Cheng, Bin

Du, Junyan

Liao, Zewen

Yang, Chupeng

Zhao, Jing

Bin Cheng

Chupeng Yang

Junyan Du

Jing Zhao

Zewen Liao

Crossref

Marine and Petroleum Geology

Determination of the series of even carbon numbered n-alk-1-enes trapped inside geomacromolecules

Institutional Repository of Guangzhou Institute of Geochemistry,CAS(GIGCAS OpenIR)

lable at ScienceDirectMarine and Petroleum Geology 51 (2014) 49e51Contents lists avaiMarine and Petroleum Geologyjournal homepage: www.elsevier .com/locate/marpetgeoDetermination of the series of even carbon numbered n-alk-1-enestrapped inside geomacromoleculesBin Cheng a, Chupeng Yang b, Junyan Du a, Jing Zhao b, Zewen Liao a,*a State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, ChinabGuangzhou Marine Geological Survey, Guangzhou 510760, Chinaa r t i c l e i n f oArticle history:Received 14 July 2013Received in revised form9 October 2013Accepted 16 November 2013Available online 3 December 2013Keywords:GeomacromoleculesEven carbon numbered n-alk-1-enesKerogenSolid bitumenAsphaltene* Corresponding author.E-mail addresses: liaozw@gig.ac.cn, zw_liao@hotm0264-8172/$ e see front matter  2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.marpetgeo.2013.11.013a b s t r a c tEven carbon numbered n-alk-1-enes, trapped inside organic geomacromolecules such as kerogen,asphaltene and solid bitumen, can survive geologic time because of the effective protection provided bythe complex matrix structures of geomacromolecules. These alkenes are presumed to be derived fromesters which are commonly present in various organisms. Subjected to either normal maturation in-crease or the impact of drastic thermal events, these trapped n-alk-1-enes gradually decrease in con-centration and eventually disappear. 2013 Elsevier Ltd. All rights reserved.1. IntroductionOrganic geomacromolecules, including humin, humic acid,kerogen, asphaltene and solid bitumen, are formed during theprocesses of biological organism deposition to diagenesis, kerogencracking and petroleum production. With large molecular weightand complex structures, those geomacromolecules are separatedby speciﬁc acid, alkali or organic solvents.The n-alk-1-enes are geologically unstable straight chain al-kenes in which the double bond is positioned at C1/C2. However,recent studies have shown that a series of even carbon numberedn-alk-1-enes are present in the trapped fractions of asphaltenes(Yang et al., 2009; Zhao et al., 2010, 2012; Tian et al., 2012) and solidbitumens (Cheng et al., 2012, 2013). They may be stable in thesegeomacromolecules owing to the protection afforded by thematrix,which inhibits transformation of the trapped n-alk-1-enes duringlater geochemical processes.However, determination of this series of n-alk-1-enes insidegeomacromolecules was still suspected and needmore veriﬁcation.As an important organic geomacromolecule, kerogen can trap somehydrocarbons in its matrix (Khaddor et al., 2008). The present workis aimed to study whether n-alk-1-enes can also be trapped insideail.com (Z. Liao).All rights reserved.kerogens, and their possible origin, geological evolution and ulti-mate fate are discussed. The geomacromolecules mentioned in thispaper are asphaltene, solid bitumen and kerogen.2. Materials and experimental approachesAmudstone (from type II kerogen, with vitrinite reﬂectance VRoof 0.52% and TOC of 11.0%), from the Upper Permian LucaogouFormation of Santanghu Basin, NW China, was selected. Fivemudstone samples were pyrolyzed in the semi-closed pyrolysissystem (details of the system refer to Liao et al., 2004) at 80 MPa for72 h, with 280 C, 380 C, 420 C, 480 C and 560 C respectively.The ﬁve pyrolysis residues and the raw mudstone were treatedwith HCl and HF, to obtain six kerogen samples. Three solid bi-tumens were collected from the drilling core of Gaoke-1, Gaoshi-1and Anping-1 wells from the Sinian Dengying Formation of SichuanBasin, China. Their equivalent vitrinite reﬂectance EVRo was be-tween 2.35 and 2.48%.All kerogens and solid bitumens were extracted with n-hexane,acetone and dichloromethane successively for 120 h, in order topreclude interference from adsorbed components. Then theextracted residues were subjected to mild oxidation to release thetrapped compounds following the procedure from Liao et al.