Twelve crude oil samples from eight Tertiary reservoir units in Trinidad were analysed by using gas chromatography combined with mass spectrometry (GC-MS)) to achieve their geochemical characterization. The specific objectives were to establish genetic relationships between the oils; determine their maturity and source lithology; and unveil the environment of deposition and extent of biodegradation. The biomarker characteristics established is an indication that the oils were generated from open marine source rocks deposited in a reducing environment. This is consequent upon, the moderate pristane/phytane ratio (1.0- 2.0); carbon preference index (CPI) and odd over even preference (OEP) close to 1; low dibenzothiophene/phenanthrene ratios; absence of 18α(H)-oleanane and; abundance of tricyclic terpanes. The oils are also characterised by high relative abundances of regular steranes, diasteranae and hopanes compared to tricyclic terpanes and correlation between the set of analysed oils also made possible. Overall, a single genetic type was established for the oils based on aliphatic and aromatic hydrocarbon fractions contained. Biodegradation was clearly evident in the majority of samples analysed as demonstrated by the presence of unresolved complex mixture and high Pr/nC17 and Ph/nC18 values. Thermal maturity assessment based on saturated and aromatic hydrocarbon maturity parameters showed that the oils are at least marginally mature and corresponds to the early oil generation window.