Lacustrine source rock deposition in response to co-evolution of environments and organisms controlled by tectonic subsidence and climate, Bohai Bay Basin, China Fang Hao a,b,⇑ , Xinhuai Zhou c , Yangming Zhu d , Yuanyuan Yang a a State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Fuxue Road No. 18, Changping, Beijing 102249, China b Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China c Department of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang 310027, China d Tianjin Branch of China National Offshore Oil Company Ltd., Tianjin 300452, China article info Article history: Received 15 September 2010 Received in revised form 15 November 2010 Accepted 27 January 2011 Available online 2 February 2011 abstract Three Paleogene syn-rift intervals from the Bohai Bay Basin, the most petroliferous basin in China, were analyzed with sedimentological and geochemical techniques to characterize the lateral source rock het- erogeneities, to reveal the environmental and ecological changes through geologic time and to construct depositional models for lacustrine source rocks under different tectonic and climatic conditions. The third (Es 3 ) and first (Es 1 ) members of the Eocene Shahejie Formation and the Oligocene Dongying Formation (Ed) display widely variable total organic carbon contents, hydrogen indices and visual kerogen compo- sitions, suggesting changes in organic facies from deep to marginal sediments. Carefully selected deep- lake facies samples from any interval, however, display fairly uniform biomarker composition. These three intervals have distinctly different biomarker assemblages, which indicate weakly alkaline, freshwa- ter lakes with a moderately deep thermocline during Es 3 deposition, alkaline-saline lakes with shallow chemocline during Es 1 deposition and acidic, freshwater lakes with deep, unstable thermocline during the deposition of the Dongying Formation. Such environmental changes corresponded to changes in sub- sidence rate and paleoclimate, from rapid subsidence and wet climate during Es 3 deposition, through slow subsidence and arid climate during Es 1 deposition to rapid subsidence and wet climate during Ed deposition and resulted in synchronous changes in terrigenous organic matter input, phytoplankton com- munity and primary productivity. The co-evolution of environments and organisms controlled by tec- tonic subsidence and climate accounted for the deposition and distribution of high quality lacustrine source rocks with distinctly different geochemical characteristics. Most rift basins experienced changes in subsidence rates and possibly changes in climates during their syn-rift evolutions. The models con- structed in this paper may have important implications for source rock prediction in other lacustrine rift basins. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Lakes and lake deposits are important to understand and predict because they host significant petroleum resources (e.g., Kelts, 1988; Katz, 1990, 1995), are used to address climate change and paleoclimate questions (e.g., Meyers, 1997, 2003; Fan et al., 2007) and are significant sources of biodiversity (Carroll and Bohacs, 1999). Modern and ancient lakes displayed a wide varia- tion in hydrogeology and water chemistry (e.g., Carroll and Bohacs, 1999). A number of studies of recent and ancient lake systems have resulted in a wide variety of lacustrine source rock models for different lakes, including the large deep anoxic lake model (Demaison and Moore, 1980), the hypersaline lake model (Kirkland and Evans, 1981), the oligotrophic meromictic lake model (Powell, 1986), the large mesosaline alkaline closed lake model (Kelts, 1988), the meromictic/oligomictic tropical/humid lake model (Talbot, 1988) and the moderately deep tropical lake model (Katz, 1990). Due to the relatively small size of the water reservoirs, lakes have higher rates of environmental change than marine systems (Kelts, 1988; Valero Garcés et al., 1995; Gonçalves, 2002), resulting in wide ranges of salinity and pH and marked variation of biota. Detailed studies in recent years confirm that lacustrine source rocks in a single rift basin may show strong vertical and horizontal 0146-6380/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2011.01.010 ⇑ Corresponding author at: State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Fuxue Road No. 18, Changping, Beijing 102249, China. E-mail addresses: [email protected](F. Hao), [email protected](X. Zhou), [email protected](Y. Zhu), [email protected](Y. Yang). Organic Geochemistry 42 (2011) 323–339 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem
Lacustrine source rock deposition in response to co-evolutionof environments and organisms controlled by tectonic subsidenceand climate, Bohai Bay Basin, China
Fang Hao a,b,⇑, Xinhuai Zhou c, Yangming Zhu d, Yuanyuan Yang a
a State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Fuxue Road No. 18, Changping, Beijing 102249, Chinab Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, Chinac Department of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang 310027, Chinad Tianjin Branch of China National Offshore Oil Company Ltd., Tianjin 300452, China
a r t i c l e i n f o a b s t r a c t
Article history:Received 15 September 2010Received in revised form 15 November 2010Accepted 27 January 2011Available online 2 February 2011
0146-6380/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.orggeochem.2011.01.010
⇑ Corresponding author at: State Key LaboratoryProspecting, China University of Petroleum, Fuxue Ro102249, China.
Three Paleogene syn-rift intervals from the Bohai Bay Basin, the most petroliferous basin in China, wereanalyzed with sedimentological and geochemical techniques to characterize the lateral source rock het-erogeneities, to reveal the environmental and ecological changes through geologic time and to constructdepositional models for lacustrine source rocks under different tectonic and climatic conditions. The third(Es3) and first (Es1) members of the Eocene Shahejie Formation and the Oligocene Dongying Formation(Ed) display widely variable total organic carbon contents, hydrogen indices and visual kerogen compo-sitions, suggesting changes in organic facies from deep to marginal sediments. Carefully selected deep-lake facies samples from any interval, however, display fairly uniform biomarker composition. Thesethree intervals have distinctly different biomarker assemblages, which indicate weakly alkaline, freshwa-ter lakes with a moderately deep thermocline during Es3 deposition, alkaline-saline lakes with shallowchemocline during Es1 deposition and acidic, freshwater lakes with deep, unstable thermocline duringthe deposition of the Dongying Formation. Such environmental changes corresponded to changes in sub-sidence rate and paleoclimate, from rapid subsidence and wet climate during Es3 deposition, throughslow subsidence and arid climate during Es1 deposition to rapid subsidence and wet climate during Eddeposition and resulted in synchronous changes in terrigenous organic matter input, phytoplankton com-munity and primary productivity. The co-evolution of environments and organisms controlled by tec-tonic subsidence and climate accounted for the deposition and distribution of high quality lacustrinesource rocks with distinctly different geochemical characteristics. Most rift basins experienced changesin subsidence rates and possibly changes in climates during their syn-rift evolutions. The models con-structed in this paper may have important implications for source rock prediction in other lacustrine riftbasins.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Lakes and lake deposits are important to understand andpredict because they host significant petroleum resources (e.g.,Kelts, 1988; Katz, 1990, 1995), are used to address climate changeand paleoclimate questions (e.g., Meyers, 1997, 2003; Fan et al.,2007) and are significant sources of biodiversity (Carroll andBohacs, 1999). Modern and ancient lakes displayed a wide varia-tion in hydrogeology and water chemistry (e.g., Carroll and Bohacs,
ll rights reserved.
of Petroleum Resources andad No. 18, Changping, Beijing
1999). A number of studies of recent and ancient lake systems haveresulted in a wide variety of lacustrine source rock models fordifferent lakes, including the large deep anoxic lake model(Demaison and Moore, 1980), the hypersaline lake model (Kirklandand Evans, 1981), the oligotrophic meromictic lake model (Powell,1986), the large mesosaline alkaline closed lake model (Kelts,1988), the meromictic/oligomictic tropical/humid lake model(Talbot, 1988) and the moderately deep tropical lake model (Katz,1990).
Due to the relatively small size of the water reservoirs, lakeshave higher rates of environmental change than marine systems(Kelts, 1988; Valero Garcés et al., 1995; Gonçalves, 2002), resultingin wide ranges of salinity and pH and marked variation of biota.Detailed studies in recent years confirm that lacustrine sourcerocks in a single rift basin may show strong vertical and horizontal
324 F. Hao et al. / Organic Geochemistry 42 (2011) 323–339
variations in hydrocarbon potentials and geochemical characteris-tics (e.g., Follows and Tyson, 1998; Justwan et al., 2006; Keymet al., 2006; Hao et al., 2009a). Several case studies have beenexamined to explain the horizontal (Follows and Tyson, 1998)and vertical (Carroll and Bohacs, 1999, 2001; Bohacs et al., 2000;Gonçalves, 2002; Harris et al., 2004) heterogeneities of lacustrinesource rocks. Carroll and Bohacs (1999, 2001) and Bohacs et al.(2000) emphasize the controls of lake types (balance filled lakes,underfilled lakes and overfilled lakes) on source rock properties.
