Quantitative Interpretation of Potential Field Data Covering Oml 29 and 11 Areas of Niger Delta, Nigeria

doi:10.21203/rs.3.rs-3399302/v1

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Quantitative Interpretation of Potential Field Data Covering Oml 29 and 11 Areas of Niger Delta, Nigeria

https://doi.org/10.21203/rs.3.rs-3399302/v1

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Hydrocarbon exploration potential of the Niger Delta, Nigeria, without doubt, contributes greatly to the economy of the nation due to its sedimentary thickness. However, the sedimentary thickness of the region has been documented but there is a paucity of information in the usage of the high-resolution aeromagnetic data to determine the sedimentary thickness of Oloibiri (OML 29) and Degema (OML 11) areas. Thus, this study was undertaken to determine the sedimentary thickness of the area using the high-resolution aeromagnetic data acquired from the Nigeria Geological Survey Agency (NGSA). Spectral depth analysis, Standard 3D Euler Deconvolution, and Source Parameter imagine (SPI) techniques were employed to estimate the sedimentary thickness of the area. A true depth of 8.41 km, 7.01 km, and 5.5 km using spectral depth analysis, Euler Deconvolution with a Structural Index (SI) of 1, and Source Parameter Imaging techniques, respectively, were revealed. These values fall within the sedimentary thickness range of the region as the data was employed. Thus, if other conditions are met, the area is plausible for hydrocarbon probing.

OML 29

OML 11

Sedimentary thickness

NGSA

Spectral analysis

SPI and Euler Deconvolution

Over the recent years, the quantitative use of potential field data like the aeromagnetic data as a reconnaissance tool in determining the depth to the basement has gained weight and has drastically improved as various techniques have effectively been developed and applied in different areas of study, with Niger Delta inclusive. The application of any geophysical technique in delineating economic deposits found underneath the earth depends on the parameter of interest that distinguishes them from their surrounding environment.

The magnetic method is a potential field method and is also nondestructive as it uses the geomagnetic field to delineate the subsurface due to differences in susceptibility. The magnetic survey method of subsurface investigation is centered on the magnetic anomalies in the magnetic field of the earth resulting from underlying rocks. The underground sources/rocks show up as anomalies in the form of digitized/gridded colour variations on the magnetic map. According to Biswas and Acharya (2016), the geomagnetic field anomalies are due to induced or remnant magnetism emanating from secondary magnetization prompted in a ferrous body by the earth’s magnetic field. The anomalies' amplitude, shape, and dimension are dependent on the inclination and intensity of the earth’s magnetic field, geometry, size, and depth of causative sources.

The magnetic survey when performed in the air, called aeromagnetic survey, is inexpensive as it requires fewer crew members, secondly, it consumes less time during the survey and covers a large area when compared to the survey conducted on land or in the ocean as land and marine survey respectively. Aeromagnetic data can reliably be used in determining the depth of the basement that underlies sedimentary cover. This quantitative approach reveals the basement morphology.

The region has been known to be the economic hub of Nigeria through the exploration and exploitation of its petroleum resources by geoscientists using different techniques known to them. The majority of the exploration works done in the region have been based on the seismic method. Nevertheless, few researchers like Okiwelu and Ude (2012) used aeromagnetic data of low resolution to map the sedimentary thickness of the reviewed area. There has been very few or no report on the usage of high-resolution aeromagnetic data, acquired between 2005 and 2012 within the study areas of Olobiri ( OML 29) and Degema ( OML 11), to determine the sedimentary thickness. Even if there are very few reports on its usage to estimate sedimentary thickness, the integration of three quantitative techniques has not been recorded within the area. Given this, this study employed spectral depth analysis, 3D Euler Deconvolution, and Source Parameter Imaging (SPI) on the high-resolution aeromagnetic data to determine the sedimentary thickness spanning the study area to compare the results obtained in this work to those done by researchers that used similar or other techniques singly or combined.

Geology Area

The area (Fig. 1) is circumscribed by Port Harcourt towards the east, Penington River in the west, southwards by Bille and Bonny, and Patani and Ahoada in the north. Geographically, the area spans longitude. 6⁰0ⁱ0ⁱⁱ E − 7⁰0ⁱ0ⁱⁱ E and latitude. 4⁰30ⁱ N − 5⁰0ⁱ0ⁱⁱ N and it is about 6050 km^{2. Figure 2} depicts the geologic map.

In Nigeria, many basic depositional cycles are distinguished. These depositional actions led to about eight sedimentary basins being formed in Nigeria where petroleum mining activities are attempted (Tuttle et al., 1999). The basins include Niger Delta Basin, Anambra Basin, Benue Trough, Benin Basin, Bida Basin, Bornu Basin, Dahomey Basin, and Sokoto Basin (Nuhu, 2009; Reijers, 1996).

