Oil-source correlation of Lower-Triassic oil seepages in Ni'erguan village，Southern Guizhou Depression，China

Fang Yuan·Yuhong Liao·Yunxin Fang，3·Ansong Geng

Oil-source correlation of Lower-Triassic oil seepages in Ni'erguan village，Southern Guizhou Depression，China

Fang Yuan1，2·Yuhong Liao1·Yunxin Fang1，3·Ansong Geng1

DOI 10.1007/s11631-015-0078-y

There are abundant bitumens and oil seepages stored in vugs in a Lower-Triassic Daye formation（T1d）marlite in Ni'erguan village in the Southern Guizhou Depression.However，the source of those oil seepages has not been determined to date.Multiple suites of source rocks of different ages exist in the depression.Both the oil seepages and potential source rocks have undergone complicated secondary alterations，which have added to the difficulty of an oil-source correlation.For example，the main source rock，a Lower-Cambrian Niutitang Formation（℈1n）mudstone，is over mature，and other potential source rocks，both from the Permian and the Triassic，are still in the oil window.In addition，the T1d oil seepages underwent a large amount of biodegradation.To minimize the inf l uence of biodegradation and thermal maturation，special methods were employed in this oil-source correlation study.These methods included catalytic hydropyrolysis，to release covalently bound biomarkers from the over mature kerogen of℈

Oil seepage·Biomarker·Carbon isotopic composition·Catalytic hydropyrolysis·Oil-source correlation

1 Introduction

The Southern Guizhou Depression is located in the southern region of the Guizhou Province in Southwest China.This depression covers an area of approximately 30，000 km2.The Southern Guizhou Depression experienced a multi-stage tectonic orogeny（Xu et al.2010）.The depression came into being in the Caledonian and Hercynian orogeny，which was followed by the uplift and deformation of the eastern part of the depression during the Indosinian and Yanshanian orogeny.Thereafter，intense tectonic deformation in the Himalayas led to the exposure of the Lower Paleozoic strata.Oil seepage occurs in many of the strata in the northern part of the depression（Fig.1）. Considerable research has been conducted on the oilsource correlation of the oil seepages in the Southern Guizhou Depression.For example，the℈1n mudstone was considered to be the source of the Middle-Cambrian oil seepages in Gedong town，the Middle-Ordovician oil seepages in Huzhuang village，the Middle-Ordovician oil seepages and the Lower Middle-Silurian oil seepages inKaitang town（Zhang et al.2007；Pan and Liu 2009；Fang et al.2011）.However，the Lower-Permian oil seepages in Lujiaqiao village and Lushan town were thought to be originated from the P1q mudstone（Zhang et al.2007；Peng et al.2011；He et al.2012；Huang et al.2013）.Nevertheless，the source of the T1d oil seepages is still under debate. Fu et al.（2007）proposed that the source of the T1d oil seepages in Ni'erguan village was the P1q mudstone because the bulk δ13C values of the T1d oil seepages are similar to those of the P1q mudstone in the Kaili area. However，after comparing the aromatic hydrocarbons of the T1d oil seepages with those in the Middle-Ordovician oil seepages in Huzhuang village and Kaitang town，He et al.（2012，2013）proposed that the℈1n mudstone was the source of all of the T1d oil seepages near Mengguan town（Fig.1）.Peng et al.（2011）argued that the T1d oil seepages in the nearby Paomuchong village of Longli county were a mixture generated from both Lower-Cambrian and Lower-Permian source rocks，as they found that the distribution of saturated hydrocarbons and regular steranes in the T1d oil seepages of Longli county were similar to those in the P1q mudstone in the Kaili area，while the bulk δ13C values of the T1d oil seepages were close to those of the Middle-Ordovician oil seepages in Huzhuang village.Moreover，Zhao（2000）thought that the T1d marlite could be the source of the T1d oil seepages because of its moderate maturity（the R0was approximately 0.7%）and high TOC content（up to 5.7%）.

Oil-source correlation is difficult in the study area because of the following three factors.First，the℈1n mudstone is over mature，resulting in low yields of extractable organic matter（EOM）.Furthermore，free biomarkers in the EOM might have been significantly altered by severe thermal maturation and may not effectively ref l ect the geochemical information of the original organic matter.Second，the reservoir（the T1d marlite）is still in the oil-generative window and has considerable petroleum generation potential.Therefore，the EOM from T1d may be mixed with oils of different origins.Third，the T1d oil seepages have undergone a large amount of biodegradation.The distribution of the biomarkers might have been altered during biodegradation and some biomarker parameters may be invalid for an oil-source correlation.Therefore，special methods must be applied in an oil-source rock correlation study.These methods include catalytic hydropyrolysis，to release covalently bound biomarkers from the kerogen of℈1n mudstone，sequential extraction，to obtain chloroform bitumen A and chloroform bitumen C from the T1d marlite，and anhydrous pyrolysis，to release the pyrolysates from the kerogen of T1d marlite.