Fig. 1. (A) Sub-basins of the Bohai Bay Basin (sub-basin classification from Allen et al. (YRM = Yellow River Mouth.
Gonçalves (2002) and Harris et al. (2004), in contrast, emphasizethe role of enhanced primary productivity at late rift stages (withrelatively low subsidence rates) triggered by increased nutrientinput/recycling.
The Bohai Bay Basin is a Cenozoic lacustrine rift basin located onthe eastern coast of China (Figs. 1 and 2). The Bohai Bay Basin is themost petroliferous basin in China, having the greatest oil produc-tion (accounting for nearly one-third of the total oil productionof China, Hao et al., 2009a), the greatest proven oil reserves and
1997)). (B) The Bozhong sub-basin showing wells from which samples were taken.
Fig. 2. Cross section showing the structural framework of the Bohai Bay Basin. Note the thick Oligocene syn-rift sediments (the Dongying Formation, Ed) and Miocene toQuaternary post-rift sediments and strong faulting in the Bozhong sub-basin. Section location in Fig. 1A.
F. Hao et al. / Organic Geochemistry 42 (2011) 323–339 325
the greatest undiscovered oil resources in the country. TheBozhong sub-basin, with proven in situ oil reserves greater than
Conglomerate Sandstone Mudstone
Potential Source Rock
Major Oil Reservoir
GEOLOGICAL AGE (Ma)
STRATA
Form.
Mio
cene
Pliocene
Quaternary
Neo
gene
Pal
eoge
ne
PY
Min
gmua
zhen
Gua
ntao
Don
gyin
gS
hahe
jieK
ongd
ian
Symbol
NmL
Ng
Ed1
Es1
Es2
Es3
Es4
Ek
Nmu
Qp
Ed2
Ed3
Basement
LITHOLOGYTECTONIC
EVOLUTION
post-rift
syn-rift
Pre-TertiaryRocks
5
10
15
20
25
30
35
40
45
50
55
60
65
Olig
ocen
eE
ocen
eP
aleo
cene
S
S
S
S
Fig. 3. Generalized stratigraphy of the Bozhong sub-basin, Bohai Bay Basin. Possiblesource rock and major reservoir intervals are marked. Form. = Formation;PY = Pingyuan.
2.5 � 109 tons (18.3 � 109 barrels) (Hao et al., 2009b; Gong et al.,2010), is one of the most petroliferous sub-basins of the BohaiBay Basin. While the most important source rock interval in otherBohai Bay sub-basins is the fourth member of the Eocene ShahejieFormation (Es4, 50.5–43.0 Ma, Fig. 3) (e.g., Fuhrmann et al., 2004;Zhang et al., 2005), Es4 has made no significant contribution tooil reserves so far found in the Bozhong sub-basin (Gong, 1997;Hao et al., 2007, 2009b,c) and factors controlling the depositionof lacustrine source rocks in the Bozhong sub-basin are not clearlyunderstood. All syn-rift intervals in the Bozhong sub-basin aredominated by sandstones and mudstones/shales. The relativelymonotonous lithologic composition and paleoenvironments re-strict the application of a conventional sedimentological approachto reveal the environmental and ecological changes through geo-logic time (Gonçalves, 2002). On the other hand, the Bozhongsub-basin experienced multiple rifting events (Hao et al., 2009b),and paleoclimate changed considerably during the syn-rift evolu-tion (Wang et al., 2010). Therefore, the Bozhong sub-basin providesan excellent natural laboratory for investigating the environmentaland ecological changes induced by changes in tectonic subsidenceand paleoclimate and for revealing the mechanisms for high qual-ity source rock deposition under different tectonic and climaticconditions. The purpose of our study is to assess the geochemicalvariability of the Eocene and Oligocene lacustrine rift sequencesof the Bozhong sub-basin, Bohai Bay Basin, to reveal environmentaland ecological changes and to construct models for high qualitylacustrine source rocks under different tectonic and climatic condi-tions by integrating geological and geochemical data.
2. Geological setting
The Bohai Bay Basin, also known as the North China Basin (Yeet al., 1985; Hsiao et al., 2004) or the Bohai Basin (Chang, 1991;Allen et al., 1997), is a Cenozoic lacustrine basin located on theeastern coast of China (Fig. 1A). The Bohai Bay Basin formed onthe North China Craton (Wang and Qian, 1992; Ge and Chen,1993) and has an area of about 200,000 km2. The mechanisms forthe formation of the Bohai Bay Basin are still controversial. Yeet al. (1985) explained the formation of the Bohai Bay Basin withMcKenzie’s (1978) two stage extension model proposing Paleogenerifting and Neogene to Quaternary thermal subsidence stages.Allen et al. (1997, 1998), in view of the prominence of the Tan-Lufault, proposed a composite pull-apart basin model to explain theformation of the Bohai Bay Basin. In recent years, more and moreworkers tend to explain the Bohai Bay Basin as a Cenozoic rift basinmodified by synchronous strike slip faulting (e.g., Hsiao et al.,2010; Qi and Yang, 2010).
326 F. Hao et al. / Organic Geochemistry 42 (2011) 323–339
The Bohai Bay Basin experienced two major tectonic evolution-ary stages (Wang and Qian, 1992; Ge and Chen, 1993; Allen et al.,1997; Hsiao et al., 2004, 2010) (Figs. 2 and 3). The basin consists ofPaleogene rifts, which were filled by a thick non-marine clasticsuccession. An unconformity at the top of the syn-rift sedimentsseparates them from Miocene–Recent strata, which were depos-ited during post-rift thermal subsidence (Allen et al., 1997). Duringthe syn-rift stage (65.0–24.6 Ma), a series of grabens and half-grabens (Fig. 2) developed along major NW and NE trending faultsets (Lu and Qi, 1997; Yang and Xu, 2004; Qi and Yang, 2010).
Table 1Biomarker parameters for rock samples from different intervals in the Bozhong sub-basin
SN Well Depth (m) Lithology Interval Pr/Ph C35/C34 G/H ETR C
1 BZ-2 3777.5 Shale Ed 2.46 0.38 0.05 0.28 02 BZ-2 3805 Shale Ed 2.32 0.41 0.05 0.28 03 BZ-2 3825 Shale Ed 2.38 0.34 0.06 0.28 04 BZ-2 3860 Shale Ed 2.63 0.33 0.05 0.23 05 BZ-2 3905 Shale Ed 2.67 0.4 0.04 0.24 06 BZ-2 3925 Shale Ed 2.75 0.38 0.05 0.24 07 BZ-2 3975 Shale Ed 2.63 0.55 0.04 0.27 08 BZ-2 4010 Shale Ed 2.68 0.49 0.05 0.29 09 BZ-2 4122.5 Shale Ed 1.79 0.5 0.03 0.24 0
10 BZ-2 4155 Shale Ed 1.66 0.47 0.04 0.26 011 BZ-7 3205 Mudstone Ed 3.04 0.36 0.04 0.22 012 BZ-6 3084.5 Shale Ed 2.66 0.32 0.05 0.17 013 BZ-8 3171.8 Shale Ed 2.75 0.51 0.06 0.22 014 BZ-8 3177.5 Shale Ed 2.11 0.43 0.05 0.23 015 BZ-9 2928.5 Shale Ed 2.88 0.46 0.05 0.26 016 BZ-3 2781 Shale Ed 1.93 0.37 0.05 0.38 117 BZ-3 3289.5 Shale Ed 2.19 0.37 0.04 0.25 118 BZ-8 3290 Shale Ed 1.9 0.43 0.05 0.25 019 BZ-9 3037.5 Shale Ed 1.82 0.48 0.07 0.25 020 BZ-3 3318 Shale Ed 2.07 0.37 0.04 0.28 121 BZ-3 3505.5 Shale Ed 2.12 0.49 0.03 0.25 022 BZ-3 3595.5 Shale Ed 1.58 0.64 0.03 0.28 023 BZ-3 3691.5 Shale Ed 1.33 0.58 0.05 0.28 0
These grabens and half-grabens coalesced to form one large basinduring the Late Oligocene and the Bohai Bay Basin entered thepost-rift stage (24.6 Ma to the present) (Figs. 2 and 3). The BohaiBay Basin consists of several sub-basins, namely the Liaohe,Bozhong, Jiyang, Huanghua, Jizhong and Linqing sub-basins (Allenet al., 1997; Fig. 1A).