An elongated rift basin is established within the Delta region, Biafra Bight, and Sao Tome and Principe within the western Nigeria continental margin (Michele et al., 1999). The Province or Basin has suspected contact with Cameroon and Sao Tome. The basin is economically viable because of its quantity of petroleum. The sedimentary thickness varies between 9–12 km (Fatoke, 2010). Various geologic formations that depict how the basin has been formed are evident within the province. The basin falls within the southwesternmost area of Benue Trough that has superior tectonics. The Volcanic Cameroun Line bound the Niger deltas other side (Michele et al., 1999).

Moving away from the South American-African plate guided the creation of Niger Delta. The opening of the South Atlantic also caused the structuring of the basin. Following such separation, several faults were built. Deposited sequentially owing to large transgression is the Akata and Agbada Formations (Fatoke, 2010). The Agbade and Benin Formations were then deposited till today during the Oligocene (Michele et al., 1999).

Three major depobelts mark Niger Delta sedimentary fill. The cycles express the movement of sediments as they change mud grains of deep origin to fluvial denser-sized grains. The changes experienced by the lithologies found within the locality are attributed to many factors. The nature of sediment is a contributing factor. Also, sediment nature is reliant on the environment deposited (Haack et al., 2000).

Three Formations are differentiated on the ratio of sand shale within the province. These formations represent deposited prograding facies. These three Formations (Fig. 3) include the Akata, Agbada, and the Benin Formation (Michele et al., 1999; Fatoke, 2010; Tuttle et al., 2015).

Consisting of thick shales, sands with lesser quantities of clay and silt, and also Paleocene in age is the Akata Formation. A thickness of 7000 meters is affiliated to the Formation.

Dated behind Eocene is the Agbada Formation. The Formation contains freshwater and sea features. The basic oil bearing facies is the Agbada Formation. 3700 meters thickness value is associated with the Agbada Formation. Younger and Oligocene in age is the Benin Formation. It contains deposition of alluvial and plains of continental flood. 2000 meters is the Formation thickness (Short and Stauble,1967).

This research made use of two sheets of geosoft digitized high-resolution aeromagnetic data (Fig. 4) acquired between 2005 and 2010 by Fugro Airborn Survey Limited on behalf of the Nigerian Geological Survey Agency (NGSA). A terrain clearance of 80m, 2km tie line direction of NE-SW and 500m line spacing in NW-SE flight direction. The International Geomagnetic Reference Field (IGRF) model for 2005 was used in removing the geomagnetic gradient which superimposes the effect/anomalies due to subsurface geology sources.

To isolate the effect of deep sources from those shallow sources, regional-residual separation was undertaken by applying the least square polynomial degree of degree one (1) using the X-Utility module of Oasis Montaj software. This art generated the residual and the regional maps. Thereafter, the residual was gridded into eighteen (18) overlapping spectral windows. On each window, the Fast Fourier Transform was performed using the Interactive Fourier (IF) filtering algorithm and this spectrally decomposed the anomalies into its power/energy and wavenumber components. Then a plot of power energy spectrum against wavenumber (Rad/km) was made for each window. On the spectral energy curve, two lines are fitted on the high and low parts of the curve. This gives rise to basement source depths as the slope of the curve is divided by two.

Euler Deconvolution was applied to the aeromagnetic data in the determination of anomalous source positions and depths around the basement. This quantitative technique relies on the Euler equation defined, according to Reid et al. (1990) as:

\(x\frac{\partial g}{\partial x}+y\frac{\partial g}{\partial y}+z \frac{\partial g}{\partial z}+ \eta g= {x}_{0 }\frac{\partial g}{\partial x}+ {y}_{0}\frac{\partial g}{\partial y}+ {z}_{0}\frac{\partial g}{\partial z}+ \eta b\) 3.9 a

Where,

\(\frac{\partial g}{\partial x}, \frac{\partial g}{\partial y}\) , \(\frac{\partial g}{\partial z}\) = x, y, and z gradients

g = perturbing field

b = field base level

\(\eta\) = Structural Index (SI). This was chosen on the bedrock of earlier knowledge of the geometry of the perturbing source. The selection of the structural index is dependent on the yet-to-be-adopted model.

\({x}_{0 }, {y}_{0}, {z}_{0}\) = new source location

The applied Euler technique made use of a 3D Euler algorithm situated on the toolbox. This method determines the depth of data automatically. The 3D Euler technique initiated on the aeromagnetic data mapped magnetic boundaries and also calculated source depths automatically. For this research gradients concerning X, Y, and Z were determined, base level of 1.0 and SI 1.0 were chosen. The structural index was chosen to model a sill/dyke using the aeromagnetic dataset. Depth values peculiar to structural indices were thus calculated.

