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

Organic petrologic and geochemical investigations were carried out on Late Aptian to Early Cenomanian (Mid-Cretaceous) sediments recovered from the pa leoshelf of the Northern Sinai (Egypt). The organic facies and thermal matu rity of sedimentary organic matter were evaluated with regard to maximum bu rial depth. Results from organic geochemical analyses (C-org/N-tot, Rock-Ev al Pyrolysis) performed on kerogen concentrates indicate the dominance of h ydrogen-depleted organic matter of either terrestrial or degraded marine or igin. Kerogen typing reveals a mixture of type III and TV. Maceral analyses conducted on concentrates support geochemical results showing variable pro portions of vitrinite, migrabitumen, graphite and traces of inertinite. Vit rinite and inertinite are considered to be of autochthonous or parautochtho nous origin indicating at least partly humid paleoclimatic conditions in th e hinterland. This assumption is also supported by the occurrence of graphi te, which points to aquatic transport from the southerly exposed Precambria n basement. The presence of migrabitumen presumably relates to migrated hyd rocarbons from underlying organic carbon-rich Jurassic strata. The organic facies of Mid-Cretaceous deposits from the Northern Sinai indicates a proxi mal fluvio-deltaic or oxic shelf environment equivalent to organic facies t ypes 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 degrees C to 425 degrees C), althou gh these values are probably depressed due to the presence of migrabitumen. Since vitrinite is very scarce in kerogen concentrates, reflectance measur ements were also performed on migrabitumen. The best correlation of both ty pes 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. Co al Geology, 11, pp. 65-79]. Reflectance values range between 0.6% to 0.7% R m, 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 lan is estimated for the Mid-Cretaceous strata of the Northern Sinai. (C) 1999 Elsevier Science B.V. All rights reserved.