Sequence Stratigraphic Analysis of “XB Field”, Central Swamp Depobelt, Niger Delta Basin, Southern Nigeria

doi:10.4236/ijg.2012.31027

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International Journal of Geosciences, 2012, 3, 237-257

http://dx.doi.org/10.4236/ijg.2012.31027 Published Online February 2012 (http://www.SciRP.org/journal/ijg)

Sequence Stratigraphic Analysis of “XB Field”, Central

Swamp Depobelt, Niger Delta Basin, Southern Nigeria

Samuel Okechukwu Onyekuru*, Emmanuel Chukwudi Ibelegbu, Julian Chukwuma Iwuagwu,

Akan Godfrey Essien, Casmir Zanders Akaolisa

Department of Geology, Federal University of Technology, Owerri, Nigeria

Email: *[email protected]

Received October 1, 2011; revised November 21, 2011; accepted December 26, 2011

ABSTRACT

Well logs and biostratigraphic data from six wells in the “XB Field”, central Swamp Depobelt, Niger Delta were inte-

grated to carry out a sequence stratigraphic analysis of depositional systems in the field. The analysis revealed four 3rd

order depositional sequ ences (SEQ1 to 4) bounded by three erosional unconformities interp reted as Sequence Bounda-

ries (SB1 to 3). Transgressive Surfaces of Erosion (TSE1 to 3) that mark the onset of marine flooding and turnarounds

from progradational facies to retrogradational facies during sequence build-up were delineated. Three 3rd order Maxi-

mum Flooding Surfaces (MFS1, MFS2 and MFS3) characterized by marker shales, high faunal abundance and diversity

were also delineated and dated 15.9, 17.4 and 19.4 Ma, respectively. The delineated sequences comprised Lowstand

Systems Tracts (progradational packages), Transgressive Systems Tracts (retrogradational packages) and Highstand

Systems Tracts (aggradational packages), which reflect depositional systems deposited during different phases of base

level changes. The Lowstand Systems Tract (LST) consists of Basin Floor Fans (BFF), Slope Fans and Channel Sands

deposited when sea level was low and accommodation space lower than rate of sediment influx. Transgressive Systems

Tract (TST) consists of retrogradational marine shales deposited during high relative sea levels and when accommoda-

tion space was higher than rate of sediment influx. Highstand Systems Tracts (HST) consisted of shoreface sands dis-

playing mostly aggradational to progradational stacking patterns. The sands of LST and HST show good reservoir qua-

lities while the shales of the TSTs could fo rm poten tial reservoir seals. The abo ve recognized sequ ences, were deposited

within the Neritic to Bathyal paleoenvironments and are dated mid-Miocene (15.9 - 20.4 Ma) in age.

Keywords: Sequence Stratigraphy; Unconformities; Progradation; Retrogradational; Aggradational;

Paleoenvironments and Reservoir

1. Introduction

The stratigraphy of the Tertiary Niger Delta is co mplica-

ted by syndepositional collapse of clastic wedges as shales

of the underlying Akata Formation are mobilized under

loads of the prograding overlying deposits of the deltaic

Agbada and fluvial Benin Formations. This situation ma kes

correlation of reservoirs of same genetic units difficult.

The understanding of the geology, structure and reser-

voir architecture and continuity of the Tertiary Niger Delta

basin fills is expected to improve immensely with the ap -

plication of a new and proven concept of sequence stra-

tigraphy. Sequence Stratigraphy is the study of rocks re-

lationships within a chronostratigraphic framework whe-

rein the succession of rocks is cyclic and generally com-

prised of genetically related stratal units [1]. The concept

explains vertical and lateral variations of sedimentary suc-

cessions in terms of relative sea level fluctuations and ba-

sin tectonics.

Sequence stratigraphic techniq ue was applied in the ana-

lysis of six wells in the “XB Field”, in the western part of

the Central Swamp Depobelt, Niger Delta Basin. The eva-

luation ensured the subdivision of the delineated vertical

sedimentary sections into genetically related depos itional

sequences bounded by surfaces of same chronological age

using wire line logs and biostratigraphic data. The vari-

ous contemporaneously deposited sediment packages (sys-

tems tracts) within the sequences were also mapped to b e

able to reconstruct the depositional sequence model of

the field, so that reservoir quality and arch itecture can be

predicted. Source and reservoir rocks within the systems

tracts were characterized and evaluated for hydrocarbon

potential and trapping mechanisms.

