Article Cite This: Energy Fuels 2017, 31, 13519−13527
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Lacustrine Source Rock Deposition in Response to Coevolution of the Paleoenvironment and Formation Mechanism of Organic-Rich Shales in the Biyang Depression, Nanxiang Basin Wenhao Li,† Shuangfang Lu,*,† Zhaozhao Tan,†,‡ and Taohua He†,‡ †
Research Institute of Unconventional Oil & Gas and Renewable Energy and ‡School of Geosciences, China University of Petroleum (East China), Qingdao, Shandong 266580, People’s Republic of China ABSTRACT: Shale oil has been found in the Paleogene lacustrine source rocks in the second (Eh32) and third beds (Eh33) of the third member of the Hetaoyuan Formation from the Biyang Depression of the Nanxiang Basin, eastern China, but the formation environments of the organic-rich shales are still unclear, which restricts the shale oil exploration. Here, this paper discusses the paleoenvironment and its control on the organic-rich shale formation. Our study suggests that shales from the Eh32 and Eh33 beds display some heterogeneities in total organic carbon (TOC) contents and oil potential as a result of their different paleoenvironments. During Eh33 deposition, the paleoproductivity was moderate to high, which was diluted by moderate to high terrigenous detrital matter (TDM) input. During Eh32 deposition, the TDM and terrigenous organic matter (TOM) inputs increased. The paleoproductivity was moderate, which was greatly diluted by the high TDM content. The redox conditions were anoxic during the two source rock intervals. Paleoproductivity is the main controlling factor of source rock formation during Eh32 and Eh33 depositions, which has a strong positive correlation with the burial of organic carbon. However, anoxia did not promote deposition of the richest source rocks. During Eh33 deposition, shales with high contents of TOC values and better oil potential could be formed in this environment on account of the moderate to high paleoproductivity. In contrast, during Eh32 deposition, high TOM input did not obviously improve the burial of organic carbon but high TDM input significantly diluted the paleoproductivity. The bottom water became semi-saline to hypersaline, but it did not enhance the anoxic environment. Shales with medium TOC values and oil potential could be formed in this environment as a result of moderate paleoproductivity.
1. INTRODUCTION Multiple parameters, including paleoclimate, paleoproductivity, redox conditions, terrigenous organic matter (TOM), and terrigenous detrital matter (TDM), could affect the organic carbon burial of the source rocks,1−3 among which the primary productivity and preservation conditions are considered to be the main controlling factors on marine source rock formation.4,5 The change of the lacustrine sedimentary environments is frequent as a result of the limited scope of lakes.6,7 Affected by the river delta system, TDM and TOM play a significant role in source rock formation in lacustrine sedimentary environments. TOM can be carried into lakes by rivers, while at the same time, nutrients transported by rivers could give rise to a surface plankton boom, leading to high biological productivity.8 However, adequate TDM can dilute productivity.9 Harris et al.10 believed that, in the late rift period, the efficient cycling of plant-derived carbon into soil carbonate is beneficial for enhancing chemical weathering and the nutrient flux, leading to a plankton boom. Shale oil has been found in the shales developed in the second and third beds of the third member of the Hetaoyuan Formation (Eh32 and Eh33). Although hydrocarbon potential and the contribution of the Hetaoyuan source rocks to the crude oils have been discussed,11,12 the organic-rich sediments in the above beds have scarcely been studied. The shale oil potential in these rocks remains unclear. In this study, influencing factors on source rock formation, including paleoproductivity, TOM and TDM inputs, and redox conditions, were discussed on the basis of organic and inorganic geochemistry data of the cores from Eh32 and Eh33 deposition. Then, the © 2017 American Chemical Society
controlling factors on shale depositions as well as the enrichment mechanisms of the two oil shale layers in the third member of the Hetaoyuan Formation of the Biyang Depression, Nanxiang Basin, were revealed. The results of this paper are aimed to provide reference for shale oil exploration in the study area.