The 18,000 km2 Bozhong sub-basin, one of the six major sub-basins of the Bohai Bay Basin (Allen et al., 1997), is located in theoffshore area of the Bohai Bay Basin (water depth from 5 m toabout 35 m) (Fig. 1A). The Bozhong sub-basin has the thickest
F. Hao et al. / Organic Geochemistry 42 (2011) 323–339 327
Tertiary and Quaternary sediments (up to 11 km, Fig. 2) in theBohai Bay Basin. The thick Oligocene syn-rift sediments (theDongying Formation) and Miocene to Quaternary post-rift sedi-ments, as well as strong late stage faulting made the Bozhongsub-basin distinctly different from other Bohai Bay sub-basins(Fig. 2). The syn-rift sediments are composed of the Paleocene–Eocene Kongdian Formation (Ek), the Eocene Shahejie Formation(Es) and the Oligocene Dongying Formation (Ed) (Figs. 2 and 3).These formations were restricted to the grabens and half-grabens(Fig. 2) and were deposited in fluvial-lacustrine environments(Allen et al., 1997, 1998; Gong, 1997). The post-rift sediments con-sist of the Miocene Guantao Formation (Ng), Miocene–PlioceneMinghuazhen Formation (Nm) and the Quaternary Pingyuan For-mation (Qp) (Fig. 3). These formations are widespread (Fig. 2), andare dominated by fluvial deposits (Gong, 1997; Yang and Xu, 2004).
The Bozhong sub-basin experienced multiple rifting events,which caused significant variations in syn-rift subsidence-sedimentation rates over time. The subsidence rate was up to600 m/Ma during the deposition of the third member of the EoceneShahejie Formation (Es3), decreased to <300 m/Ma during thedeposition of the first member of the Eocene Shahejie Formation(Es1), and increased to up to 600 m/Ma again during the depositionof the Oligocene Dongying Formation (Ed). On the other hand,paleoclimate changed considerably during the syn-rift evolutionof the Bohai Bay Basin (Wang et al., 2010). Wet, northern subtrop-ical climate dominated during the deposition of Es3. Aridityincreased from the late stage of Es3 deposition to Es2 and Es1
deposition stage and decreased thereafter, with wet, northern sub-tropical climate dominating during the deposition of the OligoceneDongying Formation (Wang et al., 2010).
3. Samples and methods
Two sample sets were analyzed in this study. One sample setconsisted of more than 300 mudstone/shale samples of differentsedimentary facies. This sample set was used for bulk geochemis-try analysis such as Rock–Eval pyrolysis, visual kerogen observa-tion and gas chromatography of saturated hydrocarbons to revealchanges in organic facies within a stratigraphic interval, from deepto lake margin facies. Another sample set consisted of 57 deep lakeshales/mudstones (Table 1). This sample set was used for gaschromatography–mass spectrometry (GC–MS) analysis to investi-gate environmental and ecological changes in the lakes throughgeologic time.
All rock samples were cleaned prior to powdering. Soxhletextraction was conducted using chloroform/methanol (87:13) for72 h and the isolated extractable organic matter was separatedinto saturated hydrocarbons, aromatic hydrocarbons and polars.
400 420 4400
200
400
600
800
1000
I
II
III
400 420 440 460 480 5000
200
400
600
800
1000
Ro=0.5%
Ro=1.0%
I
II
III
(A) Es3
Hyd
rog
en In
dex
(m
g H
C/g
TO
C)
TmaTmax (°C)
Fig. 4. Variation of hydrogen index as a function of Tmax for samples from differe
Gas chromatographic (GC) analyses of the saturated hydrocarbonfractions were achieved using a HP6890 chromatograph equippedwith a PONA fused silica column (60 m � 0.20 mm i.d., film thick-ness 0.5 lm). The oven temperature was initially held at 35 �C for5 min, programmed to 80 �C at 2 �C/min, to 300 �C at 4 �C/min andheld at 300 �C for 30 min. Helium was used as the carrier gas. GC–mass spectrometry (GC–MS) analyses of the saturate fractionswere performed with a HP6890GC/5973MSD instrument equippedwith a HP-5MS fused silica column (30 m � 0.25 mm i.d., filmthickness 0.25 lm). The GC oven temperature for analysis of thesaturate fractions was initially held at 50 �C for 2 min, pro-grammed to 100 �C at 20 �C/min and to 310 �C at 3 �C/min, andheld at 310 �C for 16.5 min. Biomarker ratios were calculated frompeak areas of individual compounds. Kerogen was isolated fromextracted samples by overnight treatment with 6 N HCl followedby concentrated HF.
4. Results
4.1. Rock–Eval pyrolysis
Rock–Eval pyrolysis is a commonly used technique to classifyorganic matter (OM) types and to assess hydrocarbon generatingpotentials (e.g., Peters, 1986). The third member of the EoceneShahejie Formation (Es3, 43.0–38.0 Ma) has total organic carbon(TOC) contents of 0.1–9.19% and Rock–Eval S2 values of 0.02–63.08 mg HC/g rock. Hydrogen indices for Es3 samples range from<50 to 1115 mg HC/g TOC, suggesting different organic mattertypes (Fig. 4A). The first member of the Shahejie Formation (Es1,35.8–32.8 Ma) displays TOC contents of 0.22–6.80%, Rock–Eval S2
values of 0.25–50 mg HC/g rock, and hydrogen indices of 15–777 mg HC/g TOC (Fig. 4B). The Oligocene Dongying Formation(Ed, 32.8–24.6 Ma) has TOC contents ranging from 0.35% to3.91%, Rock–Eval S2 values from 0.1–26.3 mg HC/g rock and hydro-gen indices from <50 to 716 mg HC/g TOC (Fig. 4C). Most sampleshave Rock–Eval Tmax lower than 445 �C, suggesting thermal matu-rity from immature to early oil generation (Peters, 1986).
4.2. Visual kerogen analysis
In accordance with the wide variations in TOC contents, Rock–Eval S2 peaks and hydrogen indices (Fig. 4), samples from differentintervals show wide variations in visual kerogen compositions(Fig. 5A and B). Kerogens from Es3 display woody organic mattercontents ranging from <1–33%, and amorphous organic matter(AOM) contents from 5–77%. Kerogens from Es1 have woodyorganic matter contents of 0–39%, and AOM contents of 0.5–78%(Fig. 5). Kerogens from the Dongying Formation have woody
460 480 500
Ro=0.5%
Ro=1.0%
(B) Es1
x (°C)
400 420 440 460 480 5000
200
400
600
800
1000
Ro=0.5%
Ro=1.0%
I
II
III
(C) Ed
Tmax (°C)
nt possible source rock intervals in the Bozhong sub-basin, Bohai Bay Basin.
Fig. 5. Variation of hydrogen indices as a function of woody organic matter contents (A), amorphous organic matter contents (B), Pristane/Phytane ratios (C) and Phytane/n-C18 ratios (D), showing source rock heterogeneities in the Bozhong sub-basin, Bohai Bay Basin.
010.11.00.1
1.0
10
Algal OM
reducing environment
Mixed OM sources
transitio
nal enviro
nment
Terrigenous O
M
Reducing
Oxidizing
Pri
stan
e/n
-C17
Phytane/n-C18
(D)
010.11.00.1
1.0
10
Pri
stan
e/P
hyt
ane
Phytane/n-C18
(C)
Ed RockEs1 RockEs3 Rock
Ed RockEs1 RockEs3 Rock
Pri
stan
e/n
-C17
Tmax (°C)
(A)
400 410 420 430 440 450 4600
1
2
3
0
2
4
6
Ph
ytan
e/n
-C18
Tmax (°C)
(B)
400 410 420 430 440 450 460
Fig. 6. Variation of pristane/phytane (A) and Pristane/n-C17 (B) as a function of Phytane/n-C18, and variation of Pristane/n-C17 (C) and Phytane/n-C18 (D) as a function of Tmax
for rock samples from the Bozhong sub-basin, Bohai Bay Basin.