The area under study is located at the western end of

the Central Swamp Depobelt of the Tertiary Niger Delta

that lies between Latitudes 5˚N and 6˚N and Longitudes

5˚E and 6˚E, covering an areal extent of about 675 km2

(Figure 1). It is part of the Tertiary Niger Delta (Akata-

Agbada) Petroleum System. The Niger Delta sedimentary

*Corresponding author.

S. O. ONYEKURU ET AL.

238

Figure 1. Showing Niger Delta depobelts, base map of “XB” field and spatial distribution of studied w ells.

deposits have been divided into three large-scale lithos-

tratigraphic units (Figure 2: (1) Basal Paleocene to Recent

pro-delta facies of the Akata Formation, (2) Eocene to Re-

cent, paralic facies of the Agbada Formation and (3) Oli-

gocene-Recent, fluvial facies of the Benin Formation,

[2-4]. These formations become progressively younger

farther into the basin, recording the long-term prograda-

tion of depositional environments of the Niger Delta onto

the Atlantic Ocean passive margin. From the Eocene to

the present, the delta has prograded southwestward,

forming depobelts that represent the most active portion

of the delta at each stage of its development [5]. These

depobelts form one of the largest regressive deltas in the

world with an area of about 30 0,000 km2 [6], a sediment

volume of 500,000 km3 [7] and a sediment thickness of

over 10 km in the basin’s depocenter [8].

2. Methodology

2.1. Data Set

Well log data suites provided for the study included

Gamma Ray (GR) Logs, Spontaneous Potential (SP) Logs,

Porosity Logs and Resistivity Logs (Figure 3). The biofa-

cies data extracted from core samples, side-wall samples

(Type 2) and ditch-cuttings (Type 3) were calibrated and

depth matched with corresponding wireline logs. The

Figure 2. Stratigraphic column showing formations of the

Niger Delta, [5,9].

S. O. ONYEKURU ET AL. 239

SSTVD

40 0 0

45 0 0

50 0 0

55 0 0

60 0 0

65 0 0

70 0 0

75 0 0

80 3 4

38 1 50.001 5 0.0 0GR_NM -12.5011 8.63SP_JN 1.00100.00LN_JN421.72 1250.45NEUT_JN

JONC-001 [S ST V D]

Figure 3. Showing representative well log suites provided for the study.

population and diversity of the benthic and planktonic

foraminifera were used for environmental and paleoba-

thymetric interpretation (Table 1). The biozone records

obtained from the wells were the palynological and fo-

raminiferal zones popularly referred to as the P- and F-

Zones. Four different pollen zones (P-Zones) and two

fauna zones (F-Zones) recognized were P720, P680, P670,

and P650 and F9500 and F9300, respectively (Table 1).

2.2. Delineation of Lithofacies and Depositional

Environments.

Gamma Ray Log values and signatures (fining and coar-

sening upward signatures) and the biofacies data helped

in determining lithofacies and depositional environments

of the different rock units in the well field. Bell shaped

log patterns on Gamma Ray Logs indicating increasing

clay contents up section or fining upward trends or an

S. O. ONYEKURU ET AL.

240

Table 1. Representative biofacies data of XB-2 well.

