Organic Geochemistry of the Paleozoic Source Rocks in the Chotts Basin, Southern Tunisia

Abstract

Abstract This paper describes a detailed geochemical evaluation of the Paleozoic source rocks in the Chotts basin- Southern Tunisia. Cutting samples collected from Middle Ordovician Azzel Formation (Fm), Late Silurian-Early Devonian FegaguiraFm and Permian ZoumitFm were analysed using Rock-Eval pyrolysis, GC and GC/MS techniques. The FegaguiraFm is the principal petroleum source rock (SR) in the basin with Total Organic Carbon (TOC) values ranging from 1 to 20%. The Petroleum Potential (PP) and the Hydrogen Index (HI) values average 8 kg HC/t rock and 225 mg/g of TOC respectively indicate that the sediments have oil and gas generating potential. The terpanes series are dominated by the tricyclic and tetracyclic terpanes comparatively to hopanes with C23, C24 and C21 tricyclic terpane as prominent compounds. The diasterane contents are relatively high confirming the shaly character of the SR. The Azzel shales has poor to moderate, occasionally good, potential for sourcing oil and gas with TOC and PP values varying from 0.80 to 4.49 % and from 0.68 to 9.20 kg of HC/t rock respectively. The HI values of 95–165 mg S2/g TOC and Tmax value of 435–448°C indicate mainly mature oil-prone kerogen. The biomarker features are characterized by high proportion of tricyclic terpanes that are dominated by C23 and C21 tricyclic terpanes. The hopanes fraction is dominated by C29 and C30 hopanes. The diasterane content are relatively high supporting the shaly character of the SR. The ZoumitFm shows fair to excellent TOC ranging from 0.06 to 6.84% and fair to good PP (reaching 4.77 kg of HC/t of rock) and both HI and Tmax values indicate mainly immature oil-prone kerogen. The biomarker analysis reveals a low content of trictyclic terpanes relative to pentacyclic terpanes. The content of C29 and C30 hopane is relatively high. The diasteranes are present in moderate to high proportions and are less abundant than regular steranes. These biomarker features indicate a marine OM associated with marly to argillaceous limestone SR, deposited in suboxic, normal salinity depositional environment.