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Hydrocarbon generating potential and accumulation contribution of the fourth member of the Shahejie Formation in the Liaodong Bay sub-basin, Bohai Bay basin
Accepted Manuscript Hydrocarbon generating potential and accumulation contribution of the fourth member of the Shahejie Formation in the Liaodong Bay sub-basin, Bohai Bay basin Jinqiang Tian, Fang Hao, Xinhuai Zhou, Huayao Zou, Bo Peng PII:
S0264-8172(16)30428-7
DOI:
10.1016/j.marpetgeo.2016.11.024
Reference:
JMPG 2747
To appear in:
Marine and Petroleum Geology
Received Date: 15 April 2016 Revised Date:
29 November 2016
Accepted Date: 30 November 2016
Please cite this article as: Tian, J., Hao, F., Zhou, X., Zou, H., Peng, B., Hydrocarbon generating potential and accumulation contribution of the fourth member of the Shahejie Formation in the Liaodong Bay sub-basin, Bohai Bay basin, Marine and Petroleum Geology (2016), doi: 10.1016/ j.marpetgeo.2016.11.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Hydrocarbon Generating Potential and Accumulation Contribution of the Fourth Member of the Shahejie Formation in the Liaodong Bay Sub-basin, Bohai Bay Basin Jinqiang Tian 1, 2*, Fang Hao 1, Xinhuai Zhou 3, Huayao Zou 4, Bo Peng 4, 5 Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China
2
U.S. Geological Survey, Central Energy Resources Science Center, Denver, CO 80225, USA
3
Tianjin Branch of China National Offshore Oil Company Ltd, Tianjin 300452, China
4
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China
5
Research Institute of Uranium Geology, Beijing 100029, China
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* Corresponding author. Tel.:﹢86 27 6788 3077. E-mail addresses: [email protected] (J.Q. Tian)
ABSTRACT
The Liaodong Bay sub-basin is one of the most petroliferous sub-basins in the
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Bohai Bay basin. The fourth member of the Eocene Shahejie Formation (E2s4, 50.5-42 Ma) which is an important source rock interval in other sub-basins was rarely studied in the Liaodong Bay sub-basin. The hydrocarbon generating potential, biomarker
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assemblage and accumulation contribution for discovered oil fields of the E2s4 source
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rock interval in the Liaodong Bay sub-basin were studied using geological and geochemical data. The E2s4 source rocks in the Liaodong Bay sub-basin display relatively high total organic carbon contents (TOC) and Rock-Eval hydrogen indices (HI) and can be considered as a set of excellent oil-prone tendency source rocks. The source rocks interval is characterized mainly by relatively low Pristane/Phytane (Pr/Ph), C19/C23 tricyclic terpane (C19/C23 TT), C24 tetracyclic terpane/C26 tricyclic terpane (C24 Tet/C26 TT), low to medium hopane/sterane (H/S), medium 4-methyl steranes/ΣC29
ACCEPTED MANUSCRIPT steranes ratio (4-MS/ΣC29 ST), C35 22S/C34 22S hopane (C35/C34 SH) and high gammacerane/αβC30 hopane (G/H), extended tricyclic terpane ratio [ETR=(C28 TT+C29 TT)/(C28 TT+C29 TT+Ts)] and were deposited in anoxic to sub-oxic
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saline-water environment with no or minor terrigenous organic matter and medium dinoflagellate input. The maturity of E2s4 source rocks in the Liaozhong and north Liaoxi sags is too high to provide oils for fields. The E2s4 source rocks in the south
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and middle Liaoxi sag displayed opportune maturity within the oil window and were
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proved by oil-source correlation having important accumulation contribution for discovered oil fields in this area. The E2s4-derived oils should be taken seriously in the future exploration in the south and middle Liaoxi sag and on adjacent uplifts.
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Key Words: Liaodong Bay Sub-basin, E2s4 Source Rocks; Biomarker Assemblage,
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Oil Source Correlation, Accumulation Contribution
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1. Introduction The Bohai Bay basin, the most petroliferous basin in China, has the largest oil production (accounting for nearly one-third of the total oil production of China) and
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the largest undiscovered hydrocarbon resource potential in the country (Hao et al., 2009a, 2011). The Liaodong Bay sub-basin, with proven oil reserves greater than 10 ×108 tons (Hao et al., 2009a; Gong et al., 2010; Jiang et al., 2010), is one of the most
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petroliferous sub-basins in the Bohai Bay basin (Figure 1). This sub-basin has been
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explored since the early 1960s (Gong, 1997, 2004; Gong et al., 2000). Several large and medium oil fields such as the SZ36-1 and JX1-1 oil fields were discovered (Figure 1B). It is more and more difficult to find new oil reserves in this sub-basin. In such a moderately to heavily explored area, clearly characterizing potential source
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rocks (Keym et al., 2006; Justwan et al., 2006), classifying discovered hydrocarbons into source-related families (Peters et al., 1994; Abrams et al., 1999; Greene et al.,
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2004; Justwan et al., 2006) and determining the geographic extent of operating petroleum systems (Magoon and Dow, 1994; Magoon et al., 2005) are very useful for
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optimizing petroleum exploration. In the past about 50 years, huge amount of works on hydrocarbon generating potential of source rocks, hydrocarbon generation history and oil source correlation were carried out (Jiang et al., 2010; Tian et al., 2011, Zuo et al., 2011; Liu et al., 2016). The members of Eocene Shahejie Formation (third member E2s3, 42-38 Ma, second and first members E2s1+2, 38-32.8 Ma) and the third member of the Oligocene Dongying Formation (E3d3, 32.8-30.3 Ma) were proved to be high-quality source rocks and their contribution for discovered oil fields have been
ACCEPTED MANUSCRIPT demonstrated (Jiang et al., 2010; Tian et al., 2011; Liu et al., 2016). Since encountered in few wells, the fourth member of the Eocene Shehejie Formation (E2s4, 50.5-42 Ma) which is the most important source rock interval in other sub-basins (Fuhrmann et al.,
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2004; Zhang et al., 2005) was rarely studied in the Liaodong Bay sub-basin. In recent years, several new wells reached to the E2s4 source rocks and some source rock samples were obtained, which make it possible to study the E2s4 source rocks. The
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purpose of this paper is to investigate the hydrocarbon generating potential and
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biomarker assemblage of the E2s4 source rock interval and to identify E2s4-derived oils in discovered oil fields in the Liaodong Bay sub-basin by integrating geological and geochemical data.
2. Geological Setting
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The Bohai Bay basin, a Cenozoic complex rifted basin formed on the basement of the North China Craton (Figure 1A) (Huang and Pearson, 1999; Hsiao et al., 2004;
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Zuo et al., 2011), is the most petroliferous basin in China. It accounts for nearly one-third of the total oil production of China. The evolution of the Bohai Bay basin
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since Cenozoic was generally divided into two stages, the synrift stage during Paleogene (65.0-24.6 Ma) and the postrift stage from Neogene to the present (24.6-0 Ma) (Allen et al., 1997; Huang and Pearson, 1999; Hu et al., 2001; Hsiao et al., 2004, 2010; Qi, 2004). The synrift sediments including the Kongdian (E1k, 65-50.5Ma), Shahejie (E2s, 50.5-32.8Ma) and Dongying (E3d, 32.8-24.6Ma) Formations from bottom to top (see Figure 3 in Liu et al., 2016) (Lu and Qi, 1997; Li et al., 2003; Wu et al., 2006) were restricted to the grabens and half grabens. These Formations were
ACCEPTED MANUSCRIPT mainly deposited in fluvial-lacustrine environments (Lu and Qi, 1997; Gong, 1997; Wu et al., 2006). The postrift sediments including Guantao (Ng, 24.6-12Ma), Minghuazhen (Nm, 12-2Ma) and Pingyuan (Qp, 2-0Ma) Formations from bottom to
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top (see Figure 3 in Liu et al., 2016) are widespread, and are dominated by fluvial deposits (Li et al., 2003; Gong, 1997, 2004; Xiao and Chen, 2003; Yang and Xu, 2004).
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The Liaodong Bay sub-basin, one of the most petroliferous sub-basins in the
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Bohai Bay basin, is located in the northeastern part of basin (Figure 1). This sub-basin consists of three sags (Liaoxi, Liaozhong and Liaodong sags) and two uplifts (Liaoxi and Liaozhong uplifts) (Figures 1B, 2). Four sets of potential source rocks including the members of Eocene Shahejie Formation (fourth member E2s4, 50.5-42 Ma, third
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member E2s3, 42-38 Ma, second and first members E2s1+2, 38-32.8 Ma) and the third member of the Oligocene Dongying Formation (E3d3, 32.8-30.3 Ma) existed in the Liaodong Bay sub-basin (Tian et al., 2011; Xu et al., 2014). Discovered oils are
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mainly distributed in the sandstone reservoirs in E2s2 and E3d2. The main cap rocks
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are mudstones in E3d3 and E3d2.
3. Samples and Methods The TOC and Rock-Eval pyrolysis were conducted on 26 E2s4 shale samples
collected from drill cuttings from five wells in the Liaodong Bay sub-basin using Rock-Eval 6 analyser. These rock samples were cleaned prior to crushing and powdering. The analytical program of Rock-Eval pyrolysis was described by Espitalié et al. (1977). Eight E2s4 rock samples were Soxhlet extracted, and the isolated
ACCEPTED MANUSCRIPT extractable organic matters were separated into saturated hydrocarbons, aromatic hydrocarbons, polar compounds and asphaltenes. In order to minimize the influence of local water inflow and/or sediment input, all the eight samples were selected from
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deep-lake facies in different wells according to the result of sedimentological analysis. 19 crude oil samples from oil fields JZ25-1 and LD4-2 and discovery SZ29-4 were also separated into fractions. Gas chromatographic (GC) and GC—mass spectrometric (GC
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—MS) analyses of the saturated fractions (from source rock extracts and crude oils)
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were conducted in HP 6890GC/5973MSD system. Detailed procedures for GC and GC —MS analysis can be found in Hao et al. (2009a). Biomarker ratios were calculated from peak areas of individual compounds. Oil-source correlation was conducted using biomarker parameters of oil samples and samples from possible source rock intervals.
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In addition, to perform this correlation, a previously published data set (Liu et al., 2016) was used in this study, which shows the biomarker compositions for the E2s3, E2s1+2 and
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E3d3 source rock intervals in the Liaodong Bay sub-basin.