DEPTH Sample TYPE ENVIRON. F. DIVERS.F. POPLN P. DIVERS. P. POPLN F.ZONE P.ZONE

4650 2 MN 11 24 2 3

4661 2 ON-BA 15 9218 3 50

4667 2 ON-BA 19 1218 6 113

4837 2 MN 6 17 1 3

4863 2 BA 15 2140 4 105

5059 2 MN 8 27 2 4

5073 2 ON-BA 11 1332 4 231

5100 2 MN-ON 10 21 2 2

5109 2 IN-MN 9 366 0 0

5333 2 ON-BA 12 379 1 4

5372 2 BA 17 1681 3 155

5428 2 MN 6 29 1 2

5640 2 ON-BA 12 3786 3 280

5682 2 MN 6 43 3 10

5774 2 IN 3 15 1 1

5920 2 IN-MN 5 701 0 0

6039 2 IN 3 7 0 0

6108 2 IN 2 2 0 0

6191 2 MN-ON 12 3235 2 16

6326 2 IN-MN 4 226 0 0

6421 2 ON-BA 17 3580 4 400

6547 2 MN-ON 10 316 1 7

6599 2 ON-BA 17 442 4 9

6679 2 MN 7 326 0 0

6771 2 IN 3 20 0 0

6846 2 ON-BA 13 1176 1 8

6864 2 ON-BA 15 601 2 5

6930 2 IN 3 10 0 0

6994 2 B 0 0 0 0

7081 2 IN 3 6 0 0

7118 2 B 0 0 0 0

7205 2 Sh.IN 1 2 0 0

7344 2 PFM 5 447 0 0

7461 2 MN 6 725 1 2

7534 2 MN 6 418 0 0

7577 2 ON-BA 24 1153 3 81

7664 2 MN 9 67 0 0

S. O. ONYEKURU ET AL. 241

upward increase in gamma ray value is a typical feature

of fluvial channel deposits ( Figure 4). Funnel-shaped log

patterns indicating decreasing clay contents up section or

a coarsening upward trend, clearly showed deltaic progra-

dation. Cylindrical (blocky or boxcar) log motif was de-

lineated as thick uniformly graded coarse grained sand-

stone unit, probably deposits of braided channel, tidal chan-

nel or subaqueous slump deposits. Serrated log motif sug-

gested intercalation of thin shales in a sandstone body,

typically of fluvial, marine and tidal processes [10].

The neutron-density logs of clean sandstone units tra-

cked each other closely or had little separations while

shale intervals had wide separations.

2.3. Stacking Patterns and Parasequences

The well log suites provid ed for the study were d isplayed

at consistent scales to enhance log trends and also to aid

recognition of facies stacking patterns and parasequences.

Parasequence stacks (vertical occurrences of repeated cy-

cles of coarsening or fining upwards sequences), gave rise

to progradational, retrogradational, or aggradational para-

sequence sets (Figure 5).

2.4. Key Stratigraphic Surfaces, Systems Tracts

and Depositional Sequences

The Maximum Flooding Surface (MFS) was recognised

on the wireline logs and biostratigraphic data as: the bo-

undary between retrogradational parasequence sets and

progradational parasequence sets; units with maximum sha-

le peaks and well-developed shales (shaliness) visible on

the GR, Resistivity and Neutron logs; a surface of maxi-

mum foraminiferal abundance and diversity.

Plots in Petrel of faunal abundance and diversity cur-

ves alongside well logs enhanced the recognition of Maxi-

mum Flooding Surfaces (MFSs) (Figure 6).

The Transgressive Surface of Erosion (TSE) which is

the first significant flooding surface in a sequence was

inferred from the presence of nick or neck on resistivity

logs caused by presence of carbonate cements probably

derived from the carbonate fauna eroded during ravine-

ment of already deposited sediments. It usually occurs at

the base of the retrogradational parasequence stacks of

the Transgressive Systems Tracts.

Sequence Boundaries (SBs) were recognized in areas

of low faunal abundance and div ersity or absence of known

bio-events, which corresponded to low Gamma Ray, high

Resistivity, SP and sonic logs responses within the shal-

lowing section. Candidate Sequence Boundaries were iden-

tified at the base of thickest and coarsest sand units be-

tween two adjacent Maximum Flooding Surfaces [11],

which naturally coincided with the shallowest environ-

ments associated with the least foraminiferal abundance

and diversity or complete absence of foraminifera. The

base of a progradational stacking pattern was also used to

define a Sequence Boundary (SB).

Systems Tracts (Lowstand Systems Tract, Transgressi-

ve Systems Tract, and Highstand Systems Tract) were re-

cognized and mapped (Figure 7), with the aid of the de-

positional sequence model [12,13].

Figure 4. Well log response character for different environments, [18].

S. O. ONYEKURU ET AL.

242

(a) (b)

Figure 5. (a) Representative parasequence stacking patterns in XB-1 well; (b) Representative parasequence stacking patterns

in XB-2 well.

Figure 6. A representative plot of wireline logs and biostratigraphic data of XB-2 well.

S. O. ONYEKURU ET AL.

243

Figure 7. Sequence stratigraphic model showing key stratigraphic surfaces and various systems tracts [12,13].

3.2. Coarse Grained Basal Sandstone

Facies (Facies 1)

The delineated MFSs and SBs were dated with marker

shales (P and F zones) and by correlation with the Niger

Delta Chronostratigraphic Chart (Figure 8) [14]. Relati-

ve ages of the surfaces mapped in the well field were de-

termined using the provided biostratigraphic report (Fig-

ure 9) [15] and correlated with the established works on

the study area [16,17].

