Using Biomarkers to Distinguish Between Oil From Al Kotlah Graben and the Surrounding Platforms, Sirt Basin, Libya

Alsharef Albaghdady and Paul Philp

The Sirt Basin contains most of Libya's producing oil fields. It is located in northern Libya, bordered on the north by Gulf of Sirt in the Mediterranean Sea. Al Kotlah Graben is a narrow graben. It trends from NNE to SSW and is aligned with the western boundary of the Abu Tumayam Trough. Al Kotlah Graben borders Al Bayda Platform at its northern boundary and the Az Zahrah-Al Hufra Platform at its southern boundary. Four crude oil samples from the Sirt Basin were selected for this study. The crude oil samples consisted of two from the Al Kotlah Graben and one each from the Az Zahrah-Al Hufrah and Al Bayda platforms. The main aims of this study were to determine the type of organic matter, depositional conditions of the source rocks and thermal maturity of the crude oil based on organic geochemical parameters. The organic geochemical analyses were determined by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). The results suggest that all of the oils are derived from source rocks rich in marine organic matter deposited under oxic to suboxic saline conditions. The oils from the graben are characterized by a low level maturity while the oils from the surrounding platforms show a relatively higher level of maturity. Based on biomarker ratios such as pristane/phytane, isoprenoids/n-alkanes, carbon preference index (CPI), 22S/22S+22R homohopane ratio, diasteranes/steranes, gammacerane index, C29 20S/20S+20R, C29/C30 hopanes and Ts/Ts+Tm, which are used as source correlation parameters, it proposed that these oils could be sourced from similar marine shale rock at different levels of maturity.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012