Introduction
In 2014, Changqing Oilfield Company discovered China’s first 100-million-ton tight oilfield in the Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin, NW China, a milestone in the tight oil exploration of the basin. Restoring the paleo-sedimentary environment and evolution characteristics of the Chang 7 Member is very important for analyzing the relationship between the evolution of the paleo-sedimentary environment and the formation of organic-rich shale. Many research methods can be used to restore paleo-sedimentary environment, among which, geochemical quantitative analysis of major elements and trace elements is the most commonly used method, and has been widely used in many basins[1,2,3,4,5,6,7]. Mg/Ca ratio method[8,9,10], MgO/CaO ratio method[10], CaO/ MgO•Al2O3 ratio method[11,12,13,14] and Sr/Cu ratio method[3, 8] are usually used for comprehensive identification of the paleo-climate. Sr/Ba ratio method[7-8, 15-17], Rb/K ratio method[16], MgO/Al2O3 ratio method[8, 15], B element method[1, 18-19], V/Ni ratio method[10], Th/U ratio method[10] and B/Ga ratio method[20] are commonly used for the paleo-salinity restoration. The elements sensitive to redox are often selected to analyze the paleo-redox conditions[21], such as V/Cr ratio[3, 21-22], Ni/Co ratio[3, 21], V/(V+Ni) ratio[23,24,25], Fe2+/Fe3+ ratio[8], U/Th ratio[21], Ce/Ce* ratio[26,27], Eu/Eu* ratio[26,27] and (Cu+Mo)/Zn ratio[28,29].
Based on comprehensive review of the restoration methods of paleo-sedimentary environment, 160 samples from the 19 outcrop sections of the Yanchang Formation around the Ordos Basin and 129 samples from 25 wells (Fig. 1 for the sections and well locations), totaling 289 were collected systematically. Among them, 243 samples were from the Chang 7 Member target reservoir, including 80, 79 and 84 samples from the Chang 71 Submember, the Chang 72 Submember and the Chang 73 Submember respectively, and a total of 46 samples from Chang 9-Chang 8 Member, Chang 6-Chang 1 Member of Yanchang Formation and other layers. Geochemical elemental test and analysis were carried out on the samples basically collected from the whole basin, and suitable methods such as the ratio of constant and trace elements were selected to restore the paleoenvironment in the sedimentary period of Chang 7 Member. Based on the analysis and test data of a large number of samples from the basin margin and intrabasin, the mudstone constant and trace elements were mainly used to qualitatively or semi-quantitatively determine the paleo-climate, paleo-salinity and paleo-redox conditions in the sedimentary period of Chang 7 Member across the whole basin, so as to more systematically restore the paleo-sedimentary environment there. At the same time, the change pattern of sedimentary environment in Chang 7 Member was examined in depth, to understand the influence of environment evolution on the development of organic-rich shale.
Fig. 1.
Fig. 1.
Structure division of and source rocks distribution in the Ordos Basin.
1. Sedimentary background
The Ordos Basin in Late Triassic was a large-scale terrestrial depression lake basin subsiding continuously, where alluvial fans, rivers, deltas, lakes and other sedimentary facies developed. The deposits in this period are a set of terrigenous clastic rock 1 000 to 1 500 m thick, in which the Yanchang Formation is well developed, and can be divided into 10 members from Chang 1 to Chang 10 from top to bottom. Among them, the Chang 7 Member is subdivided into three submembers from top to bottom, namely Chang 71, Chang 72 and Chang 73.
In the Late Triassic, Ordos Lake Basin experienced an entire evolution process of emergence, development, prosperity, decline and extinction[30]. The depositional period of Chang 7 Member of Yanchang Formation was the prosperity period of basin development, when the lake was largest, and the source rocks of semi-deep lake and deep deposited widely in the southwest of the basin (Fig. 1), with an area of about 5.6×104 km2. A set of mud-shale rich in organic matter deposited in the middle-southwest of the lake basin, which laid the hydrocarbon source foundation for the large-area low permeability- extra low permeability reservoirs and even tight oil in the Chang 7 Member.
2. Paleo-climate identification
The element enrichment characteristics of sediments in different climatic environments are different. According to the sedimentary environment characteristics of each element, the major elements such as CaO, MgO, Al2O3, K2O and other trace elements such as Sr, Ba and Rb were analyzed, and CaO/MgO•Al2O3 ratio method, Sr/Cu ratio method and Rb/Sr ratio method were adopted in combination with the previous studies on palynological assemblage characteristics to examine the paleo-climatic characteristics of the sedimentary period of the Chang 7 Member.
2.1. Identification of paleo-climate by CaO/MgO•Al2O3 ratio method
Carbonate sediment records mainly reflect the original sedimentary conditions of the lake and are basically unaffected by early diagenesis. For lakes with stable terrestrial debris input, the sedimentation of endogenous calcium carbonate directly affects the carbonate content of sediments, which contains many climate change information[11], and the sedimentary amount of calcium carbonate is proportional to temperature[12]. The carbonate minerals in lake sediments are mainly dolomite and calcite, and the CaO/MgO basically reflects the change in the ratio of calcite and dolomite in sediments[13,14]. The dolomite in freshwater lakes is mainly derived from terrestrial debris, which is generally not endogenous, while calcite comes from two sources, the exogenous calcite brought from the terrestrial source and the endogenous calcium carbonate depositing in the lake[13,14], and the change of the terrestrial debris input can be corrected by the Al2O3 content[14]. Therefore, for the sedimentary lake of Chang 7 Member with stable terrestrial debris input, the CaO/MgO• Al2O3 ratio can sensitively reflect the relative level of endogenous carbonate content, and has the significance of indicating temperature change, with high value indicating warm period and low value indicating relatively cold period[12, 14].
