Oil families and their source rocks in the Weixinan Sub-basin, Beibuwan Basin, South China Sea Baojia Huang a,b , Xianming Xiao a,⇑ , Dongsheng Cai b , R.W.T. Wilkins c , Mingquan Liu b a State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640 Guangdong, China b Research Institute, CNOOC Ltd., Zhanjiang, 524057 Guangdong, China c CSIRO Petroleum, P.O. Box 136, North Ryde, NSW, Australia article info Article history: Received 8 June 2010 Received in revised form 11 November 2010 Accepted 2 December 2010 Available online 10 December 2010 abstract Thirty-one crude oils and 15 source rocks were selected for molecular geochemical and isotopic analyses in order to establish the genetic relationships between discovered oils and petroleum source rocks in the Weixinan Sub-basin, Beibuwan Basin, South China Sea. Three groups of oils were recognized. Group I oils are only found in the upper section of the Liusagang Formation, with a moderate abundance of C 30 4-methylsteranes, low oleanane contents and lighter d 13 C values, showing a close relation to the shale occurring in the upper section of the Liusagang Formation. Group II is represented by the majority of the discoveries and is distributed in multi-sets of reservoirs having different ages. The oils are character- ized by a high abundance of C 30 4-methylsteranes, low to moderate abundance of oleanane and heavy d 13 C values, and shows a good correlation with the lacustrine shale and oil shale in the middle section of the Liusagang Formation. Group III oils occurred in the lower section of the Liusagang Formation. The oils have a lower concentration of C 30 4-methylsteranes, relatively high abundance of oleananes and their d 13 C values are intermediate. Oils of this group correlated well with the shallow lake-delta mudstone of the lower section of Liusagang Formation. These oil-source genetic relationships suggest a strong source facies control on the geographic distribution of oil groups within the Weixinan Sub-basin. The geochemical data indicate shale in the middle section of the Liusagang Formation has an excellent oil generation potential and the lower and upper sections contain dark shale and mudstone with good to fair oil potential. Future exploration or assessment of petroleum potential of the sub-basin could be improved by considering the proposed genetic relationship between the oil types and source rocks, as well as their distribution. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The Beibuwan Basin (Fig. 1a) is one of four petroliferous basins in the northern continental shelf area of the South China Sea. It covers an area of nearly 40,000 km 2 . The Weixinan Sub-basin (Fig. 1a) is located in the north part of the Beibuwan Basin. Over the past 30 years, petroleum exploration has been continuously conducted in the basin, especially in the Weixinan Sub-basin where a few oil bearing structures and several commercial oil fields have been dis- covered, such as the W10-3, W11-4, W11-4N, W11-1, W12-1, W6-8, W6-9 and W12-8 oil fields (Fig. 1b). Although data from recent exploration efforts in the sub-basin have greatly enhanced the understanding of its petroleum system, some key problems are still unresolved. Previous studies (Zhang and Huang, 1991; Gong and Li, 1997; Liu, 2004) have demonstrated there are several sets of source rocks with petroleum potential within the Eocene Liushagang Formation and the discovered oils have variable compositions. However, little work has been done on identifying oil families and establishing their relationships with the source rocks. Although shales in the middle section of Liushagang Formation are considered to be the main source rocks in the Weixinan Sub-basin, some oils are clearly derived from other sources. For example, a few commercial oil fields have been discovered in sandstones enclosed by dark shales in the upper section of the Liushagang Formation, clearly indicating an in situ oil source (Liu, 2004). The purpose of this paper is to define organic source facies and oil families and to make a correlation of the oils with their source rocks through the use of molecular and stable carbon isotopic parameters in order to achieve a better understanding of the petroleum system and its potential for additional oil discoveries, thereby reducing exploration risk in this area. 2. Geological setting The Beibuwan Basin is a Mesozoic–Cenozoic extensional basin, containing six sub-basins and three uplifts (Fig. 1a). The structural 0146-6380/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2010.12.001 ⇑ Corresponding author. Tel.: +86 020 85290176. E-mail address: [email protected](X. Xiao). Organic Geochemistry 42 (2011) 134–145 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem
Oil families and their source rocks in the Weixinan Sub-basin, BeibuwanBasin, South China Sea
Baojia Huang a,b, Xianming Xiao a,⇑, Dongsheng Cai b, R.W.T. Wilkins c, Mingquan Liu b
a State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640 Guangdong, Chinab Research Institute, CNOOC Ltd., Zhanjiang, 524057 Guangdong, Chinac CSIRO Petroleum, P.O. Box 136, North Ryde, NSW, Australia
a r t i c l e i n f o a b s t r a c t
Article history:Received 8 June 2010Received in revised form 11 November 2010Accepted 2 December 2010Available online 10 December 2010
0146-6380/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.orggeochem.2010.12.001