(2006). In short, bezene (20 mL) was used to transfer the extrac-ted residues into a 250 mL ﬂask, 30%H2O2 (4 mL) and CH3COOH(15mL) weremixed in a 50mL beaker and slowly transferred to theFigure 2. TIC of trapped fractions of solid bitumens from Sichuan Basin, China.B. Cheng et al. / Marine and Petroleum Geology 51 (2014) 49e5150ﬂask with the reactants being stirred at the same time, which wascarried out at ambient temperature (25 C) with vigorous stirringfor 24 h. The details of GCeMS analysis for the trapped fractionhave been reported by Cheng et al. (2012). No alkenes weredetermined from the extracts (by n-hexane, acetone or dichloro-methane) of any samples in the current set, and thus only alkenesthat were trapped inside geomacromolecules are discussed in thiswork.3. Results and discussion3.1. Determination of n-alk-1-enes trapped insidegeomacromolecules3.1.1. KerogensA series of even carbon numbered n-alk-1-enes were thedominant constituents detected from the trapped fraction insidethe raw kerogen (Fig. 1a) from the Santanghu mudstone, and thecomplete series of n-alkanes and sulfur (S8) were detected. Evencarbon numbered n-alk-1-enes ranged from C16eC32 with C22 andC24 as the dominant peaks. The ratio of n-alk-1-ene/n-alkane(-ene/-ane) with the same carbon number was clearly >1.From the 280 C and 380 C pyrolysis samples, only C18 and C20n-alk-1-enes were detected from the trapped fraction of kerogenresidues, with the ratio of -ene/-ane <1, while the complete seriesof n-alkanes and sulfur was still present in addition to some bio-markers. However, from the 420 C or higher temperature pyrolysissamples, no alkenes were detected. The results indicate that thisseries of even carbon numbered n-alk-1-enes trapped insidekerogen (residues) gradually decreased and eventually disappearedwith increasing pyrolysis temperature.3.1.2. Solid bitumensCheng et al. (2012, 2013) reported that a series of even carbonnumbered n-alk-1-enes were trapped inside some solid bitumens.Figure 1. TIC of trapped fraction inside kerogen (residues) from different pyrolysistemperatures.Inside the solid bitumen from the Jurassic Shaximiao Formation ofnorthwestern Sichuan Basin, China, the trapped n-alk-1-enesranged from C16eC34 with each ratio of the paired -ene/-ane >1(Cheng et al., 2012). From the Cambrian solid bitumens of north-western Sichuan Basin, the trapped even carbon numbered n-alk-1-enes ranged from C16eC22, with the ratios of -ene/-ane <1 exceptfor C22- ene/-ane (Cheng et al., 2013).In the high maturity to over mature solid bitumens from Gaoke-1, Gaoshi-1 and Anping-1 wells, only small amounts of trappedC22H44 n-alk-1-ene were detected, while more n-alkanes werepresent (Fig. 2).3.1.3. AsphaltenesSixteen asphaltenes from the Tarim Basin, NW China, wereanalyzed for trapped hydrocarbons (Yang et al., 2009), amongwhich the series of even carbon numbered n-alk-1-enes have beenrecovered from 13 samples, which generally ranged from C16eC30with the ratios of the paired -ene/-ane>1. Besides these alkenes, inthe trapped fraction 22,29,30-trisnorhop-17(21)-ene and 17a-hop-20(21)-ene were detected from 3 asphaltenes, and C23 tricyclicterp-12(13)-ene and C29 18a-30-norneohop-13(18)-enewere foundin another 3 asphaltenes (Yang et al., 2009). In addition, Zhao et al.