A geochemical survey of Brazilian marginal basins using a wide selection of source rocks and oils, ranging from Lower Cretaceous to Tertiary in age, has been undertaken. The aims were to assess the palaeoenvironment of deposition of the source rocks, to correlate reservoired oils with their putative source rocks and to understand the effects of maturity on the composition and absolute concentrations of biological markers. The geochemical analyses included evaluation of organic ca~bon contents, Rock-Eval pyrolysis data, vitrinite reflectance measurements, determination of carbon isotope ratios, elemental and visual kerogen analysis, and molecular studies involving liquid and gas chromatography, qualitative and quantitative biological marker investigations using GC-MS, metastable GC-MS .and GC-MS/MS for saturated and aromatic hydrocarbons, and UV/vis spectrophotometry, probe MS and HPLC for metalloporphyrins. The metastable ion GC-MS data were evaluated using principal component analysis. Integration of the results with geological and paleontological data provides the recognition and differentiation of seven depositional regimes, namely lacustrine freshwater, lacustrine saline water, marine evaporitic, marine carbonate, marine deltaic with carbonate influence, open marine highly anoxic with a predominance of calcareous mUdstone lithology, and open marine anoxic, with predominance of siliciclastic lithology. The analyses of the oils reveal significant differences between groups which enable a correlation with putative source rocks laid down in five of the above depositional regimes: lacustrine freshwater; lacustrine saline water; marine evaporitic: marine carbonate and marine deltaic with carbonate influence. The quantitative approach used to determine the effect of thermal maturity on the composition and concentration of the biological markers shows that care must be taken in their use as maturity indicators, since source input and mineral matrix might play an important role. On the other hand, it also shows that the concentrations decrease considerably between the onset of petroleum generation and its peak. Thus, care must also be exercised when using biological marker concentration in palaeoenvironmental assessment The metalloporphyrins in a selection of organic rich sediments have also been examined. The results suggest that their distributions can be a useful auxiliary tool in the characterisation of depositional environments of petroleum source rocks. Finally, a combined geochemical and micropalaeontological study of Cenomanian to Maastrichtian pelitic sediments from the continental margin extends the occurrence of the recognised Cenomanian-Turonian and santonian "oceanic anoxic events". As an extension, the presence of such events in the Coniacian is reported. In contrast, the Campanian-Maastrichtian appears to be a time interval when deposition under oxygenated conditions produced sediments with low organic carbon contents and poor hydrocarbon source potential. In summary, biological marker characteristics of organicrich sediments can distinguish different types of depositional environments, assessment of from analysis allow oil-source rock correlations, and the the depositional environment of the source rocks only of the oils. Thus, this thesis provides a framework of biological marker characteristics which can be compared with samples from other parts of the world.