4. Results and Discussion
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4.1. Hydrocarbon-generating Potential 26 source rock samples from the E2s4 in the Liaodong Bay sub-basin display wide
variations in TOC contents, Rock-Eval S2 peaks (Table 1, Figure 3) and hydrogen indices (HI) (Table 1, Figure 4), suggesting a considerable heterogeneity (Keym et al., 2006; Curiale, 2008). These samples show total organic carbon (TOC) contents from 0.54% to 6.60% and Rock-Eval S2 values from 0.53 mg HC/g rock to 48.81 mg HC/g rock (Table 1). 11 samples (nearly 50%) show TOC contents higher than 2.0% and
ACCEPTED MANUSCRIPT Rock-Eval S2 peaks higher than 6.0 mg HC/g rock and, therefore, can be classified as good to excellent source rocks (Wang and Chen, 1988). Only 3 samples (about 12%) show TOC contents lower than 1.0% and Rock-Eval S2 peaks lower than 2.0 mg HC/g
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rock and should be classified as poor source rocks (Table 1, Figure 3). Hydrogen indices for E2s4 samples range from less than 100 to 741 mg HC/g TOC, indicating wide variation in organic matter type (from Type Ⅲ to Type I). 11 E2s4
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samples (nearly 50%) have hydrogen indices higher than 400 mg HC/g TOC (Table 1,
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Figure 4), suggesting relatively high hydrocarbon-generating potential. 4.2. Biomarker Assemblage and Depositional Environment
Biomarkers play a significant role in revealing the environmental and organic parent material sources of source rocks and are important for oil-source and oil-oil
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correlations (Tissot and Welte, 1984; Hunt, 1996; Peters et al., 2005; Curiale, 2008). In the Liaodong Bay sub-basin, the biomarker assemblage and depositional environment of the E2s3, E2s1+2 and E3d3 source rock intervals were discussed using
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Pristane/Phytane (Pr/Ph), C35 22S/C34 22S hopane (C35/C34 SH), C19/C23 tricyclic
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terpanes (C19/C23 TT), C24 tetracyclic terpane/C26 tricyclic terpane (C24 Tet/C26 TT), gammacerane/αβ C30 hopane (G/H), extended tricyclic terpane ratio [ETR=(C28 TT+C29 TT)/(C28 TT+C29 TT+Ts)], hopane/sterane (H/S) and 4-methyl steranes/ΣC29 steranes ratio (4-MS/ΣC29 ST) (Jiang et al., 2010; Tian et al., 2011; Xu et al., 2014; Liu et al., 2016). The significance of these parameters was discussed in previous publications (Hao et al., 2011; Liu et al., 2016) and will be discussed briefly here. Pr/Ph and C35/C34 SH are proposed oxicity parameters for evaluating the redox
ACCEPTED MANUSCRIPT condition of depositional water. Low Pr/Ph and high C35/C34 SH indicate anoxic environment (Didyk et al., 1978; Sofer et al., 1984; Peters et al., 2005; Hao et al., 2009a,b). C19/C23 TT and C24 Tet/C26 TT are often used to indicate terrigenous organic
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matter input (Philp and Gilbert, 1986; Hanson et al., 2000; Preston and Edwards, 2000; Volk et al., 2005; Hao et al., 2009a,b). The G/H and ETR are usually used to indicate changes in water salinity through geological time (Fu et al., 1990; Ritts et al., 1999;
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Hanson et al., 2000, 2001; Hao et al., 2009b; Tian et al., 2011). 4-MS/ΣC29 ST is often
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used to indicate dinoflagellate (mainly Bohaidina and Parabohaidina in the Bohai Bay basin, Chen et al., 1996, 1998; Zhang et al., 2005) input (De Leeuw et al., 1983; Summons et al., 1987). The H/S ratio indicates the input of prokaryotic (bacteria) versus eukaryotic (mainly algae and higher plants) organisms to the source rocks
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(Gonςalves, 2002; Peters et al., 2005; Sepúlveda et al., 2009).
In previous publications (Jiang et al., 2010; Tian et al., 2011; Liu et al., 2016), the biomarker assemblages and depositional environments of the E2s3, E2s1+2 and E3d3
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source rock intervals in the Liaodong Bay sub-basin have been summarized. The E3d3
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source rocks are characterized by relatively high C19/C23 TT, C24 Tet/C26 TT and low G/H (<0.1), ETR (<0.3), 4-MS/∑C29 ST (<0.2) (Figure 5), and were considered to have significant contribution from terrigenous organic matter, minor contribution from dinoflagellates and freshwater conditions during deposition. The E2s1+2 source rocks are characterized by low C19/C23 TT (<0.4), C24 Tet/C26 TT (<1.5), high G/H (>0.1), ETR (mainly higher than 0.3) and medium 4-MS/∑C29 ST (0.1-0.4) (Figure 5), and were considered to have little contribution from terrigenous organic matter, medium from
ACCEPTED MANUSCRIPT dinoflagellates and saline-water conditions during deposition. The E2s3 source rocks are characterized by low to medium C19/C23 TT (<0.4), C24 Tet/C26 TT (<1.5), low G/H (<0.1) and relatively high 4-MS/∑C29 ST (majority of samples>0.4) (Figure 5), and
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were considered to have little to medium contribution from terrigenous organic matter, high from dinoflagellates as well as freshwater conditions during deposition.
In order to investigate the biomarker assemblages and depositional environments
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of the E2s4 source rocks in the Liaodong Bay sub-basin and compare conveniently with
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the other three sets of source rocks, the same parameters were used in this study. In the Liaodong Bay sub-basin, samples from the E2s4 display Pr/Ph from 0.64 to 1.38 (average 1.16, Table 2). With the exception of one sample (LD4, 3755 m), all E2s4 samples have Pr/Ph ranging from 1.0 to 1.5. The E2s4 samples display C35/C34 SH from
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0.13 to 0.60 (average 0.43, Table 2). With the exception of one sample (LD4, 3660 m), the C35/C34 SH of all E2s4 samples are concentrated in the range from 0.35 to 0.60. Such relatively low Pr/Ph and medium C35/C34 SH ratios suggest that anoxic to sub-oxic
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conditions dominated in the bottom water in lakes during E2s4 deposition. C19/C23 TT
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of E2s4 samples shows a compatibility with C24 Tet/C26 TT (Figure 5A). All E2s4 samples display C19/C23 TT lower than 0.40 and C24 Tet/C26 TT lower than 1.5 (Table 2, Figure 5A). The relatively low C19/C23 TT and C24 Tet/C26 TT of the E2s4 samples suggest a low terrigenous organic matter input. E2s4 samples display G/H ranging from 0.10 to 0.36. With the exception of one sample, all E2s4 samples display G/H higher than 0.10 (Table 2, Figure 5B, 5C). ETR of E2s4 samples shows a positive correlation with G/H (Figure 5C). E2s4 samples display ETR ranging from 0.22 to 0.55. With the
ACCEPTED MANUSCRIPT exception of 3 samples, all E2s4 samples display ETR higher than 0.30 (Table 2, Figure 5C, 5D). The relatively high G/H and ETR of E2s4 samples suggest that stratification of water column was well developed during the depression of the E2s4. E2s4 samples show
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4-MS/∑C29 ST from 0.10 to 0.39 (Table 2, Figure 5B), which suggests a medium dinoflagellate input. H/S of E2s4 samples ranged from 1.16 to 8.40 (Table 2, Figure 5D). Half of E2s4 samples have H/S ratio higher than 5.0. The low to medium H/S ratio, low
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and low terrigenous organic matter input.
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C19/C23 TT and C24 Tet/C26 TT indicate relatively high algal-, considerable bacterial-
In summary, the E2s4 source rocks are characterized by Pr/Ph (1.0-1.5), C35/C34 SH (0.35-0.60), C19/C23 TT (<0.4), C24 Tet/C26 TT (<1.5), high G/H (>0.1), ETR (mainly higher than 0.3), low to medium H/S and medium 4-MS/∑C29 ST (0.1-0.4)
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(Table 2, Figure 5, 6A) and were considered to have no or little contribution from terrigenous organic matter, medium from dinoflagellates and considerable from bacteria, and saline-water anoxic to sub-oxic conditions during deposition. Such a biomarker
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assemblage and depositional environment are very similar to that of E2s1+2 source
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rocks in the Liaodong Bay sub-basin (Figure 5). The biomarker assemblage of the E2s4 source rocks in the Liaodong Bay sub-basin (Figure 6A) is also similar to that of E2s4 source rocks in other sub-basins such as Liaohe (Figure 6B) and Jizhong (Figure 6C) sub-basins. The E2s4 source rocks in the Jiyang sub-basin display relatively higher gammacerane and C35 hopanes abundance (Figure 6D), suggesting higher salinity and more hypoxic conditions in the bottom water in lakes during deposition. In general, the E2s4 source rocks were developed in similar environments in different sub-basin in
ACCEPTED MANUSCRIPT the Bohai Bay basin. 4.3. Accumulation Contribution for Discovered Oils Compared with the other three sets of source rocks, the E2s4 source rocks were
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buried deeper and have higher maturity (Figure 2). In the Liaodong Bay sub-basin, oils in reservoirs are considered to charge mainly since the neotectonic motion (since 5.1 Ma, Hao et al., 2009a; Jiang et al., 2011). The maturity of E2s4 source rocks in the
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Liaozhong sag and north Liaoxi sub-sag was too high to provide oils for fields (Figure
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7). The E2s4 source rocks in the south and middle Liaoxi sub-sags displayed relatively low maturity from onset to the peak of oil generation (Figure 7). Therefore, it is very promising to find E2s4-derived oils in the south and middle Liaoxi depression. There are one oil field (LD4-2) in the south Liaoxi sub-sag, one oil field (JZ25-1)
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and one discovery (SZ29-4) in the middle Liaoxi sub-sag (Figure 1B). Oils in these fields and discovery have been found at depth from less than 1000 m to more than 2000 m (Table 3). It is reported in previous studies that oils at depth shallower than about
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2000 m in the Bohai offshore area were biodegraded to varying degrees, as evidenced
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by elevated density, asphaltene, and resin content, and sometimes by the occurrence of 25-norhopanes (Hao et al., 2009a,b; Tian et al., 2011). Oil sample SZ1 from the discovery SZ29-4 has the shallowest reservoirs depth among oils in the south and middle Liaoxi sub-sags. In this oil sample, the n-alkanes suffered significant biodegradation, C30 hopane suffered minor biodegradation accompanied with small amount of C29 25-norhopane, steranes and other hopanes have not suffered significant biodegradation (Figure 8). In this case, major biomarker parameters including C19/C23
ACCEPTED MANUSCRIPT TT, C24 Tet/C26 TT, G/H, 4-MS/∑C29 ST, H/S, ETR, C29 Sterane 20S/(20S + 20R) and C29 Sterane ββ/(αα + ββ) would not be obviously affected by biodegradation. Oils in the south and middle Liaoxi sub-sags display C29 Sterane 20S/(20S + 20R)
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ranging from 0.32 to 0.38 and C29 Sterane ββ/(αα + ββ) ranging from 0.36 to 0.51 (Table 3, Figure 9). Both parameters have not reached the endpoint values (Figure 9). According to the correspondence between the C29 20S/(20S + 20R) sterane ratio and the
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Ro in the Liaodong Bay sub-basin (Liu et al., 2016), the maturity of these oils is from
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the onset of oil generation to medium maturity (Figure 9).
