Geochemical Evaluation of Thermal Maturity and Depositional Palaeoenvironment of Seven Cretaceous Formations from the East Sirt Basin, Libya

Abstract

The Sirt Basin is Libya’s most important petroleum province and the world’s 13th largest 
petroleum-producing region. Several potential source rocks have been recognised within a complex 
geological setting, ranging in age from Precambrian to Eocene. Biomarker ratios, together with stable 
carbon (δ13C) and hydrogen (δD) isotopic compositions of individual hydrocarbons in source rock extracts 
(n = 21) from the East Sirte Basin were used to establish their thermal maturity and palaeoenvironmental 
conditions of deposition. Rock Eval pyrolysis data (OI range: 3 - 309 mg CO2/g TOC and HI range: 115 – 
702 mg/hydrocarbon/ g TOC) obtained from source rocks of the Cretaceous Sirte, Tagrifet, Rakb, Rachmat, 
Bahi and Nubian formations show that the organic matter (OM) is mainly dominated by Type II/III kerogen. 
Vitrinite reflectance data (% Ro range: 0.46 – 1.38) support variations in thermal maturity and indicate 
mature to post mature rocks in the Sirte and Rachmat Formations and early to mid stage maturities for the 
other formations. The Sirte Formation in the study area was found to be relatively more thermally mature 
than the Tagrifet, Rakb, Rachmat, Bahi, and Nubian Formations, reflected by δD of pristane (Pr) and phytane 
(Ph) (less depleted in D). A contribution of terrigenous organic matter to all formations except the Sirte 
Formation is evident from δD of the higher-molecular-weight n-alkanes. Anoxic and suboxic of the source 
rock deposition was identified via the pristane to phytane (Ph/Ph range 0.65 – 1.25) and dibenzothiophene 
to phenanthrene (DBT/P range 0.04 – 0.47) values.