The Coarse Grained Basal Sandstone Facies consists of

amalgamated and isolated sharp-based fining upward sand

bodies characterized by blocky to bell-shaped Gamma Ray

Log motif with little or no separation on the Neutron-De-

nsity Logs (Figure 10). The sand units are locally sepa-

rated by thin bands of shale/mudstone and lack marine

fauna. Facies 1 is interpreted as fluvial channel deposits

based on these characteristics. These channel deposits re-

present deposition in a coastal plain setting landward of

the tidal zone [19]. The blocky log pattern is common in

incised valley fills [20]. The lack of serration in the Gam-

ma Ray Log signature and absence of marine fauna sug-

gest minimal or complete absence of tidal influence.

2.5. Well Correlation

Well correlation was achieved in Petrel window with sur-

faces (SBs and MFSs) of same geologic age defined in

the study area. Marine Flooding surfaces were the best

markers or datum on which the correlation cross sections

were hung [18]. Correlation was done to determine lat-

eral continuity or discontinuity of facies, hence aiding

reservoir studies in the well field. 3.3. Shaly-Sandstone Facies (Facies 2)

3. Results, Data Analysis and Interpretations The Shaly-Sandstone Facies (Figure 11) is characterised

by the predominance of fine-medium grained sandstones

and mudstone/shale interbeds. It consists predominantly

of serrated funnel shaped Gamma Ray Log Pattern and

sometimes serrated bell to blocky shaped patterns at cer-

tain intervals. These intervals are also characterised by

high Neutron and Density Poro sity Log values with little

3.1. Lithofacies and Depositional Environments

The stratigraphic column in the study area was divided

into four (4) lithofacies, namely: 1) Coarse Grained Basal

Sandstones Facies; 2) Shaly Sandstone Facies; 3) Mud-

rock Facies; and 4) Heterolithic Facies.

S. O. ONYEKURU ET AL.

244

Figure 8. Niger Delta chronostratig r a p h i c c h a r t [14].

S. O. ONYEKURU ET AL. 245

Figure 9. Foraminiferal biofacies model for the Niger Delta (adapted from SPDC in-house).

Figure 10. Coarse grained basal sandstone Facies represented by Blocky Gamma Ray Logs.

S. O. ONYEKURU ET AL.

246

Figure 11. Shaly-sandtone Facie s de fine d by pre dominant funn el-shaped Gamma Ray Logs and low diversity forams.

or no separation. Biofacies information revealed that the

intervals exhibited low frequency and low diversity of

foraminifera belonging to the Inner-Outer Neritic (IN-ON)

depositional enviro nment.

Facies 2 is interpreted as tide dominated estuarine de-

posits based on the presence of cyclic alternation of sand-

stones and mudstones. Each funnel shape represented a

succession of coarsening—upward from mud to shallow/

marginal marine sandstones. Rhythmic alternation of high

Gamma Ray Log response and serrated funnel, bell and

blocky Gamma Ray Log motif resulted from frequent

fluctuations in current strength which is common in tidal

processes. The successions are interpreted to have been

deposited in a progr ading, estuarine environmen t.

The biofacies data showed increase in foraminiferal as-

semblages indicating progressive deeper water bathym-

etry within the mudstone units and low diversity forms at

shallow water depths within the sandstone units.

3.4. Mudrock Facies (Facies 3)

This facies is p redominantly composed of shale units with

thin siltstone intercalations displaying a retrogradational

parasequence pattern (Figure 12). The facies also exhib-

ited high frequency and diversity of foraminifera parti-

cularly those of Outer Neritic (ON) to bathyal (BA) de-

positional environments. The unit is interpreted as off-

shore deposit s.

3.5. Heterolithic Facies (Facies 4)

Facies 4 is comprised of sandstone and mudstone Heter-

oliths. The sandston e unit is recogn ised as upward –c lean-

ing units on the Gamma Ray Log and upward increasing

porosity values on the Density Log. Crescent or bow trend

in the Gamma Ray Log (Figure 13) show a cleaning–up

trend overlain by a dirtying up trend without any sharp

break. Available Biofacies data indicate that Facies 4 ac-

cumulated in proximal-fluvial marine and Inner-Middle

Neritic (IN-MN) depositional environments (i.e. Facies 4

is interpreted a shoreface deposits).

Crescent log pattern is generally the result of waxing

and waning clastic sedimentation rate [21]. The serrated

nature of the Gamma Ray Log signature is indicative of

tide/wave activity [21] and the heteroliths probably re-

flect deposition from waning storm generated flows [22].

The muddy portion characterised by high Gamma Ray

values with biofacies bathymetry in the Neritic environ-

ment indicated storm emplacement or inter-storm pelagic

sedimentation [23].

S. O. ONYEKURU ET AL. 247

Figure 12. Mudrock Facies shown by high Gamma Ray Log signature; high frequency and diversity of forams.

