Organic Geochemical Characteristics of the Upper Cretaceous Shales from Gombe Formation, Gongola Basin, Upper Benue Trough, Nigeria

Abstract

The shales from the Gombe formation were investigated by gas chromatography-flame ionization detector (GC-FID) for the n-alkanes  and isoprenoids compositions, and gas chromatography-mass spectrometry (GC-MS) for the polycyclic aromatic hydrocarbons (PAHs)  compositions within the context of the origin of organic matter, depositional environments and conditions, and thermal maturity. The n- alkane distribution in the samples indicated organic matter mainly derived from marine settings. All the samples studied relatively  showed enriched amounts of PAHs. However, the relatively low abundance and lack of the 6-ring indeno[1,2,3-cd]pyrene and  benzo[ghi]perylene in some samples suggested the absence of larger, high-temperature wildfires, either due to inadequate seasonality  of the humid-arid climate, or limited terrigenous organic matter input. The geochemical ratios computed from the n-alkanes and  isoprenoids in the shales indicated mixed inputs of terrigenous and marine organic matter but with higher contribution from marine and  deposited under oxidizing and reducing conditions with low – marginal maturity status. Also, the geochemical ratios calculated from the  PAHs indicated mixed inputs of petrogenic and pyrogenic sources for the shales but with major input from petrogenic sources. This study  showed that biomarkers were effective in evaluating the origin, depositional conditions, and maturity of organic matter in the  shales from Gombe formation, Gongola Basin, Nigeria.