Article Cite This: Energy Fuels 2017, 31, 10598-10611
pubs.acs.org/EF
Paleoenvironment and Its Control of the Formation of Oligocene Marine Source Rocks in the Deep-Water Area of the Northern South China Sea Wenhao Li*,† and Zhihuan Zhang‡ †
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 ABSTRACT: This article discusses the paleoenvironment and its control on Oligocene source rock formation in the deep water area of the northern South China Sea (SCS), including the deep water area of the Qiongdongnan (QDN) Basin and Baiyun Sag of the Pearl River Mouth (PRM) Basin. During E3y and E3l deposition, the terrigenous detrital matter (TDM) inputs were moderate to high and moderate, respectively, as indicated by TiO2 values and SiO2/Al2O3 ratios, which accordingly diluted primary production, causing low to moderate and moderate productivity, respectively. The bottom water was oxic because of abundant TDM input, which was unbeneficial for organic matter accumulation. In this environment, algal organic matter could not be preserved. Terrigenous organic matter (TOM) is the main controlling factor of Oligocene source rock formation in the deep-water area of the QDN basin. Source rocks with high organic matter content in the Yacheng Formation could not be formed, although there was moderate to high TOM input in this period, as suggested by relatively high oleanane/αβ C30hopane (OL/C30H) ratios and low ααα20RC27/ααα20RC29 sterane (C27/C29) ratios. During E3l deposition, the influx of TOM was reduced, making conditions unsuitable for high organic carbon source rock formation. In contrast to the QDN basin, there was an influx of fresh water (the ancient Pearl River) into the PRM basin, which significantly influenced the source rock formation in the Baiyun Sag. Thus, TOM and TDM inputs were abundant during E3z deposition. Increased content of TDM not only reduced primary production, causing moderate and low productivity during E3e and E3z deposition, respectively, but also contributed to oxic conditions. Although most algal organic matter could not be preserved in the oxygenated water column, part of the algal organic matter could have accumulated due to the high sedimentary rate suggested by high content of TDM, which supported Oligocene source rock formation in the Baiyun Sag. TOM mainly controlled the development of source rocks in the Enping Formation, and source rocks with relatively high organic matter abundance could be formed. However, multiple factors (besides the main factors TOM and TDM, paleoproductivity and redox conditions were also included) influenced the source rocks in the Zhuhai Formation. Organic carbon-rich source rocks could have developed in this period.
1. INTRODUCTION Preservation condition and paleoproductivity (the input of algal organic matter) are considered to be the main factors controlling marine source rock formation, and the debate on whether the high organic matter abundance of marine source rocks is dependent on preservation condition1,2 or production3 has persisted since the 1980’s. Some scholars believe that terrigenous detrital matter (TDM) input also plays an important role in organic matter accumulation.4−6 It is likely, however, that multiple paleoenvironmental parameters rather than a single factor can suggest OM accumulation in marine sediments.7−9 On the basis of the trace element data of some typical wells in the shallow water area of the Qiongdongnan (QDN) basin, Li et al.10 discussed the paleoproductivity during the sedimentary period of Oligocene source rock formation and believed that algal organic matter made little contribution to the Oligocene source rocks. However, the paleoproductivity in the deep water area of the Qiongdongnan Basin is still unknown. Li et al.11 reported that the marine productivity of the Baiyun Sag of the Pearl River Mouth (PRM) basin is limited, suggested by Al/Ti and P/Ti ratios of the marine source rocks. © 2017 American Chemical Society
