La serie bituminosa de Puertollano: evolución petrográfica y geoquímica

The Puerto/fano oil shale field comprises three oil shale beds which differ significantly in grade. Petrographically they are dominated by autochthonous material (alginite) which in the two /ower bed is found as telalginite (Botryococcus) and lamalginite, whereas in the upper bed only lamalginite occurs. Despite these differences, the elemental composition of the three iso/ated kerogens and their therma/ behaviour are very similar, indicating that both telalginite and lamalginite have a similar chemica/ composition, and may represent two physiological states of the same alga Botryococcus. The petrographical and chemical characteristics of the Puerto/fano oil sha/es suggest a deposition in a /ake with oxygen depleted bottom waters where the contribution of terrestrial derived material was relatively Jow whereas the bacteria/ activity, as indicated by the abundance of hopanes, and hopane derived compounds and the abundance of arborane/fernane derivatives was very important. Despite the short vertical distance between the three oil sha/e beds, several maturity ratios calcufated from the n-alkanes, acyclic isoprenoids and hopanes indicate an increase in thermal maturity from top to bottom and place the Puertollano oil shales at the begining of the oil window.