Article pubs.acs.org/EF
Geochemical Characteristics and Developmental Models of the Eocene Source Rocks in the Qiongdongnan Basin, Northern South China Sea Wenhao Li,*,† Zhihuan Zhang,‡ Ke Zheng,§ and Youchuan Li∥ †
Research Institute of Unconventional Oil & Gas and Renewable Energy, China University of Petroleum (East China), Qingdao 266580, China ‡ State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China § Research Institute of Geological Exploration and Development, Chuanqing Drilling Engineering, Chengdu, 610051, China ∥ CNOOC Research Institute, Beijing 100027, China ABSTRACT: Seismic data show that a set of Eocene lacustrine source rocks exists in the Qiongdongnan (QDN) Basin, northern South China Sea (SCS). The high content of C30 4-methylsteranes detected in the crude oil suggests the existence and the effectiveness of the lacustrine source rocks, and these compounds were not found in the Oligocene and Miocene source rocks. The above date indicates that the Eocene source rocks are effective and oil-prone. Lacustrine source rocks in faulted period have high organic matter abundance and better organic matter types in the SCS and other typical passive continental margin basins in the world. Lacustrine source rocks in the Eocene source rocks (E2w) from the Baiyun Sag of the Pearl River Mouth Basin have relatively high total organic content (TOC) values, which can be explained by moderate to high paleoproductivity and anoxic environment. Therefore, it can be speculated that the Eocene source rocks have good hydrocarbon potential in the QDN Basin. The developmental pattern of Eocene source rocks is revealed as follows: In mid-deep lake environment, high-quality source rocks could be formed, because of high productivity and better organic matter preservation conditions, and the central depression belt is the most favored zone beneficial to Eocene source rock formation, because of the relatively large-scale middeep lake facies. In shore-shallow lake and delta environment, high-quality source rocks could not be formed, because of the poor organic matter preservation conditions.
1. INTRODUCTION The oil and gas discovered in the northern South China Sea (SCS) are mainly from transitional facies source rocks, and partly from the marine source rocks. Natural gas in Yacheng 131 field of the Qiongdongnan (QDN) Basin is mainly originated from source rocks in the Yacheng Formation developed in transitional facies.1−4 The petroleum from the Baiyun Sag in the Pearl River Mouth (PRM) Basin of the northern SCS is mainly generated by source rocks in the Enping Formation developed in transitional facies, and partly by the marine source rocks in the Zhuhai Formation.5−8 Furthermore, the lacustrine source rocks developed in Eocene in the northern SCS are considered to be a set of oil-prone source rocks.2,9−11 The Eocene lacustrine source rocks from the Wenchang Formation in the PRM Basin have been proven to contribute greatly to crude oils.12−22 Although currently there are no wells drilling the Eocene in the QDN Basin, the seismic data reveal that there exist thick Eocene lacustrine source rocks. Because of the lack of exploratory well data, it is not clear whether the Eocene mudstones can be used as a set of effective source rocks, which directly restricts the petroleum exploration in the deep water area of the northern SCS. In this paper, the existence of the Eocene lacustrine source rocks in the QDN Basin has been revealed by the seismic data, and the effectiveness has been analyzed by the inversion of biomarker characteristics of the crude oil in the basin. Hydrocarbon potential of the Eocene © 2017 American Chemical Society
lacustrine source rocks in the QDN Basin has been predicted by the analysis of the lacustrine source rocks in rifting period in the SCS and other typical passive continental margin basins in the world. The Eocene lacustrine source rock formation in the Baiyun Sag of the PRM Basin has been analyzed to indicate the hydrocarbon potential of the source rocks in the E2w, which can be of valuable reference for Eocene source rocks in the QDN Basin. Combining the theory of sedimentology and geochemistry, the developmental model of the Eocene lacustrine source rocks has been established by using the seismic section to restore the Eocene stratigraphic profile, and the beneficial zone of excellent source rock distribution has been presented. Our objective is to provide geological and geochemical evidence for petroleum exploration in the QDN Basin of the northern SCS.