However, in continental margin basins, terrigenous organic matter (TOM) input is also a significant factor on marine source rock formation.10,11 Influenced by river-delta systems, TOM may contribute to marine source rocks in continental margin basins, as is the case in the Congo Basin12 and Niger Delta Basin.13,14 These basins are located below the estuaries of large rivers, which may bring abundant TOM along with nutrients into the basin, giving rise to ocean surface plankton booms.15 However, a large number of detrital materials transported by rivers can also dilute marine productivity.9 There are numerous rivers around the South China Sea (SCS), including Mekong River, Pearl River, Red river, and rivers from southwestern Taiwan.16 TOM is an important contributor to the marine source rock in the northern SCS. Most of the crude oil and natural gas found in the deep-water area of the northern SCS, containing the QDN basin and the Baiyun Sag of the PRM basin, were generated from Oligocene marine source rocks, especially marine source rocks with TOM inputs in the Yacheng and Enping formations, while marine Received: June 18, 2017 Revised: September 5, 2017 Published: September 6, 2017 10598
DOI: 10.1021/acs.energyfuels.7b01681 Energy Fuels 2017, 31, 10598−10611
Article
Energy & Fuels
Figure 1. Maps showing (a) the locations of the Qiongdongnan (QDN) and the Pearl River Mouth (PRM) Basin and (b) their deep-water areas.17
River, might have been transported to the northern SCS by comparing the geochemical characteristics of suspended sediments from these rivers and sedimentary rock from Taiwan. The reasons for this shift are that the Paleo-Yangtze River may have even flowed into the SCS by river capture of the paleo-Red River before the Miocene,22 and the Red River is relatively far from the SCS and mostly shielded by the Hainan Island.23,24 However, Gong et al.25 believed that TMD can transport into the QDN basin through the central submarine canyon in the basin. Although different scholars hold various views on the provenance of the SCS, the TOM and TDM played an important role in the source rock formation in the SCS. Although some scholars have discussed the provenance to the SCS, little attention has been focused on the effect of paleoproductivity, TOM and TDM on source rock formation, especially in the deep-water area of the SCS. In this study, palaeoproductivity, TOM and TDM, redox conditions in the
source rocks in the Miocene did not contribute to petroleum because of their immaturity.17−19 Marine source rocks in the northern SCS have some nonhomogeneity owing to their different influences by rivers. In addition to TOM, TDM could also be brought into the basin by rivers, which may reduce primary production. The inputs of TDM exist some heterogeneities due to the change of provenance in different sedimentary periods. Li et al.20 discussed the Nd isotopes of sediments from major rivers flowing into the SCS and proposed that pre-27 Ma sediments were dominantly derived from a southwestern to southern provenance of Indochina-Sunda Shelf and Borneo, while post23 Ma sediments were from a northern provenance of South China. They believed that the change in provenance from southwest to north was caused by a ridge jump from the north to the southwest, associated with a southwestward expansion of the ocean basin and a global rise in sea level. Liu et al.21 considered that the TDM from the Yangtze River, Chinese Loess, and Taiwan Island, rather than the Red River and Pearl 10599
DOI: 10.1021/acs.energyfuels.7b01681 Energy Fuels 2017, 31, 10598−10611
Article
Energy & Fuels
Figure 2. Stratigraphic columns of the QDN (left) and PRM (right) basins.17
facies in the Lingshui and Zhuhai Formations, respectively. During the depression period, the sedimentary environment in the northern SCS changed from the shallow-sea shelf in the late Oligocene to the deep-water slope in the Miocene. Therefore, two large depositional and subsidence centers were developed in the central depression of the QDN basin and the Baiyun sag of the PRM basin, respectively.
Oligocene are discussed, and their controls on source rock formation in the deep water area of the SCS are examined.