Thirty-one crude oils and 15 source rocks were selected for molecular geochemical and isotopic analysesin order to establish the genetic relationships between discovered oils and petroleum source rocks in theWeixinan Sub-basin, Beibuwan Basin, South China Sea. Three groups of oils were recognized. Group I oilsare only found in the upper section of the Liusagang Formation, with a moderate abundance of C30
4-methylsteranes, low oleanane contents and lighter d13C values, showing a close relation to the shaleoccurring in the upper section of the Liusagang Formation. Group II is represented by the majority ofthe discoveries and is distributed in multi-sets of reservoirs having different ages. The oils are character-ized by a high abundance of C30 4-methylsteranes, low to moderate abundance of oleanane and heavyd13C values, and shows a good correlation with the lacustrine shale and oil shale in the middle sectionof the Liusagang Formation. Group III oils occurred in the lower section of the Liusagang Formation.The oils have a lower concentration of C30 4-methylsteranes, relatively high abundance of oleananesand their d13C values are intermediate. Oils of this group correlated well with the shallow lake-deltamudstone of the lower section of Liusagang Formation. These oil-source genetic relationships suggesta strong source facies control on the geographic distribution of oil groups within the Weixinan Sub-basin.The geochemical data indicate shale in the middle section of the Liusagang Formation has an excellent oilgeneration potential and the lower and upper sections contain dark shale and mudstone with good to fairoil potential. Future exploration or assessment of petroleum potential of the sub-basin could be improvedby considering the proposed genetic relationship between the oil types and source rocks, as well as theirdistribution.
� 2010 Elsevier Ltd. All rights reserved.
1. Introduction
The Beibuwan Basin (Fig. 1a) is one of four petroliferous basins inthe northern continental shelf area of the South China Sea. It coversan area of nearly 40,000 km2. The Weixinan Sub-basin (Fig. 1a) islocated in the north part of the Beibuwan Basin. Over the past30 years, petroleum exploration has been continuously conductedin the basin, especially in the Weixinan Sub-basin where a few oilbearing structures and several commercial oil fields have been dis-covered, such as the W10-3, W11-4, W11-4N, W11-1, W12-1, W6-8,W6-9 and W12-8 oil fields (Fig. 1b). Although data from recentexploration efforts in the sub-basin have greatly enhanced theunderstanding of its petroleum system, some key problems are stillunresolved. Previous studies (Zhang and Huang, 1991; Gong and Li,1997; Liu, 2004) have demonstrated there are several sets of sourcerocks with petroleum potential within the Eocene Liushagang
ll rights reserved.
Formation and the discovered oils have variable compositions.However, little work has been done on identifying oil families andestablishing their relationships with the source rocks. Althoughshales in the middle section of Liushagang Formation are consideredto be the main source rocks in the Weixinan Sub-basin, some oils areclearly derived from other sources. For example, a few commercialoil fields have been discovered in sandstones enclosed by darkshales in the upper section of the Liushagang Formation, clearlyindicating an in situ oil source (Liu, 2004). The purpose of this paperis to define organic source facies and oil families and to make acorrelation of the oils with their source rocks through the use ofmolecular and stable carbon isotopic parameters in order to achievea better understanding of the petroleum system and its potential foradditional oil discoveries, thereby reducing exploration risk in thisarea.
2. Geological setting
The Beibuwan Basin is a Mesozoic–Cenozoic extensional basin,containing six sub-basins and three uplifts (Fig. 1a). The structural
Fig. 1. Maps showing (a) the structural divisions of the Beibuwan Basin and (b) major structures and sampled wells in the Weixinan Sub-basin.
B. Huang et al. / Organic Geochemistry 42 (2011) 134–145 135
evolution of the basin can be divided into an early (Eocene toOligocene) extensional phase which was marked by fault boundedand rifted strata, and a late (Miocene to recent) passive marginphase with relatively unstructured strata, leading to two distinc-tive structural layers (Liu, 2004).
In response to the above structural evolution, the dominantenvironments for sedimentary deposition in the Beibuwan Basinevolved from the early continental non-marine fluvial and lacus-trine deposition of the Paleocene to Oligocene, to marine trans-gression during the Miocene (Fig. 2), forming syn-rift depositsand post-rift deposits, respectively. The Eocene–Oligocene rifting
stage includes two significant syn-rift phases represented by theChangliu Formation characterized by coarse grained, matrix sup-ported conglomerate adjacent to highly active fault scarps andthe Liushagang–Weizhou formations, with interbedded sandstoneand shale more generally within the sub-basin, to a total thicknessof 3500 m. The Liushagang Formation is subdivided into upper,middle and lower sections: Liushagang-1, Liushagang-2 andLiushagang-3, respectively, each consisting of a combination ofdark shale or mudstone with sandstone. The Weizhou Formationalso contains dark mudstone, although dominated by sandstone.The top of the Weizhou Formation is marked by a significant
Fig. 2. Schematic stratigraphic column of the Weixinan Sub-basin.