As discussed above, in the Liaodong Bay sub-basin, C19/C23 TT and C24 Tet/C26 TT are usually used to distinguish E3d3-derived oils from oils generated from other three sets of source rocks. G/H and ETR can be used to distinguish E2s4- and
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E2s1+2-derived oils from E2s3- and E3d3-derived oils. 4-MS/ ∑ C29 ST and H/S (combined with other parameters) can be used to distinguish E2s3-derived oils from oils generated from other three sets of source rocks. However, there is no valid biomarker
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parameter which can be used to distinguish E2s4-derived oils from E2s1+2-derived oils.
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Fortunately, with low maturity, the E3d3 and E2s1+2 source rocks in the south and middle Liaoxi sub-sags can’t provide oils to reservoirs (Figure 10). Accordingly, just the E2s4 and E2s3 source rocks should be taken into account for oil-source correlation in the south and middle Liaoxi sub-sags. G/H and 4-MS/∑C29 ST are sufficient to identify and distinguish E2s4- and E2s3- derived oils. Oils in the oil field JZ25-1 (except sample 14A) display medium G/H (0.14-0.28) and 4-MS/∑C29 ST (0.25-0.38) (Table 3, Figure 11). These oils correlate reasonably with the E2s4 and E2s1+2 source rocks
ACCEPTED MANUSCRIPT (Figures 5B, 11). In view of the relatively low maturity (Ro<0.7%) of the E2s1+2 source rocks in the south and middle Liaoxi sub-sags (Figure 10), these oils should be interpreted to originate primarily from E2s4 source rocks in the south and middle Liaoxi
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sub-sags. Oils in the oil field LD4-2, oil samples SZ1 from the discovery SZ29-4 and 14A from the JZ25-1 oil field have higher G/H than E2s3 source rocks and higher
interpreted as a mixing of E2s4- and E2s3-derived oils.
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4-MS/∑C29 ST than E2s4 source rocks (Table 3, Figure 11). These oils should be
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The oil-source correlation above indicates that the E2s4 source rocks have important accumulation contribution to discovered oil fields in south and middle Liaoxi sub-sags and that the E2s4-derived oils should be taken into seriously consideration in the future exploration in the south and middle Liaoxi sub-sags and on adjacent uplifts.
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5. Conclusions
Based on systematic study of the characteristics and biomarker assemblages of
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the potential source rocks and reservoir oils and geological analysis, the following conclusions can be drawn:
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(1) With relatively high total organic carbon contents (TOC) and Rock-Eval
hydrogen indices (HI), the E2s4 in the Liaodong Bay sub-basin represents a set of excellent oil-prone tendency source rocks. (2) The E2s4 source rocks in the Liaodong Bay sub-basin are characterized mainly by relatively low Pr/Ph, C19/C23 TT, C24 Tet/C26 TT, low to medium H/S, medium 4-MS/ΣC29 ST, C35/C34 SH and high G/H, ETR, and were deposited in anoxic to sub-oxic saline-water environments with no or minor terrigenous organic matter and
ACCEPTED MANUSCRIPT medium dinoflagellate input. The E2s4 source rocks in the Liaodong Bay sub-basin displayed similar biomarker assemblage with the E2s1+2 source rocks in the Liaodong Bay sub-basin and the E2s4 source rocks in other sub-basins in the Bohai Bay basin.
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(3) The E2s4 source rocks in the south and middle Liaoxi depression displayed opportune maturity within the oil window and were proved by oil-source correlation having important accumulation contribution to discovered oil fields in south and middle
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Liaoxi sub-sags. The E2s4-derived oils should be taken into seriously consideration in
Acknowledgments
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the future exploration in the south and middle Liaoxi sub-sags and on adjacent uplifts.
We thank the Bohai Oil Company, CNOOC, and individuals who contributed
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samples and information for this work. The National Natural Science Foundation of China (no.41202089) and the Open Foundation of the State Kay Laboratory of
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Petroleum Resource and Prospecting, China University of Petroleum, Beijing
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(no.PRP/open-1104) provided financial support.
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ACCEPTED MANUSCRIPT Zuo, Y.H., Qiu, N.S., Zhang, Y., Li, C.C., Li, J.P., Guo, Y.H., Pang, X.Q., 2011. Geothermal regime and hydrocarbon kitchen evolution of the offshore Bohai Bay Basin, North China.
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AAPG Bull. 95, 749–769.
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Table 1. Rock-Eval Pyrolysis Data of the E2s4 Source Rocks in the Liaodong Bay Sub-basin, Bohai Bay Basin.
Depth Well
TOC
Tmax
S1
S2
(wt. %)
(℃)
(mg/g rock)
HI
Lithology (m)
(mg/g TOC)
3448
Mudstone
2.83
438
1.92
9.24
327
LD1
3450
Mudstone
2.95
440
4.20
11.60
LD1
3500
Mudstone
2.05
441
4.10
9.40
LD2
2495
Mudstone
1.31
437
0.97
3.10
LD2
2685
Mudstone
0.68
434
0.15
2.21
LD2
2792.5
Mudstone
1.09
439
0.19
2.50
LD3
3250
Mudstone
1.07
436
0.10
1.92
179
LD3
3300
Mudstone
1.63
439
0.19
3.88
238
LD3
3265
Mudstone
2.23
440
0.16
6.54
296
LD3
3315
Mudstone
LD3
3355
Mudstone
LD3
3350
Mudstone
LD3
3297.5
Mudstone
LD3
3200
Mudstone
LD3
3387.5
Mudstone
LD3
3400
Mudstone
LD3
3430
Mudstone
LD3
3417.5
LD4
RI PT
LD1
393 459 239 329
M AN U
SC
233
441
0.35
12.74
341
3.03
441
0.26
13.02
432
0.62
441
0.09
2.08
335
2.36
443
0.25
8.30
354
1.17
444
0.16
2.72
232
3.32
444
0.26
16.38
496
2.55
447
0.35
12.56
493
3.11
447
0.31
18.68
601
Mudstone
6.6
448
0.72
48.81
741
3755
Mudstone
0.75
440
0.09
0.53
71
LD4
3660
Mudstone
0.56
444
0.11
0.87
155
LD5
3755
Mudstone
0.64
421
1.47
2.89
453
LD5
3795
Mudstone
0.78
426
1.59
3.44
442
LD5
3815
Mudstone
0.63
428
1.40
2.93
465
LD5
3655
Mudstone
0.98
446
0.91
4.00
409
LD5
3690
Mudstone
1.09
447
0.80
4.56
420
LD5
3700
Mudstone
0.54
413
0.39
1.99
369
AC C
EP
TE D
3.76
ACCEPTED MANUSCRIPT Table 2. Biomarker Parameters of the E2s4 Source Rocks in the Liaodong Bay Sub-basin, Bohai Bay Basin.
C35/C34
Depth Well
C19/C23
C24 Tet/C26 G/H
Pr/Ph (m)
SH
TT
TT
ETR
4-MS/ΣC29
C29 S/
C29 ββ/
(S+R)
(αα+ββ)
H/S
ST
3448
1.06
0.56
0.13
0.62
0.32
0.49
0.39
1.73
0.09
0.27
LD2
2495
1.35
0.50
0.39
0.47
0.22
0.52
0.14
3.60
0.19
0.35
LD2
2792.5
1.09
0.37
0.07
0.64
0.23
0.49
0.19
4.42
0.24
0.37
LD4
3755
0.64
0.60
0.09
1.02
0.36
0.55
0.22
1.16
0.11
0.24
LD4
3660
1.33
0.13
0.14
0.99
0.12
0.40
0.10
5.80
0.45
0.41
LD5
3655
1.09
0.47
0.17
1.19
0.13
0.23
0.17
6.46
0.25
0.25
LD5
3755
1.36
0.48
0.30
0.91
0.12
0.22
0.21
6.87
0.24
0.32
LD5
3815
1.38
0.37
0.36
1.06
0.10
0.22
0.22
8.40
0.27
0.35
RI PT
LD1
SC
Note: Pr/Ph = Pristane/Phytane; C35/C34 SH = C35 17α(H)21β(H)22S hopane/C34 17α(H)21β(H)22S hopane; C19/C23 TT = C19 tricyclic terpane/C23 tricyclic terpane; C24 Tet/C26 TT = C24 tetracyclic terpane/C26 tricyclic terpane; G/H = gammacerane/C30 17α(H)21β(H)hopane; ETR = (C28 TT+C29 TT)/(C28 TT+C29 TT+Ts); 4-MS/ΣC29 ST = 4-methyl steranes/ΣC29 steranes; H/S = hopane/steranes [C30 17α(H)21β(H)hopane/C29 steranes αα(20S+20R)]; C29 S/(S+R) = C29 αα20S sterane/C29 αα(20S+20R)
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steranes; C29 ββ/(αα+ββ) = 5α-C29 ββ sterane/5α-C29 (αα+ββ) steranes.
Table 3. Biomarker parameters of oils from south and middle Liaoxi sub-sags, Liaodong Bay sub-basin, Bohai Bay basin.