2. GEOLOGICAL SETTING The QDN Basin, extending northeast, is located between Hainan Island and Xisha Islands. It is limited by four faults (Nos. 1, 5, 11, 12) (see Figure 1). The western part of the basin is adjacent to the Yinggehai Basin, the northern part borders the Hainan Uplift, the southeastern part is adjacent to the Xisha−Zhongsha uplifts, and the eastern part is adjacent to the Received: September 20, 2017 Revised: November 10, 2017 Published: November 11, 2017 13487
DOI: 10.1021/acs.energyfuels.7b02841 Energy Fuels 2017, 31, 13487−13493
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Figure 1. Structural location of the QDN Basin.
lithology and lithofacies. Analyzed from the seismic data, there may exist large-scale Eocene lacustrine sediments in the central depression belt of the QDN Basin, for a set of reflectors with characteristics of parallel, continuity, and strong amplitude is shown in the seismic section (Figure 3). The characteristics of low frequency, high continuance, and strong amplitude reflect a dense reflection, which is considered to be the seismic reflection of sediments in mudstones containing rich organic matter. Paralleling or subparalleling reflection structure mainly reveals the lacustrine layer relection characteristics in deep water environment. The plan of sedimentary facies derived from seismic facies indicates that lacustrine facies sediments, especially semideep sediments, are widely distributed (Figure 4). Therefore, it can be speculated that lacustrine source rocks exist in the QDN Basin. 3.1.2. Effectiveness Analysis of the Eocene Source Rocks. 4-Methylsteranes can be formed in both marine and lacustrine deposition environments and widely used as biogenic markers or in the oil-source correlation.26−29 The biogenesis of 4methylsterane is dinophyceae (dinoflagellate) or bacterium.30−33 A high abundance of C30 4-methylsteranes and a low content of gammacerane are the typical characteristics of the Eocene lacustrine source rocks in the northern SCS; examples include Eocene source rocks in the Beibuwan Basin and the Huizhou Depression in the PRM Basin.9,13,22 Abundant C30 4-methylsteranes compounds and a low content of gammacerane have been detected in the extraction mass chromatogram of crude oil in the QDN Basin (see Figure 5). However, the high content of C30 4-methylsteranes have not been found in the Oligocene and Miocene source rocks in this basin (see Figure 6). Thus, we believe that the crude oil mentioned above is generated by the Eocene lacustrine source rocks in the QDN Basin. 3.1.3. Characteristics of Lacustrine Source Rocks in Rifting Period in the SCS and Other Typical Passive Continental Margin Basins Worldwide. Through the analysis of geochemical characteristics of source rocks in passive margin basin
Zhu II Depression in the PRM Basin. The basin is composed of four structural zones: the northern depression belt (including the Yabei, Songxi, and Songdong Sags), the middle uplift zone (including the Yacheng−Songtao uplift), the central depression belt (including the Ledong, Lingshui, Songnan, and Baodao Sags), and the southern uplift belt (see Figure 1). The basin is characterized by a typical double-layer structure, sharing many similarities with passive margins.23,24 The lower structural layer formed in the rifting period consists of Eocene, Oligocene, Yacheng, and Lingshui formations. From the sedimentary period of Eocene to the Lingshui Formation, the sedimentary environment was changed from lacustrine to marine deposits. In addition, multiple uplifts and sags were formed in the lower structural layer. The upper structural layer, formed in the depression period, is characterized by Neogene and Quaternary marine strata. There are weak deformation and small fault activities in Miocene. The basement of the QDN Basin is composed of igneous rocks, metamorphic rocks, and sedimentary rocks in Pre-Tertiary. The filling sequence of the basin is mainly composed of Tertiary and Quaternary, including Eocene, Yacheng, and Lingshui Formations in the Oligocene, Sanya, Meishan, and Huangliu Formations in the Miocene, Pliocene Yinggehai Formation and Quaternary from the bottom to the top (Figure 2). Three sets of source rocks are developed in different sedimentary environments: Eocene lacustrine mudstones; Oligocene, Yacheng, and Lingshui source rocks; and Miocene marine source rocks.7
3. RESULTS AND DISCUSSION 3.1. Effectiveness and Hydrocarbon Potential of the Eocene Source Rocks in the QDN Basin. 3.1.1. Seismic Data Analysis. Seismic facies are seismic characteristics formed by a sedimentary environment,25 which can be interpreted as the sum of sedimentary facies in the seismic section. It includes the configuration of seismic reflection layer, frequency, amplitude, and continuity. In addition, amplitude, frequency, and continuity play important roles in the recognition of 13488
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Figure 2. Comprehensive stratigraphic column of the QDN Basin.7
and the former is the ideal period for source rock formation. Lacustrine shales and mudstones in the rifting period are the main source rocks, except in the Mexico Basin and the North Sea Basin. As source rocks in the rifting period in intraplate or the plate boundary, the organic matter abundance is rather high, and the type of organic matter is mainly type I or type II. The TOC values of lacustrine source rocks in Lower Cretaceous of Campos Basin have reached 2.0%−6.0%, and the organic matter types are mainly type I or type II, which are considered to be oil-prone (see Table 1). The TOC values of Eocene lacustrine source rocks in the southern SCS range from 0.8% to 8.4%, and the organic matter type is type I or type II (see Table 1). It can be considered to be a set of oil-prone source rocks. The Eocene lacustrine source rocks (E2w) in the Baiyun Sag of the PRM Basin have relatively higher TOC values, ranging from 1.11% to 1.33%, with an average of 1.22% (see Table 2), which is larger than that in the Oligocene (E3z) and Miocene (N1z) source rocks (see Table 2). Paleoproductivity parameters Al/Ti and P/Ti in the E2w source rocks are in the ranges of 21.69−26.29 and 0.20−0.39 (Table 2), respectively, with the mean value of 24.52 and 0.30, respectively, which is larger than that in Post Archean Average Shale (PAAS), which has an
Figure 3. Eocene seismic reflection characteristics revealed by seismic section across Songxi Sag in the QDN Basin.
