Organic facies and thermal maturity of Late Aptian to Early Cenomanian shelf deposits, Northern Sinai (Egypt)

Abstract

Organic petrologic and geochemical investigations were carried out on Late Aptian to Early Cenomanian (Mid-Cretaceous) sediments recovered from the paleoshelf of the Northern Sinai (Egypt). The organic facies and thermal maturity of sedimentary organic matter were evaluated with regard to maximum burial depth. Results from organic geochemical analyses (C org/N tot, Rock–Eval Pyrolysis) performed on kerogen concentrates indicate the dominance of hydrogen-depleted organic matter of either terrestrial or degraded marine origin. Kerogen typing reveals a mixture of type III and IV. Maceral analyses conducted on concentrates support geochemical results showing variable proportions of vitrinite, migrabitumen, graphite and traces of inertinite. Vitrinite and inertinite are considered to be of autochthonous or parautochthonous origin indicating at least partly humid paleoclimatic conditions in the hinterland. This assumption is also supported by the occurrence of graphite, which points to aquatic transport from the southerly exposed Precambrian basement. The presence of migrabitumen presumably relates to migrated hydrocarbons from underlying organic carbon-rich Jurassic strata. The organic facies of Mid-Cretaceous deposits from the Northern Sinai indicates a proximal fluvio-deltaic or oxic shelf environment equivalent to organic facies types C–D according to Jones [Jones, R.W., 1987. Organic facies. In: Brooks, J., Welte, D.H. (Eds.), Advances in Petroleum Geochemistry. Academic Press, London, pp. 1–80]. T max data obtained from Rock–Eval pyrolysis reveal an immature level for organic matter (350°C to 425°C), although these values are probably depressed due to the presence of migrabitumen. Since vitrinite is very scarce in kerogen concentrates, reflectance measurements were also performed on migrabitumen. The best correlation of both types of reflectance data was obtained when migrabitumen data were corrected following the approach of Jacob [Jacob, H., 1989. Classification, structure, genesis and practical importance of natural solid oil bitumen. Int. J. Coal Geology, 11, pp. 65–79]. Reflectance values range between 0.6% to 0.7% Rm, equivalent to high volatile bituminous coalification stages. Since there is no indication for an abnormal geothermal gradient along the NE-African margin, a maximum burial depth between 1.2 km and 2.0 km is estimated for the Mid-Cretaceous strata of the Northern Sinai.