Depth ID
C19/C23
Well
C24 Tet/C26
4-MS/ΣC29
G/H
(m)
TT
TT
0.50
C29 S/
C29 ββ/
(S+R)
(αα+ββ)
H/S
ST
2
JZ25-1-2
1898
0.14
0.34
0.33
8.21
0.33
0.36
3A
JZ25-1-3
1954.5
0.45
0.40
0.28
0.49
0.25
2.69
0.36
0.41
3B
JZ25-1-3
2154
0.26
0.41
0.16
0.50
0.30
5.39
0.36
0.41
4A
JZ25-1-4
2128
0.39
0.54
0.19
0.43
0.36
7.21
0.38
0.44
4B
JZ25-1-4
2158
0.30
0.56
0.20
0.40
0.35
6.44
0.35
0.42
5A
JZ25-1-5
2104.9
0.19
0.47
0.23
0.43
0.32
6.11
0.32
0.40
5B
JZ25-1-5
2362
0.34
0.36
0.22
0.47
0.33
5.48
0.34
0.41
6
JZ25-1-6
1922.5
0.26
0.45
0.15
0.51
0.30
4.89
0.35
0.41
7A
JZ25-1-7
2022.5
0.25
0.33
0.19
0.47
0.26
11.75
0.36
0.51
0.32
0.46
0.25
0.42
0.33
5.94
0.33
0.42
AC C
EP
TE D
0.24
ETR
7B
JZ25-1-7
2043.2
10
JZ25-1-10
1893
0.25
0.41
0.16
0.49
0.33
5.07
0.36
0.41
11A
JZ25-1-11
1926.8
0.24
0.44
0.20
0.48
0.33
5.57
0.34
0.43
11B
JZ25-1-11
1996
0.23
0.36
0.25
0.45
0.35
6.50
0.35
0.43
14A
JZ25-1-14
1831
0.22
0.45
0.18
0.47
0.44
10.12
0.37
0.44
14B
JZ25-1-14
1911.5
0.20
0.38
0.25
0.50
0.29
9.17
0.38
0.48
15
JZ25-1-15
1958
0.24
0.42
0.24
0.45
0.34
9.80
0.35
0.43
1A
LD4-2-1
1657.5
0.18
0.59
0.13
0.38
0.46
10.80
0.38
0.41
1B
LD4-2-1
1680
0.23
0.67
0.21
0.37
0.48
6.14
0.34
0.40
SZ1
SZ29-4-1
879.8
0.24
0.57
0.20
0.40
0.47
13.81
0.36
0.42
Note: ID = sample identifications in Figure 11.
ACCEPTED MANUSCRIPT 115°
116°
117°
118°
119° 0
41°
120° 50
121°
122°
100 km
Beijing
Bohai Bay Basin
40°
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39°
Taiwan
N Bozhong Sub-basin
Bohai Sea
Jiyang Sub-basin
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37°
SC
38°
Bohai Bay Basin 36°
(A)
10
0
Coastline Major Uplift JZ9-3
JZ20-2N
TE D
LD1
AC C
EP
N
20km
Basin Boundary
JZ20-2 JZ20-1
JZ21-1/S
LD2 JZ25-1S
SZ29-4
JX1-1 C
SZ36-1 LD4-2
LD6-2
LD10-1
Basin Boundary Extensional Fault LD3 LD4
Uplift Oil Field Discovery
(B)
Well LD5
Figure 1. (A) Sub-basins of the Bohai Bay basin (modified from Hao et al., 2009a). (B) Distribution of sags and uplifts and main oil fields in the Liaodong Bay sub-basin.
ACCEPTED MANUSCRIPT
Oil Field SZ36-1 Nm
1
3 4
Ng
Ed
Es1+2
Ed Es1+2
Es3
Es3
Es4 Es3
5
Es4 7
Liaoxi Uplift
Liaoxi Sag
Liaozhong Sag
A′
Es4
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6
SC
Depth (km)
2
Oil Field JX1-1 Qp
RI PT
A 0
Liaodong Sag
AC C
EP
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Figure 2. Cross-sections through the main oil fields showing the petroleum geology framework from sags to uplifts. Section locations are shown in Figure 1B.
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S2 (mg HC/g rock)
Good to excellent source rock 10
Fair source rock
0.1 0.1
SC
Poor source rock
M AN U
1
RI PT
100
1
10
TOC (%)
AC C
EP
TE D
Figure 3. Total organic carbon (TOC, wt.%) contents vs. Rock-Eval S2 peaks (mg HC/g rock) for the E2s4 samples from the Liaodong Bay sub-basin, Bohai Bay basin. Quality of the source rocks is evaluated according to Wang and Chen (1988).
RI PT
ACCEPTED MANUSCRIPT
I
800
Ro=0.5% II
SC
600
400
200
III 0 400
Ro=1.0%
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Hydrogen Index (mg HC/g TOC)
1000
420
440
460
480
500
Tmax (℃ ℃)
AC C
EP
TE D
Figure 4. Rock-Eval hydrogen index vs. Tmax for the E2s4 samples from the Liaodong Bay sub-basin, Bohai Bay basin. Tmax is the temperature at which the maximum amount of S2 hydrocarbons is generated. Ro = vitrinite reflectance.
ACCEPTED MANUSCRIPT
0.8
0.8
0.6
RI PT
(B)
(A) 0.6
E3d3 E2s1+2 E2s3 E2s4
G/H
0.4 E2s4+E2s3+E2s1+2
0
1
2
C24 Tet/C26 TT
0.2
E3d3 0.0 0.0
SC
E3d3 E2s1+2 E2s3 E2s4
0.2
0.0
0.4
E3d3
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C19/C23 TT
E2s4+E2s1+2
3
0.2
0.4
E2s3
0.6
0.8
1.0
0.8
1.0
(D)
(C)
E3d3 E2s1+2 E2s3 E2s4
E3d3 E2s1+2 E2s3 E2s4
EP
0.1
0.1
G/H
1.0
ETR
TE D
ETR
0.6
0.05 0.01
1.2
4-MS/Σ Σ C29 ST
0.4
0.2
0.0
0
4
8
12
16
20
24
28
32
H/S
AC C
Figure 5. Plots of various biomarker parameters, showing the differences in biomarker compositions among the four source rock intervals in the Liaodong Bay sub-basin. Abbreviations for biomarker parameters are explained in legend of Table 2.
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(A) Well LD2，2495 m，E2s4 m/z 191
C23
C21
m/z 191
αβ C30 Hopane
m/z 217
C27
C29
C20
C28 C29
C26 C22
C31-C35 Homohopanes
C24 Tet
C25
G Tm Ts
(B) Liaohe sub-basin 2743.4 m，E2s4 C23
αβ C30 Hopane
SC
C24 C28 C29
C26
C20
C25
TsTm
(C) Jizhong sub-basin 3762.5 m，E2s4 C23
αβ C30 Hopane
C24 C20 C26 C24 Tet
C22
C28 C29
TE D
C25
C19
C29 C28
4-MS
G
C22
C21
4-MS
C31-C35 Homohopanes
M AN U
C19
C28
C27
C21
C24 Tet
RI PT
C24 C19
C27 C29
C28
C31-C35 Homohopanes 4-MS
G
TsTm
(D) Jiyang sub-basin 1212.5 m, E2s4 C23 C21 C24 C20
EP
αβ C30 Hopane
C27
C29
G
C29
C19
AC C
C25 C26 C24 Tet
C28
C31-C35 Homohopanes
C28 4-MS
C22
Tm Ts
Figure 6. Representative mass chromatograms of terpane (m/z=191) and sterane (m/z=217) series of saturated fractions for the E2s4 source rocks in the Liaodong Bay (A), Liaohe (B), Jizhong (C) and Jiyang (D) sub-basins, Bohai Bay Basin. Peaks marked by solid dots are tricyclic terpanes (C19-C26). C24Tet = C24 tetracyclic terpane; G = gammacerane; C27, C28 and C29 represent C27 sterane 14α(H)17α(H)20R, C28 sterane 14α(H)17α(H)20R and C29 sterane 14α(H)17α(H)20R, respectively; 4MS = 4-methylsteranes.
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JZ9-3
10
0
20km
JZ20-2N
N
LD1
JZ20-2
JZ21-1/S
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JZ20-1
LD2
JZ25-1S
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SZ29-4
JX1-1
C
SZ36-1
LD4-2
LD6-2
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LD10-1
Ro: 0.5%~0.7% Ro: 0.7%~1.0% Ro: 1.0%~1.3% Ro: 1.3%~2.0%
LD3
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LD4
LD5
Ro: >2.0%
Basin Boundary
Oil Field
Extensional Fault
Discovery
Uplift
Well
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Figure 7. Present Ro (%) distribution at the top surface of the E2s4 in the Liaodong Bay sub-basin, Bohai Bay basin.
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αβ C30 Hopane
nC18
C31-C35 Homohopanes
nC17 Pr Ph
Gam
αβ C30 Hopane
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m/z 191
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TIC
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C31-C35 Homohopanes
G
Ts Tm
D29
m/z 177
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D29
C27
m/z 217
C29 4-MS
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C28
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Figure 8. Mass chromatograms of total ion current (TIC), hopane (m/z 191), 25norhopanes (m/z 177) and sterane (m/z 217) series for oil sample SZ29-4-1. nC17 = nheptadecane; nC18 = n-octadecane; Pr = pristane; Ph = phytane; D29 = C29 25norhopane; G = gammacerane; C27, C28 and C29 represent C27 sterane 14α(H)17α(H)20R, C28 sterane 14α(H)17α(H)20R and C29 sterane 14α(H)17α(H)20R, respectively; 4MS = 4-methylsteranes.
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0.6
Peak Oil Generation
0.5
0.4
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Ro=0.8%
0.3
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Early Oil Generation
C29 Sterane 20S/(20S + 20R)
Equilibrium
Ro=0.6%
LD4-2 SZ29-4 JZ25-1
0.2 0.2
0.3
0.4
0.5
0.6
C29 Sterane ββ/(αα ββ αα + ββ) ββ
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Figure 9. The C29 20S/(20S + 20R) vs. C29 ββ/(αα + ββ) sterane ratios of oils from fields and
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discovery in the south and middle Liaoxi sag. Labels are identified in the ID column in Table 3. Vitrinite reflectance (Ro,%) estimated after correlations by Liu et al. (2016).
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10
0
20km
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JZ25-1S
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SZ29-4
JX1-1
C
SZ36-1
LD4-2
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Ro: 0.5%~0.7% Ro: 0.7%~1.0% Ro: 1.0%~1.3%
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LD3
Ro: 1.3%~2.0%
LD4
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LD5
Basin Boundary
Oil Field
Extensional Fault
Discovery
Uplift
Well
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Figure 10. Present Ro (%) distribution at the bottom surface of the E2s1+2 in the Liaodong Bay subbasin, Bohai Bay basin.
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LD4-2 SZ29-4 JZ25-1 0.4
G/H
E2s4
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E2s4+E2s3 0.2
SZ1 14A
E3d3
E2s3 0.2
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0.0 0.0
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0.6
0.4
0.6
0.8
1.0
1.2
4-MS/Σ Σ C29 ST
Figure 11. Plot G/H vs. 4-MS/ΣC29 ST for oils from the fields and discovery in the south and middle Liaoxi sag.
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Labels are identified in the ID column in Table 3.
ACCEPTED MANUSCRIPT 1. The E2s4 in the Liaodong Bay sub-basin is a set of oil-prone tendency source rocks. 2. Biomarker assemblage of the E2s4 in the Liaodong Bay sub-basin is comparable with that in other sub-basins.
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3. South and middle Liaoxi sag and adjacent uplifts are favorable exploration areas for E2s4-derived oils.