2. GEOLOGICAL SETTING The Nanxiang Basin is located in eastern China, and the Biyang Sag lies in the east of the Nanxiang Basin. The Biyang Depression is composed of three secondary structural units, which are the northern slope belt, central deep sag belt, and southern steep slope belt, from north to south (Figure 1). It experienced an initial rifting period in the Cretaceous, a main rifting period in the Paleogene, and a depression period in the Neogene, developing the Yuhuangding Formation, Dacangfang Formation, Hetaoyuan Formation, and Liaozhuang Formation from bottom to top. The Hetaoyuan Formation is the main exploration layer characterized by mudstones, sandstones, and dolomites (Figure 2). On account of the large scale of deep to semi-deep lacustrine facies,13 mudstones and shales are widely distributed in the third member of the Hetaoyuan Formation, which can be divided into eight beds (Figure 2). The second and third beds of the third member of the Hetaoyuan Formation (Eh32 and Eh33) are the main exploration fields for shale oil in the study area.13 A previous study shows Received: September 23, 2017 Revised: October 25, 2017 Published: November 22, 2017 13519
DOI: 10.1021/acs.energyfuels.7b02880 Energy Fuels 2017, 31, 13519−13527
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Figure 1. Tectonic units of the Biyang Depression, Nanxiang Basin.
that the Eh32 source rocks have high organic matter abundance, with the total organic carbon (TOC) values ranging from 0.31 to 4.43%, averaging 1.52%, and were characterized by oil-prone type II kerogen.14
in the Eh33 deposition have better oil potential. The kerogen types of Eh32 source rocks are from I to III (Figure 3), while those of the Eh33 source rocks are mainly I and II1 (Figure 3), indicating that Eh33 source rocks have better oil potential. 4.2. Sedimentary Environment. 4.2.1. Paleoproductivity. Nutrient elements of N and P are widely used to evaluate paleoproductivity.16,17 Considering that nitrogen can be obtained from the air through marine photosynthesizers, P content rather than N content can be used as an indicator of paleoproductivity.16,18 Detrital elements, such as Ti and Al, can have a dilution effect on the absolute P content.19 Thus, P/Ti or P/Al rather than the absolute P content is used to assess paleoproductivity to mitigate this effect.20−22 Although Al and Ti originate from TDM, Murray and Leinen23 believed that the Al/Ti ratio depends upon the organism scavenging action through a gradient leaching experiment, which can be used to evaluate paleoproductivity.20 Although the controversy still exited as to whether non-hydrothermal barite has some correlation with organisms, barite was considered to be an indicator of paleoproductivity.24−26 Ratios of Ba/Ti and Ba/Al instead of the Ba content are suggested to indicate paleoproductivity to eliminate the dilution effects of other components.19,27 The ratios of P/Ti, Al/Ti, and Ba/Ti are 0.15−0.54, 18.99−28.74, and 0.09−0.21 for Eh32 shale samples, respectively (Table 2), with averages of 0.30, 23.71 and 0.15, respectively, which are larger than those in the post-Archean average shale (PAAS), with an average P/Ti ratio of 0.12, Al/Ti ratio of 16.7 and Ba/Ti ratio of 0.11,28 but far below that in the high productivity regions, with P/Ti and Al/Ti values of 2−8 and 35−41 in the equatorial Pacific Ocean.29 These parameters suggest moderate paleoproductivity during Eh32 deposition. The ratios of P/Ti, Al/Ti, and Ba/Ti are 0.13−1.81, 21.38−30.40, and 0.14−0.79 for Eh33 shale samples, respectively (Table 2), with averages of 0.59, 25.64, and 0.29, respectively, indicating moderate to high paleoproductivity during Eh33 deposition. Sterane parameters have the potential to indicate primary production in marine and lacustrine systems.30,31 4-Methylsteranes are commonly distributed in marine, evaporitic, and especially freshwater environments.32,33 The precursors of 4-methylsteranes are considered to be 4-methyl sterols, which are mostly algal in origin.34 Dinoflagellates are believed to be the main source of 4-methylsteranes.34,35 The sterane/hopane
3. SAMPLES AND METHODS A total of 28 core samples were collected in the Biyang Sag, Nanxiang Basin. A total of 14 black shales were obtained from the second bed of the third member of the Hetaoyuan Formation from well B1, while the other 14 black shales were from the third bed of the third member of the Hetaoyuan Formation from Well BY1. The samples were ground in an agate mortar and pestle to 200 mesh size for TOC and Rock-Eval analyses. The experimental procedures were reported by Li et al.15 These results are listed in Table 1. These samples were also prepared for trace and major element analyses, and the methods were also reported by Li et al.15 The results are shown in Table 2. We chose 13 samples for the analysis of gas chromatography−mass spectrometry (GC−MS). Samples were crushed into 80 meshes and then extracted in a Soxhlet apparatus with dichloromethane (CH2Cl2) for 24 h. The extracts were evaporated, handled with n-hexane, and then separated through column chromatography into the saturated, aromatic, and polar fractions. The saturated fraction was analyzed by GC−MS. The experimental conditions were listed in Li et al.15 The GC−MS results are listed in Table 3.