2. GEOLOGICAL SETTING The SCS is the largest marginal sea in the western Pacific Ocean. The QDN and PRM basins located on the northern continental shelf of the SCS were developed on the Mesozoic basement (Figure 1).17 The two basins have similar tectonic evolution characteristics, characterized by a double-layer structure. The lower structural layer was formed in the rifting stage, consisting of Eocene lacustrine deposition, Oligocene including Yacheng and Lingshui Formations in the QDN basin and Enping and Zhuhai Formations in the PRM basin (Figure 2). The upper structural layer was formed in the depression period which consists of Neogene and Quaternary, characterized by marine strata (Figure 2). In the early Oligocene, Yacheng, and Enping Formations were deposited in the QDN basin and Baiyun Sag of PRM basin, respectively, which mainly developed transitional facies in the shallow area and marine facies in the deep-water area. But the depositional processes of the Yacheng and Enping Formation present some differences. Yacheng Formation deposited coastal plain in the shallow area and neritic mudstones in the deep water area. During the sedimentary period of the Enping Formation, influenced by transgression, the sedimentary environment in the Baiyun Sag changed from lacustrine to neritic facies. In the late Oligocene, the continental margin of the northern South China Sea extensively developed marine deposits because of the increasing transgression. And the QDN and PRM basins were deposited by littoral-neritic
3. SAMPLES AND METHODS Thirty-two samples from the deep-water area of the QDN basin and twenty-eight samples from the Baiyun Sag of the PRM basin were investigated in this study (Table 1, Figure 1). Some geochemical data shown in Table 3 and Table 4 were provided by China National Offshore Oil Corporation (CNOOC) Research Institute in Beijing. TOC values of 60 samples in the two basins were measured using a Leco CS230 analyzer in State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum in Beijing. Samples were milled to 200-mesh particles by an agate mortar and pestle. And then inorganic carbon was removed. The details of the experimental procedure were reported by Li et al.11 The 32 samples from the deep-water area of the QDN basin were used for trace and major elements analyses using Philips PW2404 Xray fluorescence (XRF) spectrometer and inductively coupled plasma mass spectrometer (ICP-MS) at the Beijing Research Institute of Uranium Geology in China. The 28 samples from the Baiyun Sag of the PRM basin were for trace and major elements analyses in State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum in Beijing. The experimental conditions of these analyses were reported by Li et al.11 The sample preparation and the process of GC/MS analysis are as follows. Samples were crushed into 80 meshes and then extracted in a Soxhlet apparatus for 24 h. The extracts were evaporated, 10600
DOI: 10.1021/acs.energyfuels.7b01681 Energy Fuels 2017, 31, 10598−10611
Article
Energy & Fuels Table 1. Geochemical Data of the E3y and E3l Formation Samples in the Deep-Water Area of the QDN Basin well
depth (m)
formation
TOC (%)
SiO2 (%)
Al2O3 (%)
TiO2 (%)
Al/Ti
P/Ti
V/Cr
V/Ni
U/Th
Mo (ppm)
Y1 Y1 Y1 Y1 Y1 Y1 Y1 Y1 Y1 Y1 C1 C1 L1 L1 L1 L1 Y1 Y1 Y1 Y1 Y1 Y1 Y1 Y1 Y1 C1 C1 C1 C1 L1 L1 L1