The average CaO/MgO•Al2O3 value of Chang 7 Member is 0.112, which is lower than the 0.139 of Chang 9-Chang 8 Member, 0.148 of Chang 6-Chang 1 Member, and 0.117 of the entire Yanchang Formation (Table 1), reflecting the temperature during Chang 7 Member sedimentation at the developing peak of lake basin was slightly lower than that of the other sedimentary periods in the Yanchang Formation, which is in good agreement with the lake basin evolution of the middle-late Triassic and climate change in the Ordos Basin[31,32,33,34], that is in the early stage of lake basin formation, the climate was dry and arid, during the developing peak of lake basin in Chang 7 Member sedimentation, there was abundant rainfall, a large lake area developed, and the climate was warm and humid, and then in the shrinking period of lake basin, the temperature rose somewhat.
Table 1 Statistics on geochemical elements and element ratios of the Yanchang Formation in the Ordos Basin.
| Horizon | B/10-6 | CaO/MgO•Al2O3 | Sr/Ba | V/(V+Ni) | Th/U | Sr/Cu | Rb/K2O | Rb/Sr |
|---|---|---|---|---|---|---|---|---|
| Chang 9- Chang 8 Member | 8.1-74.3 34.13(30) | 0.057-0.295 0.139(30) | 0.21-1.94 0.44(30) | 0.56-0.91 0.76(30) | 1.34-14.54 6.38(30) | 2.52-25.24 7.09(30) | 2.86-6.21 4.18(29) | 0.07-1.66 0.62(30) |
| Chang 73 Submember | 3.1-87.6 33.25(82) | 0.001-0.514 0.101(82) | 0.06-0.94 0.38(80) | 0.51-0.97 0.79(82) | 0.28-12.38 4.48(52) | 0.88-29.37 8.19(50) | 2.52-7.37 4.41(81) | 0.01-6.72 0.69(83) |
| Chang 72 Submember | 3.05-78.9 32.10(76) | 0.002-0.313 0.112(76) | 0.07-0.88 0.34(79) | 0.68-0.97 0.78(76) | 0.28-9.77 5.26(61) | 1.75-28.22 7.20(61) | 2.52-7.57 4.30(76) | 0.12-2.15 0.63(76) |
| Chang 71 Submember | 7.25-86.4 32.67(76) | 0.027-0.239 0.125(77) | 0.13-0.91 0.33(80) | 0.59-0.91 0.77(76) | 0.96-15.67 6.10(62) | 2.54-26.70 6.71(62) | 1.70-7.11 4.11(77) | 0.07-2.28 0.70(77) |
| Chang 7 Member | 3.05-87.6 32.69(234) | 0.001-0.514 0.112(235) | 0.06-0.94 0.35(239) | 0.51-0.97 0.78(234) | 0.28-15.57 5.33(175) | 0.88-29.37 7.31(174) | 1.70-7.57 4.28(234) | 0.01-6.72 0.67(236) |
| Chang 6- Chang 1 Member | 9.95-104 46.29(16) | 0.067-0.213 0.148(16) | 0.19-0.73 0.38(16) | 0.70-0.89 0.75(16) | 3.11-9.21 6.43(16) | 2.17-23.78 7.44(16) | 3.28-5.87 4.42(16) | 0.25-2.23 0.68(16) |
| Yanchang Formation | 3.05-104 33.62(280) | 0.001-0.514 0.117(282) | 0.06-1.94 0.36(285) | 0.51-0.97 0.78(280) | 0.28-15.67 5.55(221) | 0.88-29.37 7.24(220) | 1.70-7.57 4.27(280) | 0.01-6.72 0.66(283) |
Note: The numerator in the table is the minimum to maximum value, the denominator is the average value, and the value in parentheses is the sample number.
In order to study the paleo-climate changes in various sedimentary periods of the Chang 7 Member in longitudinal direction, the CaO/MgO•Al2O3 ratio of 122 samples from outcrop sections and 121 core samples taken from the Chang 7 Member in wells of the basin (243 samples in total) were analyzed and compared by submember. The mean value of Chang 73 Submember, Chang 72 Submember and Chang 71 Submember are 0.101, 0.112, 0.125 respectively (Table 1), and the value of Chang 71 Submember is the highest, that of Chang 72 Submember in the second place, that of the Chang 73 Submember is the lowest. The low-values of CaO/MgO• Al2O3 in Chang 73 Submember are more than that in Chang 72 Submember and Chang 71 Submember, and the ratio has a slight increase trend from the Chang 73 Submember to the Chang 71 Submember (Fig. 2). In order to analyze the difference of paleo-sedimentary environment between the basin margin and intrabasin, distribution boxplots of the elemental-element ratios of samples from basin margin and basin of the three submembers of Chang 7 Member were compiled (Fig. 3). On the CaO/MgO•Al2O3 boxplots, both the median of the samples from the basin margin and the intrabasin increase from Chang 73 Submember to Chang 71 Submember (Fig. 3), reflecting the temperature gradually rose from the early to the late stage of sedimentation. The median value of the basin margin samples of Chang 73 Submember and Chang 72 Submember are smaller than that in the basin, while that is basically the same of Chang 71 Submember in the basin margin and basin. The results show that there were some differences in temperature in different regions, but the differences were small. In summary, the temperature in sedimentary period of Chang 73 Submember with the largest water body and widest water area was lower than that in Chang 72 Submember and Chang 71 Submember, and the paleo-climate of the basin margin was slightly lower than that in the basin, and the temperature during the sedimentary period of Chang 7 Member rose slightly from the early stage to the late stage.