4. RESULTS AND DISCUSSION 4.1. Analysis of Hydrocarbon Potential of Shales in Eh32 and Eh33 Depositions in the Hetaoyuan Formation. The TOC values of the shales during the Eh32 deposition range from 0.22 to 3.58% (Table 1), with an average of 1.83%, while those of the shale during the Eh33 deposition vary from 1.80 to 5.09% (Table 1), with a mean value of 3.37%. The S1 + S2 values of shales in the Eh32 and Eh33 depositions range from 0.10 to 30.11 mg/g and from 9.35 to 45.00 mg/g, averaging 11.62 and 24.47 mg/g, respectively, indicating that source rocks in the Eh33 deposition have better hydrocarbon potential. The evaluation of the oil potential is significant for shale oil exploration. Because the shale reservoir is of low permeability and produces light oil or condensates, the index S1 is used to evaluate the oil potential. S1 values in the shales during the Eh32 and Eh33 depositions range from 0.01 to 2.98 mg/g and from 0.56 to 3.45 mg/g (Table 1), with the averages of 0.80 and 1.21 mg/g, respectively, suggesting that source rocks 13520
DOI: 10.1021/acs.energyfuels.7b02880 Energy Fuels 2017, 31, 13519−13527
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Figure 2. Stratigraphic section for the Biyang Sag showing the primary lithology and depositional environments for the Hetaoyuan Formation of the Biyang Sag.14
can also be used to indicate the source of organic matter.37 When C29 sterane dominates, it suggests a high content of terrestrial higher plants, while when C27 dominates, it reveals phytoplankton blooms.38 The distribution of n-alkanes for Eh32 shale samples shows the predominance of high-molecularweight composition, while the distribution of n-alkanes for Eh33 shale samples is dominated by low-molecular-weight composition (Figure 4). The results suggest that TOM has a large contribution to shales during Eh32 deposition, while algae are the main biogenic substances of shales during Eh33 deposition. The distribution of C27−C29 regular steranes shows predominance by C29 sterane, and a low content of 4-methylsteranes has been found in the shales during Eh32 deposition (Figure 4), while a “V”-shaped C27−C29 sterane distribution and a high content of 4-methylsteranes have been found from the shales during Eh33 deposition (Figure 4). This result indicates that TOM played an important role in source rock formation during
ratios are also used to reflect the input of eukaryotic versus prokaryotic organisms.7,8,33 The 4-methylsterane index (4-methylsteranes/∑C29 steranes) is lower in shales during the Eh32 deposition, ranging from 0.01 to 0.02, while it is relatively high in shales during the Eh33 deposition, with a range from 0.06 to 0.28 and an average of 0.18. The sterane/hopane ratios in the above layers are in the ranges of 0.13−0.44 and 0.17−1.96, with mean values of 0.27 and 0.72, respectively. The parameters of the 4-methylsterane index and sterane/hopane ratio suggest that paleoproductivity during Eh33 deposition was higher than that during Eh32 deposition. 4.2.2. TOM and TDM. Biomarkers can indicate the source of organic matter.33 The carbon number distribution of n-alkanes is predominant in the n-C25−n-C35 range, indicating terrigenoussourced organic matter, while predominance occurs in the range of n-C17−n-C23, indicating phytoplankton-sourced organic matter.36 The relative content of C27−C29 regular steranes 13521