S0264-8172(16)30428-7
DOI:
10.1016/j.marpetgeo.2016.11.024
Reference:
JMPG 2747
To appear in:
Marine and Petroleum Geology
Received Date: 15 April 2016 Revised Date:
29 November 2016
Accepted Date: 30 November 2016
Please cite this article as: Tian, J., Hao, F., Zhou, X., Zou, H., Peng, B., Hydrocarbon generating potential and accumulation contribution of the fourth member of the Shahejie Formation in the Liaodong Bay sub-basin, Bohai Bay basin, Marine and Petroleum Geology (2016), doi: 10.1016/ j.marpetgeo.2016.11.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Hydrocarbon Generating Potential and Accumulation Contribution of the Fourth Member of the Shahejie Formation in the Liaodong Bay Sub-basin, Bohai Bay Basin Jinqiang Tian 1, 2*, Fang Hao 1, Xinhuai Zhou 3, Huayao Zou 4, Bo Peng 4, 5 Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China
2
U.S. Geological Survey, Central Energy Resources Science Center, Denver, CO 80225, USA
3
Tianjin Branch of China National Offshore Oil Company Ltd, Tianjin 300452, China
4
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China
5
Research Institute of Uranium Geology, Beijing 100029, China
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1
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* Corresponding author. Tel.:﹢86 27 6788 3077. E-mail addresses: [email protected] (J.Q. Tian)
ABSTRACT
The Liaodong Bay sub-basin is one of the most petroliferous sub-basins in the
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Bohai Bay basin. The fourth member of the Eocene Shahejie Formation (E2s4, 50.5-42 Ma) which is an important source rock interval in other sub-basins was rarely studied in the Liaodong Bay sub-basin. The hydrocarbon generating potential, biomarker
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assemblage and accumulation contribution for discovered oil fields of the E2s4 source
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rock interval in the Liaodong Bay sub-basin were studied using geological and geochemical data. The E2s4 source rocks in the Liaodong Bay sub-basin display relatively high total organic carbon contents (TOC) and Rock-Eval hydrogen indices (HI) and can be considered as a set of excellent oil-prone tendency source rocks. The source rocks interval is characterized mainly by relatively low Pristane/Phytane (Pr/Ph), C19/C23 tricyclic terpane (C19/C23 TT), C24 tetracyclic terpane/C26 tricyclic terpane (C24 Tet/C26 TT), low to medium hopane/sterane (H/S), medium 4-methyl steranes/ΣC29
ACCEPTED MANUSCRIPT steranes ratio (4-MS/ΣC29 ST), C35 22S/C34 22S hopane (C35/C34 SH) and high gammacerane/αβC30 hopane (G/H), extended tricyclic terpane ratio [ETR=(C28 TT+C29 TT)/(C28 TT+C29 TT+Ts)] and were deposited in anoxic to sub-oxic
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saline-water environment with no or minor terrigenous organic matter and medium dinoflagellate input. The maturity of E2s4 source rocks in the Liaozhong and north Liaoxi sags is too high to provide oils for fields. The E2s4 source rocks in the south
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and middle Liaoxi sag displayed opportune maturity within the oil window and were
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proved by oil-source correlation having important accumulation contribution for discovered oil fields in this area. The E2s4-derived oils should be taken seriously in the future exploration in the south and middle Liaoxi sag and on adjacent uplifts.
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Key Words: Liaodong Bay Sub-basin, E2s4 Source Rocks; Biomarker Assemblage,
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Oil Source Correlation, Accumulation Contribution
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1. Introduction The Bohai Bay basin, the most petroliferous basin in China, has the largest oil production (accounting for nearly one-third of the total oil production of China) and
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the largest undiscovered hydrocarbon resource potential in the country (Hao et al., 2009a, 2011). The Liaodong Bay sub-basin, with proven oil reserves greater than 10 ×108 tons (Hao et al., 2009a; Gong et al., 2010; Jiang et al., 2010), is one of the most
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petroliferous sub-basins in the Bohai Bay basin (Figure 1). This sub-basin has been
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explored since the early 1960s (Gong, 1997, 2004; Gong et al., 2000). Several large and medium oil fields such as the SZ36-1 and JX1-1 oil fields were discovered (Figure 1B). It is more and more difficult to find new oil reserves in this sub-basin. In such a moderately to heavily explored area, clearly characterizing potential source
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rocks (Keym et al., 2006; Justwan et al., 2006), classifying discovered hydrocarbons into source-related families (Peters et al., 1994; Abrams et al., 1999; Greene et al.,
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2004; Justwan et al., 2006) and determining the geographic extent of operating petroleum systems (Magoon and Dow, 1994; Magoon et al., 2005) are very useful for
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optimizing petroleum exploration. In the past about 50 years, huge amount of works on hydrocarbon generating potential of source rocks, hydrocarbon generation history and oil source correlation were carried out (Jiang et al., 2010; Tian et al., 2011, Zuo et al., 2011; Liu et al., 2016). The members of Eocene Shahejie Formation (third member E2s3, 42-38 Ma, second and first members E2s1+2, 38-32.8 Ma) and the third member of the Oligocene Dongying Formation (E3d3, 32.8-30.3 Ma) were proved to be high-quality source rocks and their contribution for discovered oil fields have been
ACCEPTED MANUSCRIPT demonstrated (Jiang et al., 2010; Tian et al., 2011; Liu et al., 2016). Since encountered in few wells, the fourth member of the Eocene Shehejie Formation (E2s4, 50.5-42 Ma) which is the most important source rock interval in other sub-basins (Fuhrmann et al.,
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2004; Zhang et al., 2005) was rarely studied in the Liaodong Bay sub-basin. In recent years, several new wells reached to the E2s4 source rocks and some source rock samples were obtained, which make it possible to study the E2s4 source rocks. The
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purpose of this paper is to investigate the hydrocarbon generating potential and
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biomarker assemblage of the E2s4 source rock interval and to identify E2s4-derived oils in discovered oil fields in the Liaodong Bay sub-basin by integrating geological and geochemical data.
2. Geological Setting
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The Bohai Bay basin, a Cenozoic complex rifted basin formed on the basement of the North China Craton (Figure 1A) (Huang and Pearson, 1999; Hsiao et al., 2004;
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Zuo et al., 2011), is the most petroliferous basin in China. It accounts for nearly one-third of the total oil production of China. The evolution of the Bohai Bay basin
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since Cenozoic was generally divided into two stages, the synrift stage during Paleogene (65.0-24.6 Ma) and the postrift stage from Neogene to the present (24.6-0 Ma) (Allen et al., 1997; Huang and Pearson, 1999; Hu et al., 2001; Hsiao et al., 2004, 2010; Qi, 2004). The synrift sediments including the Kongdian (E1k, 65-50.5Ma), Shahejie (E2s, 50.5-32.8Ma) and Dongying (E3d, 32.8-24.6Ma) Formations from bottom to top (see Figure 3 in Liu et al., 2016) (Lu and Qi, 1997; Li et al., 2003; Wu et al., 2006) were restricted to the grabens and half grabens. These Formations were
ACCEPTED MANUSCRIPT mainly deposited in fluvial-lacustrine environments (Lu and Qi, 1997; Gong, 1997; Wu et al., 2006). The postrift sediments including Guantao (Ng, 24.6-12Ma), Minghuazhen (Nm, 12-2Ma) and Pingyuan (Qp, 2-0Ma) Formations from bottom to
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top (see Figure 3 in Liu et al., 2016) are widespread, and are dominated by fluvial deposits (Li et al., 2003; Gong, 1997, 2004; Xiao and Chen, 2003; Yang and Xu, 2004).
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The Liaodong Bay sub-basin, one of the most petroliferous sub-basins in the
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Bohai Bay basin, is located in the northeastern part of basin (Figure 1). This sub-basin consists of three sags (Liaoxi, Liaozhong and Liaodong sags) and two uplifts (Liaoxi and Liaozhong uplifts) (Figures 1B, 2). Four sets of potential source rocks including the members of Eocene Shahejie Formation (fourth member E2s4, 50.5-42 Ma, third
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member E2s3, 42-38 Ma, second and first members E2s1+2, 38-32.8 Ma) and the third member of the Oligocene Dongying Formation (E3d3, 32.8-30.3 Ma) existed in the Liaodong Bay sub-basin (Tian et al., 2011; Xu et al., 2014). Discovered oils are
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mainly distributed in the sandstone reservoirs in E2s2 and E3d2. The main cap rocks
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are mudstones in E3d3 and E3d2.
3. Samples and Methods The TOC and Rock-Eval pyrolysis were conducted on 26 E2s4 shale samples
collected from drill cuttings from five wells in the Liaodong Bay sub-basin using Rock-Eval 6 analyser. These rock samples were cleaned prior to crushing and powdering. The analytical program of Rock-Eval pyrolysis was described by Espitalié et al. (1977). Eight E2s4 rock samples were Soxhlet extracted, and the isolated
ACCEPTED MANUSCRIPT extractable organic matters were separated into saturated hydrocarbons, aromatic hydrocarbons, polar compounds and asphaltenes. In order to minimize the influence of local water inflow and/or sediment input, all the eight samples were selected from
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deep-lake facies in different wells according to the result of sedimentological analysis. 19 crude oil samples from oil fields JZ25-1 and LD4-2 and discovery SZ29-4 were also separated into fractions. Gas chromatographic (GC) and GC—mass spectrometric (GC
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—MS) analyses of the saturated fractions (from source rock extracts and crude oils)
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were conducted in HP 6890GC/5973MSD system. Detailed procedures for GC and GC —MS analysis can be found in Hao et al. (2009a). Biomarker ratios were calculated from peak areas of individual compounds. Oil-source correlation was conducted using biomarker parameters of oil samples and samples from possible source rock intervals.
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In addition, to perform this correlation, a previously published data set (Liu et al., 2016) was used in this study, which shows the biomarker compositions for the E2s3, E2s1+2 and
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E3d3 source rock intervals in the Liaodong Bay sub-basin.
