Abstract
Significant thicknesses of hydrocarbon source rocks (up to 1000 m) were identified in 1–SPS–17, l–SCS–4A, –5 and –6, in the Southern Santos Basin. These marine shales of Cenomanian to Coniacian ages generally contain l–2.5% organic carbon. Pyrolysis maximum potential yields are moderate rather than good, averaging 3–4 kg hydrocarbons/tonne of rock over the hydrocarbon source intervals. Oil will be the principal product, although horizons of gas-prone kerogens are also present.
Palaeotemperature indicators (spore colour and vitrinite reflectance measurements) show that the basin is characterized by relatively low maturation gradients; calculated depths to the oil generation threshold (OGT) range from c. 3600–4500 m. The identified source rocks generally lie within ±500 m of the OGT and the presence of 1000–1500 m of Miocene and later sediments therefore restricts the time available for significant generation and migration to, at most, about 10 Ma, i.e. from late Miocene onwards. However, a variety of basin models predict substantially greater maturities and, in some instances, suggest oil generation commenced in Palaeogene times. Direct measurements of evolved hydrocarbons give support to some of the modelling predictions by apparently demonstrating hydrocarbon evolution at depths less than the palaeotemperature OGTs. The absence of significant hydrocarbon accumulations is probably a function of inadequate source richness and the short time available for generation. Migration from thick shale source beds bounded above and below by non-generative shales may also be a problem.
