Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Geochemical Investigation of Tanezzuft Formation, Murzuq Basin, Libya
(1) School of Geology and Geophysics, University of Oklahoma, Norman, OK.
A geochemical investigation was undertaken on nineteen shale samples from the upper part of Tanezzuft Formation in the Murzuq Basin, Libya, using techniques such as Rock-Eval pyrolysis (RE), gas chromatography (GC) and gas chromatography-mass spectrometry (GCMS). The distribution of the n-alkanes, isoprenoids, terpanes, and steranes have been studied and used to determine the types of organic matter, levels of thermal maturity, and nature of depositional environments.
The results from this study show a close correlation between RE data and the biomarker parameters. Based on RE data, organic matter (OM) is characterized as being poor to fair in TOC. This is because most of the hydrocarbons have already been generated and migrated away from the source rock (e.g. low values of TOC, S1, and S2). The samples studied are shown to be mainly Type III kerogen (gas prone) with some contribution of mixed type II and III kerogens (gas/oil prone).The predominance of nC17 over nC25 in the n-alkanes profiles suggests a high input of marine-derived organic matter, if this is not maturity related. However, values of Pr/nC17 vs. Ph/nC18 indicate that OM in these sediments has mixed marine/terrigenous sources. This is further supported by a predominance of C27 and C29 steranes in most of the samples studied. Pr/Ph ratios suggest that these sediments were deposited in mixed oxic to sub-oxic (high Eh) conditions, which is also indicated by the high concentrations of C30 diahopane in some samples, low ratios of C35 to C34 homohopanes as well as low quality organic matter (e.g. organic matter with low hydrogen index). The presence of pregnane and homopregnane is interpreted to be related to supersaline sedimentation conditions.
Biomarker maturity parameters indicate that most of studied rock samples were generally mature (early mature to oil expulsion), indicated by RE data and the ratios of 22S/(22S+22R), ββ/(ββ+αα), 20S/(20S+20R) and the C30-moretanes/C30-hopanes (C30βα/C30αβ) ratios. Moreover, the relatively high abundance of tricyclic terpanes relative to hopanes in most of samples analyzed was interpreted as a result of maturity rather than source input. Thus, this study confirms that hydrocarbons could be generated partly from the upper part of Tanezzuft Formation within the Murzuq Basin.