The Source Parameter Imaging (SPI), which is an automatic process just like the Euler Deconvolution, was employed in calculating source locations and the basement depths of rocks from the gridded aeromagnetic dataset. The Source Parameter Imaging technique permits the appraisal of the appropriate model automatically. The model determined automatically is exhibited on a map.

For this work, the SPI was initiated first on the menu toolbox of Oasis Montaj Software using the GX tool. Thereafter changes in X, Y, and Z of the data were determined. The determined variation aided the creation of SPI image maps.

To accomplish spectral depth analysis, the residual aeromagnetic data (Fig. 5.) were partitioned into separate windows (Fig. 6) as the regional (Fig. 7) was discarded. Some of the plots of the log of power energy spectrum against wavenumber for the various windows are in Fig. 8. From the spectral energy curves, various depth parameters like the D₁ and D₂, are highlighted in Table 1. Figure 9 exemplifies the depth of causative magnetic sources in kilometers and contour format. Standard 3D Euler Deconvolution and Source Parameter Imaging (SPI) depth maps are shown in Fig. 10 and Fig. 11. 3D depth to the basement is depicted in Fig. 12.

In the spectral depth estimate, two models, D₁ and D₂ were delineated. D₁ and D₂ reflect the depths emanating from deep-seated and shallow sources respectively. Around the basement and sediment are deep and shallow causative sources. The grid's average thicknesses were estimated and thus represented with D_a. D₁ and D₂ derived from the steep and less steep change of the spectrum also describe the depth centroid and top boundary depths of sources, respectively. The deeper sources are connected to basement sources while shallow sources are magnetic intrusions. D₁ differs from 2.50 km to 13.89 km, with a true value of about 9.97 km while D₂ lies between 1.05 km and 6.25 km, with a true depth value of 2.25 km. A maximum thickness of 13.89 km is located in window 4 placed inside the northwestern portion. Similarly, a minimum sedimentary thickness value of 2.50 km is still seen inside the southern region. Depth values for windows 1, 4, 13, 15, 16, 17, and 18 were not estimated due to noise. This is validated by the curve nature of the spectral energy.

By applying the regular Euler Deconvolution technique on the aeromagnetic residual data, the depths of various magnetic sources are accentuated. The basement magnetic depth obtained using the structural index of 1.0 varies from 2339.09 m to 11508.86 m, with 7017.14 m average. These magnetic source depth values were remarked into shallow and deeply seated magnetic sources. The shallow seated causative magnetic sources lie between 2339.09 m and 6709.62 m while sources of deep origin have thickness values lying between 7030.96 m to 11508.86 m. Colour variations highlighting the location of sources at distinct depths are observed on the map. Magnetic units with yellow, magenta, and orange colours show deeper. Various contour shapes and forms are patent on the map. Spaced and closely packed elliptical and circular contours are dominant on the map. The spaced magnetic contours indicate sources that are of deeper origin while the packed contours exemplify zones that are of shallower origin. Packed signatures are present within magnetic low regions. Magnetic low zones inside packed contours imply that the bodies within such areas are of superficial origin while magnetic low zones with relatively spaced contours are good reflections of deeper seated bodies.

Table 1 Depth Parameters Deduced from Spectral Analysis of Residual Aeromagnetic Anomalies

WINDOWS	X	Y	SLOPE		DEPTH (km)
			\({S}_{1}\)	\({S}_{2}\)	D₁	D₂
1	545000	176250	-20		-10	-
2	290000	176250	-24.5	-6.0	-12.25	-3.0
3	508750	176250	-25.0	-10.0	-12.5	-5
4	196250	176250	-27.78	-	-13.89	-
5	527500	196250	-26.75	-5.71	-13.38	-2.86
6	507500	507500	-26.67	-10	-13.34	-5
7	301000	215000	-19.33	-7.89	-9.67	-3.95
8	525000	215000	-25.5	-8.73	-12.75	-4.37
9	510000	117000	-25.0	-12.31	-12.5	-6.16
10	547500	232500	-24.5	-5.71	-12.25	-2.86
11	527500	235000	-10	-12.5	-5	-6.25
12	552500	252500	-28.57	-12.5	-14.29	-6.25
13	552500	252500	-14.29	-	-7.15	-
14	527500	252500	-18.7	-2.1	-9.35	-1.05
15	507500	252500	-5	-	-2.5	--
16	507500	252500	20	-	10.0	-
17	548000	272500	7.5	-	-3.75	-
18	527500	272500	10	-	-5	-

