Geochemical characteristics and families of the Paleozoic oil seepage and solid bitumen in the Southern Guizhou Depression, SW China

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Geochemical characteristics and families of the Paleozoic oil seepage and solid bitumen in the Southern Guizhou Depression, SW China

Ningxi Li?Guangli Wang?Bo Gao?
Xianqing Li?Shengbao Shi?Tieguan Wang

Fifteen oil seepage and solid bitumen samples in the Southern Guizhou Depression were analyzed with GC–MS.Characteristics of molecular markers and carbon isotopes are discussed systemically.The results showed that the oil seepage and solid bitumen samples in the Southern Guizhou Depression could be divided into two families:Ordovician and Siluric samples,and Permian samples.The two families are different in alkanes distribution,biomarkers,aromatic hydrocarbon composition, and stable carbon isotopes;differences mainly caused by source rock variation.

Solid bitumen·Oil families·Fossil reservoir

1 Introduction

From the 1950s,Lower Paleozoic fossil and remnant reservoirs were found in the Southern Guizhou Depression and its peripheral area in the southeast of Guizhou province(Fig.1). Superfcial drilling proved that Ordovician and Silurian strata containedoilandgasinuppercompositeanticlines.Inthe1970s, the‘‘number 8’’geological reconnaissance team discovered the Majiang fossil reservoir and Kaili remnant reservoir in which remained 3.53×108t of residual bitumen;they estimated original reserves were 16×108t(Han and Wang 1983).

As a part of the Yangtze platform,the Southern Guizhou Depressionandits peripheralareaare similar toother parts of China in terms of sedimentation,structure,generation,and accumulation which have combined to make the region a favorableareatodiscoverlargeandmediumoilandgasfelds. Analysis of the fossil and remnant reservoirs could direct the exploration of marine carbonate strata in Southern China. Previous studies have not reached consensus on oil sources andstagesofaccumulation(Maetal.2004;Zhangetal.2007; GaoandLiu2008;TengerQinandZheng2008).Inthispaper, geochemical characteristics of the Paleozoic oil seepage and solid bitumen in the Majiang and Kaili areas are analyzed in detail,andoil–oilcorrelationandfamiliesarediscussed.Four oil seepage samples and eleven solid bitumen samples of the SouthernGuizhouDepressionwerecollectedandanalyzedby GC–MS;molecular markers of samples are correlated elaborately.According to the oil-correlation,geochemical alterations via generation,migration,and accumulation of crude oil are recognized which could help analyze oil sources and direct exploration deployment.

2 Methods

2.1 Geological background

The Majiang fossil reservoir and Kaili remnant reservoir are both in known petroleum systems which have integratedsourcerock,reservoir,cap rock,and overlyingstrata(Fig.2). The fossil reservoir and remnant reservoir are located in the east of the Southern Guizhou Depression(Fig.3).The sedimentary characters are complex and include typical platform faciesandbasinfacieswithtransitionalslopefacies.Thereare several vertical groups of petroleum systems caused by tectonic activity.The petroleum system of the Majiang fossil reservoir is sourced from the Lower Cambrian basin facies mudstone,heldinLowerOrdovicianandSilurianWengxiang Group platform facies reservoirs,and sealed by the Silurian fourth member of the Wengxiang Group shore facies mudstone.Different from Majiang,the petroleum system of the Kailiremnantreservoirissourcedfromthefrstmemberofthe Wengxiang Group foreshore facies mudstone,held in the second and third members of the Wengxiang Group shore facies sandstone,and sealed by the fourth member of the Wengxiang Group mudstone.The structure of the oil trap in the southeastern Guizhou province formed by Duyun movement and Guangxi movement in the Caledonian orogeny, altered and damaged by Yanshan movement and Sichuan movement in the early Himalayan period(Han and Wang 1983;Zhangetal.2007;GaoandLiu2008;Xiangetal.2008).

Fig.1 Location of the Southern Guizhou Depression

Eight Paleozoic sections of the Majiang and Kaili areas were observed,and ffteen oil seepage and solid bitumen samples were collected(Fig.3).The sample from the Kaili H47 well is light oil;the Kaili Lushan Ordovician oil seepage is in limestone geode and Permian oil seepage occurrence isleaching.Thesampleswere crushed, extracted,and separated into four fractions(saturated hydrocarbon,aromatic hydrocarbon,non-hydrocarbon,and asphaltene).Saturated and aromatic hydrocarbon GC–MS and isotope analysis were performed.

2.2 Method

AnalysisofsaturatedandaromatichydrocarbonGC–MSwas conducted using Agilent 6890 gas chromatograph coupled with 5975i mass spectrometry,equipped with HP-5MS silica capillary column(standard is GB/T18606-2001).GC conditions:carrier gas is helium with a concentration of 99.999%; thetemperatureoftheinjectionportandtransferlineis300°C with a heating program that consists of holding at 80°C for 1 min,heating up to 310°C at a rate of 3°C per minute,and holding for 16 min.MS conditions:electron impact mode is70 eV;flamentcurrentfowis300 μA,andmultipliervoltage is 1,200 V,full scan.