328 F. Hao et al. / Organic Geochemistry 42 (2011) 323–339
Gam
(C) Well BZ-4, 3764.5m, Es 3 (Sample 48)
4-MS
(A) Well BZ-3, 3505.5m, Ed (Sample 21)
m/z 191 m/z 217m/z 191
C24 Tet
Gam
C30 hopane
4-MS
C19
C20
C22
C24
C23
C25 C26
C21
Gam
(B) Well BZ-7, 3287.5m, Es 1 (Sample 31)
4-MSC24 Tet
C19
C20
C22
C24
C23
C25 C26
C21
C19
C24 TetC20
C22
C24
C23
C25 C26
C21 C27
C29
C28
C27
C29C28
C27C29
C28
C27 RAsteranes
Fig. 7. Representative mass chromatograms of terpane (m/z = 191) and sterane (m/z = 217) series of saturate fractions for different stratigraphic intervals in the Bozhong sub-basin, Bohai Bay Basin. Peaks marked by solid dots are tricyclic terpanes (C19–C26). C24 Tet = C24 tetracyclic terpane; Gam = gammacerane; RA = re-arranged; C27, C28 and C29
represent C27 sterane 20R, C28 sterane 20R and C29 sterane 20R, respectively; 4-MS = 4-methylsteranes. Note the abundant C19, C20 tricyclic terpanes, C24 tetracyclic terpaneand C27 re-arranged steranes for Ed sample (A), abundant gammacerane for Es1 sample (B), and abundant 4-methylsteranes for the Es3 sample (C).
F. Hao et al. / Organic Geochemistry 42 (2011) 323–339 329
organic matter contents ranging from <1–39%, and AOM contentsfrom <0.1% to 76% (Fig. 5).
4.3. GC and GC–MS analysis
GC analyses were carried out on more than 300 samples. Es3
displays pristane/phytane (Pr/Ph) between 0.2 and 3.21, Pr/n-C17
between 0.31 and 4.95 and Ph/n-C18 between 0.21 and 5.53 (Figs. 5and 6). Es1 has Pr/Ph ranging from 0.16–3.55, Pr/n-C17 from 0.18–1.76, and Ph/n-C18 from 0.17–4.59 (Figs. 5 and 6). The DongyingFormation displays Pr/Ph of 0.59–3.29, Pr/n-C17 of 0.28–2.76, andPh/n-C18 of 0.13–2.64 (Figs. 5 and 6).
GC–MS analyses were conducted on 57 laminated deep lakesamples and representative m/z 191 and m/z 217 chromatogramsfor the analyzed samples are displayed in Fig. 7. The analyzed sam-ples display wide variations in the relative abundances of C19, C20
5.1. Source rock heterogeneity of different stratigraphic intervals
The third (Es3) and first (Es1) members of the Eocene ShahejieFormation and the Oligocene Dongying Formation (Ed) are poten-tial source rocks in the Bozhong sub-basin (Hao et al., 2009a,b;Gong et al., 2010). It is therefore of great significance to decipherthe heterogeneity of each interval both for petroleum reserveassessment and for oil–source rock correlation (e.g., Justwanet al., 2006; Keym et al., 2006; Curiale, 2008). Es3, Es1 and Ed alldisplay wide variations in Rock–Eval hydrogen indices (Fig. 4). Itshould be pointed out that most samples have Rock–Eval Tmax
lower than 445 �C (Fig. 4). Therefore, the wide variations inhydrogen indices could not have been caused by maturity changes.
Predictably, the Rock–Eval hydrogen indices decrease as woodyorganic matter contents increase (Fig. 5A). Several case studiesshowed that amorphous organic matter (AOM) was derived mainlyfrom algal organic matter deposited into reducing environmentsand therefore was rich in hydrogen (e.g., Powell et al., 1990; Tyson,1995; Ercegovac and Kostic, 2006). In the Bozhong sub-basin, how-ever, there is a loose correlation between Rock–Eval hydrogen indi-ces and AOM contents and samples with similar AOM contentsdisplay wide variation in hydrogen indices (Fig. 5B). Noticeably,the Dongying Formation samples usually have lower hydrogenindices than Es3 and Es1 samples with similar AOM contents(Fig. 5B), suggesting that AOM in the Dongying Formation experi-enced more intensive degradation during or immediately afterdeposition (Frimmel et al., 2004). Several Es1 and Es3 samples dom-inated by AOM have hydrogen indices no higher than 250 mg HC/gTOC (Fig. 5B), suggesting that AOM in these samples is poor inhydrogen. The loose correlation between hydrogen indices andAOM contents and the low hydrogen indices for samples domi-nated by AOM seem to indicate that AOM in the Bozhong sub-basinis complex in hydrogen contents. This is consistent with the obser-vation of Ebukanson and Kinghorn (1985) and Hao et al. (1993)who concluded that AOM could be formed both from algal organicmatter and from higher plant organic matter and therefore couldbe either hydrogen rich or hydrogen poor.
Rock–Eval hydrogen indices decrease with increasing pristane/phytane (Pr/Ph) ratios (Fig. 5C), and high abundance of phy-tane (Ph/n-C18 > 2.0) always occurs in samples with hydrogenindices >500 mg HC/g TOC (Fig. 5D). Because most samples haveRock–Eval Tmax < 445 �C (Fig. 4) and Pr/n-C17 and Ph/n-C18 showno obvious correlation with Tmax (Fig. 6A and B), the overall de-crease of hydrogen indices with Pr/Ph is caused mainly by changesin redox conditions during or immediately after deposition of
330 F. Hao et al. / Organic Geochemistry 42 (2011) 323–339
source sediments, rather than by changes in maturity levels. Threemechanisms have been proposed in the literature to explain theabnormally high abundance of phytane (high Ph/n-C18 ratios):contribution from methanogenic archaea (e.g. Rowland, 1990;Fuhrmann et al., 2004), contribution from eukaryotic phytoplankton
02
426
00
2800
3000
3200
3400
00.
61.
20
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00.
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01
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04
80
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00.
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00.
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/C23
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Ph
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/C34
ET
RG
/HC
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274M
SI
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/C29
S/H
C27
/C29
(m)
Fig. 8. Variations of major biomarker parameters reflecting depositional conditions andCrosses: Es1 samples; solid dots: Es3 samples. Abbreviations for biomarker parameters a
(Sepúlveda et al., 2009), or sulfurization of functional lipids duringearly diagenesis and subsequent temperature cleavage of the sul-fur bonds during catagenesis in sulfur rich rocks (e.g., Keelyet al., 1993; Koopmans et al., 1996). Cenozoic rocks in the Bozhongsub-basin are sulfur poor and the sulfurization–cleavage mechanism
3600
3800
4000
4200
4400
4600
Depth
/or organic matter input as a function of burial depth. Open triangles: Ed samples;re explained in Table 1.
F. Hao et al. / Organic Geochemistry 42 (2011) 323–339 331
is therefore not applicable. Contribution from eukaryotic phyto-plankton may simultaneously lead to high abundances of bothpristane and phytane, as is the case in the Levant Platform ofcentral Jordan (Sepúlveda et al., 2009). Yet in the Bozhong sub-basin Pr/Ph ratios decrease as Ph/n-C18 increases (Fig. 6C) andsamples with Ph/n-C18 greater than 2.0 display Pr/n-C17 ratiosobviously lower than the Pr/n-C17 ratios for many DongyingFormation samples that have Ph/n-C18 < 1.0 (Fig. 6D). Therefore,contribution from eukaryotic phytoplankton could not be the maincause of the high phytane abundance observed in the Bozhong sub-basin. In the Bozhong sub-basin, samples with high Ph/n-C18 ratioshave a large isotopic difference between saturate and aromaticfractions and plot above the best fit line (d13CAro = 1.14d13-CSat + 5.46) on the so called ‘‘Sofer-plot’’ (Sofer, 1984) (Guo, 2009,personal communication). The 13C depleted saturate fractions re-sulted from input from methanotrophic bacteria (Collister andWavrek, 1996). Methanotrophic bacteria thrive at the chemoclineof stratified lakes (Collister and Wavrek, 1996). The 13C depletedsaturate fractions as a result of methanotrophic input of isotopi-cally depleted lipids imply that a considerable amount of methaneshould have been generated below the chemocline. Therefore, weexplain the high Ph/n-C18 ratios for samples with high hydrogenindices as reflecting a contribution from methanogenic archaea.
Based on Rock–Eval hydrogen indices (Fig. 4), elemental compo-sitions of kerogens (not shown), visual kerogen compositions(Fig. 5A and B) and redox conditions reflected by isoprenoid hydro-carbon distributions (Figs. 5C, D and 6), four types of organic facies(Jones, 1987) can be recognized. Organic facies A has hydrogenindices >700 mg HC/g TOC and is dominated by fluorescent AOMand well preserved algal materials usually with low Pr/Ph (<1.0)and high Ph/n-C18 (>1.0 to 1.5) ratios. Organic facies B usually dis-plays hydrogen indices between 500 and 700 mg HC/g TOC. Organicfacies B is also dominated by AOM but may have woody organicmatter contents up to 15%. Organic facies BC has hydrogen indicesbetween 200 and 500 mg HC/g TOC and woody organic mattercontents of 10–45%. Organic facies C has hydrogen indices<200 mg HC/g TOC and is dominated by woody organic matter.AOM in organic facies C is fine grained and is red in color in trans-mitted light and shows no fluorescence in ultraviolet light, which isdistinctly different from AOM in organic facies A and B.
