Unconventional hydrocarbon assessment of the Silurian Formation, Ghadames Basin, Northwestern Libya

Abstract

The Silurian Tanezzuft Formation in the Ghadames Basin, Northwestern Libya, represents a critical focus of unconventional hydrocarbon exploration, leveraging its extensive organic-rich hot shales. This study undertakes a comprehensive geological, geochemical, and modeling analysis to elucidate the hydrocarbon potential of the Tanezzuft Formation. It integrates basin modeling with PetroMod software to evaluate burial histories, thermal maturity, and hydrocarbon generation, highlighting the formation's pivotal role in the region's petroleum system. Geologically, the Ghadames Basin spans across Northwestern Libya, Southern Tunisia, and Eastern Algeria, covering approximately 350,000 square kilometers. Structurally a passive continental margin, it exhibits a rich sequence of sedimentary deposits, with Paleozoic formations reaching up to 6,000 meters in thickness. Within this sequence, the Tanezzuft Formation emerges as a dual-functional unit, acting both as a source rock and an unconventional reservoir, with its basal hot shale being the primary focus. The stratigraphy reveals an intricate interplay of organic-rich shales and reservoir sandstones, further enhancing the region's hydrocarbon potential. The Tanezzuft Formation's hot shale intervals, deposited under anoxic marine conditions, are characterized by total organic carbon (TOC) values ranging from 1% to 18%, reaching up to 20% in some depocenters. The kerogen composition is predominantly Type-II, indicative of oil-prone organic matter, with significant thickness variations of up to 450 meters. This heterogeneity underscores its potential for oil and gas generation. The study dispels prior claims of Type-III kerogen presence, attributing such misinterpretations to the shale's graptolite content, which mimics Type-III characteristics. Basin modeling, a cornerstone of this research, provides critical insights into the thermal and burial history of the formation. For modeling, a 480 km long geolog'cal cross section, roughly in N-S orientation across the western Libya part of the Ghademes basin, was constructed based on available literature data. The model wa calibrated baed on present day subsurface temperature and thermal maturity data obtained on the Tanezzuft shales. After miodel was calibrated, the burial history, temperature history and hydrocarbon generation history have been evaluated. The striking future is that the first mjor burial took place prior toi Hercynian orogeny and at the depocenter, the Tanezzuft shale entered oil generation window. Uplift and erosion of the sedimentary cover led to shallowing of the Tanezzuft shales; leading to drop in temperature and frozen thermal maturity. The second burial episode took place between Triassic and Miocene leading to increasing maturity of the Tanezzuft shales and major oil and gas generation especially in the central basin. Second major uplift and erosion episode related to the Alpine orogeny commenced in the Miocene and continues at Present. This Alpine orogeny has shaped the Present-day burial and distribution of the Tanezzuft shale; shallower at the north and south parts of the basin and deeper and thus more mature in the central parts of the basin. The Tanezzuft shales exhibit maturity levels ranging between 0.6% and 2.0% Ro, with higher values correlating with increased depths toward the depocenter. Heat flow estimations and geothermal gradients corroborate the basin's suitability for hydrocarbon generation, with temperatures within the formation reaching 125-140°C, ideal for both oil and gas window maturation. Furthermore, the modeling highlights the onset of oil expulsion approximately 300 million years ago, with subsequent gas generation in deeper sections. The unconventional hydrocarbon potential of the Tanezzuft Formation is further emphasized through its mineralogical composition. The lower hot shale member demonstrates favorable brittleness, with quartz content exceeding 55%, making it amenable to hydraulic fracturing. This brittleness contrasts with the clay-rich warm shale, which is less conducive to production. Hydrocarbon generation and expulsion models reveal significant accumulations in sandstone intervals beneath the hot shales, with gas-to-oil ratios peaking in certain sections. Economic assessments estimate substantial in-place resources. The low-maturity oil in place is projected at 0.72 billion barrels, with peak maturity zones harboring approximately 14.7 billion barrels. Gas reserves are estimated at 67 trillion cubic feet at the highest maturity levels. Recovery factors ranging from 2% to 10% translate these estimates into potential yields of 0.3 to 1.48 billion barrels of oil and 1.34 to 6.7 trillion cubic feet of gas. In conclusion, this research underscores the Silurian Tanezzuft Formation's significance as a high-potential unconventional hydrocarbon resource in the Ghadames Basin. By integrating geological, geochemical, and advanced basin modeling techniques, it provides a robust framework for future exploration and development. The findings hold implications not only for regional energy strategies but also for advancing unconventional hydrocarbon methodologies globally.