Introduction

The Chad Basin Nigeria in northeastern Nigeria is part of the Chad Basin, which extends into parts of Niger, Chad, Central African Republic, and Cameroon (Avbovbo et al. 1986). For almost three decades from now researchers and scholars on petroleum potentials in Nigeria have concentrated in the South (Niger Delta and Anambra Basins), and recently in Benue trough, while little geophysical and geological works have been carried out in the Nigerian Chad Basin in the north. The Nigerian Government has on its own demonstrated renewed efforts in the search for hydrocarbon accumulation in the basin in order to increase its oil and gas reserve base. This has, however, not been successful as no oil or gas has been discovered in a commercial quantity in the region.

The subject of integrated geophysical survey has received considerable attention in the technical literatures over the past 40 years. In petroleum exploration the combination of gravity and magnetic methods for reconnaissance survey is well established. The choice of the method for a geophysical survey is guided by a number of considerations such as the objective of the survey, the geology and topography of the area to be studied (Parasnis 1986).

To meet the objectives of this study, integration of heat flow, gravity, seismic reflection and depth to basement data of the Chad Basin were carried out. Heat flow data contribute towards better understanding of geologic processes like rifting, plateau uplift and mountain building as well as providing the necessary conditions for analysis of hydrocarbon maturation in sediments. In sedimentary basin, the density contrasts between the unconsolidated overburden and basement rocks can be used to determine the thickness of the sedimentary basin or the depths to the basement rocks. The thickness of a sedimentary basin has a bearing on its hydrocarbon potential. Moreover, the structural and depositional styles within a basin can be obtained from their seismic manifestations and spatial distribution.

Origin, evolution and depositional history of Chad Basin

The Chad Basin lies within a vast area of Central and West Africa at an elevation of between 200 and 500 m above sea level and covering ~230,000 km2 (Fig. 1). The basin has been referred to as an interior sag basin (Kingston et al. 1983), due to a sagging episode that has affected it before the onset of continental separation during which a rift system junction was formed providing appropriate site for sedimentation. It, therefore, lies at the junction of basins (comprising the West African rift) which becomes active in the early Cretaceous when Gondwana started to split up into component plates (Carter et al. 1963).

Fig. 1
figure 1

Geologic map of Nigeria showing the Chad Basin (adapted from Okpikoro and Olorunniwo 2010)

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Stratigraphic descriptions of the Southern Chad Basin (Nigerian sector) are available in Carter et al. (1963), Okosun (2000) and Petters and Ekweozor (1982). Sediments are mainly continental, sparsely fossiliferous, poorly sorted, and medium to coarse-grained, feldspathic sandstones called the Bima Sandstone. A transitional calcareous deposit—Gongila Formation that accompanied the onset of marine incursions into the basin, overlies the Bima Sandstones. These are overlain unconformably by graptolitic Keri–Keri Shales while the Chad Formation overlies it to the Earth’s surface.

Genik (1992) presented a model for the regional framework and tectonic evolution of the Cretaceous-Paleogene rift basins of Niger, Chad and the Central African Republic. Both geophysical and geological interpretations of data suggest a complex series of Cretaceous grabens extending from the Benue Trough to the southwest. These data imply similar tectonic origin involving crustal thinning within and beneath the grabens and the near-surface presence of igneous intrusions within the horst/graben structures overlain by a relatively thick succession of sedimentary materials.

Materials and methods

The gravity data used for this study was modified from Cratchley as cited by Avbovbo et al. (1986). The data were obtained with the aid of a Worden prospecting gravity meter. The Bouguer gravity anomaly map is shown in Fig. 2. A qualitative interpretation of the gravity map was carried out for the study area, and result was correlated with heat flow, depth to basement and seismic data.

Fig. 2
figure 2

Bouger gravity anomaly map of Chad Basin (modified from Cratchley 1960) showing Maiduguri (Chad Basin) depression

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The heat flow data were obtained for 14 oil wells in the Chad Basin from bottom-hole temperature data (Nwankwo et al. 2009; Nwankwo and Ekine 2009). The heat flows were calculated, after correcting for drilling effects in the well-logged bottom-hole temperatures, using the Fourier one-dimensional heat flow equation

$$ Q = K\frac{{{\text{d}}T}}{{{\text{d}}z}} $$
(1)

Where \( \frac{{{\text{d}}T}}{{{\text{d}}z}} \) = geothermal gradient and K = thermal conductivity.