Both Es3 and Es1 contain organic facies A–C. The Dongying For-mation contains organic facies B to C without well developed or-ganic facies A. The wide, successive variations in hydrogenindices (Fig. 4) and visual kerogen composition (Fig. 5A and B) sug-gest strong heterogeneities of all the three intervals, from hydro-gen rich organic facies in the deep lake sediments to hydrogenpoor facies in lake marginal sediments. Such heterogeneities maybe explained as reflecting varying redox conditions from anoxicthrough transitional to oxic (Fig. 6D). It should be pointed out that,in the Bozhong sub-basin, organic facies BC and C usually have to-tal organic carbon (TOC) contents <1.0% (Hao et al., 2010). The rel-atively low hydrogen indices and TOC contents mean that thehydrocarbon generating potential of organic facies BC and C ismuch lower than that of organic facies A and B. In addition, organicfacies BC and C might contribute to condensates and hydrocarbongases, but are of minor significance for ‘‘normal’’ oils due to theoxygen rich nature of the organic matter (Price, 1989). Therefore,to understand the mechanisms for the deposition of organic faciesA and B and to predict their distribution are of great significancefor petroleum exploration in lacustrine basins.
5.2. Changes in lake water chemistry
Primary productivity and redox conditions are among the mostimportant factors controlling high quality source rock (organic fa-cies A and B) deposition (e.g., Kelts, 1988; Huc et al., 1992; Katz,
1990, 2005; Tyson, 2005). The primary productivity and redox con-ditions in a lake are largely controlled by lake water chemistry,such as salinity and acidity/alkalinity. Rapid progress in organicgeochemistry in the last 20 years makes it possible to qualitativelyestimate changes in lake water chemistry from biomarker param-eters (e.g., Volkman et al., 1998; Peters et al., 2005, p. 483–580).
The great thickness of the syn-rift succession in the Bozhongsub-basin (cf. Fig. 2) makes it impossible to construct a biomarkerprofile in a single well. In order to reveal changes in lake waterchemistry and the resultant changes in biota through geologictime, 57 samples of deep-lake facies from different intervals wereanalyzed (Table 1) and a composite biomarker profile (Figs. 8 and9) was constructed. Hao et al. (2010) showed that no terpaneand sterane parameters for these samples displayed any obviouscorrelation with the C29 bb/(bb + aa) sterane ratio (an effectivematurity parameter believed to be independent of organic matterinput; Peters et al., 2005, p. 625–630). In addition, although theselected sample set covers a relatively wide depth range (2781–4492.5 m), most parameters show no obvious correlation withburial depth (Fig. 8), and the variation trends of several parametersare even opposite to those caused by thermal maturity. Forinstance, if Pr/Ph and C19/C23 TT had been intensively influencedby thermal maturity, they would have had to increase withincreasing depth. Yet, exactly the opposite is observed (Fig. 8). Itappears that all parameters displayed in Figs. 8 and 9 are not inten-sively influenced by thermal maturity and therefore can be used toreflect organic matter input and/or depositional conditions. Care-fully selecting deep-lake facies samples based on the result ofsedimentological analysis from different wells and constructing acomposite biomarker profile could minimize the influence of localwater inflow and/or sediment input. Despite the fact that samplesfor any interval were from different wells and cover a relativelywide depth range (Table 1), all three intervals display relativelynarrow variation ranges for most parameters (Fig. 9). This on onehand suggests that the influence of local water inflow and/or sed-iment input, if any, is minor, and on the other hand implies thatthese parameters have not been significantly affected by thermalmaturity.
Pr/Ph and C35 22S/C34 22S hopane are effective oxicity parame-ters (see Peters et al., 2005, p. 499–502, 566–569 and referencestherein). Es1 has the lowest Pr/Ph ratios and the highest C35 22S/C34 22S hopane ratios (Table 1, Fig. 9A and B). In contrast, theDongying Formation displays the highest Pr/Ph ratios and thelowest C35 22S/C34 22S hopane ratios. The relatively low Pr/Phratios and high C35 22S/C34 22S hopane ratios for Es3 and Es1 sam-ples suggest that anoxic conditions prevailed in the bottom waterduring Es3 and Es1 deposition, whereas less reducing conditionsdominated in the bottom water in lakes during Ed deposition. Thisis consistent with the result of bulk geochemistry analysis. On thePr/n-C17 vs Ph/n-C18 plot, many Es3 and Es1 samples plot in areassuggesting euxinic, anoxic conditions, whereas most Ed samplesfall in the transitional to oxic fields (Fig. 6D).
The gammacerane index (gammacerane/ab C30 hopane), ex-tended tricyclic terpane ratio [ETR = (C28 + C29)/(C28 + C29 + Ts)]and C27 ba (20R + 20S) diasteranes/C27 abb (20R + 20S) sterane(C27 Dia/C27 ST) were used to reflect changes in water salinity/alka-linity through geologic time (Fig. 9C–E).
Gammacerane is believed to form by reduction of tetrahymanol(e.g., Venkatesan, 1989; ten Haven et al., 1989). The principalsource of tetrahymanol appears to be bacterivorous ciliates, whichoccur at the interface between oxic and anoxic zones in stratifiedwater columns (Sinninghe Damsté et al., 1995). Therefore, abun-dant gammacerane is usually believed to indicate the presence ofa stratified water column (e.g., Sinninghe Damsté et al., 1995;Peters et al., 2005, p. 576; Sepúlveda et al., 2009). Although astratified water column can result from either hypersalinity or a
Fig. 9. Composite organic geochemistry profile showing changes in major biomarker parameters reflecting depositional conditions and/or organic matter input from Es3
through Es1 to Ed3. Vertical lines represent the average values for the interval. SN = sample number. Abbreviations for biomarker parameters are explained in Table 1.
332 F. Hao et al. / Organic Geochemistry 42 (2011) 323–339
temperature gradient (e.g., Bohacs et al., 2000), high abundances ofgammacerane are mostly found in evaporite or high salinityenvironments (Fu et al., 1990; Chen et al., 1996; Ritts et al.,1999; Hanson et al., 2000, 2001; Holba et al., 2003; Summonset al., 2008).
Holba et al. (2001) used ETR to differentiate crude oils gener-ated from Triassic, Lower Jurassic and Middle–Upper Jurassicmarine source rocks. They observed a sharp drop in ETR at theend of the Triassic that corresponds to a major mass extinctionand implied that the mass extinction may have had an impact on
F. Hao et al. / Organic Geochemistry 42 (2011) 323–339 333
the principal biological sources of tricyclic terpanes. The effective-ness of ETR as an age related parameter was questioned by Ohmet al. (2008). In the Bozhong sub-basin, ETR increases with increas-ing gammacerane/ab C30 hopane and sterane/hopane ratios anddecreases with increasing Pr/Ph ratios (Fig. 10A–C). Similar trendswere also observed in lacustrine oils and source rocks in theJunggar Basin where high ETR occurs in crude oils and source rockswith abundant b-carotane and ETR increases as gammacerane/abC30 hopane increases (Hao et al., in press). Our observations inthe Junggar and Bohai Bay basins are consistent with those ofKruge et al. (1990a,b) and De Grande et al. (1993) who concludedthat fossil lipids of prokaryotes in saline, alkaline lakes are rich inprecursors of extended tricyclic terpanes. The close correlation ofETR with Pr/Ph, sterane/hopane and gammacerane/ab C30 hopaneratios indicate that in lacustrine environments, ETR is an effective
ET
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Fig. 10. Correlation between various biomarker parameters reflecting organic matterdifferences in biomarker compositions among the three source rock intervals. Abbreviat
indicator of the salinity and alkalinity during or immediately afterdeposition of source sediments (Hao et al., 2009a).