Fig.1 Map showing locations of oil seepages（modified from He et al.2012）

The covalently bound biomarkers released by catalytic hydropyrolysis（HyPy）were less affected by thermal maturation（Love et al.1995，1998；Murray et al.1998；Liao et al.2012；Wu et al.2012；Fang et al.2014）and contamination during outcrop exposure（Wu et al.2013）. Much of the original geochemical information about the source of the organic matter was preserved by covalently bound biomarkers，due to the protection of kerogen macromolecules.Therefore，HyPy experiments were performed for the over-mature℈1n mudstone.Three types of sedimentary organic matter（i.e.，adsorbed，included and crystal-enclosed organic matter）exist in carbonate rock（Gehman 1962；Fu and Jia 1984）.The adsorbed organic matter and chloroform bitumen A was directly extracted by chloroform.After the removal of the chloroform bitumen A and the subsequent acid treatment of the minerals，the included and crystal-enclosed organic matter in the mineral crystals and inclusions were obtained by Soxhlet extraction with chloroform；this extract is termed chloroform bitumen C（Fu and Liu 1982，Fu and Jia 1984；Xie et al.2000，2004）.Numerous previous studies have indicated that the molecular signature of the initial oil charge could be effectively revealed with a geochemical analysis of oil，bearing f l uid inclusions and the sequential extraction of oil reservoir rocks（Karlsen et al.1993；Wilhelms et al.1996；George et al.1997，1998，2004；Schwark et al.1997；Leythaeuser et al.2000，2007；Pan and Yang 2000；Pan et al.2000，2003，2005，2007；Pan and Liu 2009；Gong et al.2007）.In the present study，the T1d marlite was subjected to a practical sequential extraction method to recover the free，adsorbed（Bitm-A）and inclusion（Bitm-C）oils from the oil reservoir rocks to reconstruct the filling history of the oil reservoirs in Ni'erguan village.The T1d kerogen was heated to 320°C for 72 h in a quartz tube to acquire the anhydrous pyrolysates that ref l ect the original geochemical characteristics of the T1d marlite.

The purpose of this study was to use special methods，including catalytic hydropyrolysis，sequential extraction and anhydrous pyrolysis，to make an oil-source correlation for the T1d oil seepage.The oil-source correlation between the T1d oil seepages and the potential source rocks was based on biomarker parameters，carbon isotope composition and the petroleum geological settings.

2 General geological settings

The Southern Guizhou Depression developed many sedimentary formations from the Sinian to the Triassic.Multiple suites of source rocks exist primarily in the depression（Tenger et al.2008）.The main source rock，the Lower-Cambrian Niutitang Formation（℈1n），occurs everywhere in the depression and consists of approximately 103 m of dark mudstone with a high TOC（total organic carbon）content（average 3.16%）.This rock's kerogen is composed of sapropelic components and is at the over mature stage.The secondary source rock，the Lower-Permian Qixia Formation（P1q），mainly occurs in the Majiang-Kaili area.The P1q source rock consists of dark-grey limestone and intercalates with mudstone，usually with a total thickness of 70-175 m and a TOC content ranging from 0.55%to 1.89%.The P1q source rock with a high TOC content occurs only in the Kaili area-the TOC contents of some samples can reach 2.0%.The kerogen of P1q is at the mature stage.However，to the west，the lithology of the P1q stratum gradually shifts to grey micrite limestone.The P1q stratum near the Ni'erguan village has a total thickness of≤20 m and a TOC content of≤0.5%.Therefore，the hydrocarbon generation potential of the P1q source rocks is poor around the Ni'erguan village，although a number of researchers insist that the P1q source rocks should be considered in an oil-source rock correlation.The Lower-Triassic Daye Formation（T1d）marlite is constrained to a 20 to 60 km wide zone，which is south of the Anshun-Guiyang area and has a thickness of 30 to 123 m and a TOC content typically ranging from 0.4%to 1.2%. However，the TOC content of the marlite can reach 5.6% near Guiyang，which is very close to the Ni'erguan village. The maturity of the T1d marlite is still in the oil-generative window.Thus，the T1d marlite has considerable petroleum generation potential.Therefore，various source rocks were collected and analysed in the oil-source correlation study.

3 Samples and experimental

3.1 Samples

Two samples were collected from the geologic outcrop of the Ni'erguan village，including the Lower-Triassic Daye Formation（T1d）oil seepage and the Lower-Triassic Daye Formation marlite.The other two samples，the Lower-Permian Qixia Formation（P1q）mudstone and the Lower-Cambrian Niutitang Formation（℈1n）mudstone were collected from the Majiang-Kaili area.More details about the source rocks of the P1q mudstone and the℈1n mudstone can be found in Fang et al.（2011）.The basic geochemical characteristics of samples are listed in Table 1.

