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We report the geochemical characterisation of hydrocarbons extracted from surface sediments of the Calabar River and coastal soils, SE Niger Delta, Nigeria using gas chromatography-mass spectrometry (GC-MS). As a result, a special group of organic compounds prevalent in the entire study area was identified. It consists of aliphatic hydrocarbons (7.3-22.2% of the total lipids) with several distinctive chemical features. These include a high abundance of even numbered n-alkanes (n-C(12)-n-C(26), maximising at n-C(18), n-C(20) and n-C(22)), n-alk-1-enes (n-C(14:1)-n-C(26:1), maximising at n-C(18:1) and n-C(20:1)), giving rise to Carbon Preference Indices (CPIs) between 0.15 and 0.82. An unresolved complex mixture (UCM) occurring in the range n-C(18)-n-C(35), and the presence of hopanes indicate petroleum contamination. The predominance of even numbered n-alkanes in the Calabar River sediments are thought to be derived from inputs of different microorganisms inhabiting an oil-polluted environment and contributing to the organic matter (OM). This paper, for the first time, gives an account of the unusual predominance of even numbered n-alkanes/alkenes in surface sediments from the Niger Delta of Nigeria and thus contributes to the information on the rare occurrence of such distributions in the geosphere.