Fig. 2.
Fig. 2.
Variation characteristics of the indices of paleo-climate and paleo-redox during the deposition of Chang 7 Member in the Ordos Basin.
Fig. 3.
Fig. 3.
The boxplot of geochemical elements and element ratio of the Chang 7 Member in the Ordos Basin.
Meanwhile, in order to analyze the difference between different areas and in longitudinal direction, a well Z22 (Fig. 1 for well location) with cores taken from the entire Chang 7 Member of deep-water area of lake basin was selected for systematic sampling analysis, 27 samples were taken for geochemical elements analysis at even interval from the bottom of Chang 6 Member to the top of Chang 8 Member, with a sampling length of 125 m. In general, the CaO/MgO•Al2O3 value of Chang 7 Member samples are between 0.010 and 0.174, and the average value of the Chang 73 Submember, Chang 72 Submember and Chang 71 Submember are 0.062, 0.068 and 0.072, respectively, showing an increase trend (Fig. 4), which is consistent with the above analyzed variation trend of CaO/MgO•Al2O3 values of the Chang 7 Member in the whole basin. It suggests that during the sedimentary period of the Chang 7 Member, as the development peak of the early lake basin gradually changed to the late shrinking stage, the temperature shows a slight rising trend.
Fig. 4.
Fig. 4.
Paleo-climate indexes of the Chang 7 Member of the Yanchang Formation in Well Z22 of the Ordos Basin.
Previous research data shows that the CaO/MgO•Al2O3 values of sediments in Hongfenghu of Guizhou Province ranged between 0.043 and 0.104 under the average temperature of 15 °C from 1960 to 1990, with average value of 0.075[12,13,14]. In comparison, the average values of CaO/MgO•Al2O3 in the Chang 7 Member of the Ordos Basin was 0.112 and 0.117 in the Yanchang Formation, which were both higher than those in Hongfenghu. Therefore, it is inferred that the annual average paleo-climate during the sedimentary period in Yanchang Formation was higher than 15 °C in Hongfenghu, thus confirming that the average annual paleo-climate of the Chang 7 Member was greater than 15 °C.
2.2. Identification of paleo-climate by Sr/Cu and Rb/Sr ratio method
The content of trace elements in sediments is affected by paleo-climate, and the enrichment of trace elements in different climatic conditions is obviously different. Therefore, the ratio of wet-type elements such as Rb, Cu and dry-type elements such as Sr are often used to analyze the paleo-climate characteristics[3, 8], Sr/Cu and Rb/Sr are widely used to restore paleo-climate. Research shows that Sr/Cu less than or equal to 10 indicates warm and humid climate, and Sr/Cu greater than 10 indicates dry and hot climate[11]. The Sr/Cu of the Yanchang Formation samples are mainly distributed between 1 and 10, with an average of 7.24 (Table 1). The Sr/Cu of the Chang 7 Member is mainly less than 10 (Fig. 2), with an average of 7.31. The median of the Sr/Cu boxplots is mainly distributed between 3 and 5 (Fig. 3), which is smaller than the boundary between warm climate and dry-hot climate, indicating that the climate during sedimentary period of Chang 7 Member was warm and humid.
Rb is relatively stable in weathering, while Sr is more likely to lose due to leaching[35]. When the climate is warm, the precipitation is more plentiful and the weathering is stronger, Sr is likely to lose, and the Rb/Sr value increases. When the climate is dry, the precipitation is less, the weathering intensity is relatively lower, and more Sr remains in the parent rock, so the Rb/Sr would be lower[35]. In other words, high Rb/Sr indicates humid climate and low value indicates dry climate. The Rb/Sr of the Yanchang Formation is mainly between 0.4 and 1.5, with an average of 0.66. The Rb/Sr of the Chang 7 Member is mainly distributed between 0.2 and 1.2 (Fig. 2), with an average of 0.67 (Table 1), and the median is mainly distributed between 0.4 and 0.7 (Fig. 3). In general, the Rb/Sr value of the Yanchang Formation is higher than that of the Jurassic in the northern part of the Ordos Basin of 0.15-0.30 during transition from warm climate to the dry-hot climate[5], which reveals that the paleo-climate of sedimentary period in Yanchang Formation and Chang 7 Member were primarily warm and humid.
Comparing the vertical distribution curves of Sr/Cu and Rb/Sr, they are basically mirror to each other, with the high Rb/Sr point corresponding to the low Sr/Cu point, so corroborating each other, they indicate that the paleo-climate of sedimentary period of Chang 7 Member was mainly warm and humid.
