BELARBI, M., and A. EL OUATAOUI, ONAREP, Rabat, Morocco
The petroleum potential of Paleozoic systems in Morocco is especially related to the Silurian source rock which has always been proved to be of good quality ( TOC exceeds 10 % in some areas). However, the maturation of this source rock showed some anomalies in many areas. This phenomenon has precluded a real assessment of the Paleozoic Petroleum System in many basins. To elucidate this problem, Tadla basin was chosen as a good case study, because it has revealed a good Silurian source rock with maturation anomalies in the well KAT-1, as well as in the surrounding outcrops, which exhibit imbrications of Silurian material.
Geological setting of Tadla Basin
The Tadla basin is a depression within the Hercynian belt, which can be subdivided into three main zones.
1. The internal zone, named locally eastern Meseta, is characterized by a basin infilled with Devonian flysch and Upper Visean-Westphalian volcanic rocks.
2. The transition zone or sheet zone, which encompasses the eastern part of the central block (Central Massif Khenifra-Azrou area) and the Jebilets. In this zone, a basin of Upper Visean turbidites was developed. This was overlain by synsedimentary sheets migrating from the eastern areas.
3. The external zone, extending from the central part of the central block to Paleozoic Rehamna and Jebilets outcrops. Its whole stratigraphic sequence is from Cambrian to Westphalian times. Folding of this Hercynian belt occurred during the Namurian-Westphalian period. The Tadla basin lies between the transition and the external zone. It is bordered at the south by the north flank of the occidental High Atlas (Fig. 1).
This basin is infilled with up to 10,000 metres (32,800 ft) of Paleozoic sediments (which were folded and imbricated during the Hercynian orogeny). These were overlain by Mesozoic-Tertiary formations of 400 metres (1312 ft) thick in the main part of the basin, except at the border of the occidental High Atlas, where the thickness reaches 2,000 metres (6,560 ft).
A multidisciplinary approach, including field observations, well data study and seismic interpretation, allowed the identification of overthrust sheet style in the Tadla basin, so far overlooked.
Geological Survey and Geochemical Analysis
1. Field survey:
In the transition zone previously described, a study was carried out by Huvelin, P. (1970) and Reyberolles, M. et al. (1976) on spectacular outcrops in Khenifra-Azrou area. This study showed a development of sheets during Hercynian tectonics.
Bouabdelli, M. (1988) surveyed a section in Ahallal area (Mrirt sheet). It displays sandy silt series of Ordovician age, structurally underlain by Silurian black shales, and imbricate structures in the Ordovician quartzites with Silurian black shales outcrops, 10 to 15 metres thick, within these quartzites (Fig. 2)
Sampling of Silurian black shales from both the intervals encountered in the section revealed two distinctive maturation rates: (1) 1.27 % Ro for the shales underneath the Ordovician quartzites; and (2) 1.37 % Ro for the shales above the Ordovician imbricates.
Huvelin, P. (1970) observed the same style in the eastern Jebilets.
2. Subsurface Survey:
KAT- I well, drilled in Tadla basin, encountered the sequences of Ordovician, Silurian and Devonian age, unconformably underlain by Silurian formations.
Maturation profile of this well shows the inversion of maturation with depth. Reflectance of Silurian facies beneath the Ordovician sequence is from 0.98 to 1.2 % Ro instead of 1.4 to 1.6 % Ro, determined by extrapolation of the maturation vs. depth curve (fig. 3).
3. Geophysical survey
Tangential thrust sheets, seen in the surrounding areas of Tadla basin, have to be proven by the interpretation of seismic within the basin. These features are supported by KAT-1 well, which encountered the Silurian duplication series.
Unfortunately, seismic lines conducted in this basin are of poor quality, especially at the eastern part of the basin. The lack of good quality is rather due to this complex structural style than to the applied acquisition and processing parameters. From this point of view, any new interpretation of this seismic must take into account the tangential tectonic.
In view of these new ideas, a reinterpretation of the seismic lines was carried out. It showed at the eastern part of the basin (dip lines) the presence of the decollement zone within the Silurian plastic shales. This zone showed reflectors of high energy, separating two other zones of different dipping reflectors, which correspond to tangential structures, so far ignored or underestimated by the previous interpretations.
Seismic interpretation, combined with the study of the outcrops, and the well data, has shed a new light on the understanding of this structural style: overthrust sheet style. Geochemical analysis has confirmed this geometry which prevails in the whole area.
AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England