3515.0 3715.0 3805.0 3825.0 3925.0 3945.0 3975.0 4045.0 4105.5 4185.0 3475.0 3655.0 3695.5 3737.5 3875.5 3917.5 4246.5 4309.5 4330.5 4366.5 4411.5 4486.5 4504.5 4606.5 4687.5 3845.0 3915.0 4125.0 4215.0 3947.5 4052.5 4100.5
E3 l E3 l E3 l E3 l E3 l E3 l E3 l E3 l E3 l E3 l E3 l E3 l E3 l E3 l E3 l E3 l E3 y E3 y E3 y E3 y E3 y E3 y E3 y E3 y E3 y E3 y E3 y E3 y E3 y E3 y E3 y E3 y
0.49 1.50 0.85 0.84 0.73 0.64 0.63 0.32 0.37 0.33 0.68 0.70 0.38 0.39 0.74 0.64 0.26 0.53 0.37 0.34 0.69 0.33 0.74 0.25 0.37 0.53 0.51 0.63 0.71 0.64 0.53 0.49
60.47 58.83 55.28 55.16 56.94 57.51 59.76 60.46 56.67 53.07 68.15 65.86 39.15 42.08 58.13 55.51 65.12 53.69 56.35 59.27 56.87 57.54 60.15 54.32 58.12 67.25 65.25 62.71 62.1 57.27 59.01 55.94
11.57 18.36 16.28 17.82 15.26 15.16 13.52 13.87 18.49 19.32 12.23 13.26 11.42 12.67 12.3 11.45 14.26 20.08 19.43 17.68 17.43 16.07 16.55 19.58 16.16 12.7 13.29 14.33 16.09 11.84 14.18 16.03
0.36 0.77 0.56 0.61 0.52 0.52 0.45 0.48 0.73 0.66 0.60 0.67 0.29 0.36 0.38 0.37 0.57 0.72 0.73 0.73 0.78 0.71 0.68 0.83 0.82 0.62 0.67 0.71 0.75 0.31 0.47 0.53
28.12 21.09 25.56 25.82 26.09 25.67 26.51 25.34 22.29 25.75 17.96 17.36 35.23 30.80 28.34 27.68 22.00 24.78 23.39 21.40 19.77 19.94 21.35 20.89 17.39 17.99 17.61 17.91 19.01 33.38 26.62 26.79
0.12 0.09 0.13 0.11 0.14 0.14 0.13 0.11 0.17 0.14 0.08 0.09 0.20 0.18 0.17 0.17 0.18 0.09 0.12 0.16 0.13 0.16 0.12 0.07 0.06 0.10 0.09 0.09 0.09 0.23 0.18 0.16
1.35 1.61 1.29 1.29 1.27 1.23 1.26 1.21 1.47 1.55 0.88 1.21 1.47 1.49 1.24 1.08 1.16 1.79 1.70 1.55 1.66 1.35 1.57 1.46 1.62 1.05 0.79 0.94 1.12 1.23 1.02 1.15
1.88 3.98 2.15 2.22 2.28 2.16 2.26 3.73 3.38 3.51 3.07 2.80 1.93 2.04 1.94 1.79 3.83 4.12 3.79 3.26 4.45 3.98 3.81 3.58 4.02 2.67 2.28 2.32 2.66 1.96 2.01 2.31
0.24 0.17 0.14 0.15 0.16 0.16 0.17 0.23 0.15 0.14 0.18 0.16 0.25 0.21 0.21 0.22 0.14 0.14 0.15 0.16 0.27 0.25 0.18 0.25 0.27 0.17 0.16 0.18 0.17 0.19 0.26 0.19
2.39 1.09 0.99 1.25 1.13 1.46 2.49 0.81 0.73 0.76 0.72 0.72 0.51 0.67 2.35 2.81 0.51 1.21 0.78 1.51 2.47 2.27 1.59 0.43 1.24 0.55 0.68 0.82 0.98 1.36 4.32 1.76
deasphaltened with n-hexane, and then separated into the saturated, aromatic, and polar fractions through column chromatography using silica gel and alumina (3:1). And then the saturated fractions were analyzed by gas chromatography/mass spectrometry (GC/MS). The experimental procedure is in Li et al.11
0.23−0.24) and E3z (V/Ni: 1.15−4.18, V/Cr: 0.50−1.94, Ni/ Co: 1.91−5.65, and U/Th: 0.18−0.46) samples from the Baiyun Sag of the PRM basin (Table 2). The OL/C30H, C27/C29, S/H, TT/H, and Ga/C30H ratios of E3y source rocks are in the range of 0.39−1.66, 0.46−1.70, 0.21−0.49, 0.06−0.41 and 0.08−0.17, respectively, and they are 0.21−0.68, 0.77−1.44, 0.31−0.62, 0.11−0.68 and 0.09−0.14, respectively, in the E3l source rocks in the deep water area of the QDN basin (Table 3). The C27/C29, OL/C30H, and Ga/ C30H ratios are 0.31−5.18, 0.16−1.86, and 0.04−0.16 in the E3e marine source rocks, respectively, and they are in the range from 0.41 to 3.23, from 0.30 to 2.80, and from 0.03 to 0.15 (Table 4), respectively, in the E3z source rocks from the Baiyun Sag of the PRM basin. The correlation between the TOC values and the element and biomarker parameters in the Oligocene source rocks of the deep water area of the QDN basin and the Baiyun Sag of the PRM basin has been discussed in Figure 9 and Figure 10, respectively.