Many researchers have examined the paleo-climatic features of the sedimentary period of Chang 7 Member by using palynological assemblage features. Ji Liming et al. analyzed the palynological assemblages represented by Aratisporites- Punctatisporites and Asseretospora-Walchiites in the Chang 8 Member and Chang 7 Member of Longdong region, and found that they reflected warm temperate-subtropical climate[32,33]. Zhou Liming et al. selected the shale samples from Chang 7 Member on the section of Yaoqu Niejiahe-Xiaoqiaohe of Yao County, which was located in the semi-deep lake-deep lake sedimentary area of the southeastern basin, and analyzed the characteristics of palynological assemblage, they found the palynological assemblage was dominated by Dictyophyllidites-Punctatisporites-Taeniaesorites, and thus speculated that the climate of this area in the sedimentary period of Chang 7 Member was warm temperate-subtropical[36].
According to the above analysis of geochemical element ratios, and the paleo-climate characteristics identified by palynological assemblages by researchers, it is concluded that the climate during the deposition of the Chang 7 Member in the Ordos Basin was warm temperate-subtropical climate with temperature higher than 15 °C.
3. Paleo-water properties
In the late Triassic, the Ordos Basin was in the process of desalination. In the mid-late Triassic, the basin not only completely separated from the direct influence of seawater, but also rapidly subsided due to strong differential tectonic activities in the Indosinian, leading to the formation of a large lake. Lack of connectivity with the sea, the lake was gradually desalted by atmospheric precipitation[33]. There are many methods for studying the physical and chemical properties of paleo-water. In this study, the trace elements, Sr, Ba, B, Ga, V, Ni, Rb, Th, and U in mudstone were used to study the water properties of paleo-lake in the sedimentary period of the Yanchang Formation[37,38,39].
3.1. Paleo-salinity
Sr/Ba is an effective index to judge the salinity of paleo- lake body. Sr has higher solubility than Ba in water, so it can migrate further, and Sr/Ba can indirectly reflect terrestrial and marine sedimentations[7-8, 15-16]. According to previous studies, the Sr/Ba less than 0.5 indicates freshwater environment, while 0.5 to 1.0 indicates brackish environment, and greater than 1.0 denotes saline environment [17, 40]. The average Sr/Ba value of the Yanchang Formation, Chang 7 Member, Chang 9 Member to Chang 8 Member and Chang 6 Member and Chang 1 Member are 0.36, 0.35, 0.44 and 0.38, respectively (Table 1), all less than 0.5, indicating freshwater environment. The Sr/Ba of Chang 7 Member is mainly between 0.2 and 0.5 (Fig. 5), and the average value of Chang 73 Submember, Chang 72 Submember and Chang 71 Submember are 0.38, 0.34, and 0.33, respectively, which are relatively close. In the Sr/Ba boxplots of the two types of samples from the basin margin and the intrabasin, there is no obviously regular difference between the main distribution interval and the median value (Fig. 3), which suggests similar features of water bodies and characteristics of fresh water. Based on the Sr/Ba of the Chang 7 Member in the Ordos Basin from 0.19 to 0.69, Zhang Caili et al.[34] concluded that the whole lake during sedimentary period of Chang 7 Member was terrestrial freshwater environment.
Fig. 5.
Fig. 5.
Variation characteristics of paleo-salinity geochemical indices of the Chang 7 Member in the Ordos Basin.
Element B is often used to indicate paleo-salinity[19]. There is a linear positive correlation between B content and salinity of water. The higher the salinity of water, the more the B ions are adsorbed by sediments[19]. In general, the B content in marine environment is between 0.008% and 0.012%, while the B content in freshwater environment is less than 0.0060%.
The B content of the Yanchang Formation is 0.0034% on average (Table 1), and is mainly distributed between 0.0010% and 0.0050% in Chang 7 Member (Fig. 5), with an average of 0.0033%. The median of the samples in the boxplot of B content are similar, all less than 0.0040% (Fig. 3), indicating that the water body in the sedimentary period of Chang 7 Member was fresh water.
Rb/K2O is also commonly used to judge the paleo-salinity of the sedimentary area. Since the values of Rb and K are not in the same order of magnitude, the Rb content should be amplified by 1 000 times when calculating Rb/K2O: the value of more than 6 represents saline sedimentary environment, 4-6 brackish sedimentary environment, and less than 4 freshwater sedimentary environment[16]. The Rb/K2O value of Yanchang Formation is 4.27 on average (Table 1), and that of the Chang 7 Member is mainly distributed between 2 and 6 (Fig. 5), with an average of 4.28. The distribution range and median of the samples in the boxplot are distributed in the range of freshwater and brackish environments (Fig. 3). In the diagram of Rb/K2O and Sr/Ba, most sample points fall in the freshwater-brackish area, and some sample points are in the brackish area (Fig. 6), generally suggesting freshwater-brackish environment of Chang 7 Member.
Fig. 6.
Fig. 6.
Identification diagram of paleo-salinity of the Chang 7 Member in the Ordos Basin.