The thermal conductivity was calculated for each of the stratigraphic units for all the wells in the basin and an average values were imputed into the steady state conduction Eq. (1) to compute the heat flow values for 14 of the Wells in the basin (Table 1). The heat flow variation and the reconstructed heat flow map are shown in Figs. 3 and 4, respectively, while the depth to basement map is shown in Fig. 5. Depth to basement map of the basin was constructed from the oil wells that were drilled to basement. The seismic reflection data were obtained from the reported work of other researchers (Avbovbo et al. 1986; Okpikoro and Olorunniwo 2010). A sample of the structural interpreted seismic sections from the Chad Basin is shown in Fig. 6.

Table 1 Estimated heat flow and thermal conductivity values for wells in Chad Basin
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Fig. 3
figure 3

Heat flow distribution for wells in the Chad Basin Nigeria

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Fig. 4
figure 4

Heat flow contour map of the Chad Basin Nigeria

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Fig. 5
figure 5

Depth to basement map of Chad Basin wells in meters

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Fig. 6
figure 6

Schematic illustration of common structural traps in Chad Basin Nigeria (adapted from Okpikoro and Olorunniwo 2010)

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Results and discussion

The Bouguer gravity anomaly of the Chad Basin is negative (Fig. 2), with values ranging from −50 to −10 mgal. The gravity low is ovoid shape and trends northeast–southwest of the basin. The gravity contour map clearly reflects the general geologic structure of the subsurface. Gravity lows (−10 to −25) indicate basement uprisings while the very low gravity values (−25 to −50) are caused by a northeast southwest trending elongated depression. The general structures can be ascribed to synform structure, and to a graben. The compactness of the contours within the southern part may be related to the presence of faults within the basin. These faults have a northeast–southwest trend.

The undulation of the Bouguer gravity map may also be attributed to the presence of intrusive rocks, buried bills and basement horsts. Furthermore, the undulation of contours can also be related to the presence of folds in the sediments. The amplitude of the folds increases toward southeast and decreases northwest ward. Many normal faults cut across the folds in various directions.

The heat flow values vary from 63.63 for Masu-1 well to 105.62 m Wm−2 for Tuma-1 well. An average heat flow density of 80.6 m Wm−2 was computed for the Chad Basin. There is no distinct trend in the heat flow within the basin, although the values are relatively higher at the southwestern and northeastern axis of the basin (Fig. 3). The heat flow contour in these two areas is closely packed.

The structure in the seismic reflection data consists of faults and simple symmetrical folds, most of which have a dominant northeast–southwest trend. The fault-related features in the Chad Basin have risen from two main causes viz: those due to tensional stress which include normal faults and grabens and those due to underthrusting magmatic intrusive which includes reverse faults and horst that are closely associated with symmetrical folds. Normal faults that resulted from the tensional stress in the northwest and southeast are more abundant than reverse faults.

Comparison of the Bouguer gravity anomaly, heat flow, depth to basement maps and seismic reflection data shows that the areas with high heat flows are associated with low negative gravity values (−10 to −25). This may be associated with buried hills, crest of folds, low sedimentary thickness or depth to basement rocks. Those areas with low heat flow and very low gravity values (−30 to −50) are related to high sedimentary thickness, graben, and trough of the undulating folds (Fig. 4).

The computed heat flow values for all the oil wells in the basin suggest that its sediments are thermally mature enough to generate hydrocarbons. Moreover, gravity and depth to basement map reveal good sediment thickness for formations in the basin. The fault-related structures identified from seismic data have steep gradient planes which may not seal and can thus be multidirectional pathways for petroleum migration which eventually disseminated in the formations. Ofoegbu (1985) in his study of the magnetic feature of the nearby Benue Trough, observed a magnetic anomaly on a dyke-like body. Such anomalies are best accounted for to a great extent by the presence of igneous intrusive bodies of variable depth and occurring within the basement of sedimentary rocks. These intrusive which must have extended to the Chad Basin have adverse effects on the preservation of petroleum within the basin.

Conclusion

The results of the study show that areas of high heat flow (>80 m Wm−2) are associated with negative Bouguer gravity anomaly (−10 to −25 mgal) and low depth to basement rock (<762 m) while areas of low heat flow (<80 m Wm−2) are associated with very high negative Bouguer gravity anomaly (−30 to −25 mgal) and high depth to basement rocks. This area of high sediment thickness and high heat flow should be the target for further exploration. From the seismic interpretation, the graben in the basin is associated with low heat flow and very low negative Bouguer gravity anomalies while the closely packed contours of the heat flow and gravity maps are attributed to the presence of faults and undulating folds.

The thickness of stratigraphic units, presence of source and reservoir rocks, structural related traps and heat flow values, as observed in this study are favorable criteria for high petroleum prospect. However, if the fault planes are open they unhinder conduits for petroleum migration. The intrusive could also pose severe effects on petroleum accumulation.