Figure 13. Crescent or bow trend typified in XB-2 and XB-3

wells.

4. Results of Sequence Stratigraphic Analysis

4.1. Maximum Flooding Surface (MFS)

The first Maximum Flooding Surface (MFS1) recognized

in wells XB-3 and XB-4 (Figure 14) was dated 20.7Ma

using the Niger Delta Chronostratigraphic Chart, [14], a

regional marker, Alabamina 2 and the occurrence of the

event within P650 and F9300 biozones.

The first Maximum Flooding Surface (MFS1) recog-

nized in XB-1, XB-2, XB-5 and XB-6 wells was corre-

lated to MFS2 of XB-3 and XB-4 wells, and was dated

19.4 Ma. The surface occurred within P670 and F9300

biozone characterised by Ogara Shale marker.

MFS2 in XB-1, XB-2, XB-5 and XB-6 wells, which

correlated with MFS3 of XB-3 and XB-4 wells was dated

17.4 Ma. The MFS occurred within P680 and F9300 zone

and is an Undefined MFS.

MFS3 in XB-1, XB-2, XB-5 and XB-6 wells corre-

lated with MFS4 of XB-3 and XB-4 wells and was dated

15.9 Ma. The MFS was characterised by a regional mar-

ker, Chiloguembelina-3 and was defined within the P680

and F9500 biozones.

The summary of the recognized and identified MFSs

and the depth at which they occur in the wells are shown

in (Figures 14-19; Tables 2 and 3).

S. O. ONYEKURU ET AL.

248

Figure 14. Sequence stratigraphic summary sheet of XB-1 well showing interpreted systems tracts and constrained sur-

faces.

Figure 15. Sequence stratigraphic summary sheet of XB-2 well.

S. O. ONYEKURU ET AL. 249

Figure 16. Sequence stratigraphic summary sheet of XB-3 well.

Figure 17. Sequence stratigraphic summary sheet of XB-4 well.

S. O. ONYEKURU ET AL.

250

Figure 18. Sequence stratigraphic summary sheet of XB-5 well.

Figure 19. Sequence stratigraphic summary sheet of XB-6 well.

S. O. ONYEKURU ET AL.

251

Table 2. Contd.

DEPTH Sample TYPE ENVIRON. F. DIVERS.F. POPLN P. DIVERS. P. POPLN F.ZONE P.ZONE

7844 2 ON-BA 13 1974 4 113

7993 2 IN 7 32 2 8

8114 2 CD 2 2 0 0

8412 2 ON-BA 15 545 3 46

8522 2 IN 10 50 2 7

8702 2 IN-MN 5 1110 1 2

8803 2 MN 4 844 0 0

8831 2 MN-ON 9 533 1 1

8894 2 ON-BA 21 2067 1 6

8933 2 ON-BA 16 247 1 3

9085 2 ON-BA 12 17674 2 32

9113 2 ON-BA 22 7459 4 1664

9345 2 ON-BA 22 360 3 32

9436 2 BA 28 10075 4 2268

9450 2 ON-BA 20 9333 5 1768

9501 2 ON-BA 22 3956 4 197

9778 2 BA 31 16336 6 2568

9841 2 BA 24 11981 6 4320

9866 2 BA 29 1464 3 330

Table 3. Delineated MFS, marker fauna and biozone of studied wells.

BIOZONES DEPTH (Ft)

CHRONO

SURFACE AGE(Ma) MARKER FAUNA P-ZONEF-ZONEXB-1 XB-2 XB-3 XB-4 XB-5 XB-6

MFS1 20.7 Alabamina 2 P650 F9300 NA 9808 9740 10880 NA NA

MFS2 19.4 Ogara shell P670 F9300 10670 8361 8360 8870 11060 11040

MFS3 17.4 Unnamed P680 F9300 8270 6801 6500 7550 9850 9780

MFS4 15.9 Chiloguembelina 3 P680 F9500 6610 5324 5000 6440 8370 8350

4.2. Sequence Boundary (SB) and Transgressive

Surface of Erosion (TSE) cal erosion of the sands (ravinement) at the onset of ris-

ing sea level and beginning of a retrogradational facies

that starts with initial substrate erosion: the Transgressive

Surface of Erosion (TSE).

