Hidden Tar Sands of Nigeria Mapped With Electricity and Seismic Waves

Beneath the rolling terrain of Ondo State in southwestern Nigeria lies one of the largest unconventional hydrocarbon resources on the African continent: vast deposits of oil sand, the tar-like mixture of bitumen, sand, water, and clay that Canada built an entire industry upon. A new open-access study in the journal Discover Geoscience has now delivered the most detailed picture yet of these deposits in the Eastern Dahomey Basin, combining two complementary geophysical imaging techniques with laboratory sedimentology to map where the bitumen lies, how thick it is, and whether it could ever be worth extracting. The answer, according to the research team led by Sofiat A. Adekoya of Olabisi Onabanjo University, is cautiously encouraging: shallow, laterally continuous, and geologically favorable.

The study is notable less for any single measurement than for its integrated design. Previous investigations of the Nigerian tar sands, which stretch across the coastal Dahomey Basin into Togo, Benin, and southeastern Ghana, relied overwhelmingly on a single geophysical method, usually electrical resistivity. That approach works because bitumen is electrically resistive: hydrocarbons displace conductive pore water, so bitumen-saturated sands light up as high-resistivity anomalies. But resistivity alone is notoriously ambiguous. Dry sand, laterite, and other resistive materials can mimic the signature of oil sand, and a single dataset cannot resolve whether a resistive blob is a reservoir or an artifact. The new work tackles that ambiguity head-on by pairing resistivity imaging with seismic refraction tomography, which senses an entirely different physical property: the stiffness of the rock, expressed as the speed at which compressional waves travel through it.

The field campaign covered six traverses, each roughly 157 meters long, using an ABEM SAS1000 resistivity meter with 64 electrodes spaced 2.5 meters apart in a dipole-dipole configuration. Along the very same lines, the team deployed an ABEM Terraloc Mark 8 seismograph with matching 2.5-meter geophone spacing, striking the ground with a 10-kilogram hammer to generate seismic energy. This co-located geometry meant that every resistivity section could be compared directly, meter by meter, with its seismic counterpart. The resistivity data were inverted with a smoothness-constrained least-squares algorithm in RES2DINV over five iterations, converging to a root-mean-square error of about 3.5 percent, while the seismic first arrivals were picked in SeisImager and tomographically inverted in the Plotrefa module.

The electrical results were striking. Across all six profiles, a consistent three-layer pattern emerged: a thin, low-resistivity topsoil, a spectacularly resistive second layer, and locally a deeper clayey sand. The interpreted oil sand horizon showed resistivity values generally exceeding 3000 ohm-meters and locally soaring past one million ohm-meters, far above the 1000 to 100,000 ohm-meter range typically reported for oil sands elsewhere in the basin. On profiles 1 through 3, this resistive unit sits beneath just 1 to 4.5 meters of topsoil; on profiles 4 through 6 it begins at depths of roughly 1.5 to 3.5 meters and reaches thicknesses of about 8 to 12 meters. In several places the resistive body extends beyond the depth of investigation, hinting at a deposit considerably larger than the survey could fully image.

The seismic data told a matching story in a different physical language. Every profile revealed a three-layer velocity structure: near-surface topsoil at around 300 meters per second, an intermediate layer at roughly 1900 to 2300 meters per second interpreted as bitumen-bearing sand, and a basal unit exceeding 2800 to 3000 meters per second corresponding to consolidated, clay-interbedded sand. The intermediate velocities make physical sense. Bitumen stiffens unconsolidated sand relative to loose topsoil but leaves the layer mechanically softer than fully cemented rock, so the oil sand occupies a distinctive velocity window between its neighbors. Crucially, the intermediate-velocity zones aligned laterally with the high-resistivity anomalies on profiles 1, 3, and 4, with profile 4 showing a shallow resistive body coinciding with a seismic layer of about 2100 meters per second and 22 to 26 meters thick. Two independent physical measurements pointing to the same body at the same location sharply reduce the chance that either anomaly is a phantom.

The two methods did not agree perfectly on thickness, and the discrepancy is itself informative. Electrical resistivity tomography suggested oil sand thicknesses of roughly 5 to 30 meters, while seismic refraction yielded 12 to 34 meters for the same horizons. The authors attribute this to the fundamental physics of each technique. Resistivity represents a volumetric average sensitive to porosity, fluid saturation, and clay content, so gradational boundaries and partially saturated sands tend to be smoothed or underestimated. Seismic refraction responds to elastic stiffness and acoustic impedance, making it more sensitive to compaction and mechanical boundaries, and thus more likely to detect deeper, more continuous velocity contrasts. Differences in inversion strategy, smoothness-constrained least squares versus tomographic inversion, add further resolution differences. Rather than undermining the interpretation, the mismatch quantifies the uncertainty inherent in indirect imaging and underscores why no single method should be trusted alone.