The Th/U can be used as an index to distinguish marine and terrestrial deposits. Generally, the Th/U greater than 7 indicates terrestrial freshwater environment, 2 to 7 brackish sedimentary environment, and less than 2 marine saline environment[41]. The Th/U of Chang 7 Member is distributed between 2-8, with an average of 5.33 (Table 1), and mainly in the interval greater than 2 (Fig. 5). The average value of Chang 73 Submember, Chang 72 Submember and Chang 71 Submember are 4.48, 5.26, and 6.10, respectively, so it is speculated that the Chang 7 Member was deposited in the sedimentary environment of terrestrial freshwater-brackish. In the boxplot, the Th/U is mainly distributed in the brackish region (Fig. 3), and the distribution interval and median value of the samples of the Chang 73 Submember and the Chang 72 Submember from the intrabasin are bigger than those from basin margin, reflecting slightly higher salinity of the basin margin water. in the diagrams of Th/U and Sr/Ba, the sample points are mainly distributed in the brackish area, followed by the freshwater-brackish area (Fig. 6). Therefore, it is inferred that the water body in the sedimentary period of the Chang 7 Member was terrestrial brackish.
Based on the analysis of Sr/Ba, B, Rb/K2O, Th/U and other methods, combined with the freshwater environment of paleo- lake sedimentary environment restored by Zhang Wenzheng et al.[41,42], it is comprehensively determined that the lake water was terrestrial brackish-freshwater during deposition of Chang 7 Member.
3.2. Paleo-redox environment
To tell redox property of sedimentary environment, it is necessary to analyze the elements sensitive to redox, and it is generally considered that the major elements, V, Cd, Cr, Co, Cu, U and trace elements, Zn, Fe and Cu are more sensitive to environmental redox property[21]. Under redox conditions, V in the water can be deposited in the form of more effective organic complex than Ni[24], and the V/(V+Ni) is widely used to distinguish redox of the sedimentary environment[23].
Through probing into the paleo-sedimentary environment of the Upper Jurassic dark mudstones in the northwestern Europe region, Jomes et al. concluded that V/(V+Ni) greater than 0.77 indicated anoxic environment and extreme oxygen-poor environment, 0.60-0.77 oxygen-poor environment and secondary oxygen-rich environment, and less than 0.6 oxygen-rich environment[21]. Hatch et al. held that V/(V+Ni) less than 0.46 represented oxidizing environment, 0.46-0.57 weak oxidizing environment, 0.57-0.83 anoxic environment, and 0.83-1.00 oxygen-deficient environment[24]. Li Guangzhi et al. considered that V/(V+Ni) less than 0.5 indicated oxidizing environment, and greater than or equal to 0.5 reducing environment[43].
Although different researchers have different classification standards, they all believed that V/(V+Ni) gradually increased with the increase of the water reduction degree. The V/(V+Ni) of the Chang 7 Member of the Yanchang Formation is greater than 0.5 at minimum, and mainly from 0.70 to 0.85 (Fig. 2), with an average of 0.78 (Table 1), reflecting reducing environment of oxygen-poor and anoxia. In the boxplot, the distribution interval and median of V/(V+Ni) of samples from the basin margin are lower than those in the intrabasin (Fig. 3), which generally reflects that the reducibility in the intrabasin was stronger than the basin margin. Most values of Chang 73 Submember are distributed between 0.8 and 1.0 (Fig. 2), while the average V/(V+Ni) value of Chang 73 Submember, Chang 72 Submember and Chang 71 Submember are 0.79, 0.78 and 0.77, respectively, all indicating reducing environment of anoxia.
Comprehensive analysis of the difference in geochemical characteristics of the elements of the Chang 7 Member shows that under the terrestrial freshwater and reducing sedimentary environment, the sedimentary period from the Chang 73 Submember to the Chang 71 Submember featured slightly increase in temperature and slightly drop of reducibility. On the basis of the restoration of paleo-sedimentary environment, Well CH96 with cores taken from the whole Chang 7 Member located in the semi-deep-deep lacustrine sedimentary area was selected to systematically analyze the evolution characteristics of the paleo-sedimentary environment (Fig. 7). The results show that the sedimentary period of Chang 7 Member was temperate-subtropical climate with relatively high temperature, the temperature of Chang 73 Submember to Chang 71 Submember had an increase trend, and the sedimentary environment was an anoxic reducing freshwater sedimentary environment.
Fig. 7.
Fig. 7.
Composite columnar section of the paleo-sedimentary environment characteristics of the Chang 7 Member in Well CH96, Ordos Basin.
4. Geological significance of restoration of paleo-sedimentary environment
It can be seen from the above analysis that the climate during the sedimentary period of Chang 7 Member was warm temperate-subtropical climate, when the water system developed, the light was sufficient, and the provenance area was highly weathered, so more nutrients were brought into the lake basin by the water flow, creating favorable conditions for plankton growth and extremely high biological productivity. Due to abundant rainfall, the water body was large and deep, forming anoxic and strong reducing environment, which enabled the preservation of large amounts of organic matter. The warm climate could be helpful for growth of stable and prosperous vegetation, so the soil on the land would be consolidated, and fewer inorganic debris would be transported to the lake by rivers and winds, thus reducing the dilution effect of inorganic components on organic matter. The favorable climate conditions in the sedimentary period of Chang 7 Member have laid an excellent foundation for the formation of high organic carbon content and large-area thick-layer source rock. The Chang 73 Submember with abundant rainfall, slightly lower temperature and the strongest reducibility is most beneficial to the development and preservation of organic matter, so it has high abundance of organic matter, with an organic carbon content of up to about 20%, becoming the most important set of source rock in the Mesozoic.
