In this work, the important role of organic petrology in the reconstruction of
the evolution of a paleo-petroleum system, whose source rocks have exhausted their
ability to generate hydrocarbons due to their overmature state, is demonstrated.
Whereas geochemical screening is generally used to characterize immature-to-mature
source rocks, in the case of overmature spent source rocks, organic petrology is used to
determine the type and amount of organic matter initially contained in the rocks. In this
paper, a case study of the Cameros Basin (North-Central Spain) is presented. By means
of vitrinite reflectance measurements, a marked difference in maturity has been
determined throughout the basin. Immature to oil-window thermal conditions were
reached in the southern part of the basin, whereas overmature to dry-gas thermal
conditions were observed in the central and northern areas. In the northern sector the
organic matter shows several thermal alteration textures as a consequence of the
circulation of hydrothermal fluids during the evolution of the basin. In some of the
overmature units, the presence of micrinite residues, framboidal pyrite in the mineral
matrix and a large amount of solid bitumens suggest that these rocks originally
contained abundant organic matter and that hydrocarbons were generated during the
thermal evolution of the basin. These deposits can therefore be considered as the
original source rocks of the Cameros Basin petroleum system. Evidence of the
migration of hydrocarbons is frequently found in the form of fractures that vertically
propagate through the organic matter rich-layers. The hydrocarbon accumulations
formed by these rocks could have given rise to tar sandstone deposits that are located in
the south of the basin.
Keywords: Spent source rocks, Paleo-petroleum system, Thermal modelling,
Overmature, Hydrothermalism.