STRUCTURAL STYLES AND BASEMENT ARCHITECTURE OF THE DAHOMEY BASIN FROM GEOPHYSICAL DATA

Okoro, E.M. and Onuoha, K.M.Department of GeologyUniversity of Nigeria Nsukka

PRESENTATION OUTLINE

Introduction

Aim/Objectives of Study

Location of Study Area

Geologic Framework

Dataset and Methodology

Results and Discussion

Concluding Remarks

INTRODUCTIONThe Nigerian sector of the Dahomey Basin was until recently one of the under‐explored and poorlyunderstood Cretaceous frontier basins in the country.

For many years knowledge of the geology of the basin came primarily from boreholes drilled to thebasement and from new road cuts.

However, the integration of geophysical data with available subsurface information from shallowand deeper wells drilled for petroleum exploration is helping to throw more light on basinmorphology and structure.

The present paper provides information on structural styles and basement architecture in thebasin on the basis of aeromagnetic data coupled with results from the analysis of publiclyavailable gravity and seismic data.

The study area lie between Latitude 60 30' and 70 00' N and Longitude 20 50' and 30 30' E in theSouthwestern part of Nigeria

AIM/OBJECTIVES OF STUDY

Aim

Map the structural features and basement morphology, and estimate depth to

magnetic sources from the available geophysical data.

Objectives

Analyze structural features including faults and other discontinuities

Define the basement morphology and architecture

Determine the depth to magnetic basement in the study area

Estimate sedimentary thickness in the study area

LOCATION OF STUDY AREA

GEOLOGIC FRAMEWORK

Fig. 2. Geological Framework of the Dahomey Basin (Modified after Oladele et al., 2015)

NIGER DELTABASINGULF OF GUINEA

NIGERIA

BENINREPUBLIC

TOGO

GHANA

XX

XX

XX X

X

X

X

X

X

X

X

X

X

XX Precambrian ‐ Paleozoic

Upper Cretaceous

Paleocene

Eeocene

Neogene

Quartenary

LEGEND

0 100km

X

X

X

Legend

AgeInterpretation

Alluvium

Ilaro Fm

Oshosun Fm

Abeokuta Fm

Basement

Coastal Plain Sands(Benin Fm )

Ewekoro Fm

Recent

Oligocene ‐ Recent

Eocene 

Eocene 

Paleocene 

Cretaceous 

Precambrian 

Fig. 3. Geological  Map of Ogun State showing the study area(Modified from Olowofela et al., 2012)

GEOLOGIC FRAMEWORK

7000

’N70

30’N

7000

’N70

30’N

30 30’E 40 00’E

30 30’E 40 00’E

0 40 80km

N

Lagos State

Oyo State

OndoState

• Idogo• Ilaro

Abeokuta

Fig. 4: (A) The free air (offshore) and Bouguer(onshore) gravity field of Nigeria

(B) The Total magnetic intensity (TMI) field response(Fairhead et al., 2012)

(C) Index Map of Nigeria from NGSA showing theHRAM sheet used in this study (red box; the yellowbox covers the full area of interest for this study).

GEOLOGIC FRAMEWORK

C

DATASET AND METHODOLOGY

Dataset

HRAM data sheets 278 & 279; Residual gravity & SPI maps, Seismic section (from desk studies)

Workflow

HRAM Data

Residual gravity& SPI Maps

Seismic section

Data Enhancement  ‐RTP, 1VD, 2VD 1HD, 2HD, AS, 

Euler Deconvolution

Structural Analysis

RESULTS AND DISCUSSION

5000 0 5000 10000 15000

(meters)WGS 84 / UTM zone 32N

Total Magnetic Intensity Map(TMI)

-5.29.8

17.324.429.733.035.838.039.941.342.944.345.847.348.649.850.550.951.351.752.252.753.253.854.455.055.856.657.558.559.961.563.465.668.573.480.393.3

nT

7200

0073

0000

7400

0075

0000

7600

0077

0000

7800

00

720000730000

740000750000

760000770000

780000

-190000 -180000 -170000 -160000 -150000 -140000 -130000 -120000 -110000

-190000 -180000 -170000 -160000 -150000 -140000 -130000 -120000 -110000

6°30

'6°

40'

6°50

'7°

00'

6°30'6°40'

6°50'7°00'

2°50' 3°00' 3°10' 3°20' 3°30'

2°50' 3°00' 3°10' 3°20' 3°30'

0 1 20

3 0

40

06

606 0

06

08

0

0

00

05

0 5

0 5

100

Fig. 5: (A) Total Magnetic Intensity map of the study area (B) Reduction to Pole Color map of the study area

A B

Fig. 6. First Vertical derivative and second vertical derivative of aeromagnetic data highlighting shallow faults and fracture networks in the study area.  

