Petroleum source rocks of western and central Africa : the examples of the marine Tarfaya Basin, Morocco and the continental Congo Basin, Democratic Republic of Congo

摘要

This study comprises newly collected organic geochemical and petrological data on quality, quantity, maturity as well as the depositional environment of various organic matter-rich stratigraphic units in Morocco and the Central Congo Basin, DRC. In addition, 1D modeling was carried out for one locality in northern Morocco (Aït Moussa) and for two wells from the Central Congo Basin to obtain information on burial thermal and maturation history. For the Aït Moussa model, specific bulk kinetic parameters were used to evaluate the petroleum generation and timing. Paleozoic, Mesozoic and Cenozoic sediments were investigated from the Tarfaya Basin, southern part of Morocco/Western Sahara. Based on Corg measurements, samples with Corg >0.4 %, especially Late Cretaceous and Eocene sedimentswere considered for further geochemical and petrological analysis. Rock-Eval pyrolysis and biomarker assignment indicate a mixture of kerogen type I/II with an excellent petroleum generation potential of the Late Cretaceous and Eocene samples. More detailed analysis of Coniacian, and Late Turonian samples from a newly drilled well (Tarfaya Sondage No.2) in the Tarfaya Basin support the latter data. In contrast to the terrestrial influenced Santonian outcrop samples Coniacian and Santonian samples of Tarfaya Sondage No.2 revealed marine organic matter with a higher Corg content and a better quality of the organic matter, represented by alginite. Vitrinite reflectance measurements and various sterane and hopane ratios , as well as Tmax values of Rock-Eval pyrolysis indicate an early mature to immature range of the organic matter. Sulphur contents and Corg/TS support the theory of mainly oxygen-depleted waters, but it is still ambiguous whether the bottom waters were permanently anoxic or rather oxygen-deficient. Although these investigated source rocks of Late Cretaceous and Eocene age are excellent with respect to organic richness, kerogen quality and thickness, their low maturity poses a major problem for petroleum exploration. However, these units represent excellent unconventional petroleum ressources (i.e., oil shale). In contrast to the immature sequences of the Tarfaya Basin, the marls and limestones (Pliensbachian to earliest Toarcian) exposed at Aϊt Moussa in Boulemane Province are the only known example of an effective petroleum source rock in the Middle Atlas of Morocco. Results indicate the presence of Type I/II kerogen deposited under marine conditions with an input of predominantly algal-derived organic matter. The presence of woody particles further indicates minor terrestrial input. Organic-geochemical and biomarker analyses are consistent with the reconstruction of carbonate-rich sediments deposited under oxygen-depleted conditions. In terms of thermal maturity, the sediments have reached the oil window but not peak oil generation. Petroleum generation and migration is, however, indicated by organic geochemical and microscopic evidence, and supported by the 1D model. The calculated burial model reveals a maximum burial depth of 2.5 to 2.8 km for the Pliensbachian source rock interval. Using specific bulk kinetic parameters, the petroleum generation has occurred in two phases: Late Jurassic and Late Eocene. In contrast to carbonate- dominated, marine systems in Morocco, the sediments of the Central Congo Basin represent a more siliciclastics, terrestrial dominated system. The high Corg contents of the Loia and Stanleyville Groups (Late Jurassic to Early Cretaceous) are associated with high bioproductivity of aquatic OM (algal/phytoplankton) and its preservation. Based on biomarker analysis/n-alkane distribution a lacustrine, anoxic depositional environment can be assumed for these units. Kerogen type I is present and is the dominant microscopic constituent. Lower Paleozoic and Neoproterozoic sediments and some of the Stanleyville Group samples show hydrogen poor OM and algal derived material. All of the Mesozoic and Paleozoic source rocks are at early maturity, partly within the oil window (Stanleyville and Loia Groups), which is indicated by Tmax and vitrinite reflectance values. Finally, vitrinite reflectance data were used to calibrate 1D models for wells Dekese and Samba, which give an overview about burial-, thermal-, and maturity history of the area. Modeling revealed deepest burial during the Late Cretaceous (around 80 Ma, Santonian/Campanian transition) for both wells. For the Dekese and Samba well, an erosion of respectively 1000 m and 900 m was assumed, leading to the present maturity ranges.