Fig.2 Stratigraphy and group of source-reservoir-seal in the Southern Guizhou Depression

3 Results

3.1 Gross compositions

Saturated and aromatic hydrocarbons comprise the majority in oil seepage samples.The ratio of saturated hydrocarbon to aromatic hydrocarbon(S/A)of the Kaili Lushan Permian oil seepage is low at 0.75;three other oil seepage samples have ratios of 2 to 3.Solid bitumen’s major fractions are non-hydrocarbon and asphaltene;the S/A ratio of solid bitumen in the Kaili and Majiang areas is low. The Majiang Ordovician and Lushan Silurian solid bitumen are rich in saturated and aromatic hydrocarbon fractions,in contrast to other bitumen samples.The asphaltene fraction of Duyun Devonian solid bitumen is high at 94.49%(Table 1).

In general,oil seepages are rich in saturated and aromatic hydrocarbon fractions in comparison to solid bitumen samples(the Majiang Ordovician and Lushan Silurian samples are exceptions).The S/A ratios of upper Paleozoic samples are low at 0.24 to 0.75;lower Paleozoic samples from the Majiang area are high at 5.13 to 18.

3.2 Saturated hydrocarbons

3.2.1 n-Alkanes and acyclic isoprenoids

As the most abundant of saturated hydrocarbon,n-alkanes provide key information about geochemical character which provides insight into organic matter types,sedimentary environment,thermal evolution stage,and biodegradation(Tissot and Welte 1984;Zhang et al.2005; Yang et al.2006).

C13–C24alkanes dominate oil seepage and solid bitumen samples,although alkanes range from C11to C36with a single peak.On average the C21-/C22+ratio in the Majiang area is 2.88,higher than the Kaili area with a ratio of 1.79. CPI and OEP values are all around at 1.0 which indicates for mature stage(Table 2).

The concentration of isoprenoid alkanes is second to n-alkanes in crude oil.Pr/Ph,Ph/nC18,and Pr/nC17 could indicate organic facies and maturity(Mei and Liu 1980; Peters and Moldowan 1993).The values of Pr/Ph in the southern Guizhou Depression are distributed from 0.62 to 1.44,with an average of 1.13 in the Duyun area,which displays higher ratios than the Majiang and Kaili areas.Ph/ nC18 and Pr/nC17 exhibit the typical character of marine crude oil(Fig.4).

The correlation plate of Pr/Ph and DBT/PH built by Hughes could provide information about the lithology and sedimentary environment(Hughes et al.1995).DBT/PH values of Permian samples in the Kaili Lushan and Mjiang areas are greater than 2,and the lower Paleozoic samples are less than 1(Fig.5).

3.2.2 Steranes

C27regular sterane is from plankton;C29regular sterane is from phytoplankton and higher plants.C27,C28,and C29regular sterane distribution could refect sources of sedimentary organic matter which is signifcant to the study of oil source,so the composition of sterane series is utilized to divide samples by genetic type(Peters and Moldowan 1993).

The relative content of C27regular sterane is 13%–69%,PH-1 is too low and is caused primarily by secondary alterations and not by a different source(Fig.6). The Permian samples in the Lushan area are low,in contrast to the lower Paleozoic samples.Distinct oil source isthe main control factor,and the lower Paleozoic samples’regular sterane distribution is like an asymmetric‘‘V’’(C27＞C28＜C29).

Fig.3 Location of Majiang fossil reservoir,Kaili remnant reservoir and samples

Table 1 Fractions composition of samples

Table 2 GC-MS data of samples

Fig.4 Crossplot of ratios Ph/nC18 and Pr/nC17

Fig.5 Crossplot of ratios Pr/Ph and DBT/PH

3.2.3 Terpanes

Tricyclic terpane and hopane are very important biomarkers in crude oil.The ratio of C21/C23tricyclic terpane and C24tetracyclic terpane/C26tricyclic terpane can provide information about maturity and migration(Huang et al.1987).Tricyclic terpane content of collected samples is high and dominated by C23TT;the C21/C23tricyclic terpane ratio is less than 1.The C24tetracyclic terpane/C26tricyclic terpane ratio of the Lushan Permian samples is greater than others(Fig.7).

C3017α(H)-diahopane in crude oil is considered to be derived from hop-17(21)-ene intermediates via an allylic oxidation step at C-15 and C-16,and double bond formation and rearrangement involving the methyl group at C-14 during diagenesis(Peters and Moldowan 1993;Li et al. 2009).Depositional conditions such as Eh–pH and clay content have greater C30diaH(Philip and Gilbert 1985; Wang et al.2000;Zhao and Zhang 2001).The C30diaH/ C30H ratio of the Kaili Permian oil seepage is greater than others(＞0.5);the greater value indicates a sedimentary environment of shore shallow lake and swamp facies,with the greater C30diaH value of Kaili Permian oil seepage probably caused by a distinct sedimentary environment (Fig.8).

