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Organic Geochemical Characterization of Source Rocks (Sirt Shale) from Central of Sirt Basin, Libya.

Alsharef Albaghdady and R. Paul Philp
Organic Chemistry Group and Conoco-Phillips School of Geology and Geophysics, University of Oklahoma

The Sirt Basin contains most of Libya’s producing oil fields. It is located in north Libya, bordered on the north by Gulf of Sirt in the Mediterranean Sea. The Sirt shale is dark-brown to black laminated organic rich shale, abundant planktonic foraminifera, deposited during Maastrichtian to lowermost Campanian. The Sirt shale is the main source rock in the basin. The samples analysed in this study were collected from a well which is located south of Al Jabal oil Field on the Zaltan Platform, bounded by the Maradah and Ajdabiya Troughs. This organic geochemical study was performed on fourteen samples taken from the Sirt shale. The main aims of this study were to determine the type of organic matter, depositional conditions of these sediments and thermal maturity of Sirt shale based on organic geochemical parameters. The total organic carbon content increases with depth which may indicate facies changes. The relatively high abundance of low molecular weight n-alkanes compared to high molecular weight n-alkane suggests absence or low contributions of terrigenous organic matter. Pristane/phytane and isoprenoid/n-alkane ratios, presence of gammacerane, high pregnane ratio, regular sterane distributions and the occurrence of C30 steranes suggest the Sirt Shale was dominated by marine organic matter deposited under oxic to suboxic saline conditions. The study indicates that these rocks are mature and the main phase of oil generation has been reached. The thermal maturity of these rocks is reflected in the low ratios of moretane/ C30 hopane, 20S/ (20S+20R) C29 sterane, diasterane/regular steranes, Ts/(Ts +Tm), and C24 /C23 tricyclic terpane ratio.


AAPG Search and Discovery Article #90152©2012 AAPG Southwest Section Meeting, Fort Worth, Texas, 19-22 May 2012