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Площадь: 408403.38 км²
Eastern Niger Rift Basin (ENRB)
The "Ténéré du Tafassasset" sector in eastern Niger is a region of collapsed structures about 1000 km long from the border with Algeria in the north down to Lake Chad in the south.
It is known geologically as the "Eastern Niger Rift Basin” or “ENRB" (Fig. 1). This Mesozoic-Cenozoic intra-continental rift basin trends NNW-SSE and is included within the much larger Chad Basin Province (Brownfield, 2016).
Figure 1. Simplified isopach of the Eastern Niger Rift Basin (comprising the Kafra, Grein, Téfidet, Ténéré, Termit, and Bilma grabens/Sub-Basins) with surrounding generalized surface geology (Source: Genik, 1992; modified after Ahmed et al., 2020
North of the Agadez lineament (at about the 17th parallel; Fig. 1), which separates the ENRB into north and south sections, it comprises the Kafra, Grein, Bilma, Ténéré and Téfidet grabens (or Sub-Basins), and south of the Agadez lineament the Termit graben (or Sub-Basin) with an extension to the SW known as the N’Guel Edji graben (sometimes spelled “N’Dgel Edgi”). The N’Guel Edji graben is a relatively shallow graben about 100 km long and 40 km wide (Fig. 1; Fig. 2; Fig. 5) filled with Lower Cretaceous clastic sediments overlayed by Cenozoic lacustrine and fluvial formations. This small graben was outlined during the 1970 seismic survey (Louis, 1970, page 240). Collectively, the Ténéré and Termit grabens are sometimes called the Agadem Graben.
Figure 2. The simplified isopach of the Eastern Niger Rift Basin in relation to the main towns in Niger (Source: Google Earth and Genik, 1992; modified after Ahmed et al., 2020).
The ENRB is the most important and the most extensive petroleum basin of the West and Central African Rift System (WCARS), with over 100,000 km2 (Fig. 3). The rifts show tilted fault blocks and simple horsts & grabens (Fig. 4).
Figure 3. The West and Central African Rift System (WCARS), with the Central African Shear Zone labeled CASZ
Figure 4. The horst & grabens of the Eastern Niger Basin (Source: modified after Genik, 1992; Ahmed et al., 2020; also in Gouvernement du Niger, Ministère du pétrole, 2020).
The Eastern Niger Rift Basin is a superimposed rift basin with sedimentary structures emplaced during two rifts episodes. The Cretaceous episode is characterized by large, tilted commonly faulted blocks trending NW-SE that were reactivated in the Paleogene. In contrast, the Paleogene episode is characterized by normal faulted blocks that trend NNW-SSE. The rifting resulted in different basin structures, with the north section dominated by asymmetric half-grabens while the south section is dominated by full-grabens (Ahmed et al., 2020).
Figure 5. Cross-section through the Termit and N’Guel Edji grabens (Source: Harouna et al., 2017).
The Continental intercalaire outcrops in a very discontinuous manner between the Aïr massif and the Djado Plateau and the Téfidet Trough. Sandstone formations (e.g., Téfidet Group, Dibella Formation, etc.) overlap Precambrian rocks discordantly or, locally, Paleozoic sediments. These formations have been attributed to the Lower Cretaceous following the discovery of some paleobotanic remains and, above all, the fact that they are overlain by marine formations (Alanlara Formation or its equivalent) dated Lower to Middle Cenomanian. The thickness of the continental assemblages varies rapidly as one approaches the faulted fringes which delimit the various troughs (Téfidet, Ténéré, Kafra), where it may reach a thickness of 1000-2000 m. The infilling of these troughs began only from the Upper Aptian (Lefranc et al., 1990).
In the Late Cretaceous, the global sea level rose, leading to marine sediments transgression from the Tethys Sea and the South Atlantic on its rifted blocks. In the Termit Sub-Basin (see below), over 5 km thick sediments were deposited into a 150 km wide depression. In contrast, in the Ténéré Basin (see below), 4 to 6 km of marine, continental and lacustrine sediments were deposited (Ahmed et al., 2020). The maximum thickness of the Mesozoic-Cenozoic strata is found in the Termit Sub-Basin and is about 12,000 m (Fig. 6). The Mesozoic sequence is composed of 300-2500 m of Lower Cretaceous clastics, and 800-4200 m of Upper Cretaceous shallow marine shales, sands, silts, and minor carbonates (Donga, Yogou and Madama formations) (Liu et al., 2020) (Appendix H). The thickness of the Cenozoic continental sands and shale sediments measures 350 to 2500 meters. The Neogene and Quaternary sequences mainly consist of fluvial deposits eroded mainly in the northern section of the ENRB (Ahmed et al., 2020).
