Existence and implications of hop-17(21)-enes in the lower Cretaceous of the Saihantala Sag, Erlian Basin, China

Petroleum Science, Jul 2012

C31- to C35-hop-17(21)-enes are identified by gas chromatography-mass spectrometry (GC-MS) analysis to exist as double isomers in most samples of the Aershan Formation and members 1 and 2 of the Tenggeer Formation from well SH3. Comprehensive organic geochemistry and organic petrology study indicates that algae and bacteria are the main biological source of lower Cretaceous sediments in the Saihantala Sag, and this is in accordance with the existence of hop-17(21)-enes. The similar distributions of hop-17(21)-enes and hopanes of these samples indicate that hop-17(21)-enes were transformed into hopanes through hydrogenation during diagenesis processes. The existence of hop-17(21)-enes means that not only the formation of organic matter is related to an anoxic environment and a biological source of algae and bacteria, but also hop-17(21)-enes are direct indicators of hydrocarbon rock at an immature to low-maturity stage. High hydrocarbon conversion ratio, algae and bacteria source and a high abundance of organic matter suggest that the Saihantala Sag has the potential to generate immature to low-maturity oil, which may be of great significance for oil exploration in the Erlian Basin.

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Existence and implications of hop-17(21)-enes in the lower Cretaceous of the Saihantala Sag, Erlian Basin, China