(2010) reported a series of even carbon numbered n-alk-1-enestrapped inside the asphaltene from the ZG31 well of the centralTarim Basin.3.2. Possible origin of this series of even carbon numbered n-alk-1-enesAlexander et al. (1992, 1997) concluded that esters decomposeunder low thermal stress to yield alkenes as follows:B. Cheng et al. / Marine and Petroleum Geology 51 (2014) 49e51 51When a linear alkyl group connects to the alcohol side, then then-alk-1-enewill be selectively formed, while the resulted acids maybe integrated into the kerogen molecules again or transformed intohydrocarbons through decarboxylation. The even carbonnumbered primary alcohols can be produced through the reductionof carboxylic acid (Kunst and Samuels, 2003) or acyl-CoA (Reiserand Somerville, 1997; Cheng and Russell, 2004) which commonlyexists in the organisms. Esters with an even carbon numberedlinear alkyl chain can be readily found in the organisms.During the early stage of deposition under low thermal stress,early geomacromolecules such as humin, humic acid and kerogenbegin to form. As the depositional processes of those macromole-cules with esters proceeds, once even carbon numbered n-alk-1-enes were formed through the above reaction, they can be trap-ped inside the micropores of the geomacromolecule matrix andthen preserved over geological times. With the geochemical evo-lution of kerogens, some trapped components could then beinherited by the subsequent macromolecular fragments such asasphaltene or solid bitumen derived from the original kerogen.3.3. Thermal stability of n-alk-1-enes in the trapped fractionConcentrations of even carbon numbered n-alk-1-enes in thetrapped fraction gradually decrease and eventually disappear fromthe pyrolysis kerogen (residues) with increasing temperature(Fig. 1), indicating that maturity or abnormal thermal stress mayseriously impact the existence of these alkenes.Similarly, in the trapped fraction of the low maturity solid bi-tumens, a series of even carbon numbered n-alk-1-enes wasdetermined with the paired -ene/-ane ratios generally >1 (Chenget al., 2012, 2013). On the contrary, for the high maturity to overmature solid bitumens (EVRo as 2.35e2.48%) fromGaoke-1, Gaoshi-1 and Anping-1 well, only a little C22H44 n-alk-1-ene was foundwith the -ene/-ane ratio <1 (Fig. 2).According to the work from Yang et al. (2009), no n-alk-1-eneswere detected from the trapped fractions of Tz104-CIII, Td2-Є orYm321-Є asphaltenes. The Td2-Є and Tz104-CIII crude oils wereboth presumed to have undergone signiﬁcant thermal alterationand the Td2 oil reservoir even has experienced a temperature of>200 C (Zhang et al., 2004; Xiao et al., 2011). Though the thermalevolution of the oil reservoir Ym321 has not been reported, theneighboring Ym7 reservoir was reported to have experienced sig-niﬁcant alteration (Li et al., 2010). Therefore, high thermal stressmay account for the fact that no n-alkenes were detected fromthese 3 asphaltenes.It seems that the series of even carbon numbered n-alk-1-enescan be generally trapped inside the geomacromolecule matrixstructures. Both normal thermal maturation and abnormal thermalstress cause the alkenes to decrease in concentration and eventu-ally disappear.4. SummaryA series of even carbon numbered n-alk-1-enes were generallydetermined from the trapped fraction inside geomacromoleculessuch as kerogens, asphaltenes and solid bitumens. These n-alk-1-enes were presumed to be the degradation products of estersunder fairly low thermal impact. These alkenes generally have acomplete distribution with the ratios of paired n-alk-1-ene/n-alkane >1. Thermal maturation causes the concentration of thesealkenes to decrease and eventually disappear.AcknowledgmentsThis work has been ﬁnancially supported by the National Nat-ural Science Foundation of China (Grant 41272149) and the Na-tional Key Science Projects Program of China (Grant 2011ZX05008-002).ReferencesAlexander, R., Kralert, P.G., Kagi, R.I., 1992. Kinetics and mechanism of the thermaldecomposition of esters in sediments. Org. Geochem. 19, 133e140.Alexander, R., Kralert, P.G., Sosrowidjojo, I.B., Kagi, R.I., 1997. Kinetics and mecha-nism of the thermal elimination of alkanes from secondary stanyl and tri-terpenyl esters: implications for sedimentary processes. Org. Geochem. 26,391e398.Cheng, B., Hu, S.Z., Shen, C.B., Liao, Z.W., Liu, H., Du, J.Y., Tian, Y.K., 2013. 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Determination of the series of even carbon numbered n-alk-1-enes trapped inside geomacromolecules

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