Assessment of the thermal maturation, organofacies, and petroleum generation history of Sirte Shale Formation in Sirt Basin, Libya





Journal title

Journal of African Earth Sciences


Vol. 0 No. 196


Rajab El Zaroug


1 - 29


The current study is conducted to investigate the upper Cretaceous (Campanian) Sirte shale Formation to better understand the regional abundance of the organofacies, thermal maturation levels, and the petroleum generation and expulsion histories during the geological time in one of the most important petroliferous basins in North Africa (Sirt Basin). Rock-Eval pyrolysis analysis of 988 samples in 56 wells that penetrated the Cretaceous section in the Sirt Basin was used to screen and study the geological and geochemical characteristics of the Sirte shale Formation. Multi-dimensional basin modeling, including 1D burial histories and 2D modeling approaches, was calibrated with %VRo and pyrolysis Tmax data to establish refined regional maturity of the Sirte shale Formation. By using the Rock-Eval data, the type, occurrence, and distribution of organofacies in time and space were investigated using analytical assessment procedures and present-day mapping, such as total organic carbon (% TOCp), hydrogen index (HIp), and kerogen transformation ratio (TRp) to define the lateral and vertical special variation. Key biological marker (biomarkers) data is used to determine their special types, abundance, and host sedimentary environments. We used integrated geochemistry with basin modeling to define the Sirte shale petroleum generation and expulsion history, petroleum phase-type and quantity, and expulsion efficiency. Results indicate that the current maturation levels in the general range of 0.4–1.35 %VRo (equivalent to 405 ◦C–460 ◦C pyrolysis Tmax) that found to allow Sirte shale Formation to enter transformation levels ranging from 20% to 91%, which is equivalent to 1,615 m–3,627 m burial depth. Sirte shale Formation is dominated by two types of organofacies (B and D/E), which are characterized by %TOC up to 5% and hydrogen index from 400 to 700 mg HC/g TOC with sudden lateral and vertical variation due to the influence of the marine and transitional – terrestrial sedimentary environments. Similar indications from the biomarker data were Pr/Py ratio shows a significant existence of anoxic and dysoxic environments (generally, 0.4–2.4). Alga marine (dominant), mixed, and minor terrestrial organic matter was defined to dominate the Sirte shale Formation as indicated by Pr/nc-17 and Py-nc18 data. The petroleum generation history of the Sirte shale Formation occurred in the Oligocene time (~30 Ma). The higher expulsion efficiency for the oil phase, up to 84% detected in the deeper parts of the basin, and lower gas expulsion efficiency with its maximum up to 63%. During the geological time, two primary petroleum phases were generated and expelled (oil and gas) from the Sirte shale. The bulk fluid properties generated from Sirte Shale Formation indicate that range of GOR from 400 to 2000 scf/bbl and API in the range of 30–60.