Meanwhile, during the depositional period of Chang 73 Submember, suitable temperature, abundant precipitation, large-area freshwater lake basin and the strongest reducibility were conducive to the large-scale development, enrichment and preservation of organic matter, which created favorable conditions for enrichment of large-scale shale oil. During the sedimentary period of the Chang 72 Submember and Chang 71 Submember, the temperature increased, the evaporation became higher than before and the water body shallower. The changes in paleo-sedimentary environment led to the development of arentilla and silty sedimentary rocks in the central lake basin, so came the tight sandstone in close contact with source rock, laying foundation for the development of tight oil.
5. Conclusions
The paleo-climate during the sedimentary period of Chang 7 Member in Ordos Basin was warm temperate-subtropical climate with the temperature higher than 15 °C. The water body during the sedimentary period was terrestrial brackish water-freshwater body, and the sediments were formed under strong reduction conditions. Suitable temperature, large-area deep lake basin and strong reducing paleo-sedimentary environment lead to the large-scale development and enrichment and preservation of organic matter in the Chang 73 Submember, as a result, not only a set of high-quality source rock was formed, but also foundation for the enrichment of large-scale shale oil was laid.
Reference
Geochemical characteristics of the Jurassic Yan’an and Zhiluo Formations in the northern margin of Ordos Basin and their paleoenvironmental implications
Geochemical characteristics and environmental implications of trace elements of Zhifang Formation in Ordos Basin
Application of microelements analysis in identifying sedimentary environment: Taking Qianjiang Formation in the Jianghan Basin as an example
DOI:10.1016/S1872-5813(07)60034-6
URL
[Cited within: 5]
The compositions of microelements in clay minerals,mud rock and gypsum in lake sediments are very sensitive to the change of sedimentary environment.It may reveal lake-level change.The lithology change of sediment in saline lake in vertical is distinct,this change is controled by the balance in supply,evaporation and sediment of the river,which lead to the change of lake level.The paper mainly introduces using microelements,B,S,Ba,Ti,Fe,P,Mn etc.,in the Qianjiang Sag of the Jianghan Basin as an indication for sedimentary environment change to analyse the characteristics of change in different sequence and different systems tract,and discusses paleoclimate and sedimentary environment in different sequence.The content of B in sedimentary rock is more then in igneous rock commonly.The content of B is 135×10-6 in shale and(1～40)×10-6 in sand rock.The content of Sr is 0.8‰～1‰ in saline lake and 0.1‰～0.3‰ in freshwater.
The relationship of Late Jurassic paleoenvironment and paleoclimate with geochemical elements in Amdo Country of northern Tibet
Amdo Country of northern Tibet is close to south boundary of Qiangtang Basin and belongs to Qiangtang stratigraphic area.The late Jurassic Shamuluo Formation of the study area is mainly composed of a series of neritic clastic rock and carbonate deposits,which are in turn weathering crust sediments,tidal flat facies sediments and mixed platform facies sediments,characterized especially by a lot of organic reefs.Based on analyzing 18 samples from Shamuluo Formation,combined with the sedimentary facies analysis,the authors investigated the ancient sea- level eustacy,oxidation- reduction conditions and paleochmate of late Jurassic period in the study area.Specifically,the changes of Ni,Sr,Cu,V,Cr,Ni/Co and carbon isotopes were used to detect the changes of paleoenvironment,and the fluctuation of Mn,Na,Sr/Cu,P,Ti,oxygen isotopes and Z values were employed to deduce paleochmate.The results show that paleoenvironment evolutions experienced three stages:(1) strong oxidation environment of weathering crust;(2) semi- oxidation- reduction environment of tidal flat;(3) three times of changes between oxidation and reduction environment in platform,corresponding to the three times of reef formation;paleochmate evolutions also experienced three stages:(1) arid climate of weathering crust;(2) semi-arid and semi-humid climate of tidal flat;(3) three times of changes between arid and humid climate in platform,respectively corresponding to the three reef-forming periods.
Element geochemical characteristics of the Jurassic mudstones in the northern Ordos Basin: Implications for tracing sediment sources and paleoenvironment restoration
The geochemical characteristics of Jurassic mudstones in the Northern Ordos Basin Hangjinqi Area recorded important geological information at that time. Based on the method of X-ray fluorescence spectrometry of the major element analysis and ICP-MS trace element and rare earth element analysis,the tectonic setting and provenance attribute of Zhiluo Formation and Yan'an Formation have been comprehensively analyzed. Meanwhile,we restored the evolution of sedimentary setting by the vertical variation characteristics of geochemical parameters.The main conclusions can be drawn as follows: The Jurassic sedimentary rocks in the Northern Ordos Basin have affinities to the Precambrian metamorphotic rocks from old basement,such as gneiss,granulite,khondalite and the intrusive rock which formed in different geological time,so the provenance of the study area mainly came from it.The tectonic setting of source area is the active continental margin associated with the continental island arc. The result of paleoenvironment reconstruction is based on the vertical variation characteristics of mudstone geochemical indexes such as Sr/Cu,Rb/Sr,CIA,Sr/Ba,V/( V+Ni) and Ceanomshows that from Yan'an period to early Zhiluo period and then to late Zhiluo period,the paleoclimate was warm and humid at the beginning and tended to become increasingly dry and hot,the palaeosalinity transformed from the brackish water phase of the fresh water environment to the brackish-water phase of the fresh water environment,the redox condition belong to the reducing environment and the water column stratification is not obvious.