4. Results and Discussion
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4.1. Hydrocarbon-generating Potential 26 source rock samples from the E2s4 in the Liaodong Bay sub-basin display wide
variations in TOC contents, Rock-Eval S2 peaks (Table 1, Figure 3) and hydrogen indices (HI) (Table 1, Figure 4), suggesting a considerable heterogeneity (Keym et al., 2006; Curiale, 2008). These samples show total organic carbon (TOC) contents from 0.54% to 6.60% and Rock-Eval S2 values from 0.53 mg HC/g rock to 48.81 mg HC/g rock (Table 1). 11 samples (nearly 50%) show TOC contents higher than 2.0% and
ACCEPTED MANUSCRIPT Rock-Eval S2 peaks higher than 6.0 mg HC/g rock and, therefore, can be classified as good to excellent source rocks (Wang and Chen, 1988). Only 3 samples (about 12%) show TOC contents lower than 1.0% and Rock-Eval S2 peaks lower than 2.0 mg HC/g
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rock and should be classified as poor source rocks (Table 1, Figure 3). Hydrogen indices for E2s4 samples range from less than 100 to 741 mg HC/g TOC, indicating wide variation in organic matter type (from Type Ⅲ to Type I). 11 E2s4
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samples (nearly 50%) have hydrogen indices higher than 400 mg HC/g TOC (Table 1,
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Figure 4), suggesting relatively high hydrocarbon-generating potential. 4.2. Biomarker Assemblage and Depositional Environment
Biomarkers play a significant role in revealing the environmental and organic parent material sources of source rocks and are important for oil-source and oil-oil
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correlations (Tissot and Welte, 1984; Hunt, 1996; Peters et al., 2005; Curiale, 2008). In the Liaodong Bay sub-basin, the biomarker assemblage and depositional environment of the E2s3, E2s1+2 and E3d3 source rock intervals were discussed using
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Pristane/Phytane (Pr/Ph), C35 22S/C34 22S hopane (C35/C34 SH), C19/C23 tricyclic
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terpanes (C19/C23 TT), C24 tetracyclic terpane/C26 tricyclic terpane (C24 Tet/C26 TT), gammacerane/αβ C30 hopane (G/H), extended tricyclic terpane ratio [ETR=(C28 TT+C29 TT)/(C28 TT+C29 TT+Ts)], hopane/sterane (H/S) and 4-methyl steranes/ΣC29 steranes ratio (4-MS/ΣC29 ST) (Jiang et al., 2010; Tian et al., 2011; Xu et al., 2014; Liu et al., 2016). The significance of these parameters was discussed in previous publications (Hao et al., 2011; Liu et al., 2016) and will be discussed briefly here. Pr/Ph and C35/C34 SH are proposed oxicity parameters for evaluating the redox
ACCEPTED MANUSCRIPT condition of depositional water. Low Pr/Ph and high C35/C34 SH indicate anoxic environment (Didyk et al., 1978; Sofer et al., 1984; Peters et al., 2005; Hao et al., 2009a,b). C19/C23 TT and C24 Tet/C26 TT are often used to indicate terrigenous organic
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matter input (Philp and Gilbert, 1986; Hanson et al., 2000; Preston and Edwards, 2000; Volk et al., 2005; Hao et al., 2009a,b). The G/H and ETR are usually used to indicate changes in water salinity through geological time (Fu et al., 1990; Ritts et al., 1999;
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Hanson et al., 2000, 2001; Hao et al., 2009b; Tian et al., 2011). 4-MS/ΣC29 ST is often
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used to indicate dinoflagellate (mainly Bohaidina and Parabohaidina in the Bohai Bay basin, Chen et al., 1996, 1998; Zhang et al., 2005) input (De Leeuw et al., 1983; Summons et al., 1987). The H/S ratio indicates the input of prokaryotic (bacteria) versus eukaryotic (mainly algae and higher plants) organisms to the source rocks
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(Gonςalves, 2002; Peters et al., 2005; Sepúlveda et al., 2009).
In previous publications (Jiang et al., 2010; Tian et al., 2011; Liu et al., 2016), the biomarker assemblages and depositional environments of the E2s3, E2s1+2 and E3d3
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source rock intervals in the Liaodong Bay sub-basin have been summarized. The E3d3
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source rocks are characterized by relatively high C19/C23 TT, C24 Tet/C26 TT and low G/H (<0.1), ETR (<0.3), 4-MS/∑C29 ST (<0.2) (Figure 5), and were considered to have significant contribution from terrigenous organic matter, minor contribution from dinoflagellates and freshwater conditions during deposition. The E2s1+2 source rocks are characterized by low C19/C23 TT (<0.4), C24 Tet/C26 TT (<1.5), high G/H (>0.1), ETR (mainly higher than 0.3) and medium 4-MS/∑C29 ST (0.1-0.4) (Figure 5), and were considered to have little contribution from terrigenous organic matter, medium from
ACCEPTED MANUSCRIPT dinoflagellates and saline-water conditions during deposition. The E2s3 source rocks are characterized by low to medium C19/C23 TT (<0.4), C24 Tet/C26 TT (<1.5), low G/H (<0.1) and relatively high 4-MS/∑C29 ST (majority of samples>0.4) (Figure 5), and
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were considered to have little to medium contribution from terrigenous organic matter, high from dinoflagellates as well as freshwater conditions during deposition.
In order to investigate the biomarker assemblages and depositional environments
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of the E2s4 source rocks in the Liaodong Bay sub-basin and compare conveniently with
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the other three sets of source rocks, the same parameters were used in this study. In the Liaodong Bay sub-basin, samples from the E2s4 display Pr/Ph from 0.64 to 1.38 (average 1.16, Table 2). With the exception of one sample (LD4, 3755 m), all E2s4 samples have Pr/Ph ranging from 1.0 to 1.5. The E2s4 samples display C35/C34 SH from
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0.13 to 0.60 (average 0.43, Table 2). With the exception of one sample (LD4, 3660 m), the C35/C34 SH of all E2s4 samples are concentrated in the range from 0.35 to 0.60. Such relatively low Pr/Ph and medium C35/C34 SH ratios suggest that anoxic to sub-oxic
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conditions dominated in the bottom water in lakes during E2s4 deposition. C19/C23 TT
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of E2s4 samples shows a compatibility with C24 Tet/C26 TT (Figure 5A). All E2s4 samples display C19/C23 TT lower than 0.40 and C24 Tet/C26 TT lower than 1.5 (Table 2, Figure 5A). The relatively low C19/C23 TT and C24 Tet/C26 TT of the E2s4 samples suggest a low terrigenous organic matter input. E2s4 samples display G/H ranging from 0.10 to 0.36. With the exception of one sample, all E2s4 samples display G/H higher than 0.10 (Table 2, Figure 5B, 5C). ETR of E2s4 samples shows a positive correlation with G/H (Figure 5C). E2s4 samples display ETR ranging from 0.22 to 0.55. With the
ACCEPTED MANUSCRIPT exception of 3 samples, all E2s4 samples display ETR higher than 0.30 (Table 2, Figure 5C, 5D). The relatively high G/H and ETR of E2s4 samples suggest that stratification of water column was well developed during the depression of the E2s4. E2s4 samples show
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4-MS/∑C29 ST from 0.10 to 0.39 (Table 2, Figure 5B), which suggests a medium dinoflagellate input. H/S of E2s4 samples ranged from 1.16 to 8.40 (Table 2, Figure 5D). Half of E2s4 samples have H/S ratio higher than 5.0. The low to medium H/S ratio, low
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and low terrigenous organic matter input.
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C19/C23 TT and C24 Tet/C26 TT indicate relatively high algal-, considerable bacterial-
In summary, the E2s4 source rocks are characterized by Pr/Ph (1.0-1.5), C35/C34 SH (0.35-0.60), C19/C23 TT (<0.4), C24 Tet/C26 TT (<1.5), high G/H (>0.1), ETR (mainly higher than 0.3), low to medium H/S and medium 4-MS/∑C29 ST (0.1-0.4)
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(Table 2, Figure 5, 6A) and were considered to have no or little contribution from terrigenous organic matter, medium from dinoflagellates and considerable from bacteria, and saline-water anoxic to sub-oxic conditions during deposition. Such a biomarker
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assemblage and depositional environment are very similar to that of E2s1+2 source
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rocks in the Liaodong Bay sub-basin (Figure 5). The biomarker assemblage of the E2s4 source rocks in the Liaodong Bay sub-basin (Figure 6A) is also similar to that of E2s4 source rocks in other sub-basins such as Liaohe (Figure 6B) and Jizhong (Figure 6C) sub-basins. The E2s4 source rocks in the Jiyang sub-basin display relatively higher gammacerane and C35 hopanes abundance (Figure 6D), suggesting higher salinity and more hypoxic conditions in the bottom water in lakes during deposition. In general, the E2s4 source rocks were developed in similar environments in different sub-basin in
ACCEPTED MANUSCRIPT the Bohai Bay basin. 4.3. Accumulation Contribution for Discovered Oils Compared with the other three sets of source rocks, the E2s4 source rocks were
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buried deeper and have higher maturity (Figure 2). In the Liaodong Bay sub-basin, oils in reservoirs are considered to charge mainly since the neotectonic motion (since 5.1 Ma, Hao et al., 2009a; Jiang et al., 2011). The maturity of E2s4 source rocks in the
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Liaozhong sag and north Liaoxi sub-sag was too high to provide oils for fields (Figure
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7). The E2s4 source rocks in the south and middle Liaoxi sub-sags displayed relatively low maturity from onset to the peak of oil generation (Figure 7). Therefore, it is very promising to find E2s4-derived oils in the south and middle Liaoxi depression. There are one oil field (LD4-2) in the south Liaoxi sub-sag, one oil field (JZ25-1)
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and one discovery (SZ29-4) in the middle Liaoxi sub-sag (Figure 1B). Oils in these fields and discovery have been found at depth from less than 1000 m to more than 2000 m (Table 3). It is reported in previous studies that oils at depth shallower than about
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2000 m in the Bohai offshore area were biodegraded to varying degrees, as evidenced
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by elevated density, asphaltene, and resin content, and sometimes by the occurrence of 25-norhopanes (Hao et al., 2009a,b; Tian et al., 2011). Oil sample SZ1 from the discovery SZ29-4 has the shallowest reservoirs depth among oils in the south and middle Liaoxi sub-sags. In this oil sample, the n-alkanes suffered significant biodegradation, C30 hopane suffered minor biodegradation accompanied with small amount of C29 25-norhopane, steranes and other hopanes have not suffered significant biodegradation (Figure 8). In this case, major biomarker parameters including C19/C23
ACCEPTED MANUSCRIPT TT, C24 Tet/C26 TT, G/H, 4-MS/∑C29 ST, H/S, ETR, C29 Sterane 20S/(20S + 20R) and C29 Sterane ββ/(αα + ββ) would not be obviously affected by biodegradation. Oils in the south and middle Liaoxi sub-sags display C29 Sterane 20S/(20S + 20R)
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ranging from 0.32 to 0.38 and C29 Sterane ββ/(αα + ββ) ranging from 0.36 to 0.51 (Table 3, Figure 9). Both parameters have not reached the endpoint values (Figure 9). According to the correspondence between the C29 20S/(20S + 20R) sterane ratio and the
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Ro in the Liaodong Bay sub-basin (Liu et al., 2016), the maturity of these oils is from
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the onset of oil generation to medium maturity (Figure 9).
