--> Organic geochemistry and depositional environment of the Late Silurian-Early Devonian Fegaguira Formation in the Chotts basin (Central Tunisia)

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Organic geochemistry and depositional environment of the Late Silurian-Early Devonian Fegaguira Formation in the Chotts basin (Central Tunisia)

Abstract

Excellent hydrocarbon source rock represented by Fegaguira Formation was deposited in the Chotts basin (Central Tunisia) during the Early Silurian-Late Devonian. The organic-rich Fegaguira Formation is interpreted to be the principal petroleum source rock in the Chotts basin with possible reservoir for oil and gas shale accumulations. Cutting samples were collected from several wells and evaluated using QEMSCAN analysis, Rock–Eval pyrolysis, GC and GC/MS techniques in order to characterize the content and type of organic matter (OM), thermal maturity and mineralogical assessment. The total organic carbon (TOC) values range from 1 to 20% (mean TOC: 10 %). The petroleum potential (PP) and the hydrogen index (HI) values average 8 Kg HC/t rock and 225 mg HC/g of TOC respectively, indicating that the sediments have oil and gas generating potential. The OM is predominantly oil prone (mainly type II kerogen). Maturity parameters suggest that the Fegaguira is mature to late mature. However, samples taken from the southern side of the basin seem to be less mature than samples analyzed from the northern side of the basin. The terpanes series (m/z 191) are dominated by the tricyclic and tetracyclic terpane comparatively to hopanes with C23, C24 and C21 tricyclic terpane as prominent compounds. Extended hopanes (homohopane) up to C35 are slightly present. Series of C27-C29 steranes are abundant in the extract samples, with predominance of C29 and C27 over C28 steranes. The presence of C30 steranes, supports the marine origin of the OM. The diasterane content are relatively high confirming the shaly character of the source rock. This sedimentary rock is interpreted as laminated shale, usually deposited as mud (clay and silt) and is generally a combination of clay, silica (quartz) and locally by carbonate (calcite or dolomite). (Go to Search and Discovery to view images)