Ancient Jurassic coal seams revealed as the gas engine beneath Egypt’s Western Desert

Beneath the sun-baked plains of Egypt’s Northwestern Western Desert, a team of geologists has pieced together the complete life story of a petroleum system that has been quietly generating hydrocarbons for roughly 90 million years. By combining organic geochemistry, biomarker fingerprinting, stable carbon isotopes and one-dimensional basin modeling on samples from the Arcadia-1X well in the Meleiha Concession, researchers report that the area’s Jurassic source rocks are exceptionally rich, predominantly gas-prone, and still generating natural gas today. The study, published in Frontiers in Earth Science, resolves long-standing questions about where the oil in the producing Alam El Bueib reservoir actually came from, and points explorers toward deeper Jurassic targets that remain largely untested.

The Meleiha Concession covers roughly 700 square kilometers at the northwestern edge of the Shushan Basin, one of Egypt’s most prolific hydrocarbon provinces. The basin formed during Jurassic-Cretaceous rifting linked to the opening of the Tethys Ocean and holds sedimentary fill exceeding 10,000 meters in its deepest parts. The Arcadia field itself was discovered in 2010 and remarkably reached production just 45 days later, flowing around 3,000 barrels of oil equivalent per day. Despite this economic significance, the relationship between the Alam El Bueib oils and their Jurassic source rocks had never been rigorously constrained, and the timing of hydrocarbon generation and expulsion had not been systematically modeled, gaps the new study set out to close.

The analytical campaign centered on sixty-three samples from the Arcadia-1X well, spanning depths from 8,640 to 15,350 feet. Fifty-seven cutting samples and six conventional core samples covered the Lower Cretaceous Alam El Bueib Formation and the Jurassic succession, including the Masajid, Zahra, Upper Safa and Lower Safa intervals. Total organic carbon measurements and Rock-Eval pyrolysis were performed at the Egyptian Petroleum Research Institute, supplemented by vitrinite reflectance on seventeen samples, palynological thermal alteration indices, gas chromatography, gas chromatography-mass spectrometry biomarker analysis and stable carbon isotope measurements on oils, a condensate and source rock extracts.

The headline geochemical result is the extraordinary organic richness of the Jurassic section. Total organic carbon values in the Jurassic rocks range from 0.21 weight percent to an exceptional 75.2 weight percent at 13,190 feet depth, with an average of 10.16 weight percent. The highest values belong to coal and carbonaceous shale intervals within the Lower Safa Formation, deposited in a deltaic setting with abundant terrigenous plant input during maximum transgression. By contrast, the overlying Alam El Bueib source rocks are considerably leaner, with TOC between 0.54 and 2.30 weight percent and an average near 1.3 weight percent, classifying them as only fair to good in organic richness.

Kerogen typing tells an equally important story. Hydrogen Index values average 91 milligrams hydrocarbon per gram TOC for the Alam El Bueib and Upper Safa samples and 124 for the Lower Safa, placing the organic matter firmly in the Type III camp with minor Type II/III mixtures in the richest Lower Safa intervals. Type III kerogen, derived from higher land plants, generates mainly gas with limited oil capacity, although the most hydrogen-rich Lower Safa intervals with HI values above 200 can contribute marginal liquids. The genetic potential of the Lower Safa ranges from poor to very good, reflecting the interbedding of organic-rich shales and coals with organically lean sandstones typical of a dynamic deltaic system.

Thermal maturity assessments from three independent indicators converge on a consistent picture. Rock-Eval Tmax values for the Jurassic source rocks span 447 to 474 degrees Celsius, averaging 463 degrees, which corresponds to the peak to late oil window, with the hottest samples signaling the onset of gas generation. Vitrinite reflectance for the Jurassic intervals reaches 0.79 to 1.5 percent Ro, while the shallower Alam El Bueib rocks show 0.6 to 0.93 percent Ro, placing them in the early oil window. Palynology reinforces the story: brown spore colors yield Thermal Alteration Index values of about 3.0 to 3.1, consistent with late oil window maturity and confirming that the Jurassic section has been heated sufficiently to generate and expel hydrocarbons.