The oldest Sequence Boundary (SB1) identified in the well

field was dated 20.4 Ma. The surface represent a substan-

tial erosional surface defined before the MFS of 19.4 Ma

(Figures 14-19). SB1 is overlain in the down dip section

by a relatively thick and sharp-based sand unit identified

as incised valley fills (Basin Floor Fans) and in the up dip

areas by sharp-top facies of the uppermost prograding Hi-

ghstand parasequence (Figure 19). The thickness of the

sand units overlying SB1 in the down dip section of the

Well Field, however, varied from well to well due to lo-

SB2 and SB3 are dated 17.7 Ma and 16.7 Ma respecti-

vely, based on their relative pos itions in the stratigraphic

sections and with reference to the Niger Delta Chrono-

stratigraphic Chart [14].

Identified Transgressive Surfaces of Erosion (TSE1 to

3) lie close to the SBs marking abrupt changes from pro-

gradational facies to retrogradational facies and substan-

tially caused diminution of sand thickness deposited dur-

ing sea level fall (Figures 14-19).

S. O. ONYEKURU ET AL.

252

4.3. Stratigraphic Sequences and

Systems Tracts

Four (4) depositional sequences (SEQ1, SEQ2, SEQ3 and

SEQ4) and the accompanying systems tracts were inter-

preted and mapped in the “XB Well Field” (Figure 20),

based on log–motif s of the reference w ells (XB-1, XB-2,

XB-3, XB-4 , XB-5 an d XB-6 ) and th e sp atial di stribu tio n

of the recognized constrained surfaces (MFSs and SBs).

SEQ1 and SEQ4 formed the deepest (oldest) and top-

most (youngest) depositional seque nces respecti vely . SEQ1

is an incomplete sequence. It is enveloped on top by the

20.4 Ma SB, which was revealed only in wells XB-3 and

XB-4 that probed deeper stratigraphic sections of the

well field. Accompanying Transgressive Systems Tract

(TST) contained marine shales rich in fauna with minor

san d uni t e nveloped by the 20.7 Ma MFS. The transgress-

sive sand units have been interpreted as shoreface sands

deposited in the shelfal region during rising sea levels.

Highstand Systems Tract (HST) of the sequence, estima-

ted to be about 600 ft was deposited in the Middle Neritic

(MN) setting depicting mainly progradational-aggradatio-

nal stacking patterns.

SEQ2 is approximately 1600 ft thick and is bounded

top and bottom by 17.7 Ma and 20.4 Ma Sequence Bo-

undaries, respectively. The Lowstand Systems Tract (LST)

of this sequence formed thick sand deposits interpreted

as Basin Floor Fans (BFF), deposited in the Outer Neritic

(ON) to Bathyal (BA) depositional settings. The LST was

observed to be barren in faunal contents in most wells

and unconfor mably overlying the 20.4 Ma SB and und er-

lies a TST of about 800ft thick.

SEQ3 overlies the 17.7 Ma SB and is capped by the

16.7 Ma SB. The sequence was identified at the depth of

9200 ft in the down dip wells (XB-6 and XB-7) and from

a depth range of 6000 - 7000 ft in the up dip wells (XB-1,

XB-2 and XB-3). The sequence displayed predominantly

fluvial and tidal processes (progradational stacking pat-

tern) as shown in the parasequence stacking pattern of

the western wells (XB-1, XB-2 and XB-3). LST of this se-

quence contains reworked channel sand deposits which

were more pronounced in the down dip wells.

SEQ4 is the topmost (youngest) sequence in the study

area. It rests unconformably on the 16.7 Ma SB. The se-

quence consists of thick sand units at its base, deposited

during relative sea level lows. The sequence was depos-

ited within the Neritic paleodepositional environment.

The 15.9 Ma MFS was identified in this Sequ ence.

4.4. Well Correlation

Correlation was done using the recogn ized and identified

constrained chronostratigraphic surfaces typified by Ma-

ximum Flooding Surfaces (MFSs) and Sequence Boun-

daries (SBs; Figure 20). Correlation helped to compart-

mentalize the stratigraphic section and showed how the

Figure 20. Cross section of the studied wells showing sequences and correlated surfaces.

S. O. ONYEKURU ET AL. 253

surfaces correlated along dip and strike at certain depths

within the depositional basin, thus depicting basin ge-

ometry and depositional sequences across the well field.

The displayed correlation panel (Figure 20) indicates

that the stratigraphic column appears to be dipping in a

N-S direction and striking in the NW–SE direction. De-

position tends to be thicker in wells XB-4, XB-5 and XB-

6, which were located down dip. The occurrence of the

identified chronostratigraphic surfaces at different depths

along dip and strike lines in the studied wells shows evi-

dence of faulting in the well field.