Bennett and Olsen (2007) used C27 Dia/C27 ST as an indicator ofsource rock lithology, and showed that carbonate source rocks hadlow C27 Dia/C27 ST ratios. All analyzed samples are lacustrine shale/mudstone (Table 1) and therefore variations in the ratio cannot re-flect lithologic changes. Yet the analyzed samples display a widerange of C27 Dia/C27 ST ratios (0.20–1.12, Table 1, Fig. 9E). C27
Dia/C27 ST increases as Pr/Ph increases for samples with Pr/Ph < 2.0, but remains constant at relatively high values for sampleswith Pr/Ph > 2.0 (Fig. 10D). With the exception of three samples,C27 Dia/C27 ST decreases as the gammacerane index increases(Fig. 10E). Acidic and oxic conditions facilitate diasterene forma-tion during diagenesis (Brincat and Abbott, 2001; Peters et al.,2005, p. 533), therefore, variations in C27 Dia/C27 ST ratios in
Pr/PhE
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/H0.3 0.4 0.5 0.6
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/H
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26
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0.0 2.0 4.0 6.0 8.020
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S/H
(L)
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0.6
ranes (%)
(K)
0.3 0.4
input and/or depositional environments in the Bozhong sub-basin, showing theions for biomarker parameters are explained in Table 1.
334 F. Hao et al. / Organic Geochemistry 42 (2011) 323–339
non-marine clastic successions reflect changes in Eh and pH of thelake systems.
Es1 has the highest gammacerane index, highest ETR but thelowest C27 Dia/C27 ST (Table 1, Figs. 7, 9C–E), suggesting saline,alkaline lakes. This is supported by the fact that relatively abun-dant carbonates including dolomites were deposited in the shallowlake facies of Es1. Dolomites are known to form in concentratedalkaline lakes, but are absent from freshwater lakes (Wolfbauerand Surdam, 1974; Jones and Bowser, 1978; Fuhrmann et al.,2004). In contrast, the Dongying Formation displays the lowestgammacerane index, lowest ETR but the highest C27 Dia/C27 ST(Table 1, Figs. 7, 9C–E), suggesting freshwater, acidic lakes. Es3
has medium gammacerane index, ETR and C27 Dia/C27 ST (Table 1,Fig. 9C–E). We interpreted lakes during Es3 deposition as fresh tobrackish water, weak alkaline lakes.
In summary, considerable changes in lake water chemistry oc-curred from the middle Eocene to early Oligocene (from Es3 depo-sition to Ed deposition) in the Bozhong sub-basin. Fresh tobrackish, weakly alkaline lakes with anoxic bottom water condi-tions dominated during Es3 deposition. Saline, alkaline lakes witheuxinic bottom water conditions occurred during Es1 deposition.In contrast, freshwater, acidic lakes with dysoxic bottom waterconditions prevailed during the deposition of the DongyingFormation.
5.3. Changes in terrigenous organic matter input and primaryproducers
Terpane parameters have the potential to reflect terrigenous or-ganic matter input (e.g., Peters et al., 2005, p. 538–580). High C19
tricyclic terpane/C23 tricyclic terpane (C19/C23 TT) and C20 tricyclicterpane/C23 tricyclic terpane (C20/C23 TT) ratios indicate importantcontribution from terrigenous organic matter (e.g., Hanson et al.,2000; Preston and Edwards, 2000; George et al., 2004; Volk et al.,2005; Hao et al., 2009a). In the Bohai Bay Basin, the C24 tetracyclicterpane/C26 tricyclic terpane (C24 Tet/C26 TT) ratios increase as C19/C23 TT and Pr/Ph ratios increase (Fig. 10F,G), indicating that highabundances of C24 tetracyclic terpane (high C24 Tet/C26 TT ratios)are diagnostic of terrigenous organic matter input in lacustrinesystems (Philp and Gilbert, 1986; Hanson et al., 2000; Bohacset al., 2000; George et al., 2004). Es1 has the lowest C19/C23 TT,C20/C23 TT and C24 Tet/C26 TT ratios but the highest C23 tricyclic ter-pane/ab C30 hopane (C23 TT/C30 H) ratios (Table 1, Figs. 7, 9F–I),indicating no or minor contributions from terrigenous organicmatter. This is consistent with the strongly reducing conditionsand well developed water column stratification reflected by thelow Pr/Ph ratios and high C35 22S/C34 22S hopane, gammacerane/ab C30 hopane and ETR (Fig. 9A–D). In contrast, the Dongying For-mation displays the highest C19/C23 TT, C20/C23 TT and C24 Tet/C26
TT ratios but the lowest C23 TT/C30 H ratios (Table 1, Figs. 7, 9F–I). Such a biomarker association suggests significant contributionfrom terrigenous organic matter, which is consistent with thedysoxic conditions that prevailed during the deposition of theDongying Formation, as reflected by the high Pr/Ph ratios andlow C35 22S/C34 22S hopane, gammacerane/ab C30 hopane andETR (Table 1, Fig. 9A–D). The C19/C23 TT, C20/C23 TT and C24 Tet/C26
TT ratios for Es3 are slightly higher than those for Es1 but signifi-cantly lower than those for the Dongying Formation (Table 1,Figs. 7, 9F–H), suggesting a minor contribution of terrigenousorganic matter (Hao et al., 2009a,b).
Sterane parameters have the potential to reflect primary pro-ducers in marine and lacustrine systems (e.g., Volkman et al.,1998; Knoll et al., 2007; Sepúlveda et al., 2009). 4-Methylsteranesare commonly found in marine, evaporitic and especially freshwa-ter environments (e.g., Brassell et al., 1986; Summons et al., 1992;Peters et al., 2005, p. 530–532). The precursors of 4-methylsteranes
are presumed to be 4-methyl sterols. Apart from compounds spe-cifically identified as originating from methane oxidizing bacteria(Jahnke et al., 1999), 4-methyl steroids are mostly algal in origin(e.g. de Leeuw et al., 1983), and dinoflagellates are believed to betheir main source (De Leeuw et al., 1983; Summons et al., 1987).In the Bohai Bay Basin, abundant 4-methylsteranes appear to beassociated with abundant dinoflagellates Bohaidina and Parabohai-dina (e.g., Chen et al., 1996, 1998; Zhang et al., 2005). The 4-methylsterane index (4-methylsteranes/RC29 steranes) exhibits ageneral decrease up section (Fig. 9J), suggesting decreasing contri-butions from dinoflagellates Bohaidina and Parabohaidina. The factthat Es3 has higher 4-methylsterane abundances than Es1 (Figs. 7and 9J) supports our interpretation that freshwater lakesdominated during Es3 deposition whereas saline lakes dominatedduring Es1 deposition, since dinoflagellates Bohaidina andParabohaidina thrive in freshwater settings (Fu et al., 1990; Peterset al., 2005, p. 531).
It is usually believed that C27 steranes derive mainly from phy-toplankton and metazoa, whereas C29 steranes mainly originatefrom terrigenous higher plants (e.g., Huang and Meinschein,1979; Volkman, 1986). C27/C29 sterane ratios range between 0.42and 0.99 (average 0.76) for Es3 samples, increase to the highest val-ues in the Bozhong sub-basin for Es1 samples (0.82–1.41, average0.98), and then decrease to low values (0.48–1.0, average 0.75)for the Dongying samples (Fig. 9K). As discussed earlier, C19/C23
TT, C20/C23 TT and C24 Tet/C26 TT are effective parameters reflectingorganic matter input from terrigenous higher plants. The Es3 andEd have quite similar C27/C29 sterane ratios (Figs. 7, 9K), which ap-pears to be inconsistent with the fact that the Dongying Formationhas much higher C19/C23 TT, C20/C23 TT and C24 Tet/C26 TT ratios(Table 1, Figs. 7, 9F–H) and therefore has significant contributionfrom terrigenous higher plants whereas Es3 was dominated by al-gal organic matter. The relative C29 sterane abundances show ageneral increase with increasing C19/C23 TT and C24 Tet/C26 TT ra-tios (Fig. 10H, I). The Es3 and Ed have quite different C19/C23 TT(0.24–1.01 and 0.59–1.90 for Es3 and Ed, respectively) and C24
Tet/C26 TT (0.09–2.74 and 1.36–6.24 for Es3 and Ed, respectively)ratios. Yet they have quite similar relative abundances of C29 ster-anes (36.14–50.85% and 36.62–51.89% for Es3 and Ed, respectively,Figs. 7, 10H and I). The elevated C29 sterane abundances for the Es3
that has been confirmed to have minor terrigenous organic matterinput indicate that an additional source for C29 steranes existed.Volkman et al. (1999) and Kodner et al. (2008) confirmed thatfreshwater microalgae may be an important source for C29 ster-anes. Since freshwater lakes dominated during Es3 deposition, aplausible explanation for the enhanced C29 sterane abundances rel-ative to the low C19/C23 TT and C24 Tet/C26 TT ratios in Es3 is thecontribution of freshwater microalgae.