To ground the geophysics in actual rock, the team collected three outcrop samples, labeled ON-1 through ON-3, stripped of weathered surfaces and treated with gasoline in the laboratory to dissolve the bitumen away from the sand grains. Sieve analysis showed the cleaned sands to be medium-grained, with mean grain sizes between 1.07 and 1.38 phi, and moderately sorted, with sorting values from 1.207 to 1.342. Skewness and kurtosis statistics revealed nearly symmetrical, leptokurtic distributions in two samples, suggesting a predominantly unimodal sediment source, while one sample was strongly fine-skewed and mesokurtic, hinting at mixed sediment supply. Moderate sorting of medium sand is exactly the texture that favors porosity and permeability, which the authors interpret as indicating moderate reservoir quality, adequate storage capacity though potentially heterogeneous flow behavior.

Heavy mineral analysis added a provenance detective story. Under the microscope, the samples yielded assemblages dominated by zircon, rutile, and tourmaline, the three ultra-stable heavy minerals that survive weathering and transport when almost everything else has been destroyed. Zircon was the most abundant, averaging 17.67 grains per sample, followed by rutile at 21.33 and tourmaline at 16.0, with significant contributions from staurolite, garnet, apatite, and epidote. The resulting Zircon-Tourmaline-Rutile index ranged from 57 to 67 percent, placing the sediments in the sub-mature category, above the 50 percent threshold for immaturity but below the 76 percent that marks fully mature, extensively recycled sand. That combination points to a proximal source in felsic igneous and metamorphic rocks, most plausibly the Precambrian Basement Complex adjacent to the basin, with only moderate transport distances and limited reworking. Garnet, staurolite, and epidote are diagnostic of medium- to high-grade metamorphic terrains, reinforcing the interpretation.

Assembled together, the evidence sketches a coherent geological history. The sediments were shed from nearby crystalline highlands, carried moderate distances by rivers, and deposited in a fluvio-marine transitional environment consistent with the established setting of the Late Cretaceous Afowo Formation, whose sandy strata are known to be tar-bearing and whose interbedded shales are rich in organic matter. The basin itself was born in the Cretaceous when Africa and South America rifted apart, and its eastern arm holds more than 2500 meters of Cretaceous and younger sediment above the metamorphic basement. The oil sand of the Afowo Formation sits within this fill, and the new surveys show it occurring at depths of just 0 to 3 meters, thick enough to reach roughly 30 meters, laterally continuous across all six profiles, and with the low clay content and favorable grain size that reduce processing complexity and enhance recovery efficiency.

The authors are careful to flag the limits of their work. All geophysical interpretations remain indirect, and without borehole cores or laboratory petrophysical measurements, the lithological assignments are indicative rather than definitive. The sedimentological sample set of just three outcrops cannot capture the full spatial variability of the deposit, and questions of lateral heterogeneity and fluid saturation remain open. The team calls for borehole validation, three-dimensional geophysical imaging, and expanded sampling as the next steps. Even so, the study makes a compelling methodological argument: when electrical and elastic images of the subsurface are acquired along the same lines and interpreted together with the mineralogy of the rocks themselves, the ambiguity that has long hampered single-method exploration largely dissolves. For a country whose conventional oil reserves are gradually declining, and whose tar sands have been estimated among the largest in the world, that integrated framework may prove to be the most valuable discovery of all.

Subject of Research: Integrated geophysical and sedimentological characterization of oil sand deposits in the Eastern Dahomey Basin, Nigeria

Article Title: Integrated geophysical and geological characterization of oil sand in Eastern Dahomey Basin Southwestern, Nigeria

Article References: Adekoya, S. A., Coker, J. O., Ikhane, P. R., Oladunjoye, H. T., & Adenuga, O. A. (2026). Integrated geophysical and geological characterization of oil sand in Eastern Dahomey Basin Southwestern, Nigeria. Discover Geoscience, 4(1), Article 340. https://doi.org/10.1007/s44288-026-00633-z

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00633-z

Keywords: oil sand, bitumen, Dahomey Basin, Nigeria, electrical resistivity tomography, seismic refraction tomography, sedimentology, heavy minerals, ZTR index, reservoir characterization, geophysics, Afowo Formation