The oil seepage sample Oil-1 is stored in vugs of the Lower-Triassic Daye Formation（T1d）marlite.The T1d marlite sample NEG-3 is still in the oil window，with a TOC value of 0.87%and a Tmaxof 442°C.The℈1n mudstone sample SW-8 is over mature，with a TOC of 6.44%and a Tmaxof 609°C.The P1q mudstone sampleWC-3 is still in the oil window，with a TOC value of 2.01%and Tmaxof 449°C.

Table 1 Basic geochemical parameters of samples

3.2 Experimental

The T1d marlite（NEG-3）sample was crushed to 80 mesh and extracted with a mixture of dichloromethane（DCM）and methanol（93:7，v/v）for 72 h to collect the chloroform bitumen A（Bitm-A）.The residue of the NEG-3 sample was cleaned with dilute hydrochloric acid（1:4，v/v）to dissolve the carbonate mineral，and then extracted with a mixture of dichloromethane（DCM）and methanol（93:7，v/v）for 72 h to obtain the chloroform bitumen C（Bitm-C）. Finally，the Soxhlet extracted kerogen was sealed in a glass tube under nitrogen and heated at 320°C for 72 h for anhydrous pyrolysis.After cooling，the pyrolysates（Bitm-Py）were obtained by ultrasonic extraction，using a mixture of dichloromethane（DCM）and methanol（93:7，v/v）. After the℈1n mudstone and P1q mudstone samples were extracted，the HyPy experiment was conducted on the kerogen of℈1n mudstone sample SW-8 using procedures previously described by Love et al.（1995），Liao et al.（2012），Wu et al.（2012，2013）and Fang et al.（2014）.

Before the HyPy experiment，the kerogen of℈1n mudstone was extracted with a mixture of benzene/acetone/ methanol（5:5:2，v/v/v）for 1 week.An aqueous solution of ammonium dioxydithiomolybdate［（NH4）2MoO2S2］was added to the extracted kerogen to give a nominal loading of molybdenum of ca.5 wt%.A sample with a catalyst was loaded into the HyPy reactor tube.The system was under a constant H2pressure of 15.0 MPa and a hydrogen f l ow of 4 l/min.Next，this reactor tube was heated from an ambient temperature to 300°C（5 min）at 250°C/min.Next，we replaced the silica trap and heated the tube again from ambient temperature to 250°C at 300°C/min and from 250 to 520°C（5 min）at 8°C/min thereafter.The hydropyrolysis product was collected in a silica trap and cooled with liquid nitrogen.

The asphaltenes were precipitated from the oil seepage and source rock extracts.The maltenes were separated into saturated，aromatic and polar（NSO）fractions.The saturated and aromatic hydrocarbon fractions were analysed by GC-MS.The GC-C-IRMS system was used to measure the δ13C values of the n-alkanes in the saturated fraction. Before the GC-C-IRMS analysis，the saturated hydrocarbon fractions were depleted of branched/cyclic hydrocarbons according to the procedures of urea adduction described by Liao et al.（2004）.

3.3 Instruments

The GC-MS analysis of the saturated and aromatic biomarkers was performed using a Finnigan Trace GC Ultra gas chromatograph coupled with a Thermo Fisher DSQ II mass spectrometer.The column used was a HP-1 fused silica capillary column（60 m×0.32 mm×0.25 μm i.d.）.When the saturated hydrocarbons were analysed，the GC oven was held isothermally at60°Cfor2 min，programmed to 295°C at 4°C/min and then remained at this temperature for 30 min.The temperature programforanalysing the aromatic biomarkerswas setinitially at60°C for2 min，programmed to 290°Cata rate of3°C/min and held for25 min.The ionsource temperature was 250°C.The GC-MS system was operated in the electron impact（EI）mode with electron energy of 70 eV.

The VG Isochrom II GC-C-IRMS system was used to determine the δ13C values of the individual n-alkanes.The column used a fused a silica capillary column（DB-1；30 m×0.32 mm×0.25 μm i.d）.Helium was the carrier gas and CO2was used as a reference gas.The GC oven was set isothermally at 80°C for 2 min，programmed to 295°C at 6°C/min and then held for 25 min.The combustion furnace was held at 850°C.The standard deviation of the GC-C-IRMS for each measurement was less than 0.2‰.

