Introduction

The U.S. Geological Survey (USGS) assessed the potential for undiscovered, technically recoverable conventional oil and gas resources in Yemen as part of an ongoing assessment of conventional resources in priority provinces of the world. The assessment focused on the Mesozoic Sab’atayn, Say’un-Masila, and Jiza-Qamar Basins of Yemen (fig. 1). The Mesozoic structural framework of these basins was inherited from the orientation of basement structures developed during the Neoproterozoic (Redfern and Jones, 1995; Ellis and others, 1996; Bosence, 1997). The dominant northwest–southeast trend of the regional Neoproterozoic Najd fault system was reactivated as a transtension system in the Mesozoic as the Cimmerian terranes and Greater India terrane separated from the northeastern margin of Gondwana, opening the Neo-Tethys Ocean (Ellis and others, 1996; Ahlbrandt, 2002). These events in the Late Jurassic and Early Cretaceous formed the northwest–southeast-trending transtension basins in southern Yemen and northeast Somalia because Arabia and Somalia at that time were adjacent and part of northeastern Africa. Left-lateral transtension in the Late Jurassic (Kimmeridgian) first opened the Sab’atayn Basin in the west, then the Say’un-Masila Basin, and finally the Jiza-Qamar Basin in the east, along with their Somalian counterpart rift basins (Ellis and others, 1996; Nonn and others, 2019). Intermittent connection with the marine realm in the Tithonian led to the deposition of as much as 1,500 meters (m) of evaporites of the Sab’atayn Formation in the Sab’atayn Basin, and the evaporites form important traps and seals for oil and gas. Evaporites were not deposited in the Say’un-Masila or Jiza-Qamar Basins (Csato, 2005). The Sab’atayn, Say’un-Masila, and Jiza-Qamar Basins are structurally complex, with faulted subbasins, transfer zones, and reactivated marginal fault systems. The subbasins were sites of anoxia and the deposition of major petroleum source rocks of the Upper Jurassic Madbi Formation (Hakimi and others, 2012; As-Saruri and Sorkhabi, 2014; Hakimi and others, 2014; Hakimi and Abdullah, 2015; Hatem and others, 2016; Hakimi, Alaug, and others, 2019), Lower Cretaceous Saar Formation (Hakimi, Abdullah, and others, 2019), Upper Cretaceous Harshiyat Formation (Alaug and others, 2013; As-Saruri and Sorkhabi, 2014; Hakimi and others, 2020), and Upper Cretaceous Mukalla Formation (Alaug and others, 2013). The structurally complex subbasins affected the distribution and thickness of source rocks, and the level of thermal maturation and timing of oil and gas generation. Oligocene seafloor spreading in the Gulf of Aden and the Miocene opening of the Red Sea led to the separation of the Arabian plate from Africa, and the separation of the once-continuous Yemen and Somalia rifts by about 300 kilometers across the Gulf of Aden (Redfern and Jones, 1995; Ali and Watts, 2013; Witte and others, 2016; Nonn and others, 2019). These extensional events reactivated faults in the Yemeni basins, possibly leading to loss of oil and gas (As-Saruri and Sorkhabi, 2014).