4. RESULTS The element data of the E3y and E3l samples from the deep water area of the QDN basin and E3e and E3z samples from the Baiyun Sag of the PRM basin are listed in Table 1 and Table 2, respectively. P/Ti and Al/Ti ratios of the E3y marine source rocks are 0.06−0.23 and 17.39−33.38, respectively (Table 1), with the average values of 0.13 and 21.89, respectively. P/Ti and Al/Ti are slightly higher in the E3l marine source rocks, ranging from 0.08 to 0.20 and 17.36 to 35.23, respectively (Table 1), with mean values of 0.14 and 25.60, respectively. P/ Ti and Al/Ti ratios of the E3e marine source rocks in the Baiyun Sag of the PRM basin are 0.13−0.25 and 15.59−22.30, respectively (Table 2), with mean values of 0.19 and 20.09, and they are generally low in the E3z source rocks (P/Ti: 0.08− 0.17, Al/Ti: 12.26−18.17) of the Baiyun Sag. The V/Ni, V/Cr, and U/Th ratios and Mo contents are generally low in the E3y (V/Ni: 1.96−4.45, V/Cr: 0.79−1.79, U/Th: 0.14−0.27, Mo: 0.43−4.32) and E3l (V/Ni: 1.79−3.98, V/Cr: 0.88−1.61, U/Th: 0.14−0.25, Mo: 0.51−2.81) samples from the deep water area of the QDN basin (Table 1). The V/ Ni, V/Cr, Ni/Co, and U/Th ratios are lower in the E3e (V/Ni: 0.51−2.51, V/Cr: 0.34−0.53, Ni/Co: 1.23−5.24, and U/Th:
5. DISCUSSION 5.1. Paleoproductivity and TDM. 5.1.1. Effect of Paleoproductivity and TDM on Source Rock Formation in the Deep Water Area of the QDN Basin. Nutrient elements, nitrogen and phosphorus, are widely used to assess paleoproductivity.26−28 Tyrrell27 believes that the phosphate concentrations decided the nitrate concentrations and the input 10601
DOI: 10.1021/acs.energyfuels.7b01681 Energy Fuels 2017, 31, 10598−10611
Article
Energy & Fuels Table 2. Geochemical Data of the E3e and E3z Formation Samples in the Baiyun Sag, PRM Basina
a
well
depth (m)
formation
TOC (%)
Al (ppm)
Ti (ppm)
Al/Ti
P/Ti
V/Cr
V/Ni
U/Th
Ni/Co
terrigenous (%)
LH2 LH2 LH2 LW1 LW1 LW1 LW1 LW2 LW2 LW2 LW2 LW2 LW2 LW3 LW3 LW3 LW3 LW3 LW3 LW3 BY1 BY1 LH1 LH1 LH1 LH1 LH1 LH1
3012.5 3052.5 3287.5 2975.0 3095.0 3105.0 3117.5 3195.0 3215.5 3230.0 3242.5 3262.5 3287.5 3242.0 3308.0 3426.0 3586.0 3646.0 3702.0 3838.0 4888.0 5044.0 3000.0 3055.0 3117.5 3152.5 3200.0 3235.0
E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3e E3e E3e E3e E3e E3e
