25th International Meeting on Organic Geochemistry IMOG 2011

P-327
Organic geochemistry and organic petrology of Lower Jurassic
source rocks from Aït Moussa, Middle Atlas, Morocco
Victoria Sachse, Detlev Leythaeuser, Jan Schwarzbauer, Ralf Littke
RWTH Aachen University, Institute of Geology and Geochemistry of Petroleum and Coal, Aachen, Germany
(corresponding author:sachse@lek.rwth-aachen.de)
Proven petroleum systems of Morocco occur chiefly in
the Essaouria, Prerif and Rharb Basins. Several
source rocks have been discussed, i.e. Silurian
(eastern Essaouira), Liassic (Prerif), Oxfordian
(western Essaouira), Cretaceous (Ain Hamra) and an
intra-Miocene biogenic source rock system in the
Rharb Basin. The origin of the Cap Juby oil shows in
the offshore Tarfaya basin as well as the oil shows
encountered in Guettata wells in the onshore of
Essaouira Basin are of uncertain origin, with a
Jurassic carbonate source (Toarcian) being currently
the favored interpretation. Due to well known Toarcian
source rock systems worldwide and a similar
depositional history of Toarcian and Pliensbachian
sediments, the main objective of the present study
was to improve the characterization of the petroleum
generation potential of a Pliensbachian source rock in
the Middle Atlas using detailed molecular
geochemical analysis as well as organic petrological
studies.
The locality Aϊt Moussa (Pliensbachian to earliest
Toarcian, Boulemane province, Middle Atlas) is the
only example of an effective petroleum source rock in
the Moroccan Atlas rift basins. To get more
information about its source rock potential, Corg/Ctotalmeasurements,
sulphur measurements (TS), Rock-
Eval pyrolysis, vitrinite reflectance measurements,
fluorescence microscopy and organic-geochemical
analyses were carried out to get basic information on
their thermal maturity, the potential to generate
hydrocarbons and their depositional environment.
Investigations revealed the presence of a type-I/II
kerogen (Fig. 1), which was deposited under marine
conditions with an input of predominantly algalderived
organic matter. The presence of woody
particles indicates a small input of terrestrial organic
matter. The organic-geochemical and biomarker
analyses indicate a deposition of the carbonate-rich
sediments under oxygen-depleted conditions (Fig. 2).
Rock-Eval data as well as biomarker evaluation
revealed that oil window maturity has been reached
but not the peak of oil generation. An initial stage of
petroleum generation and migration is indicated by
geochemical and microscopic evidence. Kinetic
parameters on petroleum generation characteristics
reveal that this kerogen decomposes within a narrow
temperature interval due to a quite homogenous
structure of the algal-derived organic matter. Finally,
the kinetic parameters and the vitrinite reflectance
were used to create a 1D model which gives an
overview about burial-, thermal-, and maturity history
of the area of Aït Moussa as well as timing and
generation potential of petroleum.
Figure 1 Bulk geochemical data of Pliensbachian
samples: Rock-Eval HI and OI values.
Figure 2 Pr/nC17 vs. Ph/nC18 for selected
Pliensbachian samples (Interpretation scheme
according to Shanmungam, 1985).
Shamungam, G., 1985. Significance of coniferous rain
forests and related organic matter in generating
commercial quantities of oil, Gippsland Basin,
Australia. AAPG Bull., 69 (8), 1241-1254.
460