Journal of Petroleum Science and Engineering 192 (2020) 107322 Available online 23 April 2020 0920-4105/© 2020 Elsevier B.V. All rights reserved. Thermogenic gas generation from organic-rich shales in the southeastern Say’un-Masila Basin, Yemen as demonstrated by geochemistry, organic petrology, and basin modeling Mohammed Hail Hakimi a, b, * , Adeeb Ahmed c , Saad Mogren d , Syed Bilawal Ali Shah c , Mostafa M. Kinawy e , Aref A. Lashin b, f a Geology Department, Faculty of Applied Science, Taiz University, 6803, Taiz, Yemen b Department of Geology, University of Malaya, 50603, Kuala Lumpur, Malaysia c Department of Earth and Environmental Sciences, Bahria University, Islamabad, 44000, Pakistan d King Saud University, College of Science, Department of Geology and Geophysics, PO Box 2455, Riyadh, 11451, Saudi Arabia e Petroleum and Natural Gas Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh, 11421, Saudi Arabia f Geology Department, Faculty of Science, Benha University, P.O. Box 13518, Benha, Egypt A R T I C L E INFO Keywords: Madbi Formation Basin modeling Geochemistry Thermogenic gas potential Say’un-Masila Basin Yemen ABSTRACT The current study aimed to explore the gas-generating potential and oil-to-gas conversion in the Madbi oil-source rock system based on data from deep well within the Say’un-Masila Basin using both geochemical and petro- logical methods as well as basin models. Biomarker results for the bitumen from analyzed shale samples indicate mixed organic matter, with mainly phytoplankton, bacteria, and some terrigenous land plants, deposited under relatively anoxic marine environmental conditions. The terrigenous organic matter, and rich in lipids from phytoplankton and bacteria, suggests Types II and II/III as the original organic matter during deposition. Such fndings are not consistent with the signifcant amounts of vitrinite and inertinite that found under microscope and the low current hydrogen index (HI) values of 43–282 (Types III/IV and III/II kerogen) of the analyzed shale samples. The chemical and optical maturity indicators show that the Madbi shales throughout the studied well section are at different thermal maturity stages, consistent with late-mature stage of the oil window to gas- generation window. Therefore, this high thermal maturation is the critical factor that signifcantly modifes the original organic matter. Based on the basin models of the single well studied (Ghayl Bin Yumain-01), the initial conversion of kerogen to oil, occurring between the Late Cretaceous and Late Eocene (75–38 Ma), corresponding to peak-oil window (0.70–1.00 Easy %R o ). Furthermore, the oil was expelled during the Late Eocene to Early Oligocene; hence, the late-mature stage of the oil window (1.00–1.30 Easy %R o ) commenced during this time, with transformation ratios (TR) of 50–70%. Subsequently, the cracking of the retained oil into gas has occurred from the Early Oligocene through the present and is attributed to the high thermal maturation of the gas window (>1.30 Easy % R o ), with maximum TRs of 70–97%. 1. Introduction In eastern Yemen, the Say’un-Masila Basin contains the country’s largest productive oilfelds (Fig. 1A). Signifcant oilfelds (e.g., Sunah, Kharir, Bashir Al-Khair, Hijah, Cammal, and Tawilah) with proven oil resources have been discovered in the Say’un-Masila Basin (Fig. 1B), boosting future petroleum exploration along the basin. In the onshore Say’un-Masila Basin, scientifc research has been conducted by academic scholars and the petroleum industry for con- ventional petroleum resources. Numerous studies have thoroughly investigated the geochemical and petrological characteristics of the various Late Jurassic to Early Cretaceous potential source rocks in the Say’un-Masila Basin oilfelds. (e.g., Mills, 1992; King et al., 2003; Hakimi et al., 2010a, 2011a; Al-Areeq et al., 2001; Al-Areeq et al., 2001; Omran and Alareeq, 2018; Hakimi et al., 2019). These studies concluded that the conventional petroleum system in the Say’un-Masila Basin has * Corresponding author. Geology Department, Faculty of Applied Science, Taiz University, 6803, Taiz, Yemen. E-mail address: ibnalhakimi@yahoo.com (M.H. Hakimi). Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering journal homepage: http://www.elsevier.com/locate/petrol https://doi.org/10.1016/j.petrol.2020.107322 Received 10 December 2019; Received in revised form 20 March 2020; Accepted 19 April 2020