Relationships between wetness and maturity of coal-derived gas in China
DOI:10.1016/S1876-3804(16)30088-X
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Owing to its limited homologous, simple structure, small molecular weight and radius, natural gas could diffuse easily and migrate in a long distance, which greatly increases the difficulties of the study of source rock maturity. So in this paper we study the relationship between natural gas wetness and source rock maturity. Based on 49 coal-derived gas samples from the Ordos, Sichuan, Bohai Bay, Qiongdongnan, Junggar and Turpan鈥揌ami basins in China with integral molecular series, it is discovered that the wetness of natural gas decreases with the maturity (Ro) of source rock. The negative correlation between wetness and maturity was established. Since the wetness could be easily acquired, the Rocould be quickly calculated based on the relationship, which provides important basis for gas source definition and resource evaluation.
Compaction and hydrocarbon accumulation of Triassic Yanchang Formation Chang 8 Member, Ordos Basin, NW China: Evidences from geochemistry and fluid inclusions
DOI:10.1016/S1876-3804(17)30007-1
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Crushing, acid treatment and step wise separation and oil extraction were employed to obtain the different occurrence state hydrocarbons. All these fractions have been analyzed by Gas Chromatography-Mass Spectrometer (GC-MS). The fractions relationship and related oil charging process can be mirrored through the analysis of fractions weight content and geochemical characteristics, in combination with the research of inclusions homogenization temperature and fluorescence spectrum parameters. Experimental results reveal that there are four state hydrocarbons, i.e. free hydrocarbon, sealed hydrocarbon, hydrocarbon in carbonate cement, and hydrocarbon within inclusions caught by quartz grains and feldspar grains in the oil-rich sandstones of Chang8 Member, Triassic Yanchang Formation in the Ordos Basin. Among them, the overwhelming fraction is the free hydrocarbon, averaged 93.4%. Fluorescence spectrum parameters ofγmax,QF535andQ650/500show that the crude oil maturity of the inclusions imprisoned in feldspar, quartz, and carbonate cement increase in turn, and the parameter values of the inclusions in feldspar and quartz are similar and much different from those of carbonate cement. Through analysis of C29ββ/(αα+ββ) and C2920S/(20S+20R), together with methylphenanthrene ratio, it is revealed that thermal-evolutionary degree of the hydrocarbon within inclusions, hydrocarbon in carbonate cement, sealed hydrocarbon, free hydrocarbon reflects an upward trend, and the data of the last two type are similar. Integrated study of diagenetic sequence and thermal evolutionary degree suggest that the Chang 8 sandstones had been compacted before reservoir formation and the reservoirs have experienced three phase of charge events in which the third one played the most important role for reservoir formation.
Geochemical indicators of sedimentary environments: A summary
DOI:10.1016/S1671-2927(11)60313-1
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With the use of geochemical methods,by studying the characteristics of major and trace elements and element isotopic characteristics,the ancient sedimentary environments can be rebuilt,so as to define their sedimentary facies.Marine facies or continental facies and their paleosalinities can be determined and detected in terms of Sr/Ba ratios,borium contents-O isotopes and so on;the contents of specific elements(such as P,Sr) and element ratios(such as Sr/Cu,Mg/Ca) under particular climate conditions are the indicators of that climate;redox sensitive elements(such as Mo,U,V,Ni,Ce and Eu) are the best recorders of redox conditions;because of differences in element migration ability,the determination of water depths and the fluctuations of sea level can be made from element associations(such as Fe group,Mn group),element ratios(such as Sr/Ba,Sr/Ca) and isotopes(87Sr/86Sr).Thus,we can also identify their provenances,source rocks' lithologies and tectonic settings,as well as hydrothermal deposition or not.
The whole-rock cerium anomaly: A potential indicator of eustatic sea-level changes in shales of anoxic facies
DOI:10.1016/0037-0738(95)00020-8
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The whole-rock cerium anomaly, tested for outer shelf-upper slope stratigraphic sections from the middle Ordovician through the lower Silurian of Scotland, is proposed as an empirical technique to develop a eustatic 3rd-order or finer-scale sea-level curve. This interval was chosen as it straddles the well-documented Late Ordovician glaciation and can be defined by graptolite zones. The anomaly is calculated from neutron activation analysis of low-carbonate, phosphate-free, fossil-free field-identified shales of the graptolite facies by comparison of the normalized cerium content with the linearized trend of the normalized composition of other rare earth elements in order of atomic number. For sections originally deposited in the main pycnocline below the surface mixed layer, values of the anomaly for a given sample would indicate its position on the redox curves developed for the early Paleozoic by Wilde (1987). Changes in the anomaly that are positive with time would indicate a lowering of sea level as the apparent depth on the redox curve would reflect more oxic conditions. Relative changes negative with time would indicate a rise in sea level as the apparent depth reflects more anoxic conditions. Depending on the vertical sample spacing and the time interval sampled, resolution of the order of 1 m.y. might be achieved. Thus the Vail et al. (1977) curves of the 3rd order (1 to 10 m.y.) or of finer scale could be obtained by this technique with the proper choice of section. Accordingly, for the early through middle Paleozoic when the main pycnocline was anoxic, this geochemical technique could be used to develop eustatic sea-level curves and additionally offer an independent calibration for seismic stratigraphy as well as an indicator of glacial-interglacial climatic sequences or eustatic changes due to fluctuations in global ridge crest volumes.