4 Results and discussion

4.1 Geochemical characteristics of samples

4.1.1 The geochemical characteristics of potential source rocks

The total yields of the hydropyrolysates（H-SW-8）by HyPy from℈1n mudstone kerogen SW-8 were 40 times greater than those of the extracts（SW-8），which suggests that HyPy can release a higher amount of soluble organic matter from over mature kerogen than the Soxhletextraction from the corresponding source rock can（Liao et al.2012；Fang et al.2014）.Fang et al.（2014）already carefully compared the covalently bound biomarkers and the free biomarkers in the EOM of SW-8 and determined that the covalently bound biomarkers released by HyPy were more stable and provided more dependable information for oil-source correlation.The GC-MS TIC trace of the saturated hydrocarbon in the extracts（SW-8）showed the unimodal arrangement with a maximum at n-C18（Fig.2）.However，the GC-MS TIC trace of the saturated hydrocarbon fraction of H-SW-8 showed a high relative abundance of branched alkanes（Fig.2）.The distribution of C27-C29-αααR steranes in the H-SW-8 was C27＞C29≥C28，which was similar to that of the extracts（Fig.3）.However，in the extracts，the abundance of pregnane and homopregnane were higher than those of regular steranes and the pregnane/regular steranes ratio was greater than that of the hydropyrolysates（Fang et al.2014）.The abundance of pregnane and homopregnane was increased relative to that of the regular steranes，with an increase in thermal maturity（Huang et al.1994）.Therefore，the thermal maturity of the hydropyrolysates is lower than that of the extracts.In the hydropyrolysates，the distribution of terpanes was characterized by a high content of C30hopane and a low content of tricyclic terpanes，while the distribution in the extracts was the opposite（Fig.3）.With an increase in thermal maturity，the abundance of the tricyclic terpanes gradually increased relative to that of the hopanes，as the thermal stability of the tricyclic terpanes is higher than that of the hopanes（Peters et al.1995）.This finding also implies that，unlike the free biomarkers，the hydropyrolysates are insensitive to thermal alteration（Liao et al.2012）.Therefore，the covalently bound biomarkers released by HyPy could retain more effective information concerning the original organic input than the free biomarkers could.The GC-MS TIC trace of the saturated compounds in the Soxhlet extract of the P1q mudstone（WC-3）showed a unimodal arrangement and a maximum of n-C18（Fig.2）.The chromatogram of m/z 217 was dominated by pregnane and homopregnane.The distribution of C27-C28-C29regular steranes was characterized by C29＞C28＞C27（Fig.3），while the chromatogram of m/z 191 was predominantly characterized by C23tricyclic terpane（Fig.3）.Thus，the characteristics of the℈1n mudstone extract and the P1q mudstone extract are markedly different.

Fig.2 TICs of saturated hydrocarbons in the hydropyrolysates，Soxhlet extracts，sequential extracts，anhydrous pyrolysates and oil seepage.1 n C17=C17-n-alkanes，2 n C18=C18-n-alkanes，3 Pr=Pristane，4 Ph=Phytane

There was an unresolved complex mixture（UCM）hump in the GC-MS TIC trace of saturated hydrocarbons in the chloroform bitumen A from T1d marlite（Bitm-A）.Additionally，the concentration of pristane and phytane was higher in the entire sample relative to n-C17and n-C18，respectively（Fig.2）.This could imply that the chloroform bitumen A has undergone slight biodegradation（Peters and Moldowan，1993）.However，the n-alkane distribution still had a unimodal arrangement and a maximum at n-C22.The distribution of C27-C28-C29-αααR steranes had an order of C29＞C27＞C28.The pentacyclic terpanes were characterized by a high abundance of C30hopane，while the C23tricyclic terpane was predominat in the tricyclic terpanes（Fig.3）.Nevertheless，in the chloroform bitumen C（Bitm-C）of the T1d marlite，there was no unresolved complex mixture（UCM）hump，and the distribution of the n-alkanes showed an unimodal arrangement and a maximum at n-C17（Fig.2）.Chloroform bitumen C generally enters the mineral crystal inclusions or gaps during early digenesis，and was therefore less affected by the thermal alteration and modern sediment pollution than free oil and adsorbed oil because of the mineral crystal's protection（Spiro 1984）. Therefore，bitumen C might preserve information about the original organic matter（Spiro 1984；Wen and Zhang 1997；Li et al.2008）.By comparing the geochemical characteristics of chloroform bitumen A and chloroform bitumen C，we can fi nd the differences in the source of the organics and might be able to con fi rm the time of hydrocarbon fi lling（Pan et al.2003；Pan and Liu 2009；Jin et al.2012）. In Bitm-C，the abundance of C27-αααR sterane was slightly higher than that of the C28-αααR and C29-αααR steranes. Additionally，the terpane distribution was characterized by a high abundance of C23tricyclic terpane and C29hopane relative to C30hopane（Fig.3）.Therefore，Bitm-C was distinctly different from Bitm-A in the distributions of both steranes and terpanes.Anhydrous pyrolysis of kerogen releases strongly bound constituents from kerogen，such as n-alkanes and biomarkers（Tissot and Welte 1984；Brian 1984；Colin and Wang 1988）.Anhydrous pyrolysis at 320°C for 72 h is a type of mild pyrolysis，during which abundant hydrocarbons can be released without signi ficantly affecting the biomarker distribution and the carbon isotopes（Fu and Qin，1995；Xiong and Geng，2000）.The GC-MS TIC trace of saturates in the pyrolysates（Bitm-Py）had a maximum at n-C17（Fig.2）.The C27-αααR sterane was most abundant among the C27-C29-αααR steranes in Bitm-Py.The C29hopane was the most abundant compound in the distribution of terpanes in Bitm-Py，We found the same results for Bitm-C（Fig.3）.Therefore，remarkable differences exist between Bitm-A，Bitm-C and Bitm-Py. The C29-αααR sterane was the most abundant compound among the C27-C29-αααR steranes in Bitm-A，but in Bitm-C and Bitm-Py，the distribution of C27-C29-αααR steranes was dominated by the C27-αααR sterane.C30hopane was higher than C29hopane in Bitm-A，but C29hopane was higher in Bitm-C and Bitm-Py.