1.08 0.69 0.93 1.11 1.05 0.97 1.01 0.64 0.66 0.91 0.89 1.08 1.22 0.67 0.70 0.80 0.79 0.72 0.68 0.66 1.48 1.34 0.42 0.51 0.47 0.58 0.32 0.40
72200 56100 50400 32700 52800 67100 62800 35600 77700 89000 74600 75100 58700 45600 53800 60000 73700 67800 94900 82100 99500 101000 68600 87200 70500 63200 95400 68400
5160 3540 4110 1990 3480 4130 3640 2640 4350 5860 5310 4950 4410 2830 3600 4140 4850 4160 5720 4520 5920 5560 4400 3910 3270 2930 5080 3300
13.99 15.85 12.26 16.43 15.17 16.25 17.25 13.48 17.86 15.19 14.05 15.17 13.31 16.11 14.94 14.49 15.20 16.30 16.59 18.16 16.81 18.17 15.59 22.30 21.56 21.57 18.78 20.73
0.09 0.10 0.08 0.16 0.10 0.10 0.10 0.17 0.13 0.09 0.10 0.10 0.09 0.15 0.12 0.11 0.10 0.11 0.12 0.11 0.14 0.15 0.25 0.18 0.18 0.19 0.13 0.18
1.64 1.94 1.46 1.17 1.84 1.74 1.68 1.32 0.65 1.45 1.39 1.70 1.25 0.50 1.48 1.69 1.40 1.51 1.42 1.73 0.90 1.02 0.51 0.34 0.38 0.37 0.53 0.43
3.77 3.30 4.18 1.34 3.76 3.78 3.18 2.19 1.15 3.84 3.44 4.13 3.72 1.57 2.87 4.02 3.52 3.15 2.96 3.61 1.91 2.39 2.51 0.51 0.61 0.51 0.80 0.85
0.19 0.19 0.20 0.46 0.22 0.23 0.23 0.33 0.19 0.21 0.21 0.19 0.20 0.20 0.19 0.19 0.19 0.19 0.19 0.20 0.18 0.22 0.23 0.24 0.23 0.24 0.23 0.24
3.11 3.31 1.91 5.65 2.56 3.06 3.32 3.22 3.42 2.41 2.40 2.29 2.35 3.14 2.19 2.09 2.33 2.80 2.61 2.64 3.41 2.63 1.23 5.24 5.21 5.08 3.31 3.80
86.07 59.05 68.56 33.19 58.05 68.89 60.72 44.04 72.56 97.75 88.57 82.57 73.56 47.21 60.05 69.06 80.90 69.39 95.41 75.40 98.75 92.74 73.39 65.22 54.55 48.87 84.74 55.05
Note: % terrigenous = (Tisample/Tishale) × 100. Terrigenous % is calculated assuming 5995 ppm Ti in the terrigenous component.32,33
Table 3. TOC Values and Some Biomarker Parameters of the E3y and E3l Formation Samples in the Deep-Water Area of the QDN Basina well
depth (m)
formation
TOC (%)
OL/C30H
C27/C29
S/H
TT/H
Ga/C30H
Y1 Y1 L1 Y2 Y2 Y1 C1 C1 L1
3535.0 3875.0 3812.5 3792.0 3930.0 4426.5 3705.0 4025.0 3989.5
E3 l E3 l E3 l E3 l E3 l E3 y E3 y E3 y E3 y
0.53 0.63 0.60 0.47 0.64 0.67 0.69 0.59 0.61
0.26 0.21 0.50 0.25 0.68 1.12 1.66 0.41 0.39
1.15 0.88 1.21 0.77 1.44 1.15 0.46 0.71 1.70
0.62 0.46 0.31 0.51 0.36 0.49 0.21 0.34 0.25
0.68 0.65 0.11 0.13 0.21 0.14 0.06 0.41 0.11
0.10 0.11 0.14 0.12 0.09 0.08 0.12 0.17 0.13
a Note: OL/C30H = oleanane/C30hopane; C27/C29 = ααα20RC27/ααα20RC29 sterane; S/H = steranes/hopanes; TT/H = tricyclic terpanes/ hopanes; and Ga/C30H = gammacerane/C30 hopane.
