Characteristics of the Upper Cretaceous Yogou transitional formation and source rock potential, Niger rift basin

Abstract

A High-resolution Field-emission scanning electron microscopy (FE-SEM) technique was used to investigate the complex Upper Cretaceous Yogou transitional Formation and source rock potential. The source rock consists of complex interfingering laminae of claystone, sandstone and shale of marine-transitional environments. Thermal maturity from Rock-Eval pyrolysis placed it between immature to early mature oil windows with Tmax ranging between 419 0C and 440 0C. Organic matters in the studied location are mainly bitumen and kerogen in a ratio of 3:1. Both the organic matters have micro to nano-scale organic pores, indicating a relatively high thermal maturity phase of the organic matter constituents. Several forms of pyrites exist in the Formation; euhedral, detrital, framboids and sunflower pyrite. The framboidal pyrites are large to medium sizes with diameters ranging from 63.1 to 3.27 μm. With increasing depth, the frambiods diameter decreases but increase in abundance. Based on the framboidal pyrite distributions, the lower Yogou source rock has relatively high organic content than the upper Yogou source rock; the high abundance of detrital pyrites at 2590 m depth is evidence of paleo-erosional reworked sediments that may constitute a surface of unconformity or marine to terrestrial transition zone. The pyrites with high abundance at depth 2605 m are all recrystallized, supporting the post formation erosion/exposure to air and/or water. The petrographic investigation also revealed the presence of well-preserved juvenile pollen of several microns in dimensions in the upper Cretaceous source rock formation. Based on these well preserved fossils and the relative abundance of pyrite at the depth of 2585 m, a euxinic to anoxic environment prevailed in the low section of the Yogou Formation, whereas the upper section is dominated by oxic to anoxic environment, where pyrite and organic matters are relatively low. Micro-cracks running parallel to bitumen/host rock boundary are paths for primary oil expulsion and migration. Where these micro cracks are not present may be due to differences in facies and deformation styles between the brittle clasts and the ductile clays present in the Formation. The newly documented juvenile pollens are important discovery that will help in reconstructing the paleo-history of the Niger Rift Basin.