The oil itself proved to be a light, high-quality crude with an API gravity of 35 degrees and an extraordinarily low sulfur content of just 0.01 percent, a signature of terrigenous organic matter in clastic reservoirs. Saturates dominate the composition at 76.12 percent, with resins and asphaltenes at only 5.6 percent, and carbon preference index and odd-even predominance values near unity confirming a mature, non-biodegraded oil. A pristane/phytane ratio of 4.03, well above 3.0, points to oxic deposition of terrestrial organic matter, and the oil plots squarely in the Type III kerogen and humic coal field on the Shanmugam diagram.

Biomarker and isotope evidence then tied the oil to its Jurassic parent. The oil is dominated by C29 steranes at 47.9 percent of the sterane assemblage, a classic higher-plant signature, and shows a C24 tetracyclic/hopane ratio of 0.14, a low gammacerane index of 0.06 and a norhopane/hopane ratio of 0.55, all indicating a non-marine, oxic, clastic source. The Lower Safa source rock extract mirrors these characteristics, with C29 steranes at 42.3 percent and comparable triterpane parameters. On the Sofer crossplot, the oils cluster tightly in the terrestrial field of the Northwestern Desert, while the source extracts sit slightly offset toward lighter isotopic values, exactly the pattern expected from carbon isotope fractionation during generation and expulsion. The Khatatba condensate at 13,050 feet shares the oils’ isotopic composition despite higher API gravity, suggesting it formed through evaporative fractionation in the presence of a gas cap rather than representing a separate, more mature charge.

One-dimensional basin modeling with PetroMod reconstructed the system’s temporal evolution in four tectonic phases. Jurassic rifting from roughly 170 to 150 million years ago deposited the organic-rich Khatatba Formation; Early Cretaceous passive subsidence buried the source to depths of 2,500 to 3,000 meters; Late Cretaceous Syrian Arc inversion loaded the section further and, crucially, created the structural traps; and Tertiary subsidence drove the rocks to maximum burial near 4,500 meters. The model shows the Jurassic source rocks entered the oil window during the Late Cretaceous around 85 to 90 million years ago, reached peak oil generation during the mid-Eocene to Oligocene around 35 to 40 million years ago, and began expelling oil between roughly 30 and 35 million years ago at temperatures of 143 to 160 degrees Celsius, as constrained by light hydrocarbon ratio thermometers. Gas generation ignited during the Miocene and continues to the present day in the deepest intervals exceeding 1.35 percent Ro.

The near-synchronous timing of trap formation and oil generation is the study’s most consequential finding for exploration strategy. Because Syrian Arc structures were already in place when the Jurassic source rocks began generating oil, migrating hydrocarbons could be efficiently captured, with fault systems acting as vertical conduits from the deeply buried source at 3,900 to 4,700 meters up into the Alam El Bueib reservoirs at 2,600 to 3,200 meters. The produced oils show no biodegradation or water washing, preserved by effective Masajid and intraformational shale seals, deep burial above 80 degrees Celsius and freshwater reservoir conditions. The authors conclude that the Meleiha petroleum system is fundamentally gas-prone, with the Lower Safa Formation as its principal source, and that significant remaining potential lies in deeper Jurassic intervals where favorable structural and stratigraphic conditions coincide with still-generating mature source rocks.

Subject of Research: Source-rock evaluation, oil-source correlation and one-dimensional basin modeling of the Jurassic-Cretaceous petroleum system in the Meleiha Concession, Northwestern Western Desert, Egypt

Article Title: Integrated source-rock evaluation, oil-source correlation, and one-dimensional basin modeling, Meleiha concession, Northwestern Western Desert, Egypt

Article References: El-Gendy, N. H., Barakat, M., EL-Bastawesy, M., Elsaid, A. A., Arafat, M., Farouk, S., & Al-Kahtany, K. (2026). Integrated source-rock evaluation, oil-source correlation, and one-dimensional basin modeling, Meleiha concession, Northwestern Western Desert, Egypt. Frontiers in Earth Science, 14, 1934620. https://doi.org/10.3389/feart.2026.1934620

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1934620

Keywords: petroleum geochemistry, source rock, Khatatba Formation, Western Desert, Egypt, basin modeling, biomarkers, vitrinite reflectance, Shushan Basin, oil-source correlation, natural gas, Alam El Bueib