136 B. Huang et al. / Organic Geochemistry 42 (2011) 134–145
regional unconformity surface, which defines the boundarybetween the syn-rift and post-rift (passive) sedimentary fillingof the Beibuwan Basin. The post-rift marine sediments of theMiocene to Pleistocene, consisting mainly of sandstone interbed-ded with mudstones with a total thickness of about 3000 m(Fig. 2), act as an excellent regional seal for hydrocarbon fluids(Huang et al., 2008).
Available geological and geochemical data have indicated petro-leum source rocks are well developed in the Liushagang Formation(Zhang and Huang, 1991; Liu, 2004) and petroleum generated fromthese source rocks migrated into the sandstones within the Liusha-gang Formation or migrated upwards through faults into sand-stone bodies in the Weizhou, Xiayang and Jiaowei formations(Liu, 2004). Sandstone reservoirs in these formations are all pro-spective exploration targets in this sub-basin.
3. Samples and experimental conditions
TOC and Rock–Eval analysis data were obtained for about 150core and cutting samples from 30 wells and a total of 31 crude
oil samples were collected for analysis during drill stem tests inthe Weixinan Sub-basin (Fig. 1b). Selected source rocks wereextracted using dichloromethane in a Soxhlet apparatus for 72 h.After asphaltene was precipitated and group compositionsfractioned using column chromatography for both oils and rockextracts, their saturate fractions were analyzed using a Hewlett–Packard 5890II equipped with a 50 m � 0.32 mm i.d. HP-5 fusedsilica capillary column. The carrier gas was helium at a flow rateof 1.5 ml/min. The samples were injected using a splitless injectormaintained at 300 �C. The GC oven temperature was programmedfrom 35 to 300 �C at 5 �C/min and maintained at the final temper-ature for 30 min.
Gas chromatography–mass spectrometric (GC–MS) analyseswere carried out using a Thermo DSQ II-Trace/MS220-5327.Helium was used as carrier gas, and the oven temperature wasprogrammed from 50 to 300 �C at 4 �C/min. Samples were rou-tinely analyzed in full scan mode (m/z 50–500). The stable carbonisotope (d13C) analysis of whole oils and source rock kerogens wascarried out using a Finnigan MAT-251 mass spectrometer, with ananalytical precision of ±0.03‰ based on the PDB standard.
B. Huang et al. / Organic Geochemistry 42 (2011) 134–145 137
4. Results and discussion
4.1. Bulk composition
The oils from the Weixinan Sub-basin have a density rangingfrom 0.820–0.969 g/cm3 and a low sulfur content of less than0.5% (Table 1), typical of oils of non-marine origin (Gransch andPosthuma, 1974). Most of the oils have a saturated hydrocarbonpredominance (55–80%); only a few oil samples (W521-1 andW141-2) especially from the W5-2 and W1-4 oilfields have a lowabundance of saturates (Table 1). Those low saturate oils have ahigh content of non-hydrocarbon + asphaltene (usually >32%),low content of n-alkanes and low wax, indicating they have beenmoderately biodegraded (Fig. 3). In general, an oil with a saturatedhydrocarbon content over 60% is not effected by biodegradation inthe Weixinan Sub-basin and biodegradation becomes obvious assaturated hydrocarbon content decreases from 60% to 40%. Thebiodegraded oils were not used for oil-source correlation in thisstudy.
4.2. Oil families
On the basis of the saturated hydrocarbon biomarker distribu-tions and stable carbon isotopic data of the studied oils from theWeixinan Sub-basin, they can be largely classified into three com-positional groups (Table 1, Figs. 4–6).
4.2.1. Group I oilThe oil samples from the Liushagang-1 section reservoir of W6-
8, W6-9 and W11-1N oil fields belong to this group. The oil is char-
Table 1Basic data and selected geochemical parameters for crude oils from the Weixinan Sub-bas
a Fr. = Formation; Sats. = saturates, Arom. = aromatics, Res. = resins, Aph. = asphaltenesC30 hopane ratio, calculated from the m/z 191 mass fragmentograms; C27/C29 = C27/C29 20calculated from the m/z 217 mass fragmentograms; 20S/20R = 20S/(20S + 20R) ratio foWZ = Weizhou, LS1 = Liushagang-1, LS2 = Liushagang-2, LS3 = Liushagang-3, C = Cretaceo
acterized by a moderate abundance of C30 4-methylsteranes(Fig. 6) with the ratio of C30 4-methylsteranes to C29 steranes of0.72–2.24, low content of oleanane and lighter carbon isotope val-ues (�28.30‰ to �27.10‰) compared with other group oils (Table1, Figs. 4a and 5). The C30 4-methylsterane is a distinctive constit-uent of the Eocene Wenchang Formation lacustrine sediments inthe Pearl River Mouth Basin (Robison et al., 1998; Zhu et al.,1999; Huang et al., 2003) and its biological precursors may berelated to certain dinoflagellates thriving in freshwater lakes(Brassell et al., 1988; Goodwin et al., 1988). According to an inves-tigation made by Fu et al. (1985) on the Paleocene–Eocene oil shalefrom the onshore Maoming Basin, dinoflagellates with high hydro-carbon source potentials occur dominantly in deep freshwaterlacustrine facies. The moderate abundance of C30 4-methylsteranesof Group I oils indicates its source rocks deposited in a shallowerlake environment. Their C29 steroid isomerization ratio C29 (20S/20S + 20R) ranges from 0.28 to 0.44 (Fig. 4b) and does not reachits thermal equilibrium value of about 0.50 (Peters et al., 1986; Liet al., 1995), further indicating the source rock has not been buriedto a great depth.
