Geochemical characteristics of the Yogou Formation of the Termit Basin (Niger)

Background. Hydrocarbon deposits in Niger have been actively explored since the 1970s. However, oil production only commenced in 2011 with the discovery of the Agadem oil field. The available hydrocarbon resources of this region are currently underestimated; thus, it is relevant to forecast the territory for oil and gas content using modern integrated geological and geophysical methods and studies. To that end, the available hydrocarbon systems in the region, including source rock formations, reservoir rocks, sealing rock, reservoir traps should be determined. Additionally, the way these elements are related to each other should be studied. This article examines the source rock formations of the Termit Basin in the Republic of Niger. The region is of considerable interest due to the oil and gas prospects of the Yogou Formation.

Aim. To examine the geochemical characteristics of the Yogou Formation through identifying the organic matter source of the Yogou Formation mudstones, as well as specifying the predominant phase composition of hydrocarbons generated by mudstones of the Yogou Formation.

Materials and methods. Samples of the Yogou Formation mudstones from nine wells located in the Agadem Block of the Termit Basin were used. The results of pyrolytic, petrographic, and biomarker studies of the Yogou Formation source rock kerogen obtained from stock materials, published pa- pers, and reports provided by the Ministry of Petroleum Resources of the Republic of Niger, were analyzed.

Results. Petrographic analysis performed by the China National Petroleum Corporation revealed that the kerogen of the source rocks of the Yogou Formation has a maceral composition including vitrinite, inertinite, and liptinite, particularly leptodetrinite, sporinite, cutinite, and resistinite. Analysis of the biomarker content of the extracted samples of the Yogou Formation showed that C27-C28-C29 steranes are present in the extracts with a predominance of C29 steranes over the common C27 and C28 steranes. According to the initial hydrogen index values determined by Rock-Eval pyrolysis, the predominant source for organic matter was higher vegetation.

Conclusion. The organic matter of the rocks of the Yogou Formation was originally derived from higher terrestrial vegetation. Consequently, the source rocks will generate more gaseous hydro-arbons.

1. Burke K., Dessauwagie T.F.J., Whiteman A.J. Geological history of the Benue valley and adjacent areas. In: Dessauwagie T.F.J., Whiteman A.J. (Eds.), African Geology. Ibadan: Ibadan University Press, 1972. P. 197—205.

2. Burke K.S. Chad Basin: An active intracontinental basin. Tectonophysics. 1976. Vol. 36. P. 197—206.

3. CNPC. Geological and geophysical studies in the Agadem Block, Niger. 2013.

4. Czochanska Z., Gilbert T.D., Philp R.P., Sheppard C.M., Weston R.J., Wood T.A., Woolhouse A.D. Geochemical application of sterane and triterpane biomarkers to describe oils from the Taranaki Basin, New Zealand. Organic Geochemistry. 1988. Vol. 12. P. 123—135.

5. Fairhead J.D., Green C.M., Masterton S.M., Guiraud R. The role of plate tectonics, stress variations and stratigraphic unconformities in the evolution of the West and Central African rift system and Atlantic continental margins. Tectonophysics. 2013. Vol. 594. P. 118—127.

6. Faure H. Geologic exploration of the post-Paleo­zoic sedimentary formations of eastern Niger. Mém. Bureau of Geological and Mining Research. 1966. No. 47. P. 629—630.

7. Genik G.J. Regional framework, structural and petro­leum aspects of rift basins in Niger, Chad and Central African Republic ( CAR). Tectonophysics. 1992. Vol. 213. P. 169—185. https://doi.org/10.1016/B978-0-444-89912-5.50036-3.

8. Genick G.J. Petroleum Geology of Rift Basins in Niger, Chad, and Central Africa. AAPG Bull. 1993. Vol. 77. P. 1405—1434.

9. Harouna M., Philp R.P. Potential petroleum parent rocks in the Termite Basin, Niger. Journal of Petroleum Geology. 2012. Vol. 35, No. 2. P. 165—185.

10. Huang W.Y., Meinschein W.G. Sterols as environ­mental indicators. Geochim. and Cosmochim. Acta. 1979. Vol. 43. P. 739—745.

11. ICCP. The New Vitrinite Classification ( ICCP System 1994). Fuel. 1998. Vol. 77. P. 349—358.

12. ICCP. The New Inertinite Classification (ICCP System 1994). Fuel. 2001. Vol. 80. P. 459—471.

13. ICCP. Classification of liptinite–ICCP System 1994. International Journal of Coal Geology. 2017. Vol. 169. P. 40—61.

14. Lafargue E., Marquis F., Pilot D. Application of Rock- Eval in hydrocarbon exploration and soil contamina­tion studies. Journal of the French Petroleum Institute. 1998. Vol. 53, No. 4. P. 421—437.

15. Lai H., Li M., Liu J., Mao F., Xiao H., He W., Yang L. Organic geochemical characteristics and sediment­ation patterns of Upper Cretaceous marine parent rocks in the Termite Basin, Niger. Paleogeogr. Paleocl. 2018. Vol. 495. P. 292—308.

16. Liu B., Wan L., Mao F., Liu J., Lü M., Wang Y. Hydrocarbon potential of Upper Cretaceous marine source rocks in the Termit Basin, Niger. Fart. Geol. 2015. Vol. 38. P. 157—175.

17. Liu B., Pan X.H., Wan L.K., Su Y.D., Mao F.J., Liu J.G., Lü M.S., Wang Y.H. Structural evolu­tion and main factors controlling Paleogene hy­drocarbon accumulation in the Termit Basin, Eastern Niger. Acta PetroleiSinica. 2012. Vol. 33. P. 394—403.

18. Olade M.A. Evolution of the Benue Trough in Nigeria (aulacogen): A tectonic model. Geol. Magazine. 1975. Vol. 112. P. 575—583.

19. Peters K.E., Walters C.C., Moldowan J.M. Handbook of Biomarkers. Vol. 1: Biomarkers and isotopes in the en­vironment and human history. Cambridge: Cambridge University Press. 2005. P. 75—76.

20. Peters K.E., Moldowan J.M. Handbook of biomarkers: Interpreting molecular fossils in petroleum and an­cient sediments. Englewood Cliffs. NJ: Prentice Hall. 1993. P. 227—228.

21. Peters K.E., Cassa M.R. Applied Source Rock Geochemistry. The petroleum system — from source to trap: AAPG Memoir 60. 1994. P. 93—120.

22. Petters S.W. Stratigraphic evolution of the Benue Trough and its implications for the Upper Cretaceous paleogeography of West Africa. Journal Geology. 1978. Vol. 86. P. 311—322.

23. Wang L., Liu J., Mao F., Lv M., Liu B. Petroleum geo­chemistry of the East Niger Basin termite field. Vt. Petrol. Geol. 2014. Vol. 51. P. 167—183.

24. Zhou Lihong, Su Junqing, Dong Xiaowei, Shi Buqing, Sun Zhihua, Qian Maolu, Lu D., Liu Aiping. Controlling factors of hydrocarbon accumulation in the Termite rift basin, Niger. Petroleum Exploration and Development. 2017. Vol. 44. P. 358—367.