Understanding the regional migration routes and field charging history of the Oued Mya-Hassi Messaoud petroleum system, Algeria
This thesis describes a detailed geochemical evaluation of the Oued Mya-Hassi Messaoud ridge petroleum system, located in the central Sahara-Algeria. The study area consists of several fields: (1) the Devonian and the Lower Triassic fields located in the Oued Mya basin which are generally small and in direct contact with the Silurian source rock in the basin, (2) the broad low-relief structural traps developed in the Cambro-Ordovician quartzite reservoirs on the adjoining El-Agreb-Hassi Messaoud ridge, 50 to 60 km east of the Oued Mya basin. Utilising conventional biomarker facies and maturity parameters and aromatic hydrocarbons, it is shown that all the oils from the Devonian, the Lower Triassic and the Cambrian fields across the whole petroleum system are similar to the Silurian source rock strata which are widely recognised to be the main source in the region. The oils in the study area are discriminated into three main groups; I) oils from the Hassi Messaoud, Hassi Guettar and El-Agreb fields located in the Hassi Messaoud ridge, II) oils from the Devonian and the Lower Triassic fields located in the Oued Mya basin and III) oils from El-Baguel and Mesdar Cambrian fields. The oils from the El-Baguel and Mesdar Cambrian fields are believed to be generated from the Silurian source rocks located in the Berkine basin, east of the Hassi Messaoud ridge. The biomarker and aromatic hydrocarbon maturity parameters as well as the pyrrolic nitrogen compounds (alkylcarbazoles and benzocarbazoles) data suggest that the oils from Mokh-El-Kebch, N'goussa, Guellala northeast, Guellala and Draa Temra Triassic fields have undergone the least migration distances and were predominantly charged vertically from the underlying Silurian source rocks which are in direct contact with the Devonian and the Lower Triassic reservoirs. The oils from the Haoud Berkaoui and the Benkahla fields appear to have experienced both vertical followed by lateral migration (probably —20 km). Charging of the Hassi Messdoud reservoirs occurred predominantly from the west-northwest and east sectors. The oil experienced long lateral migration distances (40-60 km) starting from the Silurian source rock strata in the northeast of the Oued Mya basin and the Silurian source rock strata located around the Dzabat area east of the field. Once in the Hassi Messaoud reservoirs, the oil migrated upwards towards the crest located in the centre, then downwards to the north, south and finally west parts of the field. Charging of the Hassi Guettar and El-Agreb fields occurred from the north (i.e. from the Hassi Messaoud field). It seems that El-Agreb oils have experienced the longest migration distances across the whole petroleum system (120 to 140 km). Finally, the charging of the Mesdar and El-Baguel fields took place laterally from the southsoutheast starting from the Silurian source likely located in the Berkine basin east of the Hassi Messaoud ridge. The oil in the El-Baguel field seems to have experienced longer migration distance then the oil from the Mesdar field. The study of the petroleum fluid inclusions in the Hassi Messaoud field revealed a reasonable model of temperature and charge history of the Hassi Messaoud field. The modelling results suggest that the Hassi Messaoud sandstone member was at a depth of 2.8 to 3.3 km during the time of petroleum migration into the reservoirs and petroleum inclusion formation. The temperature range at this depth was between 106°C to 118°C from Late Cretaceous to Palaeogene. The distribution of biomarkers and aromatic hydrocarbons in the palaeo-oils extracted from the petroleum inclusions in 6 samples from Hassi Messaoud field and 1 sample from Haoud Berkaoui field revealed that: (1) the palaeo-oils trapped in petroleum inclusions of both fields and the present-day produced oils are similar, likely generated from the Silurian, main source rocks in the region; (2) the palaeo-oils are less mature than the present-day produced oils; and (3) the palaeo-oil trapped in the Haoud Berkaoui Triassic reservoir is even less mature than the palaeo-oils trapped in the Hassi Messaoud reservoirs which may suggest an earlier oil charging of the Haoud Berkaoui field compared to the charging of the Hassi Messaoud field.