C28 steranes are associated with specific phytoplankton types(e.g., diatoms, Grantham and Wakefield, 1988; Volkman et al.,1998) that contain chlorophyll-c (Knoll et al., 2007). In the Bozhongsub-basin, an overall trend of increasing C28/C29 sterane ratio withincreasing C27/C29 sterane ratio (Fig. 10J) was observed. C28/C29
sterane ratios show moderate values for Es3 (0.49–0.82, average0.62), increase to relatively high values between 0.42 and 0.92(average 0.77) for Es1 and then decrease to low values for theDongying Formation (0.30–0.84, average 0.51) (Table 1, Fig. 9L).This suggests an enhanced contribution from chlorophyll-ccontaining phytoplankton relative to C29 producing organisms(Knoll et al., 2007) in Es1.
The sterane/hopane ratios reflect input of eukaryotic (mainly al-gae and higher plants) versus prokaryotic (bacteria) organisms tothe source rocks (e.g., Peters and Moldowan, 1993; Gonçalves,2002; Peters et al., 2005, p. 524; Sepúlveda et al., 2009). In theBozhong sub-basin, sterane/hopane ratios increase as C28 steraneabundances increase and decrease as C29 sterane abundances
F. Hao et al. / Organic Geochemistry 42 (2011) 323–339 335
increase (Fig. 10K and L), indicating that high sterane/hopaneratios in the Bozhong sub-basin were caused by contribution fromchlorophyll-c containing phytoplankton rather than C29 producingorganisms (Knoll et al., 2007). Sterane/hopane ratios show low val-ues for Es3 (0.10–0.37, average 0.16), increase to relatively highvalues for Es1 (0.16–0.57, average 0.31), and drop to extremelylow values for the Dongying Formation (0.06–0.24, average 0.11)(Table 1, Fig. 9M). The sterane/hopane ratios for Es3 are comparablewith those for the organic-rich lacustrine source rocks dominatedby algal organic matter in the Camamu Basin (Gonçalves, 2002)and in the Congo Basin (Harris et al., 2004). These sterane/hopaneratios, combined with low C19/C23 TT, C20/C23 TT and C24 Tet/C26 TTratios, suggest important contribution from bacterial biomass (e.g.,Gonçalves, 2002; Peters et al., 2005, p. 524; Sepúlveda et al., 2009).The relatively high sterane/hopane ratios for Es1 co-occur with rel-atively high C23 TT/ab C30 hopane ratios and high C27 and C28 ster-ane abundances (Table 1, Figs. 9 and 10K), and therefore indicatereduced contribution from bacteria but enhanced primary produc-tion rates (Gonçalves, 2002). The extremely low sterane/hopane ra-tios for the Dongying Formation are in accordance with the highC19/C23 TT, C20/C23 TT and C24 Tet/C26 TT ratios and reflect a signif-icant contribution from terrigenous organic matter and heavy bac-terial degradation (e.g., Tissot and Welte, 1984, p. 122; Frimmelet al., 2004; Peters et al., 2005, p. 524).
5.4. Models for source rock deposition under different tectonic andclimatic conditions
Stratigraphic and subsidence studies show that the Bozhongsub-basin experienced rapid subsidence during Es3 deposition(up to 600 m/Ma), reduced subsidence rate during Es2 and Es1
deposition (no higher than 300 m/Ma) and another episode of ra-pid subsidence during Ed deposition (up to 600 m/Ma). The paleo-climate also changed considerably, from humid climate during Es3
deposition through arid climate during Es2 and Es1 deposition tohumid climate again during the deposition of the Dongying Forma-tion (Wang et al., 2010). It is evident that the changes in waterchemistry indicated by biomarker parameters (Fig. 9A–E) wereconsistent with changes in subsidence rate and climates. Moreimportantly, both parameters reflecting terrigenous higher plantinput (Fig. 9F–I) and parameters associated with primary produc-ers (Fig. 9J–M) co-vary with parameters reflecting depositionalconditions (Fig. 9A–E), which strongly suggest the co-evolution ofecological systems with environments. In other words, changesin tectonic subsidence and climate induced environmentalchanges, which in return resulted in changes in ecological commu-nities in the lake systems. These observed environmental and eco-logical changes enable us to construct models for the deposition ofhigh quality lacustrine source rocks under different tectonic andclimatic conditions.
During Es3 deposition, the intense activity of border faults andrapid subsidence, together with high water inflow under wet cli-mate but relatively low sediment input, resulted in a deep, fresh-water lake and generally regressional or aggradational basin fill(Fig. 11A). The low area/depth ratio of the lake probably hamperedefficient water mixing, favoring stable water column stratification(Gonçalves, 2002). The stable water column stratification can beinferred from the moderate gammacerane indices for the Es3 sam-ples (Fig. 9C) deposited in freshwater lake (e.g., Sinninghe Damstéet al., 1995). The water column stratification resulted probablyfrom a temperature gradient and a moderate-depth thermoclinewas expected (Bohacs et al., 2000), forming a thick and relativelypermanent anoxic water column (Fig. 11A). The oxygen depletionand the low sulfate availability resulted in the dominance of bacte-rial fermentation and methanogenesis in the water column/sediments under the thermocline which was evidenced by the
abnormally high abundances of phytane (cf. Fig. 6) believed to bederived from methanogenic archaea (e.g., Rowland, 1990;Fuhrmann et al., 2004; see the earlier section). Methanogenesisin the euxinic bottom water and/or sediments must have sustainedmethanotrophic bacteria at the interface between oxic and anoxiczones (Collister and Wavrek, 1996; Fig. 11A), which can be evi-denced from the occurrence of the large isotopic difference be-tween saturate and aromatic fractions for Es3 samples with highhydrogen indices (Guo, 2009, personal communication). The highwater inflow, which carried dissolved inorganic carbon and nitrateinto the lake, might have sustained moderate to high primary pro-ductivity. In the freshwater lake, dinoflagellates Bohaidina andParabohaidina thrived as evidenced from the high 4-methylsteraneindices (Fig. 9J), and freshwater microalgae and chlorophyll-c con-taining phytoplankton might also be important members of thecommunity. The contribution of freshwater microalgae causedthe enhanced C29 sterane abundances (Figs. 9K, 10H, I) in the ab-sence of significant terrigenous higher plant input. The moderateto high primary productivity and stable water column stratificationaccounted for the deposition of the laminated, high quality sourcerock in Es3, the most important source rock in the Bozhong sub-basin (Gong, 1997; Hao et al., 2009c).
During Es1 deposition, the low water inflow and intensive evap-oration in arid climate, together with the low subsidence rate, ledto the formation of saline, alkaline, shallower lake (Fig. 11B). Lowsediment input resulted in regressional or aggradational basin-fill,and culminated in carbonate deposition (Fig. 11B). The weak dis-turbance by water inflow and high salinity favored water columnstratification and the establishment of a shallow, stable chemo-cline (Fig. 11B). The stable water column stratification and euxinicbottom water conditions can be evidenced from the fact that Es1
has the lowest Pr/Ph, the highest C35 22S/C34 22S hopane, highestETR and especially the highest gammacerane indices (Fig. 9A–D)in the Bozhong sub-basin. The stable water column stratificationand euxinic bottom water conditions can also be inferred fromthe high Ph/n-C18 ratios (cf. Fig. 6) and the large isotopic differencebetween the saturate and aromatic fractions for the Es1 samples(Guo, 2009, personal communication), which are believed to beassociated with methanogenic archaea (Rowland, 1990; Fuhrmannet al., 2004) and methanotrophic bacteria (Collister and Wavrek,1996), respectively. The combination of relatively high nutrientconcentration in the saline waters (Fuhrmann et al., 2004), abun-dant CO3 ions under alkaline conditions (Fuhrmann et al., 2004)and the excellent water column illumination due to very low sed-iment input (Kelts, 1988) might lead to high primary productivityin the saline lake (Fig. 11B), which appears to be supported by thefact that the Es1 has the highest sterane/hopane ratios in theBozhong sub-basin (Fig. 9M). In the warm, saline lake, chloro-phyll-c containing phytoplankton bloomed, as indicated by the rel-atively high C28/C29 sterane ratios (Table 1, Fig. 9L). DinoflagellatesBohaidina and Parabohaidina might also be important members ofphytoplankton community, as indicated by the moderate 4-methylsterane indices (Fig. 9J). This scenario accounts for thedeposition of high quality source rock in Es1, which was confirmedto have independently sourced commercial oil accumulations (Haoet al., 2010) and made important contribution to many commercialaccumulations in the Bozhong sub-basin (Hao et al., 2009a,b). Dueto smaller thickness and lower thermal maturity, Es1 is of lesserimportance as source rock than Es3 in the Bozhong sub-basin.
