of the wells were 
sunk around this closure. 
Similarly, surface map 4, represents overpres-
sure zone 4, delineated at depth 2 744.73 m 
(9 005.02 ft). The map shows a massively fault-
ed zone penetrated by the regional growth 
faults F1& F2, antithetic fault F3 and synthetic 
faults F4 (Figure 16). This zone most proba-
bly represents the lower portion of the Agba-
Figure 14: 3D grid showing geometry of faults and positions da Formation because of its proximity to the 
of surfaces 2, 3 and 4 across the faults. over pressured Akata shale. Tuttle [20] proposed 
Nton, M. E., Ayeni, M. D. 
RMZ – M&G | 2015 | Vol. 62 | pp. 105–115
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the top of Akata shale as the most pressurized OBL 4 showed overpressure intervals, while 
zone in the Niger Delta. It implies that pressure none is evident in well OBL_5. Owing to the rel-
regime would be higher in this zone due to its ative positions of the wells across the regional 
proximity to Akata Formation than zones in fault and the rate of sedimentation in the Niger 
shallower depths. Weber [28] reported the oc- Delta, sealing faults that penetrated the basin 
currence of overpressure in some sand units could have also influenced overpressure devel-
isolated by faults in the Gulf of Guinea. Similar opment in the study wells. 
occurrence may be observed in sand reservoirs Arising from pressure conditions in wells 
in well OBL 1, at depth greater than 2 895 m OBL_1, OBL_2 and OBL_4; evaluation of pore 
(9 500 ft), which seem to be isolated due to the pressure development, well bore stability, 
intense influence of the major, antithetic and formation strength and related uncertainties 
synthetic faults; thus resulting in high pore should be carried out prior to drilling other 
pressure. There is need for caution when drill- wells in the study area. It is suggested that a 
ing such zones. Of all the wells sunk around the standard drilling, well design and well opera-
antithetic fault F3, all indicate overpressure in- tion program should be aimed at ensuring safety 
terval except in OBL 5 well (Figure 10). The rea- of lives, environment and cost effectiveness [29]. 
son for this in OBL_5 may be due to the dissipa- As part of the assessment and releases for oil 
tion of pressure through the faults to shallower mining lease in developed nations, Canada pre-
reservoirs up dip. cisely; pore pressure prognosis and formation 
Faults are known to possess capacity for over- strength assessment are important criteria that 
pressure development where sealing units are must be met [30]. 
present [9]. Studies have shown that the sealing Faults play an important role in hydrocarbon 
ability of a fault is dependent on presence of accumulation in the Niger Delta basin; as many 
over 25 % shale/clay smears along the fault [28]. of the hydrocarbon reservoirs within this basin 
Generally, the soft over pressured Akata shale are structurally controlled [19]. However, there 
in the Niger Delta basin rises up to fill the fault may be some risk involved in drilling through 
zones, thus enhancing the sealing capability of the overpressure zones, penetrated by the re-
faults [28]. Apart from under compaction, over- gional faults F1, F2 and other minor faults. 
pressure development in wells OBL 1, OBL 2 These faults may serve as conduits for pres-
and OBL 4 may also be attributed to sealing sure communication from deeper formations 
faults. to shallower depths especially in gas -charged 
zones, thereby posing problems when a shal-
low formation is being drilled. Often times 
Summary and recommendations when this happens, the reservoir may not be 
encountered at all leading to loss of capital. 
Bore hole logs comprising gamma ray, densi- Sometimes, wells may be re-cemented, to drill 
ty and sonic logs in four wells offshore west- later or completely abandoned as reported by 
ern Niger Delta, Nigeria were integrated with Mohamad [31] and O‘Connor [32]. Fault gouges 
3D-Seismic section for pore pressure detection around the fault zones may cave-in into the 
and structural studies. At the upper sections of well bore during drilling, thereby causing the 
the wells, high sand: shale ratio was delineat- drill string to get stuck due to the instability or 
ed while at the deeper sections shale ratio in- incompetent nature of lithologies around the 
creased. Two regional, antithetic and synthetic fault zones. 
faults characterized the study area. They are It is recommended therefore, that evaluation of 
observed to penetrate beyond the four over- pore pressure development, well bore stability, 
pressure intervals represented as surfaces and formation strength and related uncertainties 
stratigraphic zones; thus showing the magni- should be carried out. Comprehensive studies 
tude and influence of these faults in overpres- on regional sealing potential of the faults across 
sure development within the study area. Of the the basin would help reduce drilling risk and in 
four wells analysed, wells OBL_1, OBL_2 and planning of drilling activities. 
Pore pressure detection and risk assessment of obl oil field, offshore Niger delta, Nigeria
UNIVERSITY OF IBADAN LIBRARY
114
Acknowledgements  [11] Whiteman, A. (1982): Nigeria: Its Petroleum Geology, 
Resources and Potential. London, Graham and Trot-
The authors are grateful to Chevron Nigeria man, Vols. 1 & 2, 394 p.
Limited for provision of data set for this study. [12] Short, K. C., Stauble, A. J. (1967): Outline of Geology 
We appreciate the invaluable assistance of of Niger Delta. American Association of Petroleum 
Schlumberger Nigeria limited, in the provision Geologists Bulletin, 51, pp. 761–779.
of Petrel software for data interpretation. Many [13] Doust, H., Omatsola, M. E. (1990): Niger Delta, In: 
thanks to the anonymous reviewers for useful Edwards J. D. and Santogrossi, P. A. (eds.), Diver-
suggestions and criticisms that have improved gent / Passive margin basins, AAPG. Memoir 48, 
the quality of this paper. Tulsa, American Association of Petroleum Geolo-
gists, pp. 239–248.
[14] Stacher, P. (1995): Present understanding of the 
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