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Source Rock Potential of the Tarfaya Basin SW Morocco

Abstract

The Tarfaya Basin located in southwestern Morocco is one of the mid Atlantic potential petroleum basins and has attracted many researchers and companies to investigate the conventional and nonconventional petroleum resources. This research demonstrates the source rock potential in the pre- and post-rift successions. Geochemical evaluations of three onshore wells and outcrop samples prove good to excellent petroleum source rock potential within the late Albian, Upper Cretaceous and Eocene successions. On the other hand, outcrop samples within the Ordovician, Devonian, Carboniferous and Lower Cretaceous except for the Albian have poor source rock potential. The samples were assessed for source rock quality and thermal maturity by elemental analysis, Rock-Eval pyrolysis and microscopic methods. Furthermore, selected samples were studied by gas chromatography-mass spectrometry in order to obtain information on kerogen type and depositional environment. All of the samples are thermally immature based on microscopic and pyrolysis results. The samples from the Ordovician, Devonian, Carboniferous and Lower Cretaceous have TOC values of less than 0.50 wt.-%. In contrast, the late Albian shows excellent source rock quality in core samples taken close to the Atlantic coast with TOC ranging from 2.12 to 4.37 wt.-% and HI exceeding 410 mgHC/gTOC. Microscopic analysis demonstrates dominance of alginite (phytoplankton-derived) kerogen and lack of vitrinite indicating a marine depositional environment. On the other hand, outcrop samples collected from the eastern part of the basin show poor source rock potential for the Albian. The difference is explained by changes in the depositional environment from marine to non-marine to the eastern hinterland. Moreover, the organic matter-rich Upper Albian beds might correlate with the oceanic anoxic event (OAE 1c) that occurs at this age.

Cenomanian outcrop samples show variable petroleum source rock potential. Close to the coastline, the Cenomanian is rich in marine organic matter with TOC contents averaging 4 wt.% and excellent kerogen quality attested by average HI values of 600 mgHC/g TOC. Furthermore, the samples are poor in vitrinite and rich in liptintie which suggests excellent oil prone source rocks. The overlying Upper Cenomanian and the Lower Turonian witnessed one of the most significant Oceanic Anoxic Events in the Earth history (OAE 2). The inland outcrop samples from the Cenomanian/Turonian boundary are of terrestrial or lagoonal environment, whereas the well samples close to the coast represent fully marine conditions. These well samples are characterized by high TOC and HI values that average 8 wt% TOC and 725 mgHC/gTOC, respectively. The organic richness fluctuates between 1 wt % and more than 6 wt% during the late Turonian. The high HI values indicate oil prone source rocks. The Coniacian and Santonian intervals partly represent the agewise less well defined OAE 3 and are also rich in TOC having high HI values. Several outcrop samples from the Eocene show variable source rock qualities. The TOC ranges from 0.05 to 7.20 wt% and HI varies from 89 to 694 mgHC/gTOC.

Kinetic experiments on some of the Cretaceous samples indicate a temperature of an onset of petroleum generation between 101-115 °C. Therefore, theses samples if buried at approximate depth of 4000 m will start to generate oil assuming a geothermal gradient of 25 to 30 °C/km. The results suggest that there is a great oil source potential for the late Albian to the Eocene especially in the coastal and offshore areas. In the coastal areas, maturity is not sufficient for petroleum generation, but the thick sediments might act as oil shales for retorting. In some offshore areas, sufficient temperature and maturity might be reached for petroleum generation. Furthermore, the results provide important information on depositional environment and oceanic anoxic events during the Cretaceous.