Fig.3 M/z 217 and m/z 191 mass chromatograms of the hydropyrolysates，Soxhlet extracts，sequential extracts，anhydrous pyrolysates and oil seepage.1 TT21=C21-tricyclic terpane，2 TT23=C23-tricyclic terpane，3 Tet24=C24-tetracyclic terpane，4 Ts=18α（H）-C27-trisnorhopane，5 Tm=17α（H）-C27-trisnorhopane，6 H29=C29-αβ hopane，7 H30=C30-αβ hopane，8 C21=C21-pregnane，9 C22=C22-homopregnane，10 C27D=C27-βαS diasterane，11 C27=C27-αααR sterane，12 C28=C28-αααR sterane，13 C29=C29-αααR sterane

4.1.2 The geochemical characteristics of the T1d oil seepage

The GC-MS TIC trace of saturates in the T1d oil seepage（Oil-1）showed that most n-alkanes were removed and thatthere was a broad unresolved complex mixture（UCM）hump and high peaks，indicating pristane and phytane（Fig.2）.This suggests that the oil seepage Oil-1 was heavily biodegraded（Peters and Moldowan 1993）.The concentration of regular steranes was higher than that of pregnane and homopregnane，but the distribution of steranes was dominated by the C29-αααR sterane（Fig.3），indicating that the distribution of C27-C28-C29-αααR steranes might have been altered to some extent by biodegradation.Similar to Bitm-A，Oil-1 had a normal pattern ajd C30hopane predominated in the pentacyclic terpanes，while C23tricyclic terpane predominated in the tricyclic terpanes（Fig.3）.

4.2 Oil-source correlation of the T1d oil seepages with possible potential source rocks

4.2.1 Steranes

The distribution of the C27-C28-C29regular steranes was widely used in oil-source correlations due to the stability of regular steranes in the oil window（Peters et al.1989）.The distribution of the C27-C28-C29regular steranes in the hydropyrolysates H-SW-8 was dominated by the C27-αααR sterane（Fig.4A），while in the EOM from the P1q mudstone WC-3，it was dominated by the C29-αααR sterane.For the T1d marlite，the C27-αααR sterane was most abundant in the distribution of C27-C29-αααR steranes in chloroform bitumen C（Bitm-C）and the kerogen pyrolysates（Bitm-Py）. Nevertheless，the distribution of the C27-C29-αααR regular steranes in the T1d oil seepage（Oil-1）was dominated by the C29-αααR sterane.The C29/（C27+C28+C29）ratio was higher than 0.54，much higher than that in the potential source rocks mentioned above.Diasteranes are thought to have a higher bioresistance than the C27-29regular steranes（Rullko¨tter and Wendisch 1982；Goodwin et al.1981）.The C27DβαRdiasterane/C27ααRsterane ratio in Oil-1 was 0.94，which was higher than those in the other samples（Fig.4B）. The broad unresolved complex mixture（UCM）hump of the T1d oil seepage indicates that biodegradation had partly consumed the C27-αααRand C28-αααRsteranes in Oil-1 but had not yet affected the C29-αααR sterane and the C27DβαS diasterane.Therefore，the biodegradation may have invalidated the biomarkerparameters based on the regularsteranes in the T1d oil seepage for the oil-source correlation.The regular sterane characteristics of the T1d oil seepage for the oil-source correlations should be cautiously applied.

It is believed that pregnane and homopregnane were derived from hormones，pregnanol，pregnanone or the thermal cracking of the C27-C29-regular steranes（De Leeuw and Bass 1986；Huang et al.1994）.Pregnane and homopregnane are highly resistant to biodegradation and may preserve original information about the source organic matter（Wenger and Isaksen 2002）.Table 2 shows that the pregnane/homopregnane ratio（C21/C22）of H-SW-8 and WC-3 is 1.78 and 1.51，respectively.The C21/C22ratio in Bitm-C and Bitm-Py is 2.93 and 2.66，respectively.The ratio in Oil-1 is 2.12，lower than Bitm-C and Bitm-Py，but higher than H-SW-8 and WC-3.Therefore，the heavily biodegraded oil seepage Oil-1 may have multiple sources.