5. Discussion

5.1. Depositional Sequence Architecture

Depositional systems in the “XB” Well field comprise

Lowstand Systems Tracts (LSTs), Transgressive Systems

Tracts (TSTs) and Highstand Systems Tracts (HSTs).

The LSTs are represented by coeval facies dominated

by deposition basinward of the shelf-edge during maxi-

mum regression and are characterized by deep-water de-

position from gr avity flows and/or traction processes wi-

thin shelf-edge or canyon-head delta. The sediments as-

sociated with LSTs recognized in the study area are the

Fluvial Channel Sands and Slope Fans (SF).

Fluvial Channel Sands are associated with erosion of

canyons into slopes and incision of fluvial valleys into

the shelf. Siliciclastic sediments sometimes bypassed the

shelf and slope through the valleys and canyons to feed

the Basin Floor Fans (BFF). These turbidites present ex-

cellent reservoir qualities. Channel sands seen in SEQ2

lie unconformably on 20.4 Ma SB i n well s XB -1 (810 0 ft),

XB-2 (9200 ft) and XB -4 (9600 f t). Chan nel sands ob ser-

ved in SEQ3 overlaid the 17.7Ma SB in wells XB-3

(7700 ft), XB-4 (7500 ft) and XB-6 (10,500 ft).

Slope fans are made up of turbidity-levee channels and

overbank deposits. They overlie the Basin Flo or Fans (BFF)

and are downlapped by the overlying Lowstand wedge.

Slope fans identified in wells XB-1, XB-2 and XB-3 oc-

curred at depths 7400 ft, 7500 ft and 8144 ft, r espectiv ely

(Figure 21). The fans were observed to overlie the 17.7

Ma SB in XB-1 and XB-2 Wells and 16.7Ma SB in XB-3

Well.

Figure 21. Distribution of depositional systems in “XB” field.

S. O. ONYEKURU ET AL.

254

. The Slope Fan system is commonly characterized by

crescent log motif in individual levee channel units, thi-

ckening and thinning of individual overbank sands and

fining upwards of individual channel sands from a sharp

base. Slope fans are formed as the rate of eustatic sea le-

vel fall becomes less than the rate of rise associated with

subsidence [24].

5.2. Transgressive Systems Tract (TST)

The Transgressive Systems Tract developed in response

to sea level rise and when sedimentation rate was not

able to keep pace with the rate of sea level rise, thus ma-

rine facies retrograde landward to flood the shelf; deltaic

progradation ceases and much of the sand is trapped up

dip in estuaries. The upper boundary of the TST defines

the MFS. Condensed sections, characterized by faunal

abundance and diversity peaks are developed near this

surface [25]. Transgressive Systems Tracts were charac-

terized by transition from upward shallowing to upward

deepening and transgressive erosional surfaces (TSE) on

the shelf.

The TSTs capping the LST Facies in the studied well

field were observed to be very thick and contained main-

ly marine shales with minor transgressive sands.

5.3. Highstand Systems Tracts (HST)

The rate of sea level rise decreased during the develop-

ment of Highstand Systems Tracts. HSTs are character-

ized by intervals of coarsening and shallowing upwards,

with both fluvial and deltaic sands near the top of the unit

prograding laterally into Neritic shales. In the studied wells,

the in tervals are very thi ck. This ma y be attribu ted to very

high rates of subsidence, high sediment input and insta-

bility similar to sediment pattern in the Gulf Coast [26].

5.4. Reservoir Potential of the XB-Well Field

Six (6) potential reservoirs (R1, R2, R3, R4, R5 and R6)

delineated in the “XB Field” were mainly the channel

sands and shoreface sands of LSTs and HSTs, respect-

tively, that disp layed low Gamma Ray and high Resistiv-

ity values (Figure 22).

Figure 22. Low Gamma ray and high resistivity values of potential reservoirs in “XB”-field.

S. O. ONYEKURU ET AL. 255

5.5. Reservoir Continuity

The lateral continuity of sand bodies determines reser-

voir’s area of coverage and helps to calculate accumula-

ted hydrocarbon volume.

Reservoir R1, was identified within the HST of SEQ1

and traceable to XB-2 and XB-3. It can be mapped by the

20.4Ma SB that caps the sequence. Reservoir R2 is found

within the shoreface facies of the HST of SEQ2. This

reservoir has a lateral continuity extending from well XB-

2 to XB-3 (Figure 22). Reservoir R3 represents the thick

channel sand facies of the LST and is correlated to res-

ervoir R6. Both reservoirs are underlain by the 16.7Ma

SB. Reservoir R4 occurs within the LST of SEQ2 in

XB-5 and XB-6 Wells, while reservoir R5 was identified

in the sandy facies interbedded in the predominantly

shale facies of the TST.

