Source rock potential and depositional environment of Upper Cretaceous sedimentary rocks, Abu Gharadig Basin, Western Desert, Egypt: An integrated palynological, organic and inorganic geochemical study

Abstract

The Abu Gharadig Basin in the northern part of the Western Desert, Egypt is among the most important petroleum provinces in Egypt. Here, Cenomanian to Santonian rocks of Bahariya Formation and Abu Roash A-G members from the GPT-3 well were investigated for their depositional environment, kerogen quality, petroleum generation potential and thermal maturity by geochemical, petrological and palynological methods. The sediments of the Bahariya and Abu Roash formations excluding the Abu Roash “F” Member represent variable shallow marine environments with poor preservation of marine organic matter, whereas there is an excellent preservation in the Abu Roash “F” Member. The basal part of this member is characterized by anoxic, carbonate-rich depositional conditions, high TOC values up to 7%, high HI up to 700 mgHC/gTOC and liptinite-dominated organic matter. Depletion of iron leads to sulfur incorporation into organic matter which is reflected in high thiophene/benzene ratios. Above, sediments are enriched in “terrestrial elements” Fe, Ti, as well as K and Mn. This pattern and the occurrence of the fresh/brackish water algae Botryococcus suggests a regression phase during deposition. The upper part of the Abu Roash “F” Member is again characterized by fully marine, suboxic conditions and a lower thiophene/benzene ratio. The source rock section demonstrates lower thermal maturity compared to the above and below sections based on all microscopic, pyrolysis and biomarker data. This indicates thermal maturity retardation/suppression most likely due to a different organic facies and early diagenetic transformation of kerogen as indicated by the geochemical and palynological data. The source rock heterogeneity indicates a relatively shallow marine environment where the changes in bottom water conditions are very sensitive to sea level fluctuations. Residual oil characterization reveals two reservoir compartments, which are in the Abu Roash “D” and “C” members.The Abu Roash “F” source rocks are compared with time-equivalent sediments in the Tarfaya Basin in Morocco, which represent more distal settings. The more proximal group (A) is characterized by S/Fe ratios between 0.57 and 0.67 indicating suboxic conditions. It has TOC and HI values not exceeding 4% and 600 mgHC/gTOC, respectively, at variable CaCO3 contents. The more distal Tarfaya sediments (group B) have higher TOC and HI as well as high organic sulfur contents. Group A is higher in terrigenous element concentrations. The Tmax of immature source rocks ranges from 410 to 415°C in organic‑sulfur rich samples and from 420 to 430°C in organic‑sulfur poor samples revealing an important impact of organic facies on maturity parameters. Furthermore, there is an inverse relationship between S/Fe and Al2O3/TOC which might become an important tool in paleoenvironmental reconstructions as well as petroleum source rock studies.