4.2.2 Terpanes

The bioresistance of the terpanes was higher than that of the regular steranes.The abundances of the terpanes was high relative to that of the regular steranes in the T1d oil seepage，indicating biodegradation may not have affected the distribution of the terpanes（Seifert and Moldowan 1979；Aquino et al.1983；Peters and Moldowan 1993）in the T1d oil seepage.Therefore，the characteristics of terpanes could be used for oil-source correlation.Tricyclic terpanes are widely found in oils and source rock extracts，especially in crude oils from marine sources.The tricyclic terpanes possibly originated from prokaryotic cell membranes and are generally dominated by C23tricyclic terpane（Ourisson et al.1982；Connan et al.1980；Aquino et al. 1983）.The T1d oil seepage（Oil-1），chloroform bitumen A（Bitm-A）and chloroform bitumen C（Bitm-C），all contained high concentrations of tricyclic terpanes，with similar distributions of tricyclic terpanes（Table 2）.For Oil-1，Bitm-A and Bitm-C，the ratio of C21tricyclic terpane to C23tricyclic terpane（TT21/TT23）ranged from 0.52 to 0.59，while the ratio of C23tricyclic terpane to C24tricyclic terpane（TT23/TT24）ranged from 1.16 to 1.31（Fig.5A，B）. The TT21/TT23ratios in the EOM from the P1q mudstone（WC-3），℈

Fig.4 Triangular plot of the distribution of C27-C28-C29αααR steranes and C27DβαR diasterane/C27ααR sterane ratio in the hydropyrolysates，Soxhlet extracts，sequential extracts，anhydrous pyrolysates and oil seepages

1n kerogen hydropyrolysates（H-SW-8）and the T1d kerogen pyrolysates（Bitm-Py）are 0.75，0.60 and 0.71，respectively，while it is 0.52 in Oil-1（Fig.5A）.Therefore，the TT21/TT23ratio of the T1d oil seepage might have a close relationship with℈1n mudstone.A simulation study by Liao et al.（2012）indicated that the TT23/（TT23+TT24）ratio is a reliable index in bitumen-source and bitumenbitumen correlations when oils have undergone heavy biodegradation and subsequent severe thermal alteration. Table 2 shows the TT23/TT24ratios in oil seepages，bitumen C，potential source rock extracts，kerogen hydropyrolysates and kerogen pyrolysates.The TT23/TT24ratio of Oil-1 was much closer to that of T1d kerogen pyrolysate Bitm-Py（TT23/TT24=1.13）than those of H-SW-8 and WC-3（Fig.5B），indicating that the T1d oil seepage Oil-1 might have a close relationship with the T1d mudstone. Therefore，the distribution of tricyclic terpanes shows that the T1d oil seepage might come from multiple source rocks.

The ratio of C23tricyclic terpane to C30pentacyclic terpane（TT23/H30）was commonly used as a source-related biomarker parameter to identify the different organic inputs from bacteria，algae or other prokaryotic organisms（Peters et al.2005；Liao et al.2012）.The TT23/H30ratios in the chloroform bitumen A（Bitm-A）from the T1d marlite and the T1d oil seepage Oil-1 were 0.62 and 0.52，respectively. The TT23/H30ratio in H-SW-8 was only 0.14，lower than that in Oil-1（Fig.5C）.Nevertheless，the TT23/H30ratios in the other potential source rock samples were greater than 1.0.Hence，the TT23/H30ratio of Oil-1 falls between those of H-SW-8 and Bitm-Py，indicating that the T1d oil seepage should originate from multiple source rocks.

Table 2 Biomarker parameters of the hydropyrolysates，the Soxhlet extracts，the sequential extracts，the pyrolysates and the oil seepage

The H29/H30ratios in crude oils were usually below 1.0，except for crude oils derived from evaporite or carbonate. The crude oils generated from source rocks containing rich terrestrial organic matter also exhibited a relative predominance of C29hopane.Therefore，the H29/H30ratio might contain information regarding the original organic input and the depositional environment of the source rocks（Clark and Philp 1989；Brooks 1986）.Figure 5D shows that the T1d kerogen pyrolysate Bitm-Py has the highest H29/H30ratio of 1.50，while the Bitm-C has second highest H29/H30value，1.12.Nevertheless，the H29/H30ratio for the T1d oil seepage is 0.49，which is very similar to those of the extracts and hydropyrolysates from the potential source rocks，but is significantly lower than those of Bitm-Py and Bitm-C（Fig.5D）.Therefore，some differences should be evident in the source organic input of the T1d oil seepage and the T1d marlite.

A number of maturity parameters are listed in Table 2. Most of the maturity parameters were similar for all of the samples，such as 22S/（22S+22R）-H31.However，the Ts/（Ts+Tm）ratios of all of the samples，except Bitm-Py，were very close.Bitm-Py had a much lower Ts/（Ts+Tm）ratio of 0.11（Fig.5E），possibly because the covalently bound biomarkers were protected from secondary alterations，such as biodegradation and thermal maturation，by the kerogen macromolecular structure.Ts and Tm are thought to be bonded to the kerogen macromolecules in early diagenesis.They can avoid the isomerization common in free biomarkers（Love et al.1998；Liao et al.2012）. The MPI indices of Oil-1，Bitm-A，Bitm-C and Bitm-Py had similar values，all within the range of 0.76-0.99（Fig.5F）.However，the MPI indices of WC-3 and H-SW-8 were 1.58 and 2.03，respectively（Fig.5F），both greater than that of Oil-1 and Bitm-Py，implying that the T1d oil seepage may correlate with the T1d marlite.