5.6. Reservoir Geometry

It has been noted that known reservoir rocks in the Niger

Delta are Eocen e to Pliocene in age. The y are often stacked

and range in thickness from less than 15 m to 10% being

greater than 45 m thick [2]. The thicker reservoirs likely

represent composite bodies of stacked channels [5]. The

various reservoirs in the wells (Figure 23), which are

stacked channel sands, range between 80 (21 m) to 160

feet (42 m) thick.

Figure 23. Stacked channel re servoir sands and thic knesses in XB-2 and 3 wells.

S. O. ONYEKURU ET AL.

256

Based on reservoir quality and geometry in the “XB”

Field, the most important reservoir types are the point

bars of distributary channels and coastal barrier bars in-

termittently cut by sand-filled channels [6].

5.7. Source Rock Potential

Several thick shale units of the TST identified in the

studied wells were considered potential source rocks for

the hydrocarbons found in the reservoirs in the “XB”

Field.

5.8. Trapping Mechanisms

Most known traps in the Niger Delta basin are structural,

although stratigraphic traps are not uncommon. The struc-

tural traps develo ped during synsedimentary d eformation

of Agbada paralic sequences [2,27]. A variety of struc-

tural trapping elements, including those associated with

simple rollover structures, clay filled channels, structures

with multiple growth faults, structures with antithetic

faults, and collapsed crest structures, have been described

[5]. On the flanks of the delta, stratigraphic traps are like-

ly as important as structural traps [28]. The primary seal

rock in the Niger Delta is the interbedded shale within

the Agbada Formation. The shale provides three types of

seals-clay smears along faults, interbedded sealing units

against which reservoir sands are juxtaposed due to fault-

ing, and vertical seals [5].

Results of well correlation showed that delineated con-

strained surfaces were not laterally continuous. These trun-

cations were inferred to be caused by sydepositional faults

in the field. These faults constitute the major traps for

hydrocarbon accumulation.

Also, the shale of TST and shale units within the HST

could form top and bottom seals for hydrocarbons in the

reservoir sand. The reservoir rocks of the LST and HST

and the seals from prodelta shales of the TST can com-

bined to constitute stratigraphic trap s for hydrocarbon ac-

cumulation in the well field.

6. Summary and Conclusions

Six wells whose petrophysical well logs and biostrati-

graphic data were made available for a sequence strati-

graphic study, provided a rare opportunity to interpret de-

positional facies and systems of the “XB” field located

within the central Swamp depobelt, Niger Delta basin.

Analysis of the vertical succession of depositional fa-

cies revealed four third order depositional sequences of

mid-Miocene in age, bounded chronologically by 20 Ma

SB, 17.7 Ma SB and 16.7 Ma SB (Type 1 Sequence Bo-

undaries).

The depositional sequences experienced major flood-

ing episodes characterized by high faunal population and

diversity. SEQ1, which is the oldest sequence, experien-

ced a transgressive episode marked by 20.7 MFS (Ala-

bamina 2). SEQ2 and SEQ3 experienced other transgres-

sion episodes marked by faunal diversity and abundance

which formed the 19.4 MFS (Ogara shale) and 17.4 MFS

(Unnamed MFS). The youngest sequence, SEQ4 in the

well field experienced maximum flooding event defined

by 15.9 MFS (Chiloguembelina 3).

In each sequence, the lowermost sections were marked

by deposits arising from relatively low sea level, forming

channels and slope complexes. The middle sections were

deposited during a generally high relative sea lev el, while

the uppermost sections were deposited during gradual

drops in relative sea level lowering (Highstand). These

inferred variations in relative sea level defined third order

depositional systems that comprised Lowstand Systems

Tract (LST) at the base of the section, Transgressive Sys-

tems Tract (TST) in the middle of the section and the

Highstand Systems Tract (HST) at the top of each section.

In terms of hydrocarbon exploration, the sand units of

the LST and HST formed the basin floor fans, channel

and shoreface sands of the delta. The high resistivity log

values revealed that they are potential good hydrocarbon

reservoirs. The shales of the TST in which most of the

MFS were delineated could form seals to the reservoir

units. A combination of the reservoir sands of the LST

and HST and the shale units of the TST can form good

stratigraphic traps for hydrocarbon and hence should also

be targeted during hydrocarbon exploration.

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