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Energy & Fuels Table 1. Geochemical Data of the Shales during Eh32 and Eh33 Deposition in the Biyang Depression, Nanxiang Basin well
depth (m)
lithology
formation
TOC (%)
S1 (mg/g)
S2 (mg/g)
S1 + S2 (mg/g)
HI (mg/g)
Tmax (°C)
B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1
2023.20 2024.40 2028.00 2029.31 2031.11 2109.35 2110.00 2110.73 2112.00 2115.11 2116.12 2117.48 2119.60 2120.88 2430.55 2433.90 2435.10 2435.50 2436.70 2438.00 2438.90 2440.00 2442.38 2443.30 2444.30 2447.60 2450.40 2451.10
shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale
Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33
3.48 3.24 0.57 1.62 0.88 0.24 0.33 0.22 0.71 0.96 3.51 3.08 3.25 3.58 2.30 2.56 2.42 1.80 5.09 4.65 4.40 4.06 3.39 3.12 2.76 4.48 2.76 3.45
1.42 2.98 0.08 0.62 0.10 0.03 0.01 0.02 0.04 0.23 1.39 1.15 1.49 1.68 1.65 1.16 1.00 2.07 0.69 0.86 0.93 0.82 1.16 0.76 3.45 0.77 1.10 0.56
25.47 16.48 0.65 7.17 1.55 0.07 0.29 0.10 1.18 3.10 28.72 19.69 23.46 23.49 7.70 16.17 10.67 8.73 44.31 38.77 31.45 34.73 20.43 18.78 8.78 41.72 13.85 29.54
26.89 19.46 0.73 7.79 1.65 0.10 0.30 0.12 1.22 3.33 30.11 20.84 24.95 25.17 9.35 17.33 11.67 10.80 45.00 39.63 32.38 35.55 21.59 19.54 12.23 42.49 14.95 30.10
731.90 508.64 113.44 442.59 175.94 28.81 88.41 45.45 165.50 322.58 818.23 639.29 721.85 656.15 334.78 631.64 440.91 485.00 870.53 833.76 714.77 855.42 602.65 601.92 318.12 931.25 501.81 856.23
440 434 438 438 437 433 434 432 444 440 447 443 444 444 439 447 440 444 447 443 447 446 446 449 435 451 441 447
Table 2. Inorganic Geochemical Data of the Shales during Eh32 and Eh33 Deposition in the Biyang Depression, Nanxiang Basin
a
well
depth (m)
lithology
formation
Al (%)
Ti (%)
P/Ti
Al/Ti
Ba/Ti
Mo (ppm)
V/(V + Ni)
Sr/Ba
terrigenous (%)a
B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1 BY1
2023.20 2024.40 2028.00 2029.31 2031.11 2109.35 2110.00 2110.73 2112.00 2115.11 2116.12 2117.48 2119.60 2120.88 2430.55 2433.90 2435.10 2435.50 2436.70 2438.00 2438.90 2440.00 2442.38 2443.30 2444.30 2447.60 2450.40 2451.10
shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale shale
Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33
6.85 7.32 7.61 7.01 6.73 9.95 10.10 8.30 8.89 7.55 7.27 6.96 8.60 8.60 8.12 3.88 5.38 4.93 5.01 7.44 7.76 9.09 7.64 5.84 8.28 1.51 9.11 7.59
0.24 0.29 0.32 0.29 0.28 0.52 0.48 0.41 0.39 0.33 0.29 0.27 0.35 0.35 0.38 0.15 0.20 0.18 0.19 0.30 0.32 0.35 0.31 0.22 0.34 0.05 0.37 0.32
0.19 0.38 0.28 0.41 0.49 0.15 0.23 0.22 0.23 0.26 0.54 0.43 0.16 0.26 0.18 1.81 0.31 0.30 0.69 0.23 0.24 0.48 0.22 0.81 1.53 1.03 0.27 0.13
28.74 25.25 23.89 23.91 23.66 18.99 21.03 20.14 22.70 23.23 25.50 25.88 24.37 24.64 21.38 26.36 27.28 27.23 26.75 25.20 24.55 25.90 24.77 26.26 24.13 30.40 24.92 23.90
0.15 0.17 0.16 0.20 0.21 0.10 0.11 0.09 0.15 0.16 0.17 0.14 0.12 0.12 0.22 0.36 0.27 0.19 0.35 0.19 0.18 0.21 0.17 0.69 0.17 0.79 0.19 0.14
16.92 16.70 41.00 3.87 1.95 0.92 0.87 1.02 2.52 3.44 7.99 26.60 7.17 17.59 44.97 12.52 19.18 11.90 22.71 25.18 11.55 11.89 26.13 11.45 11.53 4.91 16.68 11.11
0.71 0.76 0.71 0.77 0.74 0.76 0.79 0.70 0.77 0.85 0.75 0.67 0.79 0.75 0.70 0.68 0.73 0.70 0.74 0.62 0.72 0.74 0.61 0.72 0.75 0.82 0.76 0.72
1.77 1.87 1.79 1.67 1.51 0.64 1.17 1.04 1.03 1.61 1.08 1.56 1.41 1.18 0.57 1.93 0.87 1.00 2.43 0.59 0.73 0.82 0.61 0.44 1.21 0.55 0.57 0.74
