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A Novel Geochemical Concept for Predicting Source Rock Organofacies: Implications for Conventional and Unconventional Resources Initial Assessment


A detailed organic and inorganic geochemical analysis of Cretaceous samples from the Abu Gharadig Basin (AB) in Egypt and the Tarfaya Basin (TB) in Morocco reveals a new geochemical concept that depends on the changes in the kerogen properties and source rock lithology from proximal to distal settings. In the AGB, the Cenomanian-Turonian Boundary section shows two source rocks attributed to two sea transgression cycles. Moreover, the TB samples also show two source rock types. This type is moderately rich in terrestrial elements such as Al, Fe, Mn, Ti, and TOC does not exceed 5% whereas HI is lower than 600 mgHC/gTOC. Based on biomarker data and TOC-Fe-S relationships, this source rock type was deposited in suboxic conditions. The second source rock type (B) includes the upper Cenomanian to Turonian from the TB as well as the first transgression cycle from the Abu Roash-F Member from AB. This type is characterized by high carbonate contents, high TOC, and HI values that are greater than 5% and 600 mgHC/gTOC, respectively. It is low in terrestrial element contents and high in organic sulfur. The biomarker and elemental data imply an anoxic depositional environment. These source rocks were deposited in more distal settings compared to source rock type-A. The study shows that the depositional environment changed from suboxic to anoxic toward the distal settings as denoted by the S/Fe ratios that increase from 0.57 in type-A to more than 1.5 in type-B. Moreover, the terrestrial element flux declines toward the distal settings. This led to excess S due to limited Fe availability to form pyrite, which results in S incorporation into organic matter. All samples are immature based on organic microscopy and biomarkers, however, they show two different Tmax value ranges. It ranges from 410 to 415°C in organic-sulfur rich samples (type-B) and from 420 to 430°C in organic-sulfur poor samples (type-A). These changes in properties were examined using various geochemical ratios and parameters. The investigation discloses an inverse relationship between S/Fe and (Al2O3 or TiO2)/TOC. Other relationships between various geochemical parameters are found. The new concept was validated in other basins with large data set. These relationships can become a vital tool in hydrocarbon exploration and paleoenvironmental studies.