4.2.2. Group II oilThis group includes oils from the Xiayang Formation, Jiaowei
Formation, Weizhou Formation, Carboniferous strata and some res-ervoirs in the Liushagang-3, Liushagang-2 and Liushagang-1 sec-tions in the Weixinan Sub-basin (Tables 1). The oils are black tobrown in color, with a medium density (0.821–0.887 g/cm3). Mostof them have high wax (20.5–24.4%) and low sulfur contents. Threeoil samples from shallow reservoirs influenced by biodegradationhave a relatively high density (0.942–0.969 g/cm3), high asphaltene
in.a
.% Res.% Aph.% ol/C30 C27/C29 4MSI 20S/20R d13Coil%0 Oil group
. Saturates, aromatics, resins, asphaltenes are normalized percent. ol/C30 = oleanane/R steranes; 4MSI = ratio of C30 4-methylsteranes to C29 regular steranes (20R + 20S),r aaa-C29 steranes; Formation name: Q = Quaternary, JW = Jiaowei, XY = Xiayang,us. nd = not determined.
WZ61S11812.1mWZ Formationoil
W521 830 893m XY Formation oil
Pr
Ph
n-C27
n-C17
Fig. 3. Gas chromatograms of aliphatic fractions of a biodegraded oil and a normal Group II oil from the Weixinan Sub-basin.
138 B. Huang et al. / Organic Geochemistry 42 (2011) 134–145
content, and low wax content (Table 1). The carbon isotope ratios ofthis group of oils vary from �24.56‰ to �26.88‰. Although theratios are mostly heavier than those of Group III oils, their valuesoverlap considerably (Table 1, Fig. 5).
Biomarker data are a valuable aid in defining a better groupidentification of the oils. Oils of group II without biodegradationshow a Pr/Ph ratio of 1.5–2.6, indicating an origin from type I/IIkerogen deposited under suboxic conditions. These oils are differ-entiated from oils of other groups by a high abundance of C30
4-methylsteranes (Fig. 6), with the ratio of C30 4-methylsteranesto C29 regular steranes of 2.34–5.2 and a relatively low content ofoleanane which is derived from angiosperms (Peters andMoldowan, 1993). Thus, Group II oils were most likely derivedfrom source rocks deposited in a freshwater lacustrine setting.These source rocks probably reached the peak stage of oil genera-tion based on the ratio C2920S/(20S + 20R) of 0.44–0.63 (Figs. 4band 6, Table 1). It was found that both samples of a shallow oil(936–940 m) and a deep oil (2913–2988 m) from the W12-8 andW12-2 reservoirs have a nearly identical composition and thermalmaturation level (Table 1), which suggests the oil generated fromdeeply buried source rocks underwent vertical migration to chargeshallow reservoirs.
4.2.3. Group III oilFour oil samples produced from the Liushagang-3 reservoirs
belong to this group. Similar to most of the Group II oils, theseoils have a low to medium density, high wax and low sulfurcontents (Table 1). The oils have a relatively high content ofoleanane, with ol/C30 ratio (oleanane/C30 hopane) of 0.26–0.35,a high pristane/phytane ratio of 2.03–2.48, and a moderate abun-dance of C30 4-methylsteranes (Figs. 4 and 6). These oils areslightly depleted in their 12C values compared to the Group I oil(Table 1, Fig. 5).
The relatively high pristane/phytane ratio, lesser abundance of4-methylsteranes and the presence of a relatively high content ofoleanane indicate that the Group III oils were derived from source
rocks with a more important contribution from terrigenous organicmatter.
4.3. Possible source rocks
As indicated by the results of regional geological and geochem-ical studies, there are two important sets of organic rich strata inthe Weixinan Sub-basin: the Eocene Liushagang Formation andthe Oligocene Weizhou Formation. Some of the major geochemicalparameters of these source rocks are presented in Figs. 7 and 8.According to the results, the three sets of dark shales and mud-stones in the Luishagang Formation are all good source rocks, butthe organic rich mudstone within the Weizhou Formation has alower oil potential.
