Source rock characterization and biomarkers analysis of Adar and Galhak Formations, Rawat central sub-basin, White Nile basin, Sudan

Abstract

Geochemical analysis was conducted on shaly-cutting samples from various formations in Rawat Central Sub-Basin using Rock-Eval pyrolysis, vitrinite reflectance, and GC-MS to determine the characteristics of different source rocks in the basin, including their hydrocarbon potential, maturity, and organic facies. It can be inferred that most samples exhibit characteristics of gas-prone source rocks. Only Adar and the shaly upper section of the Campanian Galhak formation are promising in having excellent source rocks with TOC reaching up to 12%, HI (563–919 mg/g TOC) Tmax (443–445 °C), and S2 (77 mgCO2/g rock) with a potential to generate oil. The maturity data of both Tmax and vitrinite reflectance revealed that these source rocks are within the early mature to early peak mature with dominant kerogen type of a mixed type II-III kerogen. Few samples reflect a pure type I and type II kerogen.The biomarkers data revealed that three possible organic faces can be identified. Slightly brackish anoxic to sub-anoxic deep lacustrine/marine shale to marginal marine shale (coastal plain estuary) characterized by (Pr/Ph < 1, High Hopanes, C23 TT, Pr/Ph < 1, C25TT/C26TT and C24TeT/C26TT < 1, C27 ∼ C29 sterane, high Gammacerene and β-carotane) (Organofacies I). Mixed facies of distal lacustrine to coastal plain proximal lacustrine (Organofacies II) characterized by (High waxiness, Pr/Ph > 2, C25TT/C26TT and C24TeT/C26TT > 1, high C28 sterane). Sub-oxic terrestrial to coastal plain estuary/bay facies characterized by (Pr/ph ∼1, C25TT/C26TT and C24TeT/C26TT > 1, low Gammacerane, High C29 sterane) (Organofacies III). Lower Galhak formation was deposited in slightly brackish, clay-rich, deep lacustrine to mixed marginal marine shale estuary representing Organofacies I with an average terrestrial input forming type II & III kerogen. For Galhak formation, the depositional environment ranged from proximal sub-oxic sallow lacustrine to distal sub-oxic lacustrine (Organofacies II) with a high terrestrial input in the Lates. Adar formation was deposited in a clay-rich, sub-oxic, terrestrial (Fluvial) to coastal plain (estuary/bay) (Organofacies III) with an average terrestrial input generating a mixed type II/III kerogen. The relatively low 20S/(20S + 20R) and ββ/(ββ + αα) C29 suggest an early maturation in Adar formation and early to peck mature source rocks in Galhak formation.