Chad basin

From SEG Wiki
Jump to navigation Jump to search
ADVERTISEMENT
Figure 1.Location of Chad basin in Africa map[1]

Geological setting and origin

Figure 2.Stratigraphic column of Chad basin. The column shows the main formations of the Chad basin and their characteristics. Taken from Ali, & Orazulike (2011).

The Chad basin is an intracontinental basin located in Central-West Africa, with an area of 230,000 Km^2, covers parts of Republic of Niger, Chad, Sudan and the northern portions of Cameroon and Nigeria.[2]The formation of the Chad basin took place during the Early Cretaceous when the African and South American lithospheric plates separated and the Atlantic opened.[3]The Chad basin belongs to the fault system  “West and Central Africa Rift System” (WCARS) meaning that it is genetically related with other basins. The main stratigraphic units of the Chad basin can be found in the Figure 2.

Petroleum geology

Source rocks

Figure 3.Diagram of the Hydrogen index vs Oxygen index of samples from the Chad basin. Showing mainly type III kerogen. Taken from Olugbemiro & Abaa(1997).

The source rocks of the Chad basin are Fika and Gongolia formations. This stratigraphic units have a Total Organic Carbon (TOC) that ranges between 0.5% and 4.41%, therefore they have appropriate percentage of carbon to be considered a source rock.[4].The type of carbon of this formation is Kerogen type III and II, this can be found in the Figure, where the Hydrogen Index(HI) and the Oxygen Index are plotted.

Trap

Figure 4.Most common structural traps in the Chad basin. Taken from Okpikoro & Olorunniwo (2010).

The traps of the Chad basin were mainly formed due to tensional stress directed Northwest and Southeast and perpendicular to the main axis of the basin[5]. The main structural traps of the Chad basin are structural, where we can find normal faults, grabens, horst, reverses and folds. The representation of the most common structural traps can be found in the Figure.

Seal

The Gombe and Kerri-Kerri are considered as a seal rocks, however, are not regionally widespread and are  have experienced intense weathering[6]. The magmatic sill intrusion may form a low permeable layer that can act as a local seal .The Fika Shale may contain a local seal and function as a self-sourcing and self-trapping system. The Plio-Pleistocene Chad Formation in the serves as an overburden rock together with a less reported post-Chad Formation which measures up to over 800 m in some depressions.[7]

Reservoirs

The Bima Formation is considered as reservoir rock due to its has high predominance of sandstones and gas-retention capacity directly linked to the thermal influence of the basement. This unit is a fluvial deposit that contains medium to coarse-grained feldspathic sandstone facies, mudstone and carbonate rock[8]

Maturation

The maturation of the source rocks are in general mature. The Fika formation goes from Immature to mature (>435°C) in the oil window, and the Gongila Formation is mature located in the oil window and some parts are considered postmature that belong the Gas window(>470°C).

Oil migration & generation

Figure 5.Oil generation in the Chad basin. The three zones(Early, Main, Late) are classified depending on the maturation. Figure taken from Ahmed et al., (2022).

There are three zones of oil generation in the Chad basin, they can be divided depending on their maturation in Early Oil zone, Main Oil zone, and Late Oil zone. The Early Oil zone is located in the Fika formation, whereas Main Oil Zone and Late Oil Zone is in the Gongolia formation. The Figure shows the location of these zones in the stratigraphic cross section of the Chad basin.

Figure 6.Diagram of the oil and gas migration in the Chad basin. The green arrows show the path of the oil and gas migration in the basin, and the red shows the accumulation, mainly in the fold traps. Taken from Ahmed et al.,(2022).

The oil migrations take place in the structural traps of the Chad basin, in this case there is vertical migration of oil and gas through the faults planes, the direction and accumulation of oil and gas can be found in the Figure

Petroleum exploration and potential

Table 1.Chad Provinces assessment results for undiscovered, technically recoverable oil, gas, and natural gas liquids. Table taken from Brownfield et al.,(2010).

The drilling of oil and gas started in 1979,with the drilling of 23 wells in nearly 14 years with little or no success by Frontier Exploratory Services (F.E.S), the failure of this extraction stage can be explained because of the focus on the upper cretaceous sediments, that are located in the Fika shale formation, that is located in the Immature Oil zone.[9]An Assessment of Undiscovered Oil and Gas Resources of the Chad Basin by the US geological survey found that •The mean volumes are estimated at 2,315 million barrels of oil (MMBO), 14,648 billion cubic feet of gas (BCFG), and 391 million barrels of natural gas liquids. The Table 1 shows the distribution of the findings.

Further reading

File:Assessment of petroleum potential of the Chad basin.pdf [1]

References

  1. https://www.earthbyte.org/Resources/ICONS/AFR/ChadBasin/AFR.ChadBasin.html
  2. Burke, Kevin. "The Chad Basin: an active intra-continental basin." Developments in geotectonics. Vol. 12. Elsevier, 1976. 197-206.
  3. Originated during the Early Cretaceous when the African and South American lithospheric plates separated and the Atlantic opened.
  4. Olugbemiro, R. O., Ligouis, B., & Abaa, S. I. (1997). The Cretaceous series in the Chad basin, NE Nigeria: source rock potential and thermal maturity. Journal of Petroleum Geology, 20(1), 51-68.
  5. Avbovbo, A. A., E. O. Ayoola, and G. A. Osahon. "Depositional and structural styles in Chad Basin of northeastern Nigeria." AAPG bulletin 70.12 (1986): 1787-1798.
  6. Olugbemiro, R. O., Ligouis, B., & Abaa, S. I. (1997). The Cretaceous series in the Chad basin, NE Nigeria: source rock potential and thermal maturity. Journal of Petroleum Geology, 20(1), 51-68.
  7. Ahmed, K. S., Liu, K., Moussa, H., Liu, J., Ahmed, H. A., & Kra, K. L. (2022). Assessment of petroleum system elements and migration pattern of Borno (Chad) Basin, northeastern Nigeria. Journal of Petroleum Science and Engineering, 208, 109505.
  8. A. Tukur, N.K. Samaila, S.T. Grimes, I.I. Kariya, M.S. Chaanda Two member subdivision of the Bima sandstone, upper Benue trough, Nigeria: based on sedimentological data J. Afr. Earth Sci., 104 (2015), pp. 140-158
  9. Adepelumi, A. A., Alao, O. A., Ako, B. D., & Oseikpe, R. E. (2012). MODELING OF HYDROCARBON POTENTIAL AND THERMAL MATURITY OF GONGILA SHALE, CHAD BASIN, NORTHEASTERN NIGERIA. Oil Shale, 29(2).