of the southern SCS, the Atlantic rift Basin, the North Sea Basin, the Mexico Basin, and the North West Shelf of Australia, it is revealed that the source rocks were developed in rifting period and depression stage (post-rifting period) (see Table 1), 13489
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Figure 4. Plan of sedimentary facies in the QDN Basin.
Figure 5. Distribution characteristics of some biomarkers in the crude oil in the QDN Basin. [Legend: C30H = C30 hopane, OL= oleanane, Ga = gammacerane, and C30 4-MST = C30 4-methylsterane.]
average Al/Ti ratio of 16.7 and P/Ti ratio of 0.12.34 It indicates that the lake productivity during the Eocene was moderate to high in the Baiyun Sag of the PRM Basin. The Al/Ti and/Ti ratios in the lacustrine source rocks from the E2w are obviously greater than that in the Oligocene and Miocene source rocks (see Table 2). V/Cr and V/Ni ratios are commonly used to indicate redox conditions.35,36 V/Cr and V/Ni ratios in the E2w source rocks range from 2.41 to 3.94 and from 5.59 to 9.08 (Table 2), respectively, with average values of 3.34 and 7.79,
respectively, which are much larger than those observed in the E3z and N1z source rocks. It indicates that the redox conditions were mainly anoxic in the Eocene. The moderate to high paleoproductivity and relatively anoxic environment give rise to Eocene oil-prone source rocks. From the above analysis, it is indicated that the Eocene lacustrine mudstones in the QDN Basin must have relatively high hydrocarbon potential, which can be considered as a set of oil-prone source rocks. 13490
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Figure 6. Mass chromatograms showing the distribution of n-alkane series (TIC), terpenoids (m/z 191), and steroids (m/z 217) in saturated hydrocarbons of core extracts in Oligocene and Miocene source rocks in the QDN Basin. [Legend: C30H = C30 hopane, OL = oleanane, and Ga = gammacerane.]
Table 1. Developmental Characteristics of Source Rocks in the Main Continental Margin Basins in the World formation
lithology
total organic content, TOC
type of organic matter
Ro
tectonic evolution stage
sedimentary environment
Upper Miocene
mudstone
Southern South China Sea 0.5%−2.0% II−III
Lower Miocene
coals, mudstones, shales
0.4%−3.8%
II−III
mature stage
Oligocene to Lower Miocene Paleocene to Lower Oligocene
coal, mudstones, shales
0.4%−3.8%
II−III
mature stage
late depression stage early depression stage late rifting stage
mudstones
0.8%−8.4%
I, II
mature stage
early rifting stage
Upper Jurassic
shales
3.0%−10.0%
mature stage
late rifting stage
Middle Jurassic
coals, shales
80.0%
Lower Cretaceous
mudstones, shales
Eocene to Miocene
shales
Late Jurassic - Early Cretaceous
shales
II−III Campos Basin 2.0%−6.0% I, II Niger Delta Basin 2.6% II−III Congo Basin 2.0%−3.0% I, III
Plaeocene to Eocene
shales
1.5%−2.7%
Upper Cretaceous
>1.0%
II
mature stage
Upper Jurassic
carbonatite, siliceous clastic rocks lime mudstones
1.0%−2.0%
II
over mature stage
Paleozoic and Mesozoic
coals, mudstones
North Sea Basin II
Mexico Basin III
mature stage
early rifting stage
transitional and marine facies transitional facies
mature stage
early rifting stage
deep lacustrine facies
mature stage
drift stage
delta and transitional facies
mature stage
rifting stage
deep lacustrine facies
mature stage
late depression stage late depression stage late depression stage
delta and marine facies
rifting stage
fluvial−delta facies
North West Shelf of Australia III over mature stage
13491
transitional and marine facies transitional, delta and marine facies transitional, delta and marine facies lacustrine facies
marine facies marine facies
DOI: 10.1021/acs.energyfuels.7b02841 Energy Fuels 2017, 31, 13487−13493
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Table 2. Distribution Characteristics of TOC, Al/Ti, P/Ti, V/Cr, and V/Ni Ratios in the Eocene, Oligocene, and Miocene Source Rocks Revealed by Well LW4-1-1 in the Baiyun Sag of the PRM Basin, Northern SCS well
depth (m)
formation
TOC (%)
Al/Ti
P/Ti
V/Cr
V/Ni
LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1 LW4-1-1