38.43 51.47 55.52 59.14 45.34 91.17 80.81 60.38 65.49 58.39 48.73 47.25 54.43 68.13 58.72 25.94 36.53 37.81 31.61 47.41 54.46 59.29 49.68 39.12 55.66 35.01 62.19 52.93
Terrigenous (%) = (Tisample/Tishale) × 100. Terrigenous (%) was calculated assuming 5995 ppm of Ti in the terrigenous component.23,28 13522
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Energy & Fuels Table 3. Some Biomarker Parameters of the Eh32 and Eh33 Shales from the Biyang Depression, Nanxiang Basina
a
well
depth (m)
formation
lithology
Pr/Ph
Ga/C30H
S/H
4MSI
C21−/C22+
B1 B1 B1 B1 B1 B1 BY1 BY1 BY1 BY1 BY1 BY1 BY1
2024.40 2028.00 2031.11 2110.00 2110.73 2115.11 2430.55 2435.10 2438.90 2440.00 2443.30 2447.60 2451.10
Eh32 Eh32 Eh32 Eh32 Eh32 Eh32 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33 Eh33
shale shale shale shale shale shale shale shale shale shale shale shale shale
0.37 0.47 0.47 0.86 0.39 0.36 0.36 0.46 0.45 0.45 0.44 0.37 0.38
1.31 0.75 0.52 0.67 1.22 1.13 0.33 0.23 0.22 0.25 0.31 0.29 0.71
0.14 0.41 0.44 0.21 0.13 0.29 1.96 0.20 0.39 0.17 0.19 0.63 1.51
0.01 0.01 0.02 0.01 0.01 0.01 0.18 0.28 0.28 0.19 0.15 0.09 0.06
0.42 0.49 0.77 0.88 0.84 0.32 0.44 0.51 0.51 0.68 0.68 0.68 0.56
Ga/C30H, gammacerane/C30 hopane; S/H, steranes/hopanes; 4MSI, 4-methylsterane/C29 steranes; and C21−/C22+, ∑n-C21−/∑n-C22+.
environment was indicated when the V/(V + Ni) ratio was less than 0.54.40 Mo is considered to be the best indicator to distinguish between anoxic and euxinic environments.41 The Mo content in the range of 5−40 ppm implies an euxinic environment.42,43 V/(V + Ni) ratios from the shales during Eh32 deposition range from 0.67 to 0.85 (Table 2), averaging 0.75, while those from the shales during Eh33 deposition are between 0.61 and 0.82 (Table 2), with a mean value of 0.71, indicating an anoxic environment in these periods. The Mo contents from the Eh32 and Eh33 shales are in the ranges of 0.87−41.00 and 4.91−44.97 ppm (Table 2), respectively, with average values of 10.61 and 17.27 ppm, respectively, which are higher than those of the detrital matter (1 ppm)27 and plankton (2 ppm).44 This result suggests that shales from Eh32 and Eh33 were both developed in anoxic environments. The ratio of Pr/Ph is also a good indicator of redox conditions.33 The Pr/Ph ratios from shales during Eh32 and Eh33 depositions are lower, ranging from 0.36 to 0.86 (Table 3), suggesting an anoxic environment. The water salinity is an important factor for source rock formation. An anoxic environment could be formed in bottom water as a result of the high salinity content at the sediment− water interface.45 Gammacerane is commonly used to reveal the water salinity, and a high content of gammacerane is considered to be related to a saline environment and stratified water.15,46,47 Gammacerane is widely distributed in shales during Eh32 and Eh33 depositions. The gammacerane/C30 hopane ratio (Ga/C30H) from shales during Eh32 deposition varies from 0.52 to 1.31 (Table 3), with an average value of 0.93, while it ranges from 0.22 to 0.71 during Eh33 deposition (Table 3), with a mean value of 0.33, indicating that shales during Eh32 deposition developed in semi-saline to hypersaline environments and shales from Eh33 formed in brackish to semisaline environments. The Sr/Ba ratio is also an indicator of salinity, the high content of which indicates a saline environment.48 The Sr/Ba ratios of the shales during Eh32 deposition range from 0.64 to 1.87 (Table 2), with an average of 1.38, while it varies from 0.44 to 2.43 during Eh33 deposition (Table 2), with a mean value of 0.93. This result indicates that the water salinity is larger during Eh32 deposition than during Eh33 deposition. However, Sr/Ba ratios have no correlation with V/(V + Ni) ratios during Eh32 and Eh33 depositions (Figure. 