Figure 6. Stratigraphic columns of selected rift sub-basins within the Eastern Niger Basin (Source: Brownfield, 2016). Refer to Fig. 5-25 for the position of the three sub-basins.
The Continental Terminal in eastern Niger (Dollé and Homodji formations) is comparable to western Niger: Siderolitic facies, with ferruginous oolites interstratified levels near the base of the formations (Lang et al., 1990).
Oil Exploration and Development History
The stratigraphic and structural knowledge of the ENRB (surface and sub-surface) is largely due to the long history of petroleum exploration in the region. Oil exploration in Niger began during the 1950s. For the first 20 years it consisted mainly of reconnaissance surveys of the southern Hoggar, Djado, Talak and Lake Chad areas and the south of Iullemmeden Basin. From 1970 to 1980, geophysical surveys were carried out in the Séguédine, Agadem and Termit areas (Louis, 1970, for instance), in the southern and SW parts of the Iullemmeden Basin and in the Djado Basin. Wells Séguédine-1 and Tiffa-1 were drilled in 1973-74 in the Kafra graben by Texaco Niger Inc. Well Madama-1 in the Termit graben, drilled in 1975, resulted in the first oil discovery in Niger. Four more wells were drilled in the Termit Basin in 1977 by Esso Exploration & Production Niger Inc. (Iaguil-1, Yogou-1, Moul-1 and Donga-1). Well Yogou-2 was drilled by Esso in 1980, and eleven further wells were drilled in this area by Elf Aquitaine in 1982-94 (Trakes-1, Sokor -1 to -5, Goumeri-1, Araga-1, Faringa-1, Agadi-1 and Karam-1), leading to the discovery of more than one million tons of oil in the Termit Basin. In 1997 and 1998, a further three wells were drilled by Esso (Boujamah-1, Sountellane-1 and Soudana-1), followed by three wells by Petronas/Texaco (Achigore-1, Jaouro-1 and Gani-1) in 2004 and early 2005. In 2006 Petronas/Texaco abandoned the Agadem permit in the Termit Sub-Basin, deeming it uneconomical. Therefore, once again, the permit became the state's property. These earlier investigations by Esso Exploration and Production Niger Inc., Petronas/Texaco, Elf Aquitaine, etc., had acquired more than 15,000-line km of seismic data together with some 30,000 km of aeromagnetic lines and drilled more than 20 exploration wells.
In 2007, the Agadem block was re-allocated to China National Petroleum Corporation (CNPC) through a public tender with a signed agreement between the CNPC and the Niger government (2008) to construct a 20,000 barrels/day capacity refinery in Niger on a share capital arrangement of 60% China and 40% Niger. Between 2008 and 2012, CNPC acquired the first-ever 3D seismic lines (13,000 km2) over the Agadem permit and more than 18,000 km2 of 2D seismic lines. 127 exploration wells were drilled, with 97 being discovery wells. In 2011, the first oil was produced from the Sokor and Goumeri fields. The refinery was launched in the Zinder region, with well completions, construction of surface facilities, and a 462.5 km-long pipeline from the oil fields to the refinery realized. The exploration success of CNPC in the Agadem block raised the estimated reserve to 3.5 billion barrels of oil from the 328 million barrels earlier recorded (over 1 billion barrels of recoverable oil at a 30% recovery rate estimated) and about 17 billion m3 of gas from 10 billion (13 billion m3 recoverable). Eleven wells in Sokor yield daily crude production of 12,000 stb/d, and ten wells in Goumeridaily oil production of about 8000 stb/d. In 2013, 20% of the CNPC Agadem license was sold to China Petroleum Corporation (CPC) Taiwan and the same year, portions of the Agadem acreage (R1/R2) were mandatorily relinquished.
Between 2014 and 2018, Savannah Petroleum Plc. was awarded the CNPC relinquished R1/R2 acreage. Another area, R3/R4, was also acquired in 2015, putting the total acreage at 13,655 km2. Savannah Petroleum has acquired a 36,948 km Full Tensor Gravity (FTG) survey (2015) and an 806 km2 3D seismic survey of a portion of R3 following its phase development plan. Three wells drilled (Bushiya-1, Amidigh-1 and Kunama-1) so far in the R3 acreage indicated light oil shows. Recently, a fourth well (Eridal-1) also in the R3 acreage has proven successful. The R1/R2 acreage covers 30.5% of the CNPC Agadem area and contains an estimated net reserve of 812 MMBBL. (Harouna & Philp, 2012; Harouna et al., 2017; Ahmed et al., 2020).
Data source: Stratigraphic Lexicon. The Sedimentary Formations of The Republic of Niger, Africa. Jacques LeBlanc, 2022
Следующий Бассейн: Земля Франса-Иосифа