Organic geochemical characterization of crude oils and source rocks from Concession 6, central Sirt Basin, Libya

Abstract

The Sirt Basin contains most of Libya's producing oil fields and is in northern Libya, south of the Gulf of Sirt in the Mediterranean Sea. The main aims of this study were to characterize the type of organic matter and depositional conditions of the source rocks and thermal maturity of the crude oils in this region using a variety of geochemical techniques including gas chromatography (GC), gas chromatography/mass spectrometry (GCMS), stable carbon and hydrogen isotopes, total organic carbon (TOC), and Rock Eval analysis. Twelve crude oil samples from Cambrian-Ordovician Gargaf, Upper Paleocene Zaltan, and Middle Eocene Gialo reservoirs and seventy-nine source rock samples representing Sirt Shale samples from nine wells were collected from the Zaltan Platform and surrounding troughs in the central Sirt Basin. Organic geochemical parameters indicate that these oils were sourced from marine organic matter deposited under oxic to suboxic conditions. The oils were divided into three groups based on their whole oil carbon and hydrogen isotopic composition and thermal maturity as manifested by several saturate and aromatic hydrocarbon maturity parameters. Based on the geochemical data, it is proposed that there are at least two sources for the crude oils in the Zaltan Platform area of the central Sirt Basin. The study also indicates that the Sirt Shale source rocks from Maradah Trough are dominated by marine organic matter and have reached the main phase of oil generation.

Organic Geochemistry and Oil-Source Correlation of Oil Seepages and Potential Source Rocks in Qiangtang Basin, China

“…The C 27 and C 28 steranes come from marine organic and lacustrine algae, respectively, while C 29 steranes are generally derived from land plant organic matter. , Therefore, the relative abundances of ααα20RC 27 -C 29 of regular steranes are usually used to differentiate the type of organic matter. , ααα20RC 27 -C 28 -C 29 of regular sterane distribution patterns of oil shale samples from the Quse Formation show a V-shape (Figure b). In this study, the relative contents of C 27 of regular sterane are roughly equivalent to C 29 , and C 27 /C 29 ranges from 1.02 to 1.09 (avg.…”

Geochemical Characteristics of Three Oil Families and Their Possible Source Rocks in the Sub-Sag A of Weixinan Depression, Beibuwan Basin, Offshore South China Sea

“…The terrestrial/aquatic ratio (TAR), which is defined as (nC 27 + nC 29 + nC 31 )/(nC 15 + nC 17 + nC 19 ), 27 is in the range of 1.09−2.96 (average = 2.03) in the A1 oil family compared to the A2 oil family (0.59−0.90; average 0.73) and the A3 oil family (0.57−0.94; average 0.78). Excluding the influence of thermal maturity on the TAR ratios (Figure 6a), it possibly indicates that the A1 was derived from a source rock with a larger input of terrigenous OM (e.g., higher plants) compared to the A2 and A3 oils.…”

“…To our knowledge, the δ 13 C variation is dependent on some factors, such as biological OM, the depositional condition, , thermal maturity, and biodegradation. , Just to get a better understanding of its effectiveness for oil family classification, these controlling factors are discussed below.…”

Organic geochemical characteristics of Eocene crude oils from Zhanhua Depression, Bohai Bay Basin, China

“…High abundance of gammacerane is typically related to highly reducing and hypersaline conditions during the deposition of organic matter (Damsté et al 1995;Peters et al 2005;Summons et al 2008;Albaghdady 2013). The average ratios of gammacerane to C 30 hopane (Ga/C 30 H) of oil samples from Es 2 (0.34) and Es 4 (0.21) are higher than that of Es 1 (0.16) and Es 3 (0.12), suggesting source rocks of oils from Es 2 and Es 4 were likely deposited in the brackishsaline lake and that from Es 1 and Es 3 were deposited in fresh-water lake conditions.…”

“…The proportions of regular C 27-29 steranes (%) have been used to indicate types of photosynthetic eukaryotes. C 27 steranes are mainly derived from algae, diatom, and zooplankton in lacustrine deposits, C 28 steranes are believed to be derived from lacustrine algae, while C 29 steranes typically constitute the sterols of vascular plants (Volkman 2003;Peters et al 2005;Albaghdady 2013). The distribution of C 27 , C 28 , and C 29 aaa(20R) steranes is plotted on a ternary diagram (Fig.…”