6a), suggesting that water salinity had nothing to do with redox conditions. The Sr/Ba ratios of the shales during Eh32 deposition had a certain positive correlation with Mo contents, while this correlation was not found in the shales
Figure 3. Plot of HI versus Tmax outlining the kerogen type for the source rocks in the Eh32 and Eh33 depositions in the Hetaoyuan Formation of the Biyang Depression, Nanxiang Basin.
Eh32 deposition, while algae organic matter was the main biogenic substance for shales during Eh33 deposition. The contents of Al and Ti are used to assess TDM input.19,39 The contents of Al and Ti in Eh32 shale samples are 6.73−10.10 and 0.24−0.52%, respectively (Table 2), with average values of 7.98 and 0.34%, respectively, while they are 1.51−9.11 and 0.05−0.38% for Eh33 shale samples, respectively (Table 2), with mean values of 6.54 and 0.26%, respectively. These results suggest that the amount of TDM input during Eh32 deposition is greater than that during Eh33 deposition. The Ti content in sediments is used to calculate the amount of terrigenous detritus,23,28 and the formula is terrigenous (%) = (Tisample/ Tishale) × 100 (PAAS with Tishale = 5995 ppm). The proportions of the terrigenous detritus from the shales during Eh32 and Eh33 depositions are in the ranges of 38.43−91.17 and 25.94−62.16%, respectively, with average values of 58.91 and 46.17%, respectively. The Al/Ti and P/Ti ratios of the shales decrease with the increase in the proportion of terrigenous detritus [terrigenous (%)] (panels a and b of Figure 5), suggesting that TDM diluted primary production during Eh32 and Eh33 depositions. 4.2.3. Redox Conditions. The ratio V/(V + Ni) can be used to indicate redox conditions. An anoxic environment was implied when the V/(V + Ni) ratio was over 0.54, and an oxic 13523
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Figure 4. Chromatograms showing the distribution of n-alkanes (TIC), terpenoids (m/z 191), and steroids (m/z 217) in saturated hydrocarbons of the Eh32 and Eh33 shales from the Biyang Depression, Nanxiang Basin. C30H, C30 hopane; Ga, gammacerane; C27, ααα20RC27 sterane; C28, ααα20RC28 sterane; C29, ααα20RC29 sterane; and 4-MS, 4-methylsteranes.
Figure 6. Relationship between (a) Sr/Ba and V/(V + Ni) ratio and (b) Sr/Ba and Mo content in the shales during Eh32 and Eh33 depositions in the Hetaoyuan Formation of the Biyang Depression, Nanxiang Basin.
Figure 5. Relationship between (a) terrigenous (%) and Al/Ti ratio and (b) terrigenous (%) and P/Ti ratio in the shales during Eh32 and Eh33 depositions in the Hetaoyuan Formation of the Biyang Depression, Nanxiang Basin. 13524
DOI: 10.1021/acs.energyfuels.7b02880 Energy Fuels 2017, 31, 13519−13527
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Figure 7. Relationship between (a) Al/Ti and TOC value, (b) C21−/C22+ and TOC value, (c) Mo content and TOC value, and (d) V/(V + Ni) and TOC value in the shales during Eh32 and Eh33 depositions in the Hetaoyuan Formation of the Biyang Depression, Nanxiang Basin. C21−/C22+ = ∑n-C21−/∑n-C22+.