2632.5 2682.5 2712.5 2782.5 2847.5 2907.5 2975.0 3095.0 3105.0 3117.5 3192.5 3207.5 3220.0 3232.5 3247.5
N1 z N1 z N1 z N1 z N1 z N1 z E3 z E3 z E3 z E3 z E2 w E2 w E2 w E2 w E2 w
0.56 0.56 0.63 0.68 0.71 0.63 1.11 1.05 0.97 1.01 1.17 1.11 1.25 1.33 1.26
14.00 15.38 15.39 17.45 16.48 17.20 16.43 15.17 16.25 17.25 26.29 24.54 25.46 24.64 21.69
0.11 0.10 0.16 0.13 0.11 0.12 0.16 0.10 0.10 0.10 0.29 0.34 0.39 0.28 0.20
1.52 1.28 1.50 1.65 1.77 1.86 1.17 1.84 1.74 1.68 3.38 2.41 3.55 3.40 3.94
2.17 1.88 2.14 1.96 2.52 1.62 1.34 3.76 3.78 3.18 8.08 5.59 9.08 8.16 8.05
3.2. Developmental Model of the Eocene Source Rocks in the QDN Basin. Figure 7 shows a section through
Eocene is restored through the above section, which is shown in Figure 8. Combined with the seismic facies in Eocene in the QDN Basin, the distribution characteristics of Eocene sedimentary facies in different structure belts can be revealed. Delta facies is mainly distributed in the Yacheng salient and slope, while shore-shallow lacustrine and semideep lake facies are mainly developed in the northern and central depression belts (Delta facies are distributed in the Yabei and Yanan Sags). Eocene source rocks are mainly controlled by ancient lake productivity and organic matter preservation conditions, and these factors are obviously variant in different sedimentary environments. Therefore, the developmental model of Eocene source rocks in the QDN Basin can be concluded as follows (Figure 8): source rocks in shore-shallow lacustrine and semideep lake facies were mainly developed in the Yabei, Yanan, and Ledong Sags. There were abundant planktonic algaes in semideep lake environment, where the organic matter preservation conditions were perfect (reductive water body), which was beneficial to source rock formation. The central depression belt where semideep lake facies was widely distributed was the most favorable area for the formation of the source rocks. However, the water body was oxic with strong hydrodynamic conditions in a shore-shallow lake environment, where excellent source rocks could not be formed. Delta facies was developed in the Yacheng salient and slope, and there was
Figure 7. Stratigraphic section derived from seismic section across the Yabei, Yanan, and Ledong Sags in the west of the QDN Basin.
the Yabei, Yanan, and Ledong Sags in the West QDN Basin. The section reveals a large thickness of Eocene in the Ledong Sag of the central depression belt. The stratigraphic profile in
Figure 8. Developmental model of the Eocene lacustrine source rocks in the QDN Basin. 13492
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abundant terrigenous organic matter in this environment, but the hydrodynamic conditions were strong and organic matter could not accumulate in the bottom water. Thus, high-quality source rocks could not be formed in this environment.
4. CONCLUSIONS (1) The seismic data in the QDN Basin reveals that a set of lacustrine source rocks may exist in the Eocene. The Eocene source rocks are effective and oil-prone, as proven by the high content of C30 4-methylsteranes in the crude oil, and these compounds have not been found in the Oligocene and Miocene source rocks. (2) The Eocene lacustrine source rocks could have rather good hydrocarbon potential in the QDN Basin; in view of that, lacustrine source rocks in the rifting period have been proven to have good hydrocarbon potential in the typical passive continental margin basins in the world. (3) There were high lake productivity and excellent organic matte preservation conditions in the semideep lake environment, where excellent source rocks could be formed. The central depression belt of the basin was the most favorable area for the Eocene source rock formation.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 15253220962. E-mail:
[email protected]. ORCID
Wenhao Li: 0000-0001-5916-4163 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was financially supported by National Natural Science Foundation of China (Nos. 41402122 and 41172134). The authors would like to thank CNOOC for providing the geochemical and seismic data.
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