3.3 Fluorene series

The aromatic hydrocarbon fraction is a signifcant component in crude oil and provides geochemical informationabout sedimentary environment,sources,and maturity(Li and He 2008).Fluorene,dibenzothiophene,and dibenzofuran are similar in structure;Lin and Huang found that dibenzothiophene and dibenzofuran were related to oxidation or deoxidation environments(Lin et al.1987).In general,a greater value of dibenzothiophene indicates that crude oil and source rock formed in a marine saline environment with strong deoxidation.

Fig.6 Ternary diagram of C27, C28and C29sterane composition

Fig.7 Crossplot of ratios C21/ C23tricyclic terpane and C24tetracyclic terpane/C26tricyclic terpane

Fig.8 Crossplot of ratios Ts/ (Ts+Tm)and C30diaH/C30H hopanes

Contents of fuorene less than 30%,and greater dibenzothiophenevaluesrefectthedeoxidationenvironment.The dibenzofuran content of the Kaili Ordovician and Silurian samples is greater than others with a range of 15%to 27% (Fig.9).

3.4 Carbon isotope

Hydrocarbons inherit the carbon isotopes of the source organic matter and are affected by fractionation via thermal evolution processes.Generally speaking,differences in the stable carbon isotope(δ13C)fraction between samples ofthe same source crude oil affected by thermal evolution would not be greater than 3‰ (Peters and Moldowan 1993;Stahl 1977),whereas δ13C differences greater than 3‰imply different oil sources.δ13C of the Kaili Permian oil seepage is-28‰,making it distinct from Ordovician and Silurian Oil seepage(Fig.10).Solid bitumen samples display a similar pattern,except for the Duyun and Kaili Lushan Ordovician and Silurian samples—a difference that is caused by variations in maturity(Fig.11).

Fig.9 Ternary diagram of fuorene,dibenzothiophene and dibenzofuran composition

Fig.10 Stable carbon isotope of oil seepage

Fig.11 Stable carbon isotope of solid bitumen

4 Discussion

Oil correlation shows that oil seepage and solid bitumen of the Southern Guizhou Depression can be divided into two families.One is the Permian samples which have a lower S/A ratio and a greater ratio of DBT/PH.Lower content of C27regular sterane indicates less plankton input.A greater ratio of C24FT/C26TT and C30diaH/C30H indicates a distinct sedimentary environment. Greater contentofdibenzothiophene,lower content of dibenzofuran,and heavier stable isotopes refect that the Permian samples have a different source rock from the lower Paleozoic samples.

The second family is O–S oil seepage and solid bitumen with a ratio of DBT/PH less than 1,representing characteristic marine facies hydrocarbons.C27–C28–C29regular sterane is distributed like an asymmetrical‘‘V’’,indicating mainly sources are plankton.The ratio of C21TT/C23TT less than 1,C24FT/C26TT less than 2,and C30diaH/C30H less than 0.5 generally indicate an oxygen-poor and lowclay environment.Richness in dibenzothiophene and dibenzofuran refects a reduced environment;stable isotope are light besides individual samples affected by maturity.

The differences in the geochemical characteristics between the O–S and P oil seepage and solid bitumen are caused by different source rocks.O–S samples are generated from Lower Cambrian source rock and the Permian samples may be from Lower Permian source rock.CPI and OEP are balanceable and the maturity of solid bitumen is higher.

5 Conclusion

The oil seepage and solid bitumen in the Southern Guizhou Depression can be divided into two families:Permian samples and lower Paleozoic samples.The two families are different in alkane distribution,biomarkers,aromatic hydrocarbon composition,and stable carbon isotopes due to variations in source rock.

AcknowledgmentsWe thank Dr Jin Zhijun,Dr Liu Dongsheng, and Zhou Dikang for their help with outcrop investigations;and two anonymous reviewers for comments and suggestions.This work was funded by the State Key Project of Petroleum(2008ZX05005-001-009HZ),the NationalNaturalScience Foundation ofChina (41172126),the State Key Laboratory of Petroleum Resources and Prospecting(PRP2010-01)and the Science Foundation of China University of Petroleum(LLYJ-2011-05 and KYJJ-2012-01-01).

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Received:6 December 2013/Revised:23 December 2013/Accepted:24 February 2014/Published online:30 December 2014 ?Science Press,Institute of Geochemistry,CAS and Springer-Verlag Berlin Heidelberg 2014

N.Li·G.Wang·S.Shi·T.Wang

State Key Laboratory of Petroleum Resources and Prospecting, and Department of Organic Geochemistry and Environmental Science,College of Geosciences,China University of Petroleum, Beijing 102249,China

N.Li(?)

School of Energy Resources,China University of Geosciences, Beijing 100083,China

e-mail:liningxi@139.com

B.Gao

Exploration and Development Research Institute,Sinopec, Beijing 10083,China

X.Li

China University of Mining&Technology,Beijing 10083, China