WHAT WE KNOW ABOUT THE NOGAL SEDIMENTARY BASIN

Fig 1. Simplified geological and structural map of northern Somalia, adapted from Abbate et al. (1993b) and Ali & Watts (2015).

The Nogal Basin in northern Somalia (Fig. 1) is one of the least explored onshore sedimentary basins in East Africa, yet available technical evidence indicates a complete and working petroleum system with significant undiscovered oil potential. A key reference for understanding this potential is an independent technical assessment completed in 2007 by Sproule International Limited, a Canadian petroleum consulting firm, using internationally recognized evaluation standards.

The basin is interpreted as part of a failed rift system related to the opening of the Gulf of Aden, as illustrated schematically in the conceptual model shown in Fig. 2. Prior to tectonic separation approximately 18 million years ago, northern Somalia and Yemen formed a continuous geological province, which explains the strong analogies between the Nogal Basin and productive Jurassic–Cretaceous petroleum systems in Yemen. Sedimentary thickness in Nogal exceeds 10,000 feet in places, providing multiple stratigraphic levels capable of hosting hydrocarbons.

Fig 2. Conceptual model of an aulacogen (failed rift).

Exploration in the basin has been extremely limited. Only five wells were drilled, mostly during the late 1980s and early 1990s, before civil unrest halted operations; Fig. 3 shows the locations of these wells and their stratigraphic correlations. Importantly, some of these wells did not reach the primary Jurassic targets. Despite this, oil shows were recorded in shallower intervals, and surface oil seeps were observed along basin-bounding faults. These observations confirm that hydrocarbons were generated and migrated within the basin—an essential prerequisite for any viable petroleum system.

Fig 3. Well correlation across the Nogal Basin showing stratigraphic continuity and lithological variations interpreted from wireline logs (Ali & Watts, 2015)

The assessment identified all key petroleum system elements. The Jurassic Uarandab Formation, composed of organic-rich shales and marls, was interpreted as the main source rock. The primary reservoir target is the Jurassic Gabredarre Formation, with secondary targets in the Upper Cretaceous Gumburo and Jesomma formations. Trapping is dominated by large, fault-bounded structural closures identified on vintage 2D seismic data; Fig. 4 shows a representative seismic line with multiple normal faults defining these closures.

Fig 4. Interpreted seismic profile across the Nogal Basin showing the Nogal-1 well tie and six mappable Jurassic–Miocene stratigraphic sequences (Ali & Watts, 2015).

Reservoir properties used in the probabilistic analysis reflect ranges considered realistic for a frontier rift basin. Porosity values vary from moderate to good across the Cretaceous and Jurassic reservoirs, with net pay thicknesses commonly on the order of several tens to a few hundred feet and water saturations indicating the potential for hydrocarbon charge. Oil formation volume factors were assumed within standard ranges for light to medium crude oils. These values were derived from partial well logs, historical operator reports, and regional analogs, and were considered geologically reasonable for the Nogal Basin.

Reservoir areas were modeled separately using log-normal distributions across seven fault-bounded structures, with sizes ranging from approximately 160 acres to more than 14,000 acres. Only structural traps were considered in the analysis, including tilted fault blocks and rollover anticlines with three-way dip closure against major faults. Stratigraphic trapping was not included due to limited well control, although it remains a possibility for future exploration.

A Monte Carlo probabilistic model with 10,000 iterations was applied to estimate undiscovered oil-in-place volumes. The analysis focused exclusively on oil, as gas was considered unlikely to be economic in the onshore northern Somalia context. The probabilistic analysis indicates gross undiscovered oil-in-place volumes in the Nogal Basin ranging from approximately 2.2 billion barrels at the low case to about 10.4 billion barrels at the high case, with a best estimate of roughly 4.3 billion barrels. These volumes are explicitly reported as undiscovered and un-risked oil-in-place. No probability of geological success was applied, and no economic screening was conducted.

Finally, if the Nogal Basin is to move beyond speculation, the next steps are clear. What is missing is not geological justification, but modern data. Updated seismic acquisition, reprocessing of legacy lines, and deeper wells that properly test the primary Jurassic targets are essential before any serious judgment can be made. Without this, discussion around the basin will remain theoretical. In my view, the future of the Nogal Basin depends less on discovering something new and more on finally testing what decades of evidence already suggest.