Figure 8. Depositional models for the two source rock intervals (a, Eh33 depositional stage; b, Eh32 depositional stage) showing the environmental changes and their control of source rock formation in the Biyang Depression, Nanxiang Basin.
during Eh33 deposition (Figure. 6b), indicating that water column stratification is likely to form in a hypersaline
environment during Eh32 deposition and that it is beneficial to Mo enrichment in this lentic environment. 13525
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Energy & Fuels 4.3. Correlation between the Burial of Organic Carbon and the Sedimentary Environment. The Al/Ti ratios in the shales during Eh32 and Eh33 depositions have positive correlations with TOC values (Figure 7a), suggesting that paleoproductivity controls the burial of organic carbon. The C21−/C22+ ratios also have a positive correlation with TOC values (Figure 7b), indicating that algal organic matter instead of TOM controls the burial of organic carbon, although the TOM input was adequate during the Eh32 deposition period. The Mo contents and V/(V + Ni) ratios have nothing to do with the TOC values of the source rocks (panels c and d of Figure 7), revealing that redox conditions are not the main factors influencing source rock formation during the Eh32 and Eh33 depositional periods. Thus, the relatively high content of TOC values in shales during the Eh33 deposition depends upon moderate to high paleoproductivity, although TDM had a dilution effect on paleoproductivity during this period (panels a and b of Figure 5). The moderate TOC values in shales during the Eh32 deposition are caused by moderate paleoproductivity, which was greatly diluted by the high TDM input. 4.4. Developmental Models of Shales. During the Eh33 deposition period, paleoproductivity was moderate to high, as suggested by the Al/Ti, P/Ti, and Ba/Ti ratios (Table 2 and Figure 8a). The moderate to high TDM input diluted the primary production to a certain extent. Most algal organic matter could be accumulated at the bottom of the water column, where there was brackish to semi-saline and an anoxic environment. Although multiple parameters, including moderate TOM input, moderate to high paleoproductivity conditions, and an anoxic environment, determined the burial of organic carbon, paleoproductivity controlled the source rock formation in this period. Shales with a high content of TOC values and better oil potential could be formed in this environment (Figure 8a). During the Eh32 deposition period, the paleoproductivity was moderate, which was greatly diluted by the high TDM supply (Figure 8b). The bottom water was semi-saline to hypersaline and anoxic, which was beneficial to organic matter accumulation. Although TOM input was high, paleoproductivity was the main factor controlling the burial of organic carbon. Shales with medium TOC values and oil potential could be formed in this environment (Figure 8b).
water did not enhance the anoxic environment. Shales with medium TOC values and oil potential could be formed in this environment.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-18661856596. E-mail: lushuangfang@upc. edu.cn. ORCID
Wenhao Li: 0000-0001-5916-4163 Shuangfang Lu: 0000-0002-0758-4075 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (41402122), the Research Project Funded by SINOPEC (P15028), and the Fundamental Research Funds for the Central Universities (15CX05046A).
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5. CONCLUSION (1) Shales from the Eh32 and Eh33 depositions display widely variable TOC contents and oil potential. During Eh 3 3 deposition, the paleoproductivity was moderate to high, with moderate TOM input and moderate to high TDM input. However, during Eh32 deposition, the paleoproductivity was moderate, with high TOM and TDM inputs. The redox conditions were anoxic during the two source rock intervals. (2) Paleoproductivity controls the burial of organic carbon in the shales during Eh32 and Eh33 depositions, while anoxia did not trigger intense organic carbon deposition. The increased TOM input during Eh32 did not promote the burial of organic carbon, but the enhanced TDM input had a larger dilution effect on paleoproductivity in this period. (3) During Eh33 deposition, algal organic matter could be accumulated as a result of the brackish to semi-saline bottom water and anoxic environment. Shales with a high content of TOC values and better oil potential could be formed in this environment. In contrast, during Eh32 deposition, the organic matter originated from both algae and TOM and the semi-saline to hypersaline bottom 13526
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