Al/Ti ratio depends on the number of organism scavenging action, which can assess primary production. There is a strong positive correlation between P/Ti and Al/ Ti ratios of the marine source rocks in Oligocene Yacheng and Lingshui Formations in the deep-water area of the QDN basin (Figure 3a), suggesting that P/Ti and Al/Ti ratios can act as proxies for primary production. P/Ti and Al/Ti ratios of marine source rocks in the Yacheng Formation are 0.06−0.23 and 17.39−33.38, respectively (Table 1), with the averages of 0.13 and 21.89, respectively. The P/Ti and Al/Ti ratios are slightly greater than that in Post Archean Average Shale (PAAS) with an average P/Ti ratio of 0.12 and Al/Ti ratio of 16.733 but far below that in the high productivity regions in the equatorial Pacific Ocean with the P/Ti and Al/Ti values of 2−8 and 35−
of phosphorus controlled the ocean primary production; some organisms could obtain nitrogen from the air when the NO3−/ PO43− ratio was lower, but there is no alternative source once phosphate depletes. The changing concentration of phosphates will affect the concentration of nitrate; therefore, phosphorus determines the capacity of primary productivity. Considering that Ti and Al originate from TDM, which can have a dilution effect on the absolute phosphorus content,29 P/Ti or P/Al rather than the absolute phosphorus content is used to suggest the paleoproductivity to eliminate this effect.6,9,30,31 The gradient leaching experiment by Murray and Leinen32 suggests that mineral phase residue has more than 95% of Ti and approximately 50% of Al combined with biogenesis, thus the 10602
DOI: 10.1021/acs.energyfuels.7b01681 Energy Fuels 2017, 31, 10598−10611
Article
Energy & Fuels
Table 4. TOC Values and Some Biomarker Parameters of the E3e and E3z Formation Samples in the Baiyun Sag, PRM Basina
a
well
depth (m)
formation
TOC (%)
OL/C30H
C27/C29
Ga/C30H
LH2 LW1 LW1 LW2 LW2 LH3 LH3 LH3 LW3 LW3 LH3 LH3 LH3 LH3 LH3 LH3 LH3 LH3 LH3 LH3 LH3 LH3 LH3 LH1 LH1 LH1
3287.5 3095.0 3117.5 3195.0 3242.5 2811.5 2856.5 2916.5 3511.5 3530.0 2955.5 2988.5 3009.5 3036.5 3054.5 3075.5 3087.5 3120.5 3144.5 3174.5 3192.5 3216.5 3228.5 3055.0 3152.5 3235.0
E3z E3z E3z E3z E3z E3z E3z E3z E3z E3z E3e E3e E3e E3e E3e E3e E3e E3e E3e E3e E3e E3e E3e E3e E3e E3e
0.93 1.05 1.01 0.64 0.89 0.79 0.65 0.48 0.71 0.79 1.03 1.07 1.11 1.13 1.34 1.23 1.27 1.15 1.25 1.15 1.17 1.25 1.28 0.51 0.58 0.40
0.46 0.62 0.30 0.62 0.87 0.90 2.14 2.32 1.34 2.80 0.78 0.84 1.01 0.94 1.86 1.73 1.74 0.97 0.91 1.06 0.90 1.19 0.95 0.16 0.18 0.17
1.07 3.23 2.87 2.17 0.86 1.12 1.27 1.07 0.62 0.41 0.31 0.69 0.75 0.85 0.97 0.99 1.07 1.01 1.07 1.21 1.44 1.11 1.37 5.18 4.96 4.60
0.09 0.05 0.12 0.10 0.04 0.15 0.12 0.12 0.03 0.06 0.06 0.04 0.10 0.10 0.08 0.10 0.09 0.10 0.05 0.11 0.11 0.12 0.11 0.15 0.15 0.16
Note: OL/C30H = oleanane/C30hopane; C27/C29 = ααα20RC27/ααα20RC29 sterane; and Ga/C30H = gammacerane/C30 hopane.
35.23, respectively (Table 1), with average values of 0.14 and 25.60, respectively, suggesting moderate primary production. The Al/Ti and P/Ti ratios suggest that paleoproductivity is relatively higher in the sedimentary period of Lingshui Formation than in the period of Yangcheng Formation. This phenomenon can be explained by the sedimentation rate or TDM input. Source rocks in the Lingshui Formation have lower TiO2 (0.29−0.77%, average value 0.52%) and a higher SiO2/Al2O3 ratio (2.75−5.57, mean value 3.99) (Table 1, Figure 3b), suggesting a relatively lower input of TDM and slow sedimentation rate. However, source rocks in the Yacheng Formation have relatively higher TiO2 (0.31−0.83%, average value 0.66%) and a lower SiO2/Al2O3 ratio (2.67−5.30, mean value 3.83) (Table 1), indicating that the TDM input is slightly greater in this period, which could have diluted primary production. 5.1.2. Effect of Paleoproductivity and TDM on Source Rock formation in the Baiyun Sag, PRM Basin. P/Ti and Al/ Ti ratios of the marine source rocks in the Enping Formation are in the range of 0.13−0.25 and 15.59−22.30, respectively (Table 2), with mean values of 0.19 and 20.09, respectively, suggesting moderate paleoproductivity. P/Ti and Al/Ti ratios in the source rocks of Zhuhai Formation are 0.08−0.17 and 12.26−18.17, respectively, averaging 0.11 and 15.59, respectively, indicating low paleoproductivity. Al and Ti contents are used to indicate input of TDM.29,35 Figure 4b shows a strong correlation between Al and Ti content. The contents of Al and Ti in the source rocks of Enping Formation are in the range of 63200−95400 ppm and 2930−5080 ppm, respectively (Table 2), with mean values of 75550 and 3815 ppm, respectively. Al and Ti content in the source rocks of Zhuhai Formation are 32700−101000 ppm and
Figure 3. Relationship between (a) Al/Ti and P/Ti and (b) TiO2 and SiO2/Al2O3 in the marine source rocks of Yangcheng and Lingshui Formations, deep-water area of the QDN basin.