Pet.Sci. Existence and implications of hop-17(21)-enes in the lower Cretaceous of the Saihantala Sag, Erlian Basin, China Luo Qingyong 0 Yu Shijun 1 Liu Yan 0 Zhang Yanqi 0 Han Hui 0 Qi Lin 0 Zhong Ningning 0 0 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum , Beijing 102249 , China 1 Jiangxi Division of Geological Prospecting Center , Sinoma, Shang-rao, Jiangxi 334000 , China C31- to C35 MS) analysis to exist as double isomers in most samples of the Aershan Formation and members 1 and 2 of the Tenggeer Formation from well SH3. Comprehensive organic geochemistry and organic petrology study indicates that algae and bacteria are the main biological source of lower Cretaceous sediments in the Saihantala Sag, and this is in accordance with the existence of hop-17(21)-enes. The similar distributions of hop-17(21)-enes and hopanes of these samples indicate that hop-17(21)-enes were transformed into hopanes through hydrogenation during diagenesis processes. The existence of hop-17(21)-enes means that not only the formation of organic matter is related to an anoxic environment and a biological source of algae and bacteria, but also hop-17(21)-enes are direct indicators of hydrocarbon rock at an immature to low-maturity stage. High hydrocarbon conversion ratio, algae and bacteria source and a high abundance of organic matter suggest that the Saihantala Sag has the potential to generate immature to low-maturity Biomarkers; hop-17(21)-ene; biological source; immature oil; Erlian Basin 1 Introduction Hopanoids are the most abundant group of complex organic molecules in the geosphere (Ourisson et al, 1984) . They are ubiquitous and have a resistant carbon skeleton. For these reasons, hopanoids are widely applied as biomarkers in ancient sediments and oils (Peters and Moldowan, 1993) . They have the potential to provide information regarding the original organic matter, maturity, depositional condition and second change. Although extensive research has been conducted on hopanes (Peters and Moldowan, 1993; Moldowan and McCaffrey, 1995; Farrimond and Telnes, 1996) , very few studies of hop-17(21)-enes has reported. Currently, agreement has been reached for hop-17(21)-enes on their necessary depositional conditions (hypersaline and (Ten Haven, 1985; Palmer and Zumberge, 1981; Zhao et al, 2003; Zhang and Zhang, 1999) . The biological source of hop17(21)-enes may be related to bacteria ( Bechtel et al, 2002 ; well SH3 of the Saihantala Sag provides an opportunity to investigate the geological significance of hop-17(21)-enes, including their biological source, maturity significance and depositional environments. This paper aims to contribute to the understanding of the geological significance of hop17(21)-enes through comprehensive organic geochemistry and organic petrology. 2 Geological Setting The Erlian Basin, a Mesozoic and Cenozoic faulted basin, developed on the basis of the Hercynian Fold Belt between Inner Mongolia and the Greater Hinggan Mountains, is located in the northeast of the Inner Mongolia Autonomous Region (Zhao et al, 1996) . The Erlian Basin underwent the formation of a Mesozoic fold belt, the development of an early-middle Jurassic intermontane basin, a late Jurassic-toearly Cretaceous rift valley, and the late Cretaceous-to-present extinction of the rift valley (Ma, 2005) . The Saihantala Sag is located in the Tenggeer depression of the Erlian Basin (Fig. 1). Based on tectonic characteristics, the Saihantala Sag can be belt, the Bayan fault tectonic belt, the Duruji hollow belt, the Chagan slope belt and the Buhe nose-shaped tectonic belt (Fig. 1). Well SH3 is an exploration test well located in the Duruji hollow belt of the Saihantala Sag (Fig. 1). Well SH3 includes Quaternary, Tertiary, lower Cretaceous (the Saihantala s u o e c a t e r C a u h n a y a B Depth m 800 1000 1200 1400 1600 1800 2000 2200 2400 Strata unit Flash 2000EA were: the combustion tube was filled with reduction copper, chromic oxide and silver-containing cobalt oxide; the flow velocity of the carrier gas He was 100 mL/ minute; the flow velocity of the combustion gas O2 was 250 mL/minute; the flow velocity of the sweeping gas He was the same as that of the combustion gas; and the reactor temperature was 980 ?C. The pressure was 2?10-8 13C values are expressed relative to Pee Dee Belemnite (PDB) standard. 4 Results and discussion H o p - 1 7 ( 2 1 ) - e n e s , n e o h o p - 1 3 ( 1 8 ) - e n e s , h o p a n e s , identified in samples from well SH3 by using standards available in software library databases and through comparison with published mass chromatogram data (Schwark et al, 1998) (Fig. 3 and Table 1). enes The C30 to C35 series of hop-17(21)-enes and hopanes are but no hop-17(21)-enes were found in sample SH3-2225.08. (1990) and Bechtel et al (2002 ; 2004; 2007) considered that hop-17(21)-enes are directly contributed to sediments by bacteria. Maceral studies have shown that sapropelinites, which are mainly in the shape of line and debris, dominate the maceral. The distribution of amorphous sapropelinites occurs along bedding (Fig. 4) with many mineral-bituminous groundmasses (Fig. 4(a) and (d)). These indicate that the remains of algae underwent microbial degradation during the early diagenesis, some of which formed amorphous sapropelinites and some of which were dispersed to mineral groundmasses forming organic-inorganic complexes, i.e., mineral-bituminous groundmasses. Furthermore, according to geochemical data, the hydrogen index (HI) is high (Fig. 2 13C values of EOM are lower than -28? (Fig. 5). These indicate that algae and bacteria are the main source of biological material in the lower Cretaceous of the Saihantala Sag. So the likelihood of hop-17(21)-enes coming from bacteria is high. Samples in the Saihantala Sag reveal an extended hop17(21)-enes distribution with a maximum content of C31 and then smoothly declining to C35, i.e., C31>C32>C33>C34>C35 (Fig. 3). The distribution of C31 to C35 the same as that of hop-17(21)-enes. Comparable distributions of C31 to C35 hop-17(21)-enes and hopanes indicate hopanes may have originated from hydrogenation of hop-17(21)-enes. Hop-17(21)-enes are commonly found in modern sediments and low thermal evolution strata. The C=C double bond of hop-17(21)-enes is thermally unstable, and hop17(21)-enes are either transformed into stable hopanes or degraded (Ensminger et al, 1977) . As a result, hop-17(21)enes were regarded to be the diagnostic biomarker for sediments of an immature to low-maturity stage (Ten Haven et al, 1985; Boon et al, 1983 ; Kohnen et al, 1991 ; De las ?-31 , 13 C-32 -28 -29 -30 -33 -34 -35 Saturated hydrocarbons Aromatics Nonhydrocarbons Asphaltenes Fig. 5 13C of the lower Cretaceous source rocks from well SH3 Acknowledgements This work was supported by the National Science and Technology Major Project of China (2008ZX05018-002). The authors would like to thank Associate Professor Wang C.J. and Dr. Cui J.W. for helpful discussion and suggestions. We thank the two anonymous referees for their useful comments and suggestions. Bechtel A , Markic M , Sachsenhofer R F , et al. 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Qingyong Luo, Shijun Yu, Yan Liu, Yanqi Zhang, Hui Han, Lin Qi, Ningning Zhong. Existence and implications of hop-17(21)-enes in the lower Cretaceous of the Saihantala Sag, Erlian Basin, China, Petroleum Science, 2012, 154-160, DOI: 10.1007/s12182-012-0195-8