Distribution and type of organic matter in Cretaceous to Tertiary source rocks in Soroosh and Nowrooz fields, Persian Gulf

Detailed organic petrography analysis were conducted on drill core and cutting samples from the Nahr Umr (Middle Albian), Burgan (Lower Albian) and Ratawi (Hauterivian) formations on the Soroosh and Nowrooz fields, NW part of the Iranian sector of the Persian Gulf. The thermal maturity of the Middle Albian Nahr Umr Formation appears to have reached the top of the oil window. The underlying Burgan (Kazhdumi) Formation shows vitrinite reflectance values less than those within the Nahr Umr Formation. The Burgan organic matter is terrestrial in origin and appears to be formed in a coal measures part of the section. Lamalginite is not prominent within the Burgan samples that were examined. The vitrinite found is texturally immature. The association of low reflectance values and textural immaturity could be due to early impregnation with oil, presumably from deeper horizons within the section. The Ratawi samples are lithologically dominated by carbonate and contain abundant lamalginite macerals. Much of the lamalginite is unusual in having a diffuse form. Most of the lamalginite macerals are associated with free oil inclusions. The Ratawi Formation is within the zone of active oil generation at present. In textural terms, the closest similarity to the Ratawi is the organic matter in the Miocene Monterey Formation from the west coast of the USA. The lamalginite differs from that found in lacustrine facies, and from that present in some prominent algal- rich Mesozoic source rocks such as the Kimmeridge Clay.