Fig.5 Biomarker parameters of terpanes in the hydropyrolysates，Soxhlet extracts，sequential extracts，anhydrous pyrolysates and oil seepage

The above oil-source correlation study based on the distribution of tricyclic terpanes and pentacyclic triterpanes suggests that the T1d oil seepage could not have been generated from a single source rock.Not only the T1d marlite，but also the℈1n mudstone might have contributed to the oil seepage in the T1d marlite.Furthermore，because of the characteristics of the steranes and terpanes，the possibility of a contribution from the P1q source rock cannot be excluded.

4.2.3 Triaromatic steroids

It is commonly known that triaromatic steroids are the aromatization products of monoaromatic steranes and have highest bioresistance among the commonly used biomarkers（Peters and Moldowan 1993；Ludwig et al. 1981）.The long chain triaromatic steroids（C26-C28-triaromatic steroid）could have originated from marine Acritarchs（Zhang et al.2002；Peters et al.2005）.The distribution of long chain triaromatic steroids can ref l ect the original organic input（Ludwig et al.1981；Mackenzie et al.，1982）.The short chain triaromatic steroids（C20-C21-triaromatic steroid）did not originate entirely from the degradation of their long chain homologues（C26-C28-triaromatic steroid），ergo they may have different organic matter inputs than those of the long chain homologues（Mackenzie et al.1982；Riolo et al.1986；Beach et al. 1989）.Therefore，it is appropriate to consider the distribution of the long chain triaromatic steroids in an oilsource correlation.Figure 6 shows that the distribution of long chain triaromatic steroids in the T1d oil seepage Oil-1 is dominated by the C28-triaromatic steroid，which is different from the distributions in the Bitm-Py，but similar to those of the Lower Ordovician oil seepages（KT-1，KT-2）in Kaitang village in the Kaili area，which were demonstrated to have originated from the℈1n mudstone by Fang et al.（2011）.Therefore，the T1d oil seepage should have a close relationship with the℈1n mudstone.By comparing the distribution of the triaromatic steroids，He et al.（2012）also proposed that the T1d oil seepages in Mengguantownship，which was very close to Ni'erguan village，originated from the℈1n mudstone.Additionally，the triaromatic steroids were not detected in the P1q mudstone，indicating that the P1q source rock cannot be the sole source rock for the T1d oil seepage.

Fig.6 Distributions of long chain triaromatic steroids in the sequential extracts，pyrolysates and oil seepages.TA26S C26S-triaromatic steroid，TA26R+27S C26R-triaromatic steroid+C27S-triaromatic steroid，TA28S C28S-triaromatic steroid，TA27R C27R-triaromatic steroid，TA28R C28R-triaromatic steroid

Fig.7 Bulk δ13C values of the whole oil seepages，sequential extracts and anhydrous pyrolysates

4.2.4 Bulk δ13C values of whole oil samples and extracts The bulk δ13C for both the potential source rock kerogens and the oil seepage are shown in Table 1.The bulk kerogen δ13C values of the T1d marlite（NEG-3），℈1n mudstone（SW-8）and P1q mudstone（WC-3）were-26.8‰，-32.5‰and-28.7‰，respectively.The bulk δ13C value of the T1d oil seepage（Oil-1）was-30.5‰，while the chloroform bitumen A（Bitm-A）and chloroform bitumen C（Bitm-C）from T1d marlite had similar δ13C values of -29.6‰and-29.4‰，respectively，approximately 1‰heavier（enriched in13C）than that of Oil-1.The bulk δ13C value of whole oil changes very little and could be a useful index in an oil-source correlation of heavily biodegraded oils（Kvenvolden et al.1995；Sun et al.2005）.If the kerogen pyrolysates，chloroform bitumen C，chloroform bitumen A and oil seepage originated from the same source rock，the difference between them in the bulk δ13C should be within 2‰（Tissot and Welte，1984）.However，the δ13C value of Bitm-Py was-27.8‰，2.7‰heavier than that of the T1d oil seepage（Fig.7）.Therefore，the T1d oil seepage could not be entirely derived from the T1d marlite，but might partly originate from the℈1n mudstone whose bulk δ13C of kerogen is-32.5‰.Additionally，the bulk δ13C values of two oil seepages（Hu47，MJ-02）derived from the℈

1n mudstone（Zhang et al.2007）were-30.8‰and-31.9‰，respectively（Table 3；Fig.7），close to that of the T1d oil seepage.This implies that the T1d oil seepage could mainly originate from the℈1n mudstone.The bulk δ13C values of the Lower Permian oil seepages（MJ-03，LJ-22）that are believed to have originated from the high quality P1q source rocks are about-27.5‰（Table 3；Fig.7），which is significantly heavier than that of the T1d oil seepage（Oil-1）.Therefore，the T1d oil seepage cannot have a close relationship with the P1q source rocks.

