Organic geochemistry and petrology of a lower Jurassic (Pliensbachian) petroleum source rock from Aït Moussa, Middle Atlas, Morocco

Abstract

Organic-rich silty marls and limestones (Pliensbachian to earliest Toarcian) exposed at Aït Moussa in Boulemane Province are the only known example of an effective petroleum source rock in the Middle Atlas of Morocco. In this study, petrological and organic-geochemical analyses (vitrinite reflectance measurements, Rock-Eval pyrolysis, GC-MS) were carried out in order to evaluate the maturity, quality and quantity of the organic matter (OM) and to investigate the depositional environment of these source rocks. Results indicate the presence of Type I/II kerogen which was deposited under marine conditions with an input of predominantly algal-derived organic matter. The presence of woody particles indicates minor input of terrestrial material. Organic-geochemical and biomarker analyses are consistent with deposition of carbonate-rich sediments under oxygen-depleted but not anoxic conditions. In terms of thermal maturity, the sediments have reached the oil window (0.5-0.6 %VRt) but not peak oil generation, although petroleum generation and migration are indicated by organic geochemical and microscopic evidence. Kinetic parameters derived from an investigation of petroleum generation characteristics show that the kerogen decomposes within a narrow temperature interval due to the fairly homogenous structure of the algal-derived organic matter. The kinetic parameters together with vitrinite reflectance data were used to construct a ID model of the burial, thermal and maturation history of the Aït Moussa locality. The model suggested that deepest burial (approx. 3200 m) for the Pliensbachian succession took place in the Eocene (approx. 40 Ma). Two phases of hydrocarbon generation occurred, the first in the Late Jurassic/Early Cretaceous (approx. 150 Ma), and the second at the time of deepest burial (Eocene). © 2012 The Authors. Journal of Petroleum Geology © 2012 Scientific Press Ltd.