As discussed above, in the Liaodong Bay sub-basin, C19/C23 TT and C24 Tet/C26 TT are usually used to distinguish E3d3-derived oils from oils generated from other three sets of source rocks. G/H and ETR can be used to distinguish E2s4- and
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E2s1+2-derived oils from E2s3- and E3d3-derived oils. 4-MS/ ∑ C29 ST and H/S (combined with other parameters) can be used to distinguish E2s3-derived oils from oils generated from other three sets of source rocks. However, there is no valid biomarker
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parameter which can be used to distinguish E2s4-derived oils from E2s1+2-derived oils.
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Fortunately, with low maturity, the E3d3 and E2s1+2 source rocks in the south and middle Liaoxi sub-sags can’t provide oils to reservoirs (Figure 10). Accordingly, just the E2s4 and E2s3 source rocks should be taken into account for oil-source correlation in the south and middle Liaoxi sub-sags. G/H and 4-MS/∑C29 ST are sufficient to identify and distinguish E2s4- and E2s3- derived oils. Oils in the oil field JZ25-1 (except sample 14A) display medium G/H (0.14-0.28) and 4-MS/∑C29 ST (0.25-0.38) (Table 3, Figure 11). These oils correlate reasonably with the E2s4 and E2s1+2 source rocks
ACCEPTED MANUSCRIPT (Figures 5B, 11). In view of the relatively low maturity (Ro<0.7%) of the E2s1+2 source rocks in the south and middle Liaoxi sub-sags (Figure 10), these oils should be interpreted to originate primarily from E2s4 source rocks in the south and middle Liaoxi
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sub-sags. Oils in the oil field LD4-2, oil samples SZ1 from the discovery SZ29-4 and 14A from the JZ25-1 oil field have higher G/H than E2s3 source rocks and higher
interpreted as a mixing of E2s4- and E2s3-derived oils.
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4-MS/∑C29 ST than E2s4 source rocks (Table 3, Figure 11). These oils should be
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The oil-source correlation above indicates that the E2s4 source rocks have important accumulation contribution to discovered oil fields in south and middle Liaoxi sub-sags and that the E2s4-derived oils should be taken into seriously consideration in the future exploration in the south and middle Liaoxi sub-sags and on adjacent uplifts.
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5. Conclusions
Based on systematic study of the characteristics and biomarker assemblages of
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the potential source rocks and reservoir oils and geological analysis, the following conclusions can be drawn:
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(1) With relatively high total organic carbon contents (TOC) and Rock-Eval
hydrogen indices (HI), the E2s4 in the Liaodong Bay sub-basin represents a set of excellent oil-prone tendency source rocks. (2) The E2s4 source rocks in the Liaodong Bay sub-basin are characterized mainly by relatively low Pr/Ph, C19/C23 TT, C24 Tet/C26 TT, low to medium H/S, medium 4-MS/ΣC29 ST, C35/C34 SH and high G/H, ETR, and were deposited in anoxic to sub-oxic saline-water environments with no or minor terrigenous organic matter and
ACCEPTED MANUSCRIPT medium dinoflagellate input. The E2s4 source rocks in the Liaodong Bay sub-basin displayed similar biomarker assemblage with the E2s1+2 source rocks in the Liaodong Bay sub-basin and the E2s4 source rocks in other sub-basins in the Bohai Bay basin.
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(3) The E2s4 source rocks in the south and middle Liaoxi depression displayed opportune maturity within the oil window and were proved by oil-source correlation having important accumulation contribution to discovered oil fields in south and middle
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Liaoxi sub-sags. The E2s4-derived oils should be taken into seriously consideration in
Acknowledgments
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the future exploration in the south and middle Liaoxi sub-sags and on adjacent uplifts.
We thank the Bohai Oil Company, CNOOC, and individuals who contributed
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samples and information for this work. The National Natural Science Foundation of China (no.41202089) and the Open Foundation of the State Kay Laboratory of
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Petroleum Resource and Prospecting, China University of Petroleum, Beijing
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(no.PRP/open-1104) provided financial support.
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Table 1. Rock-Eval Pyrolysis Data of the E2s4 Source Rocks in the Liaodong Bay Sub-basin, Bohai Bay Basin.
Depth Well
TOC
Tmax
S1
S2
(wt. %)
(℃)
(mg/g rock)
HI
Lithology (m)
(mg/g TOC)
3448
Mudstone
2.83
438
1.92
9.24
327
LD1
3450
Mudstone
2.95
440
4.20
11.60
LD1
3500
Mudstone
2.05
441
4.10
9.40
LD2
2495
Mudstone
1.31
437
0.97
3.10
LD2
2685
Mudstone
0.68
434
0.15
2.21
LD2
2792.5
Mudstone
1.09
439
0.19
2.50
LD3
3250
Mudstone
1.07
436
0.10
1.92
179
LD3
3300
Mudstone
1.63
439
0.19
3.88
238
LD3
3265
Mudstone
2.23
440
0.16
6.54
296
LD3
3315
Mudstone
LD3
3355
Mudstone
LD3
3350
Mudstone
LD3
3297.5
Mudstone
LD3
3200
Mudstone
LD3
3387.5
Mudstone
LD3
3400
Mudstone
LD3
3430
Mudstone
LD3
3417.5
LD4
RI PT
LD1
393 459 239 329
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SC
233
441
0.35
12.74
341
3.03
441
0.26
13.02
432
0.62
441
0.09
2.08
335
2.36
443
0.25
8.30
354
1.17
444
0.16
2.72
232
3.32
444
0.26
16.38
496
2.55
447
0.35
12.56
493
3.11
447
0.31
18.68
601
Mudstone
6.6
448
0.72
48.81
741
3755
Mudstone
0.75
440
0.09
0.53
71
LD4
3660
Mudstone
0.56
444
0.11
0.87
155
LD5
3755
Mudstone
0.64
421
1.47
2.89
453
LD5
3795
Mudstone
0.78
426
1.59
3.44
442
LD5
3815
Mudstone
0.63
428
1.40
2.93
465
LD5
3655
Mudstone
0.98
446
0.91
4.00
409
LD5
3690
Mudstone
1.09
447
0.80
4.56
420
LD5
3700
Mudstone
0.54
413
0.39
1.99
369
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3.76
ACCEPTED MANUSCRIPT Table 2. Biomarker Parameters of the E2s4 Source Rocks in the Liaodong Bay Sub-basin, Bohai Bay Basin.
C35/C34
Depth Well
C19/C23
C24 Tet/C26 G/H
Pr/Ph (m)
SH
TT
TT
ETR
4-MS/ΣC29
C29 S/
C29 ββ/
(S+R)
(αα+ββ)
H/S
ST
3448
1.06
0.56
0.13
0.62
0.32
0.49
0.39
1.73
0.09
0.27
LD2
2495
1.35
0.50
0.39
0.47
0.22
0.52
0.14
3.60
0.19
0.35
LD2
2792.5
1.09
0.37
0.07
0.64
0.23
0.49
0.19
4.42
0.24
0.37
LD4
3755
0.64
0.60
0.09
1.02
0.36
0.55
0.22
1.16
0.11
0.24
LD4
3660
1.33
0.13
0.14
0.99
0.12
0.40
0.10
5.80
0.45
0.41
LD5
3655
1.09
0.47
0.17
1.19
0.13
0.23
0.17
6.46
0.25
0.25
LD5
3755
1.36
0.48
0.30
0.91
0.12
0.22
0.21
6.87
0.24
0.32
LD5
3815
1.38
0.37
0.36
1.06
0.10
0.22
0.22
8.40
0.27
0.35
RI PT
LD1
SC
Note: Pr/Ph = Pristane/Phytane; C35/C34 SH = C35 17α(H)21β(H)22S hopane/C34 17α(H)21β(H)22S hopane; C19/C23 TT = C19 tricyclic terpane/C23 tricyclic terpane; C24 Tet/C26 TT = C24 tetracyclic terpane/C26 tricyclic terpane; G/H = gammacerane/C30 17α(H)21β(H)hopane; ETR = (C28 TT+C29 TT)/(C28 TT+C29 TT+Ts); 4-MS/ΣC29 ST = 4-methyl steranes/ΣC29 steranes; H/S = hopane/steranes [C30 17α(H)21β(H)hopane/C29 steranes αα(20S+20R)]; C29 S/(S+R) = C29 αα20S sterane/C29 αα(20S+20R)
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steranes; C29 ββ/(αα+ββ) = 5α-C29 ββ sterane/5α-C29 (αα+ββ) steranes.
Table 3. Biomarker parameters of oils from south and middle Liaoxi sub-sags, Liaodong Bay sub-basin, Bohai Bay basin.
Depth ID
C19/C23
Well
C24 Tet/C26
4-MS/ΣC29
G/H
(m)
TT
TT
0.50
C29 S/
C29 ββ/
(S+R)
(αα+ββ)
H/S
ST
2
JZ25-1-2
1898
0.14
0.34
0.33
8.21
0.33
0.36
3A
JZ25-1-3
1954.5
0.45
0.40
0.28
0.49
0.25
2.69
0.36
0.41
3B
JZ25-1-3
2154
0.26
0.41
0.16
0.50
0.30
5.39
0.36
0.41
4A
JZ25-1-4
2128
0.39
0.54
0.19
0.43
0.36
7.21
0.38
0.44
4B
JZ25-1-4
2158
0.30
0.56
0.20
0.40
0.35
6.44
0.35
0.42
5A
JZ25-1-5
2104.9
0.19
0.47
0.23
0.43
0.32
6.11
0.32
0.40
5B
JZ25-1-5
2362
0.34
0.36
0.22
0.47
0.33
5.48
0.34
0.41
6
JZ25-1-6
1922.5
0.26
0.45
0.15
0.51
0.30
4.89
0.35
0.41
7A
JZ25-1-7
2022.5
0.25
0.33
0.19
0.47
0.26
11.75
0.36
0.51
0.32
0.46
0.25
0.42
0.33
5.94
0.33
0.42
AC C
EP
TE D
0.24
ETR
7B
JZ25-1-7
2043.2
10
JZ25-1-10
1893
0.25
0.41
0.16
0.49
0.33
5.07
0.36
0.41
11A
JZ25-1-11
1926.8
0.24
0.44
0.20
0.48
0.33
5.57
0.34
0.43
11B
JZ25-1-11
1996
0.23
0.36
0.25
0.45
0.35
6.50
0.35
0.43
14A
JZ25-1-14
1831
0.22
0.45
0.18
0.47
0.44
10.12
0.37
0.44
14B
JZ25-1-14
1911.5
0.20
0.38
0.25
0.50
0.29
9.17
0.38
0.48
15
JZ25-1-15
1958
0.24
0.42
0.24
0.45
0.34
9.80
0.35
0.43
1A
LD4-2-1
1657.5
0.18
0.59
0.13
0.38
0.46
10.80
0.38
0.41
1B
LD4-2-1
1680
0.23
0.67
0.21
0.37
0.48
6.14
0.34
0.40
SZ1
SZ29-4-1
879.8
0.24
0.57
0.20
0.40
0.47
13.81
0.36
0.42
Note: ID = sample identifications in Figure 11.