4.2.5 The δ13C values of individual n-alkanes

The stable carbon isotopic composition of the individual nalkanes is one of the most important pieces of information for an oil-source correlation（Bjorøy et al.1991；Liu et al. 2006:Yu et al.2012）.It is commonly accepted that the δ13C values of n-alkanes change only slightly during biodegradation（Xiong and Geng 2000；Sun et al.2005）. Mild thermal maturation may lead to a slight enrichment in13C.Typically，such a variation in the δ13C value is within 2-3‰（Bjorøy et al.1992；Liao et al.2004，2012；Liao and Geng 2009）.The δ13C value for the individual nalkanes in the T1d oil seepage（Oil-1）ranged from-29‰to-31.5‰，which is very similar to those in the chloroform bitumen A（Bitm-A）（Fig.8；Table 4）.The δ13C value of the individual n-alkanes（C18-C27）in Bitm-C ranged from-29‰to-30‰，heavier than those in both Oil-1 and Bitm-A.This indicates that exchange between free oil（Oil-1）and adsorbed oil（Bitm-A）might easily occur，but that such an exchange would be difficult between free oil and oil trapped in an inclusion.The δ13C values of the individual n-alkanes in Bitm-Py were all around-28‰，which was 1-2‰heavier than those in Bitm-C.However，the difference in the δ13C of the individual n-alkanes between Bitm-Py and Oil-1 was over 2‰（Fig.8），indicating that the T1d oil seepage could not be entirely derived from the T1d marlite.The δ13C values of the individual n-alkanes in the EOM from the P1q mudstone（WC-3）were the heaviest，ranging from-25‰to -28‰（Fig.8），while the δ13C values of individual nalkanes in the hydropyrolysates（H-SW-8）ranged frp，-29‰to-31‰（Fang et al.2014），very similar to those in Oil-1（Fig.8），suggesting that the T1d oil seepage could be mostly generated from the℈1n mudstone.

Table 3 Basic information of oil seepages from other literatures

Fig.8 δ13C values of individual n-alkanes in the hydropyrolysates，Soxhlet extracts，sequential extracts，anhydrous pyrolysates and oil seepage

1n mudstone and the partially mixed oils derived from the T1d marlite.

4.3 The history of hydrocarbon generation of source rocks

Table 4 The δ13C values of individual n-alkanes of the hydropyrolysates，the Soxhlet extracts，the sequential extracts，the pyrolysates and the oil seepage

Fig.9 Hydrocarbon generation history of source rocks in Guiyang area（modified from Feng et al.2008）

5 Conclusions

（1）Our oil-source correlation results indicate that the heavily biodegraded T1d oil seepage was primarily generated from the℈1n mudstone in the Late Permian-Early Triassic and partially mixed with oil generated from T1d marlite，but the oil generated from the P1q may have been neglected；

（2）Our oil-source correlation in Ni'erguan village demonstrates that such methods as catalytic hydropyrolysis，anhydrous pyrolysis and sequential extraction are effective in a complicated oil-source correlation in the Southern Guizhou Depression.

Acknowledgments We would like to extend our thanks to Dr. Liangliang Wu and Dr.Bin Cheng of Guangzhou Institute of Geochemistry for their useful suggestions and help.Our thanks also go to Dr.Yankuang Tian and Mr.Huashan Chen of Guangzhou Institute of Geochemistry for their help with the GC-MS and GC-C-IRMS analyses.This work was supported jointly by the National Science and Technology Major Project of China（Grant Nos:2011ZX05008-002 and 2011ZX05005-001）.

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Received:28 November 2014/Revised:25 May 2015/Accepted:19 October 2015/Published online:28 October 2015

©Science Press，Institute of Geochemistry，CAS and Springer-Verlag Berlin Heidelberg 2015

✉Ansong Geng asgeng@gzb.ac.cn

1The State Key Laboratory of Organic Geochemistry，Guangzhou Institute of Geochemistry，Chinese Academy of Sciences，Wushan，Guangzhou 510640，People's Republic of China

2University of Chinese Academy of Sciences，Beijing 100039，People's Republic of China

3The Key Laboratory of Marine Mineral Resources，Guangzhou Marine Geological Survey，Ministry of Land and Resources，Guangzhou 510075，People's Republic of China

1n mudstone，sequential extraction，to obtain chloroform bitumen A and chloroform bitumen C from the T1d marlite，and anhydrous pyrolysis，to release pyrolysates from the kerogen of T1d marlite.Using the methods above，the biomarkers and n-alkanes released from the oil samples and source rocks were analysed by GC-MS and GC-C-IRMS.The oil-source correlation indicated that the T1d oil seepage primarily originated from the℈

1n mudstone and was partially mixed with oil generated from the T1d marlite.Furthermore，the seepage also demonstrated that the above methods were effective for the complicated oil-source correlation in the Southern Guizhou Depression.