4.3.1. Liushagang-3 sectionThe Liushagang-3 section in the lower part of the formation
contains approximately 200 m of interbedded sandstones andshales. The shales occur in thin layers. They are rich in organicmatter, with a TOC content of 0.4–4.35%, and S1 + S2 value of0.66–22.64 mg/g rock. The organic matter is dominated by typeII1 and II2 kerogens, with a small component of type III kerogen.The measured vitrinite reflectance values of the Liushagang-3 sec-tion shales from the slope area of the sub-basin with a burialdepth of 1200–3100 m range from 0.5–0.95%, within the oil win-dow. However, this section has a burial depth of 4500–5300 m inthe central area, at which the calculated Ro value is in the rangeof 1.5–2.0%.
The extracts of the Liushagang-3 shale have the moderate ratioof C30 4-methylsteranes to C29 regular steranes of 0.65–1.41, andrelatively abundant oleananes. There is a relative predominanceof the C29 relative to the C27 compound (Fig. 9). High relativeamounts of C29 steranes are generally thought to indicate a highercontent of terrigenous organic matter (Peters et al., 1986), support-ing the geological interpretation that the Liushagang-3 strata were
(a)
(b)
Fig. 4. Cross plots of (a) ol/C30ab-hopane ratio vs. C30 4MSI and (b) ol/C30ab-hopane ratio vs. C29 20S/20(S + R) for oil samples collected from the Weixinan Sub-basin,illustrating the three different genetic oil groups. ol/C30ab-hop = oleanane/C30ab-hopane ratio, C30 4MSI = C30 4-methylsterane to C29 regular steranes (20R + 20S) ratio.
Fig. 5. Cross plot of C30 4MSI vs. whole oil d13C values, showing three differentgenetic oil groups defined by biomarker parameters. 4MSI = C30 4-methylsterane toC29 regular steranes (20R + 20S) ratio.
B. Huang et al. / Organic Geochemistry 42 (2011) 134–145 139
deposited nearshore in a lake and deltaic environment with a dom-inant terrigenous input during deposition.
4.3.2. Liushagang-2 sectionDark shales are widely developed in the middle section of the
Liushagang Formation, with a total thickness of 300–500 m andoil shales occur on the bottom and top of the section, with a totalthickness of 10–60 m, representing a period of thriving lake devel-opment and anoxic deposition in the Weixinan Sub-basin. Theseshales have a TOC content ranging from 0.4–5.94%, with an averageof 2.33%. The dominant kerogens are types II1 and I. Most of theshales have a burial depth of 2000–4000 m, with a measured vitr-inite reflectance of 0.5–1.2%, indicating a main oil generation stage.In the deepest area, the burial depth of this section reaches 4600 m,with a calculated Ro value of 1.8% (Fig. 10).
Clear differences exist between the biomarkers of extracts ofthe Liushagang-2 and Liushagang-3 source rocks. As shown inFig. 9, the shale extracts of Liushagang-2 have a relatively higherratio of C30 4-methylsteranes to C29 regular steranes, and lowerabundance of oleananes, indicating a higher proportion of algal or-ganic matter. This is consistent with kerogens of types II1 and I(Fig. 8).
The Liushagang-2 section entered the oil generation thresholdat a burial depth of approximately 2500 m, in the central Weixinan
M/Z 191 M/Z 217
W11722063.6mLS1 Sectionoil
WZ114N6 2222.5-2235.0m LS1 Section
oil
Group I
W611 1967-2052.4m Carboniferous oil
W1032 1968.5-1978m LS3 Section
oil
W114A1899mJW Formationoil
Group II
W11813104-3270mLS3 Section oil
C30
OLTm
Ts
Group III
C30-4m St
Fig. 6. m/z 191, 217 and 369 mass fragmentograms of three genetic group of oils in the Weixinan Sub-basin (ol: oleanane; C30: C30 4-methylsteranes; Ts: 18a- trisnorhopane,Tm: 17a trisnorhopane).
140 B. Huang et al. / Organic Geochemistry 42 (2011) 134–145
Sub-basin reaching the main stage of oil generation during theMiocene to Pliocene.
4.3.3. Liushagang-1 sectionThe Liushagang-1 section in the upper part of the formation
consists of dark gray shales and sandstone. The shales have aTOC value ranging from 0.54–3.81% with an average value of1.8%, and a S1 + S2 value mostly >2 mg/g rock with an average valueof 4.8 mg/g rock (Fig. 7). According to the data of Rock–Eval anal-ysis, the organic matter in these samples is predominantly typeII1–II2.
Biomarkers from the rock extracts have some important differ-ences to those noted from the other two source facies. As com-
pared with source rocks from Liushagang-3 and Liushagang-2,the rock extracts have a relatively low content of C30 4-methylster-anes, and low abundance of oleananes (Fig. 9).
