--> --> Geochemical Investigation of the Upper Cretaceous and Tertiary Succession in the Zelten Platform and Ajdabiya Trough, Sirt Basin, Libya

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Geochemical Investigation of the Upper Cretaceous and Tertiary Succession in the Zelten Platform and Ajdabiya Trough, Sirt Basin, Libya

Abstract

This study is based on organic geochemistry and organic petrographic analysis carried out on 181 rock samples collected from four wells (5p1-59, Q1-59, MM1-59, RRR1-59). From Upper Cretaceous to Tertiary succession within the Ajdabya Trough and the margin of the Zelten Platform. The recorded geochemical values varied in these wells from one formation to another depending on lithology, paleoenviroment and tectonic events. The Sirt Shale is here considered as the most important with regard to hydrocarbon source potential, since it was found to reflect a total organic carbon content ranging from 1.16 to 4.98 % (average 2.61), with Hydrogen index (HI) mostly below 520 (ranging from 142.5 to 520) type–II kerogen (oil prone). In general, the Sirt Formation shows variation in thickness a decrease toward the west from about 1213 ft (367.5 m) in well 5p1-59 in Ajdabya Trough to about 380 ft (115 m) in well RRR1 −59 in Zelten Platform), organic facies analysis show that the Sirt Shales are dominaned by amorphous (AOM) organic matter deposited in anoxic marine condition. Maturity evaluation based on Tmax oC, SCIand Ro % suggesting the maturity increases from immature to reach mature for well succession. From the sequence stratigraphic point view, the Sirt shale source rocks in the well RRR1-59 was divided into two Parasequence sets (Aggradational parasequence and Retrogradational parasequence). The second source rocks in the area is the Rachmat Formation of Upper Cretaceous age, they have total organic carbon ranging from 0.76 to 2.46 average 1.69 with mixed kerogen of type II /III (oil and gas prone). The rest of Tertiary formations such as Gialo & Augila formations is dominated by carbonates sediments and have very low total organic carbon contents less than 1%, the HI is ranging from 32 to 50, the organic facies analysis suggesting that these formations were deposited in oxic marine condition.