La serie bituminosa de Puertollano: evolución petrográfica y geoquímica

Abstract

The Puertollano 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 lower 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 isolated kerogens and their thermal behaviour are very similar, indicating that both telalginite and lamalginite have a similar chemical com position, and may represent two physiological states of the same alga Botryococcus . The petrographical and chemical characteristics of the Puerto Llano oil shales suggest a deposition in a lake with oxygen depleted bottom waters where the contribution of terrestrial derived material was relatively low whereas the bacterial 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 shale beds, several maturity ratios calculated 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