The Liushagang-1 source rocks are well developed over thewhole Weixinan Sub-basin, and have a current burial depth of2000–3400 m in the major part of the sub-basin, with a Ro valuebetween 0.4% and 1.0%. Because these source rocks have a rela-tively low maturity, they have only in part entered into the mainstage of oil generation (Fig. 10).
4.3.4. Weizhou FormationThe Weizhou Formation contains dark gray shales and coals.
These shales have TOC values ranging from 0.32–3.69% with an
0
10
20
30
40
TOC (%)
S 1+S 2
(mg/
g)
0
10
20
30
40
0 2 4 6 8 10
0 2 4 6 8 10
S 1+S 2
(mg/
g)
TOC (%)
LS1LS2LS3
(b) WZ Formation
(a) LS Formation
Fig. 7. Cross plots of TOC vs. Rock–Eval S1 + S2 values for potential source rocks from the Liushagang (a) and Weizhou formations (b).
0
100
200
300
400
500
600
700
800
900
Tmax (oC)
HI
(mgH
C/g
TO
C)
I
II1
II2
III
0
100
200
300
400
500
600
700
800
900
400 420 440 460 480400 420 440 460 480
I
II1
II2
III
Tmax (oC)
HI
(mgH
C/g
TO
C)
(b)(a)
LS1LS2LS3
Fig. 8. Modified Van Krevelen diagrams showing the Hydrogen Index–Tmax relationships for source rocks from the (a) Liushagang and (b) Weizhou formations.
B. Huang et al. / Organic Geochemistry 42 (2011) 134–145 141
average value of 0.57%. The kerogen types for most samples aretypes III and II2, with a minor portion of type II1 (Fig. 8). Distinctivebiomarker features of the Weizhou Formation rock extracts are thenear absence of C30 4-methylsteranes (Fig. 9) and a relatively high ra-tio of C29/C27. This indicates that the Weizhou Formation has a great-er terrigenous matter input compared to the Liushagang Formation.
The current burial depth of Weizhou Formation reaches locally3060 m in the depositional center, but is mostly 1500–2800 m inthe Weixinan Sub-basin, with a measured reflectance of 0.30–0.70%, in the immature to low mature stage of oil generation(Fig. 10). Therefore, the Weizhou Formation has made no signifi-cant contribution to the reservoirs in the Weixinan Sub-basin.
M/Z 217
C27
C29
C30-4mSt
M/Z 191
olTmTs
W11-8-1 3392–3394m LS3 Section shale
W111N2 2424m LS2 Section shale
W1121 2922–2940m LS1 Section
shale
W12222788–2790m LS2 Sectionoil-shale
C30
C30
olTm
Ts
C30-4m St
C27
C29
W12132436mWZ Formationshale
Fig. 9. Selected m/z 191, 217 mass fragmentograms of source rock extracts from the Liushagang and Weizhou Formations.
142 B. Huang et al. / Organic Geochemistry 42 (2011) 134–145
4.4. Oil-source rock correlation
It is clear from the above discussion that the three groups of oilsproduced from the Weixinan Sub-basin were derived from differ-ent source facies. Applying the relative abundance of C30 4-methyl-steranes, oleanane and stable carbon isotopic data as parameters totrace the relationship between the oils and the source rocks in thissub-basin, the oil-source correlation can be established. Figs. 6
and 9 show m/z 191 and 217 mass fragmentograms for the threeoil groups and their correlative source rocks. Group I oils can becorrelated to the shallow lake source facies of the Liushagang-1 sec-tion based on their moderate abundance of C30 4-methylsteranesand relatively low oleanane. Group II oils and the medium-deepwater lacustrine shales and oil shales from the Liushagang-2section are both high in C30 4-methylsteranes but low to moderatein oleanane, suggesting a clear genetic relationship. Group III oils
60 40 20 0
Age (my)
Dep
th (
m)
CL
LS3
LS2
LS1
WZ
XY
JW
DL
Q|
WL
Paleogene Neocene FormationQ
0
1000
2000
3000
4000
5000
6000
Ro=0.6%
Ro=1.3%
Q=Quaternary
WL=WangliujiaoDL=Dengliujiao
JW=JiaoweiXY=XiayangWZ=Weizhou
LS1=Liushagang-1LS2=Liushagang-2
LS3=Liushagang-3CL=Changliu
Fig. 10. Burial history curves for source rocks at the depocenter of the Weixinan Sub-basin. The location of the well used for this modeling work is shown in Fig. 1b. Oilwindow is defined as Ro = 0.6–1.35%. Formation names: CL = Changliu, LS = Liushagang; WZ = Weizhou; XY = Xiayang; JW = Jiaowei; DL = Dengliujiao; Q = Quaternary.