As a consequence of rapid subsidence and high water inflow,deep, freshwater lakes occurred during the deposition of theOligocene Dongying Formation (Fig. 11C). The lakes at this stageseemed to be hydrologically open. This, together with the rapidpropagation of delta due to efficient sediment supply, led to a deep,unstable thermocline with dysoxic bottom water conditions(Fig. 11C). The unstable water column stratification can be inferred
Saline, anoxic
∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗High Productivity
CH4
Low Water Inflow and Sediment Supply
Saline water, oxic
Carbonate
Organic Facies A and B
Organic FaciesBC and C
(B)
• Low water inflow and sediment supply, intensive evaporation, abundant carbonate deposition, regressional or aggradational basin-fill.
• Shallow, stable chemocline, euxinic bottom conditions (methanogenesis).• Very low terrigenous organic matter input, high primary productivity, blooming of
chlorophyll-c containing phytoplankton.
evaporation
Freshwater, Oxic
Fresh to brackish, anoxic,
∗ ∗ ∗ ∗ ∗ ∗ ∗TOM TOM
Moderate to High Productivity
MBCiliates
CH4
High Water Inflow and Moderate Sediment Supply
Organic Facies A and B
Organic FaciesBC and C
(A)
• High water inflow and moderate sediment supply, regressional or aggradationalbasin-fill.
• Moderate-depth, stable thermocline, euxinic bottom conditions (methanogenesis).• Low to moderate terrigenous organic matter input, moderate to high primary
productivity, thriving of dinoflagellates Bohaidina and Parabohaidina.
Freshwater, oxic
∗ ∗ ∗ ∗ ∗ ∗ ∗TOM
Dysoxic to anoxic
Moderate to High Productivity?
High Water Inflow and Efficient Sediment Supply
Organic Facies B + BC
Organic Facies BC and C
(C)
• High water inflow and efficient sediment supply, progradational basin-fill. • Deep, unstable thermocline.• High terrigenous organic matter input, algal organic matter preferentially oxidized.
Fig. 11. Depositional models for the three source rock intervals showing the environmental and ecological changes induced by changes in tectonic subsidence and climateand their control on organic facies distributions. (A) Deep, freshwater lake at Es3 deposition stage; (B) Shallower, saline-alkaline lake at Es1 deposition stage; (C) Freshwaterlake at Ed deposition stage. TOM = terrigenous organic matter; MB = Methanotrophic bacteria. See text for further discussion.
336 F. Hao et al. / Organic Geochemistry 42 (2011) 323–339
from the very low gammacerane indices (Fig. 9C). The dysoxicbottom water conditions are indicated by the relatively high Pr/Ph,low C35 22S/C34 22S hopane and low ETR (Fig. 9B, D), and can
also be inferred from the absence of high Ph/n-C18 ratios (cf.Fig. 6) which indicated that methanogenesis in the bottomwater/sediments did not occur (Fig. 11C). High water inflow, high
F. Hao et al. / Organic Geochemistry 42 (2011) 323–339 337
sediment input and abundant terrigenous organic matter inputmight provide efficient nitrate to support moderate to high pri-mary productivity (Harris et al., 2004). However, algal organic mat-ter might be preferentially degraded or oxidized in the oxic ordysoxic conditions, which appears to be supported by the extre-mely low sterane/hopane ratios observed in the Ed samples(Fig. 9M). The oxidation/degradation of algal organic matter, to-gether with dilution by terrigenous organic matter (Sachsenhoferet al., 2003), led to the deposition of the Dongying Formationsource rock with significant higher plant organic matter input(Fig. 11C). Source rock in the Dongying Formation has been con-firmed to have independently sourced a small commercial oil accu-mulation close to the generative kitchens (Hao et al., 2010), andmade important contribution to the Penglai 19-3 oil field, the larg-est offshore oil field so far found in China (Hao et al., 2009c).
In summary, changes in tectonic subsidence and climate duringthe syn-rift evolution of the Bozhong sub-basin caused changes inhydrological status and water chemistry of the lakes, whichinduced changes in the phytoplankton community and primaryproductivity. The synergetic evolution of environments and organ-isms in the lake systems (Fig. 11) accounted for the deposition ofthe three source rock intervals with different hydrocarbon generat-ing potentials (Figs. 4 and 5) and distinctly different biomarkerassemblages (Figs. 6–10).
6. Conclusions
Based on our bulk geochemical study of more than 300 samplesand molecular geochemical observation on 57 samples, the follow-ing conclusions can be drawn.
(1) The third (Es3) and first (Es1) members of the Eocene ShahejieFormation and the Oligocene Dongying Formation (Ed) alldisplay widely variable total organic carbon contents,Rock–Eval hydrogen indices, woody and amorphous organicmatter contents, Pr/Ph and Ph/n-C18 ratios, suggesting stronglateral heterogeneities in organic facies from deep water tomarginal lake sediments. Amorphous organic matter, whichis usually believed to originate mainly from algal organicmatter in reducing environments, may be complex in originand therefore may be either rich or poor in hydrogen.
(2) Despite the strong lateral heterogeneities indicated by bulkgeochemical analyses, the deep-lake facies samples fromany interval display fairly narrow variation ranges for mostmolecular parameters reflecting organic matter originsand/or depositional environments. This implies that ‘‘effec-tive oil source rocks’’ (high quality source rocks that havegenerated and expelled a considerable amount of oil andmade a considerable contribution to commercial oilaccumulations) in different intervals might not be asheterogeneous as that reflected by the wide variation inTOC contents, visual kerogen composition and hydrogenindices, which is particularly important for oil-source rockcorrelation. The deep-lake facies from the three intervals,although all are dominated by shales/mudstones, havedistinctly different biomarker associations, suggesting thatmolecular geochemical analysis is a powerful tool to con-struct the basin fill history.
(3) The distinctly different biomarker assemblages for the threesyn-rift intervals indicate weakly alkaline, freshwater lakeswith moderately deep thermocline during Es3 deposition,alkaline-saline lakes with shallow chemocline during Es1
deposition and acidic, freshwater lakes with deep, unstablethermocline during the deposition of the Dongying Forma-tion. Such environmental changes corresponded to changes
in subsidence rate and paleoclimate, from rapid subsidenceand wet climate during Es3 deposition, through slow subsi-dence and arid climate during Es1 deposition to rapid subsi-dence and wet climate during Ed deposition and resulted insynchronous changes in terrigenous organic matter input,phytoplankton community and primary productivity. Thesynergetic evolution of environments and organisms con-trolled by tectonic subsidence and climate accounted forthe deposition and distribution of high quality lacustrinesource rocks with distinctly different geochemical character-istics. The development of source rocks at three syn-riftstages under different tectonic and climatic conditions isresponsible for the great petroleum reserves and the diversegeochemical characteristics of petroleum in the sub-basin.
Acknowledgments
This study was supported by the National Natural ScienceFoundation of China (Grant No. 90914006) and Program forChangjiang Scholars and Innovative Research Team in theUniversity (IRT0658). We appreciate the collaboration and enthusi-astic support of Qinglong Xia, Lixin Tian, Shui Yu, Yonghua Guo andShike Zhou at the Tianjin Branch of China National Offshore OilCompany Ltd, Zaisheng Gong, Yunhua Deng, Jingfu Wu, YimeiSun and Gongcheng Zhang at the CNOOC Research Center, XiongqiPang, Chunjiang Wang, Xiuxiang Lü, Liangjie Tang, Jiafu Qi,Nansheng Qiu, Zhenxue Jiang and many other co-workers at ChinaUniversity of Petroleum, Sitian Li, Renchen Xin and Qiugen Zhengat China University of Geosciences at Beijing. We thank TianjinBranch of China National Offshore Oil Company Ltd. for kindly pro-viding part of the data in this study. L.R. Snowdon (Editor-in-Chief)is gratefully acknowledged for his constructive review of an earlierversion of this manuscript. We thank Barry J. Katz, Maowen Li andKenneth E. Peters (Associate Editor) for their thorough and criticalreviews and suggestions to improve the manuscript.
Associate Editor—Kenneth E. Peters
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