The source parameter imaging (SPI) technique exemplifies sources that are spatially and evenly distributed. Generally, magnetic source depths vary from − 9702.05 m to 11852.68 m with a true value of 5450 .65 m. The magnetic units were remarked into two basic magnetic models: deep and shallow models. The deep sources fall within 5077.53 m to 11852.68 while the superficial sources vary between 4693.27 m and − 9702.05 m. The negative values found beside the last magnetic field value show that such a body is of shallower origin and possibly an igneous outcrop while the positive values represent deeper magnetic sources. Ako et al. (2014) stated that such negative depth values expressed in the SPI legend indicate buried source depths which may be deep-seated basement rocks or near surface intrusive while the positive values depict outcropping magnetic bodies. The red, yellow, and magenta colours generally elaborate about points immersed by deep sources while the blue colour pronounces areas of shallow-lying magnetic sources. Depth models obtained when spectral analysis, Euler deconvolution, and Source Parameter Imaging (SPI) methods were employed on the residual aeromagnetic data are almost akin to one another. These three independent techniques gave depth values that are akin to the ones attained by previous researchers who employed the old and new aeromagnetic datasets. The thickest sedimentary cover of about 13 km is existent in window 6. This is at variance with that acquired by Ali et al. (2012). They obtained the maximum thickness of sediment to be 10 km and this perfectly agrees with the thickness of windows 2 and 16. The spectral analysis result, however, shows an unequal distribution of sedimentary thickness values as against the claim by Ako et al. (2014) who stated that the sedimentary thickness increases southward. Oladele and Ojo (2013) estimated the depth to gravity/magnetic sources of parts of Niger Delta, to be 14.1 km. Eke et al. (2016) discriminated the depth of gravity sources using quantitative techniques. They obtained anomalous source depths to generally lie between 2,000 m and 10,600 m. Eke and Okeke (2016) estimated source depths of gravity and magnetic sources by utilizing the least square method. They obtained that depths of sources with density contrast vary from 1,000 m to 3, 500 m while source magnetic depths range from 2,183 m to 4,385 m. Depths to various magnetic sources, covering the lower Imo River basin were determined by Chikwendu et al. (2012). In their study, the shallow depth magnetic layer varies from 0.238 to 0.905 km while the deeper magnetic layer varies from 9.786 km to 14.238 km. To determine sediment thickness within parts of Offshore Niger Delta, Emujakporue and Ofoha (2015), established a thickness of 9466 m via spectral depths estimate of magnetic data.

The basement depth map depicts magnetic bodies found at different depth intervals. Magnetic bodies with low-intensity values have thickness values limiting between 2 and 9 km. Magnetic bodies with such thickness are found around the northern region. Sources with high-intensity values have thickness values that range between 10 and 13.5 km. Figure 12 illustrates the basement shape and size (morphology). The basement slopes from the North West and South West towards the center where possible sediments get trapped and settled. Alternating undulations were not noticed on the basement map due to no high differentiation of the basement. The non-undulating nature of the area signifies that a sedimentary terrain with little or no volcanic rocks. At the edge of the basement map towards the northeastern region, a geologic syncline is evident. This suggests that a trap exists.

The analysis applied quantitatively on the aeromagnetic dataset revealed two source models, D₁ and D₂, using spectral computation, Euler Deconvolution and SPI methods. D₁ is a reflection of sediment thickness values of Olobiri (OML 29) and Degema (OML 11), Nigeria. Standard Euler Deconvolution technique highlighted magnetic depth values varying from 2.3 to 11.5 km, with 7.01 km real value obtained using 1 as the SI. Magnetic source depths vary from − 9702.05 m to 11852.68 m with true a value of 5.5 km as SPI was applied. Thus, true sedimentary values of 8.41 km, 7.01 km, and 5.5 km for various causative magnetic sources using spectral depth analysis, Euler Deconvolution, and Source Parameter Imaging techniques fall within the sedimentary thickness range of the Niger Delta region as the high-resolution aeromagnetic data was employed. This consequently supports the assertion that hydrocarbon is plausible within the region if other conditions are met.

Competing of interests

The authors declare that no competing interest of any kind will the publication of this work.

Acknowledgment

We sincerely acknowledge the Nigerian Geological Survey Agency (NGSA) for providing the data used in this research work

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No competing interests reported.

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Quantitative Interpretation of Potential Field Data Covering Oml 29 and 11 Areas of Niger Delta, Nigeria

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Abstract

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Introduction

Geology Area

Materials and Methods

Results

Discussion

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Competing of interests

Acknowledgment

References

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