B. Huang et al. / Organic Geochemistry 42 (2011) 134–145 143
show a strong affinity to the shallow lake to delta source facies ofthe Liushagang-3 section because both have moderate C30
4-methylsterane and relatively high oleanane values.These oil-source correlations are further supported by the d13C
values of the oils and source rock kerogens. A non-biodegraded oilusually displays a similar or little lighter (1–2‰) d13C value with itssource kerogens (Tissot and Welte, 1984; Sofer, 1984). Fig. 11 alsopresents the genetic relationship between the three groups of oilsand their corresponding source kerogens based on their d13Cvalues.
Geological observations can give support to geochemical oil-source correlations in the Weixinan Sub-basin. For example, thereservoir yielding oil samples from the W6-8 Oilfield identifiedas Group I (Table 1), occurs within the Liushagang-1 shale, indicat-ing it is the only source of the oil.
Although the above simple oil-source correlation is dominant inthe Weixinan Sub-basin, mixing of oils sourced from the differentsections can be also traced from the geochemical data. For exam-ple, a few oil samples have their C30 4MSI values between GroupII oil and Group I oil (Fig. 4a). These oil samples were taken fromtraps related to boundary faults which cut through the Lisha-gang-1 and Lishagang-2 sections.
4.5. Implications for exploration
Exploration activities have mainly focused on oils derived fromthe Liushagang-II section and structure traps in the Weixinan Sub-basin over past 30 years (Liu, 2004). Recent discoveries of litholog-ical oil traps have inspired new interest in the central area of the
sub-basin (Huang et al., 2008). Future exploration or assessmentof petroleum potential of the Weixinan Sub-basin can be assistedby considering the proposed genetic relationships between theoil types and source rocks.
Provided that reservoirs within Liushagang-I, Liushagang-II andLiushagang-III sections are not cut through by faults, they usuallytrap oils from their own source rocks (Huang et al., 2008), thereforeall sandstone bodies within the Liushagang Formation should befavorable exploration targets. Burial history results show thatwithin the basin center area where the Liushagang-1 section ismore deeply buried and strong thermal maturation occurred, thereis greater oil potential. Compared with the Liushagang-2 and Liu-shagang-3 sections, the reservoirs within the Liushagang-1 sectionare at a fairly shallow burial depth and have therefore undergone arelatively low degree of catagenesis and should have good reser-voir properties (Liu, 2004). So the Liushagang-1 section will repre-sent an important target, especially in the central area, for futuresubtle oil-reservoir exploration in the Weixinan Sub-basin.
As discussed above, the Liushagang-2 section contains thethickest and most widespread source rock in the Weixinan Sub-ba-sin and a major part of the source rock is now in the main stage ofoil generation. Some boundary faults cut through this section toconnect very shallow strata including Jiaowei, Xiayang and Weiz-hou formations (Liu, 2004). The combination of the faults and sandbearing strata forms good pathways for oil migration (Fig. 12).These observations not only explain the current distribution ofgroup II oils, but also indicate that shallow strata, especially instructures close to the faults in the edge area of the sub-basin,would provide other favorable places for oil accumulation.
-29 -28 -27 -26 -25 -24
LS2 source rock
LS1 source rock
LS3 source rock
δ 13 C%o
Group I oil /
Group II oil /
Group III oil /
LS1 source rock
LS2 source rock
Group II oil
LS3 source rock
Group III oil
Group I oil
Fig. 11. Correlation of three groups of oils with corresponding source rock kerogens based on their d13C values.
dept
h (m
)
0
2000
4000
6000
WZ
LS2
LS3CL
LS1
XYJW
Upper Miocene -Q
C
A A’NW
Oil Heavy oil Source rock Sandstone
Fig. 12. Conceptual model of petroleum generation and migration in the Weixinan Sub-basin, showing reservoirs in relation to source rocks and migration pathways. Thelocation of the section is shown in Fig 1b. See Fig. 10 for abbreviations.
144 B. Huang et al. / Organic Geochemistry 42 (2011) 134–145
Although the source rocks within the Lishagang-3 section havelower oil potential compared with other two sections, the group IIIoil which has been discovered from within this formation proves itis also prospective. Further exploration should investigate sand-stone bodies within the section, as well as the trap structures re-lated to boundary faults which extend to the Lishagang-3 section.Since the reservoir within Lishagang-3 section has evolved to themiddle-late stage of diagenesis caused by its deep burial in thecentral area of the Weixjnan Sub-basin (Fig. 12), its low porosityand permeability would be a negative for further exploration.
5. Conclusions
(1) Crude oils produced in the Weixinan Sub-basin can be lar-gely classified into three groups based on their biomarkerdistributions and carbon isotopic values.
(2) The Eocene Liushagang Formation contains three differentpetroleum source rock facies. They are the shallow lake shalefacies of the Liushagang-1 section in the upper part, the med-ium-deep lake shale facies of the intermediate Liushagang-2section, and the shallow lake to delta shale facies of theLiushagang-3 section in the lower part of the formation.