41.34 P/Ti and Al/Ti ratios of marine source rocks in the Lingshui Formation range from 0.08 to 0.20 and from 17.36 to 10603
DOI: 10.1021/acs.energyfuels.7b01681 Energy Fuels 2017, 31, 10598−10611
Article
Energy & Fuels
(bacteria) organisms to the source rocks.36,41 High content of S/H and TT/H ratios are often indicative of organic-rich source rocks dominated by algal organic matter, such as the source rocks in the Congo Basin.42 In contrast, low S/H and TT/H ratios may be associated with source rocks mainly contributed by TOM.41 C27/C29, S/H, and TT/H ratios of source rocks in the Yacheng Formation are in the range of 0.46−1.70, 0.21−0.49, and 0.06−0.41, respectively (Table 3), with the average values of 1.01, 0.32, and 0.18, respectively. C27/C29, S/H, and TT/H ratios of source rocks in the Lingshui Formation are 0.77−1.44, 0.31−0.62, and 0.11−0.68, respectively (Table 3), with average values of 1.09, 0.45, and 0.36, respectively. These data suggest that the input of TOM is a greater contributor to source rocks in the Yacheng Formation. 5.2.2. TOM Input in the Baiyun Sag, PRM Basin. Oleanane is widely distributed in the Enping and Zhuhai Formations in the Baiyun Sag of the PRM basin (Figure 6), suggesting that the formation of the marine source rocks was influenced by TOM. Although the high content of bicadinanes are detected in the source rocks of Enping Formation in the Panyu Low Uplift, northern slope of Baiyun Sag,11,17 these compounds have not been found in the Oligocene source rocks in the deep-water area of the Baiyun Sag. The distribution characteristics of ααα20RC27, ααα20RC28, and ααα20RC29 regular steranes are similar in the Enping and Zhuhai Formations which distributed as “V” type. The C27/C29 ratios of the source rocks in the Enping Formation are in the range of 0.31−5.18 with a mean value of 1.72, while they are 0.41−3.23 with an average of 1.47 in the source rocks of Zhuhai Formation. The OL/C30H ratio varies in the source rocks of these two formations. The OL/ C30H ratios in the mudstones of Enping Formation range from 0.16 to 1.86 with an average value of 0.96, while they are from 0.30 to 2.80 with a mean value of 1.24 in the source rocks of Zhuhai Formation (Table 4), indicating moderate to high and high contents of TOM input in the sedimentary period of Zhuhai and Enping Formations, respectively. 5.3. Redox Conditions. 5.3.1. Redox Conditions in the Deep Water Area of the QDN Basin. In an oxic environment, V5+ exists in form of vanadate.43 However, V5+ is reduced to form VO(OH)2 or organometallic ligands, vanadium porphyrin, or other forms under anoxic or euxinic environment, which can be enriched in sediments. Cr exists as Cr4+ in the chromate anion in an oxic environment.44 Cr4+ is reduced to Cr3+ in anoxic conditions, which is enriched in the sediment as Cr2O3 or Cr(OH)3 or other forms.45,46 On the basis of the behaviors of V and Cr, Jones and Manning47 proposed that a V/Cr ratio >4.25 suggests an anoxic to euxinic environment, 2−4.25 implies suboxic conditions, and