Total Petroleum System and Assessment Units

The USGS defined the Mesozoic–Cenozoic Composite Total Petroleum System (TPS)—encompassing the Sab’atayn, Say’un-Masila, and Jiza-Qamar Basins in Yemen—which consists of source rocks in the Madbi Formation, Saar Formation, and Harshiyat and Mukalla Formations. Organic-rich shales of the Madbi Formation are the major source rocks in Yemen and are present in the Sab’atayn Basin and Say’un-Masila Basin, and possibly in the Jiza-Qamar Basin (As-Saruri and Sorkhabi, 2014). Shales of the Madbi Formation contain Type II and subordinate lacustrine Type I and terrestrial Type III organic matter, have total organic carbon (TOC) content as much as 20 weight percent (wt. pct.), have hydrogen index (HI) values as much as 1,100 milligrams of hydrocarbon per gram of TOC (mg HC/g TOC), and are as much as 600 m thick (Hakimi and others, 2012; As-Saruri and Sorkhabi, 2014; Hakimi and others, 2014; Al-Areeq and Maky, 2015; Hakimi and Abdullah, 2015; Hakimi and others, 2015b; Hatem and others, 2016; Hakimi, Alaug, and others, 2019). The Upper Jurassic Meem Shale Member of the Madbi Formation contains Type II organic matter, has TOC content as much as 10 wt. pct., has HI values as much as 300 mg HC/g TOC, and is as much as 600 m thick (As-Saruri and Sorkhabi, 2014; Hakimi, Alaug, and others, 2019). The Lam Member of the Madbi Formation contains Type II organic matter, has TOC content as much as 8 wt. pct., has HI values as much as 800 mg HC/g TOC, and is as much as 900 m thick (As-Saruri and Sorkhabi, 2014). Lower Cretaceous shales of the Saar Formation are a major source rock in the Jiza-Qamar Basin, where the shales contain Type II and Type III organic matter, have TOC content as much as 10 wt. pct., and have HI values as much as 300 mg HC/g TOC (Hakimi, Abdullah, and others, 2019). Shales of the Upper Cretaceous Harshiyat Formation potentially occur in the three basins and have mixed Type II and III organic matter (with subordinate Type 1), TOC content as much as 18 wt. pct., and HI values as much as 923 mg HC/g TOC (Hakimi and others, 2020). Shales of the Upper Cretaceous Mukalla Formation in the Jiza-Qamar Basin contain mainly mixed Type III and II source rocks, have TOC content as much as 8 wt. pct., have HI values as much as 350 mg HC/g TOC, and are as much as 500 m thick (Alaug and others, 2013). Coal beds of the Upper Cretaceous Mukalla Formation in the Jiza-Qamar Basin may be an important source of gas, and some coal beds with a high liptinite content may be a source rock for oil (Alaug and others, 2013; Hakimi and others, 2015a).

Reservoir rocks in the Sab’atayn Basin are fluvial-deltaic, nearshore marine, turbidite sandstones; platform-margin carbonates; and fractured basement rocks (As-Saruri and Sorkhabi, 2014). Reservoir rocks in the Say’un-Masila Basin are Upper Jurassic fluvial-deltaic to turbidite sandstones; platform carbonates; and Lower Cretaceous Qishn sandstones, platform carbonates, and fractured basement rocks. Reservoir rocks in the Jiza-Qamar Basin are nearshore marine sandstones of the Qishn Formation (Ahlbrandt, 2002). Traps in these complex extensional systems are mainly structural, but stratigraphic traps are present in the deepwater clastic systems and platform-margin carbonate reservoirs. Seals are mainly formed by Tithonian salt in the Sab’atayn Basin, whereas seals are low-permeability carbonates of the Lower Cretaceous Qishn Formation in the Say’un-Masila and Jiza-Qamar Basins, where the carbonates overlie reservoir sandstones of the Qishn Formation (Hakimi and others, 2012).

Three conventional assessment units (AUs) were defined within the Mesozoic–Cenozoic Composite TPS: the Sab’atayn Basin Reservoirs AU, Say’un-Masila Basin Reservoirs AU, and Jiza-Qamar Basin Reservoirs AU. The assessment input data for the three conventional AUs are summarized in table 1 and Schenk (2025).

Undiscovered Resources Summary

The USGS quantitatively assessed undiscovered conventional oil and gas resources in three AUs in Yemen (table 2). The estimated mean resources are 261 million barrels of oil (MMBO), or 0.26 billion barrels of oil, with an F95–F5 range from 95 to 558 MMBO; 4,514 billion cubic feet of gas (BCFG), or 4.5 trillion cubic feet of gas, with an F95–F5 range from 1,034 to 11,878 BCFG; and 122 million barrels of natural gas liquids (MMBNGL), or 0.1 billion barrels, with an F95–F5 range from 29 to 323 MMBNGL.

For More Information

Assessment results are also available at the USGS Energy Resources Program website, https://www.usgs.gov/programs/energy-resources-program.

Yemen Assessment Team

Christopher J. Schenk, Tracey J. Mercier, Phuong A. Le, Andrea D. Cicero, Sarah E. Gelman, Jane S. Hearon, Benjamin G. Johnson, Jenny H. Lagesse, and Heidi M. Leathers-Miller