(3) Groups I, II and III oils are correlated to source rocks in theLuishagang-1, Luishagang-2 and Luishagang-3 sections,respectively.
(4) The proposed oil-source relationships indicate a strong con-trol of mature source rocks on the oil distribution within theLiushagang Formation. Shallow oil accumulations have beenstrongly controlled by faults cutting through the sourcerocks and reservoirs.
Acknowledgements
The authors are indebted to Dr. Andrew Murray and twoanonymous reviewers for their insightful comments and sugges-tions that have significantly improved the manuscript, and toCNOOC-Ltd. for making available the data. This work was finan-cially supported by the Earmarked Fund of the State Key Labora-tory of Organic Geochemistry of the Chinese Academy of Sciences(Grant No. SKLOG2009A01). This is contribution No. IS1273 fromGIGCAS.
Associate Editor—Andrew Murray
References
Brassell S.C., Sheng G.Y., Fu J.M., Eglinton G., 1988. Biological markers in lacustrineChinese oil shales. In: Fleet A.J., Kelts K., Talbot M.R. (Eds.), Lacustrine PetroleumSource Rocks. Blackwell, pp. 299–308.
B. Huang et al. / Organic Geochemistry 42 (2011) 134–145 145
Fu J.M., Xu F., Chen D., Liu D., Hu C., Jia R., Xu S., Brassell S.C., Eglinton G., 1985.Biological markers of organic input in Maoming oil shale. Geochemistry 2, 119–127 (in Chinese).
Gong Z.S., Li S.T., 1997. Continental Margin Basin Analysis and HydrocarbonAccumulation of the Northern South China Sea. Science Press, Beijing (inChinese).
Goodwin N.S., Mann A.L., Patience R.C., 1988. Structure and significance of C30 4-methyl steranes in lacustrine shales and oils. Organic Geochemistry 12, 495–506.
Gransch J.A., Posthuma J., 1974. On the origin of sulphur in crudes. In: Tissot B.,Bienner F. (Eds.), Advances in Organic Geochemistry 1973. Editions Technip,Paris, pp. 727–739.
Huang B.J., Xiao X.M., Zhang M.Q., 2003. Geochemistry, grouping and origins ofcrude oils in the western Pearl River Mouth Basin, offshore South China Sea.Organic Geochemistry 34, 993–1008.
Huang B.J., Xu X.D., Li L., 2008. Geochemistry, Petroleum Potentials of Source Rocksand Hydrocarbon Accumulation in the Weixinan Sub-basin, Beibuwan Basin,South China Sea. CNOOC Internal Report (in Chinese).
Li M., Larter S.R., Mei B., Wu T., Chen Y., 1995. Maturity assessment of ‘‘immatureoils ’’ produced from the Shahejie Formation of the Liaohe Basin, NE China. In:Grimalt J.O., Dorronsoro C. (Eds.), Organic Geochemistry: Developments andApplications to Energy, Climate, Environment and Human History. EuropeanAssociation of Organic Geochemists, San Sebastian, pp. 421–423.
Liu M.Q., 2004. Assumed gas accumulation systems in the Beibuwan basin. ChinaOffshore Oil and Gas 16, 93–97 (in Chinese).
Peters K.E., Moldowan J.M., Schoell M., Hempkins W.B., 1986. Petroleum isotopicand biomarker composition related to source rock organic matter anddepositional environment. In: Leythaeuser D., Rullkötter J. (Eds.), Advances inOrganic Geochemistry 1985. Organic Geochemistry 10, 17–27.
Peters K.E., Moldowan J.M., 1993. The Biomarker Guide, Interpreting MolecularFossils in Petroleum and Sediments. Englewood Cliffs, Prentice Hall.
Robison C.R., Elrod L.W., Bissada K.K., 1998. Petroleum generation, migration, andentrapment in the Zhu 1 depression, Pearl River Mouth Basin, South China Sea.International Journal of Coal Geology 37, 155–178.
Sofer Z., 1984. Stable carbon isotope compositions of crude oil, application to sourcedepositional environments and petroleum alteration. Bulletin of AmericanAssociation of Petroleum Geologists 68, 31–49.
Tissot B.P., Welte D.H., 1984. Petroleum Formation and Occurrence, second ed.Springer-Verlag, Berlin.
Zhang Q.X., Huang B.J., 1991. Genetic types and generation history of natural gasesfrom major basins in the South China Sea. China Offshore Oil and Gas 4, 5–13 (inChinese).
Zhu W.L., Li M.B., Wu P.K., 1999. Petroleum system of Zhu III sub-basin, Pearl RiverMouth Basin, South China Sea. Bulletin American Association of PetroleumGeologists 83, 990–1003.