Structural Framework

Fig. 7. First horizontal derivative and second horizontal derivative of aeromagnetic data highlighting discontinuities in the study area.  

Fig. 8. Analytical signal color map showing basement blocks of different magnetizations.

Basement Block Pattern

A

B

E

D

C

Fig. 9. SPI derived depth to magnetic basement imaging the Lagos and Badagry grabens (Oladele and Ayolabi, 2014).  

Fig. 10. Residual gravity map of the Dahomey Basin (Oladele and Ayolabi, 2014)

Fig. 11. Geological model along W – E profile in Figure 9 showing basement block pattern (horst and graben) of the Dahomey Basin (After Oladele and Ayolabi, 2014).

Fig. 12. Fracture zone at the Okitipupa Ridge/Benin Hinge Line 

Source: Lekoil Technical Report on Block OPL‐325, Niger Delta 

SENW

Fig. 13. (A) Map of the aeromagnetic residualshowing the location of the modeled cross‐section (B) Geological model along NW ‐ SEtraverse across the shoreline into the AtlanticOcean (After Oladele et al., 2016).

Fig. 14. Composite section showing offshore and onshore seismic data (After Tyrrell et al., 2017).

Half graben

Basement

5000 0 5000 10000 15000

(meters)WGS 84 / UTM zone 32N

EULER DECONVOLUTION MAPSI= 0

(m)Depth

< 500500 - 1000

1000 - 1500> 1500

7200

0073

0000

7400

0075

0000

7600

0077

0000

7800

00

720000730000

740000750000

760000770000

780000

-190000 -180000 -170000 -160000 -150000 -140000 -130000 -120000 -110000

-190000 -180000 -170000 -160000 -150000 -140000 -130000 -120000 -110000

6°30

'6°

40'

6°50

'7°

00'

6°30'6°40'

6°50'7°00'

2°50' 3°00' 3°10' 3°20' 3°30'

2°50' 3°00' 3°10' 3°20' 3°30'

0 1 20

3 0

4 0

06

60

6 0

06

08

0

0

00

05

05

0 5

100

Depth to Magnetic Sources

5000 0 5000 10000 15000

(meters)WGS 84 / UTM zone 32N

EULER DECONVOLUTION MAPSI= 1

(m)Depth

< 500500 - 1000

1000 - 1500> 1500

7200

0073

0000

7400

0075

0000

7600

0077

0000

7800

00

720000730000

740000750000

760000770000

780000

-190000 -180000 -170000 -160000 -150000 -140000 -130000 -120000 -110000

-190000 -180000 -170000 -160000 -150000 -140000 -130000 -120000 -110000

6°30

'6°

40'

6°50

'7°

00'

6°30'6°40'

6°50'7°00'

2°50' 3°00' 3°10' 3°20' 3°30'

2°50' 3°00' 3°10' 3°20' 3°30'

0 1 20

3 0

40

06

60

6 0

06

08

0

0

00

05

05

0 5

100

Fig. 15. Euler Deconvolution maps of the study area for structural indices of 0 (A) and 1 (B).

A B

CONCLUDING REMARKS

Geologic imprints on the underlying basement rocks in the study area have been investigated using aeromagnetic data,

integrated with gravity and seismic data from public sources

Information on the structural styles and basement architecture have been provided with the help of vertical and

horizontal derivatives and analytical signal processing while the depth to various magnetic sources were predicted using

the standard Euler method

Faults and fracture networks in the study area were trending NE ‐ SW, NW ‐ SE and E – W

Depth to magnetic sources in the study area ranges between 0.5km to greater than 1.5km, suggesting low thickness

sediments in this part of Dahomey Basin

However, recent studies have shown that depth to basement varied from 0.18km in the onshore areas to 9km offshore .

A maximum depth of 6.38km (Oladele et al., 2015), 6.03km (Opara, 2011), 4km Oladele et al. (2016) and 3.5km (Kaki et

al. (2013) to the magnetic basement have also been predicted

Interpretation of the analytical signal map indicated high susceptibility basement uplift and low susceptibility

sediment filled depression, reflecting the rugose nature of the underlying basement

This suggests a horst and graben architecture on the basement of the study area

Successful exploration program in the Dahomey Basin requires the integration of data sources, coupled with

sound knowledge of the subsurface geological complexities

CONCLUDING REMARKS

NAPE: Opportunity

PTDF: Sponsorship

NGSA: Provision of Research Dataset

Geosoft: Software

Basin Analysis Reservoir Geology‐RG: Technical Support

Department of Geology UNN: Favorable Disposition

ACKNOWLEDGEMENT

Thanks for Listening…

Cretaceous in Nigeria Workshop – Abuja 2019