Source Rock Characterization of the Late Cretaceous Brown Limestone of Egypt

Abstract

The Brown Limestone of Egypt is a primary source of hydrocarbons in the Gulf of Suez. It was deposited during the Late Cretaceous prior to Miocene rifting and development of the Gulf of Suez. Organic-rich, age-equivalent units to the Brown Limestone can be found across much of the northeastern margin of ancient Gondwana, and occur presently from Libya to Syria. The Brown Limestone was deposited during the initial marine transgression across the stable platform. Near-shore deposits of the Brown Limestone contain abundant phosphorites and glauconites, in addition to the organic-rich shales and marls. Offshore facies, such as found in the Gulf of Suez, while also organic-rich, do not contain concentrated phosphorite or glauconitic horizons. The association of phosphorites, glauconites, and abundant organic matter indicates a zone of high primary productivity, which along with the development of low oxygen levels, led to preservation of organic-rich, oil-prone shales and marls. Organic enrichment, hydrocarbon generation potential, and the type of organic matter varies in response to the depositional setting. The most hydrogen-enriched facies occur in the Gulf of Suez, while landward facies, in Cretaceous time, vary from strongly oil-prone to nonsource. Based on molecular and isotopic geochemical data, all of the oils from the Gulf of Suez appear to have been generated from common or very similar sources. After accounting for differences in maturity, extracts from the Brown Limestone appear to be very similar to the examined oils. However, several potential sources have been identified in the Gulf of Suez and the oils cannot be unambiguously correlated back to the Brown Limestone. These sources range in age from Late Cretaceous to Miocene and contribution from the Eocene limestones is probable.