ACCEPTED MANUSCRIPT 115°
116°
117°
118°
119° 0
41°
120° 50
121°
122°
100 km
Beijing
Bohai Bay Basin
40°
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39°
Taiwan
N Bozhong Sub-basin
Bohai Sea
Jiyang Sub-basin
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37°
SC
38°
Bohai Bay Basin 36°
(A)
10
0
Coastline Major Uplift JZ9-3
JZ20-2N
TE D
LD1
AC C
EP
N
20km
Basin Boundary
JZ20-2 JZ20-1
JZ21-1/S
LD2 JZ25-1S
SZ29-4
JX1-1 C
SZ36-1 LD4-2
LD6-2
LD10-1
Basin Boundary Extensional Fault LD3 LD4
Uplift Oil Field Discovery
(B)
Well LD5
Figure 1. (A) Sub-basins of the Bohai Bay basin (modified from Hao et al., 2009a). (B) Distribution of sags and uplifts and main oil fields in the Liaodong Bay sub-basin.
ACCEPTED MANUSCRIPT
Oil Field SZ36-1 Nm
1
3 4
Ng
Ed
Es1+2
Ed Es1+2
Es3
Es3
Es4 Es3
5
Es4 7
Liaoxi Uplift
Liaoxi Sag
Liaozhong Sag
A′
Es4
M AN U
6
SC
Depth (km)
2
Oil Field JX1-1 Qp
RI PT
A 0
Liaodong Sag
AC C
EP
TE D
Figure 2. Cross-sections through the main oil fields showing the petroleum geology framework from sags to uplifts. Section locations are shown in Figure 1B.
ACCEPTED MANUSCRIPT
S2 (mg HC/g rock)
Good to excellent source rock 10
Fair source rock
0.1 0.1
SC
Poor source rock
M AN U
1
RI PT
100
1
10
TOC (%)
AC C
EP
TE D
Figure 3. Total organic carbon (TOC, wt.%) contents vs. Rock-Eval S2 peaks (mg HC/g rock) for the E2s4 samples from the Liaodong Bay sub-basin, Bohai Bay basin. Quality of the source rocks is evaluated according to Wang and Chen (1988).
RI PT
ACCEPTED MANUSCRIPT
I
800
Ro=0.5% II
SC
600
400
200
III 0 400
Ro=1.0%
M AN U
Hydrogen Index (mg HC/g TOC)
1000
420
440
460
480
500
Tmax (℃ ℃)
AC C
EP
TE D
Figure 4. Rock-Eval hydrogen index vs. Tmax for the E2s4 samples from the Liaodong Bay sub-basin, Bohai Bay basin. Tmax is the temperature at which the maximum amount of S2 hydrocarbons is generated. Ro = vitrinite reflectance.
ACCEPTED MANUSCRIPT
0.8
0.8
0.6
RI PT
(B)
(A) 0.6
E3d3 E2s1+2 E2s3 E2s4
G/H
0.4 E2s4+E2s3+E2s1+2
0
1
2
C24 Tet/C26 TT
0.2
E3d3 0.0 0.0
SC
E3d3 E2s1+2 E2s3 E2s4
0.2
0.0
0.4
E3d3
M AN U
C19/C23 TT
E2s4+E2s1+2
3
0.2
0.4
E2s3
0.6
0.8
1.0
0.8
1.0
(D)
(C)
E3d3 E2s1+2 E2s3 E2s4
E3d3 E2s1+2 E2s3 E2s4
EP
0.1
0.1
G/H
1.0
ETR
TE D
ETR
0.6
0.05 0.01
1.2
4-MS/Σ Σ C29 ST
0.4
0.2
0.0
0
4
8
12
16
20
24
28
32
H/S
AC C
Figure 5. Plots of various biomarker parameters, showing the differences in biomarker compositions among the four source rock intervals in the Liaodong Bay sub-basin. Abbreviations for biomarker parameters are explained in legend of Table 2.
ACCEPTED MANUSCRIPT
(A) Well LD2，2495 m，E2s4 m/z 191
C23
C21
m/z 191
αβ C30 Hopane
m/z 217
C27
C29
C20
C28 C29
C26 C22
C31-C35 Homohopanes
C24 Tet
C25
G Tm Ts
(B) Liaohe sub-basin 2743.4 m，E2s4 C23
αβ C30 Hopane
SC
C24 C28 C29
C26
C20
C25
TsTm
(C) Jizhong sub-basin 3762.5 m，E2s4 C23
αβ C30 Hopane
C24 C20 C26 C24 Tet
C22
C28 C29
TE D
C25
C19
C29 C28
4-MS
G
C22
C21
4-MS
C31-C35 Homohopanes
M AN U
C19
C28
C27
C21
C24 Tet
RI PT
C24 C19
C27 C29
C28
C31-C35 Homohopanes 4-MS
G
TsTm
(D) Jiyang sub-basin 1212.5 m, E2s4 C23 C21 C24 C20
EP
αβ C30 Hopane
C27
C29
G
C29
C19
AC C
C25 C26 C24 Tet
C28
C31-C35 Homohopanes
C28 4-MS
C22
Tm Ts
Figure 6. Representative mass chromatograms of terpane (m/z=191) and sterane (m/z=217) series of saturated fractions for the E2s4 source rocks in the Liaodong Bay (A), Liaohe (B), Jizhong (C) and Jiyang (D) sub-basins, Bohai Bay Basin. Peaks marked by solid dots are tricyclic terpanes (C19-C26). C24Tet = C24 tetracyclic terpane; G = gammacerane; C27, C28 and C29 represent C27 sterane 14α(H)17α(H)20R, C28 sterane 14α(H)17α(H)20R and C29 sterane 14α(H)17α(H)20R, respectively; 4MS = 4-methylsteranes.
RI PT
ACCEPTED MANUSCRIPT
JZ9-3
10
0
20km
JZ20-2N
N
LD1
JZ20-2
JZ21-1/S
SC
JZ20-1
LD2
JZ25-1S
M AN U
SZ29-4
JX1-1
C
SZ36-1
LD4-2
LD6-2
TE D
LD10-1
Ro: 0.5%~0.7% Ro: 0.7%~1.0% Ro: 1.0%~1.3% Ro: 1.3%~2.0%
LD3
EP
LD4
LD5
Ro: >2.0%
Basin Boundary
Oil Field
Extensional Fault
Discovery
Uplift
Well
AC C
Figure 7. Present Ro (%) distribution at the top surface of the E2s4 in the Liaodong Bay sub-basin, Bohai Bay basin.
ACCEPTED MANUSCRIPT
αβ C30 Hopane
nC18
C31-C35 Homohopanes
nC17 Pr Ph
Gam
αβ C30 Hopane
SC
m/z 191
RI PT
TIC
M AN U
C31-C35 Homohopanes
G
Ts Tm
D29
m/z 177
TE D
D29
C27
m/z 217
C29 4-MS
EP
C28
AC C
Figure 8. Mass chromatograms of total ion current (TIC), hopane (m/z 191), 25norhopanes (m/z 177) and sterane (m/z 217) series for oil sample SZ29-4-1. nC17 = nheptadecane; nC18 = n-octadecane; Pr = pristane; Ph = phytane; D29 = C29 25norhopane; G = gammacerane; C27, C28 and C29 represent C27 sterane 14α(H)17α(H)20R, C28 sterane 14α(H)17α(H)20R and C29 sterane 14α(H)17α(H)20R, respectively; 4MS = 4-methylsteranes.
ACCEPTED MANUSCRIPT
RI PT
0.6
Peak Oil Generation
0.5
0.4
SC
Ro=0.8%
0.3
M AN U
Early Oil Generation
C29 Sterane 20S/(20S + 20R)
Equilibrium
Ro=0.6%
LD4-2 SZ29-4 JZ25-1
0.2 0.2
0.3
0.4
0.5
0.6
C29 Sterane ββ/(αα ββ αα + ββ) ββ
TE D
Figure 9. The C29 20S/(20S + 20R) vs. C29 ββ/(αα + ββ) sterane ratios of oils from fields and
AC C
EP
discovery in the south and middle Liaoxi sag. Labels are identified in the ID column in Table 3. Vitrinite reflectance (Ro,%) estimated after correlations by Liu et al. (2016).
ACCEPTED MANUSCRIPT
10
0
20km
RI PT
JZ9-3
JZ20-2N
N
LD1
JZ20-2 JZ20-1
SC
JZ21-1/S
LD2
JZ25-1S
M AN U
SZ29-4
JX1-1
C
SZ36-1
LD4-2
LD6-2
LD10-1
Ro: 0.5%~0.7% Ro: 0.7%~1.0% Ro: 1.0%~1.3%
TE D
LD3
Ro: 1.3%~2.0%
LD4
EP
LD5
Basin Boundary
Oil Field
Extensional Fault
Discovery
Uplift
Well
AC C
Figure 10. Present Ro (%) distribution at the bottom surface of the E2s1+2 in the Liaodong Bay subbasin, Bohai Bay basin.
ACCEPTED MANUSCRIPT
LD4-2 SZ29-4 JZ25-1 0.4
G/H
E2s4
SC
E2s4+E2s3 0.2
SZ1 14A
E3d3
E2s3 0.2
M AN U
0.0 0.0
RI PT
0.6
0.4
0.6
0.8
1.0
1.2
4-MS/Σ Σ C29 ST
Figure 11. Plot G/H vs. 4-MS/ΣC29 ST for oils from the fields and discovery in the south and middle Liaoxi sag.
AC C
EP
TE D
Labels are identified in the ID column in Table 3.
ACCEPTED MANUSCRIPT 1. The E2s4 in the Liaodong Bay sub-basin is a set of oil-prone tendency source rocks. 2. Biomarker assemblage of the E2s4 in the Liaodong Bay sub-basin is comparable with that in other sub-basins.
AC C
EP
TE D
M AN U
SC
RI PT
3. South and middle Liaoxi sag and adjacent uplifts are favorable exploration areas for E2s4-derived oils.