Geochemical characteristics and paleoenvironmental analysis of dark fine grained rocks of Wawukuang and Shuinan formations in Jiaolai Basin
The strata of Wawukuang and Shuinan Formations in Laiyang Group,Jiaolai Basin are mainly composed of shale and sandstone with some limestone.This paper conducted the element geochemistry test of the rock samples in the study area by using WD-XRF,ICP-MS and other instrument and discussed the palaeoenvironment of Wawukuang and Shuinan Formations based on the content of major,trace and rare earth element and the ratio of related elements in the samples.After research,it came to the following conclusions:The contents of SiO2,MgO and CaO are relatively higher in Wawukuang and Shuinan Formations,and the content of Al2 O3 in some samples of Shuinan Formation amounts to more than 10%.Both the sets of strata show the obvious enrichment of light rare earth elements.By using a series of environmental criteria such as Sr/Ba,Rb/K,V/Ni,Th/U,U/Th,Eu/Eu*,Ce/Ce*,and Sr/Cu,and by combining with previous studies and field section characteristics,it is found that the strata of both Wawukuang and Shuinan Formations show obvious marine sedimentary characteristics formed in dry,hot and oxidized environments.
Recent temperature records of annually laminated sedimentsin Hongfeng lake, Guizhou
Lake sedimentation is a large pool for past global change. Carbonate geochemistry as an index has interested many scien-tists , and can provide a variety of high-solution information on short-or long-term climatic or environmental changes.Through detail comparison of profile of carbonate concentration and CaO/MgO A12O3 ratio in the core which is calibrated according to mass-depth from varve-counting dating with recent annual average temperature data we find that the agreement between the two cuvyes is astonishingly good, meanwhile, average molal ratio of CaO and CO2 in the sediments is nearly 1:1, this suggests that almost all carbonate minerals are calcite, which contains many climatic and hydro chemical information. They probably refect the relationship between calcite precipitation and climate change.According to calcite phychemical equibrium of pore water at the sediment-water interface during early diagensis process, we calcuate the Fd (carborate loss flux due to dissolution and upward diffusion at the interface) and Fs (calcite sedimentary flux). We define r as carborate perturbative extent at the interface:r = Fd/Fs (1)r is about 17%, we, therefore, can conclude that calcite unsaturated dissolution and perturbative process at the sediment-wa-ter interface is very small, much smaller than we imagine before.The paper finally suggests that carbonate concentration and CaO/MgO Al2O3 ratio can be used as a high-solution temper-ature index in recent lake sediments.
Chemical elements in sediments of Lake Erhai and palaeoclimate evoluton
Sediment particle size distribution and its environmental significance in Lake Erhai, Yunnan province
DOI:10.1007/BF03052905
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A closed or semi-closed plateau lake, whose sediment records can provide us with plenty of fine and high resolution information, is a sensitive indicator of climatic and environmental changes. During the reconstruction of various short-time-scale climatic and environmental changes, the geochemical records in plateau-lake sediments are superior to other natural files. Based on fine dissection of the vertical profile of sediment particle sizes, this paper reveals the quasi-periodical changes of sediment particle sizes, which indicates the quasi-periodical fluctuations of the regional climate. A synthetic analysis of multiple indexes shows that sediment particle size is a more sensitive and more effective index of climatic and environmental changes than other geochemical indexes. High content of >20 m sediment particles and low content of 2 10 m sediment particles indicate a warm-dry climate and conversely a cold-humid climate, and their ratio can be used as an effective index of climatic changes. The basic climate succession in the region of Lake Erhai is characterized as being alternatively warm-dry and cold-humid and it has been developing into a warm-dry climate as a whole. There exist at least 2 time-scale quasi-periodical fluctuations of the regional climate in Lake Erhai. At present, the region of Lake Erhai is at the end of the warm-dry period and at the beginning of a cold-humid period, so the temperature will go down and the water level will rise.
Geochemical characteristics and environmental significance of Dengying formation of Upper Sinian in Qiannan Depression
DOI:10.1016/S1003-9953(10)60145-4
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The dolomite deposited in the Dengying Formation of Upper Sinian in the Qiannan Depression.Based on the actual measurement the Yangtiaozhai section,Majiang,we used the geochemical analyses to discuss the sedimentary geochemistry characteristics and the palaeosedimentary environment significance of Deng-ying Formations.The results indicated:(1)CaO and MgO remarkably positively correlated with each other;SiO2 negatively correlated with CaO and MgO;Sr negatively correlated with MgO,and positively correlated with Al2O3,K2O,Na2O,TiO2,P2O5,TFe;V,Al2O3,K2O,TiO2,TFe and P2O5 positively correlated with each other;Mn weakly positively correlated with TFe.(2) The content of Sr and the value of 1,000 Sr/Ca increased with the depth of sedimentary water,regarded as the mark of the palaeobathymetric and succession boundary.(3)The analytic results of the ratios of Sr/Ba,MgO/A12O3,MgO/CaO and V/(V+Ni) revealed that the palaeosedimentary environments of Dengying Formation is ocean sediments which was formed in reducing environment of continental sea in hot arid climate,and with the detrital material of terrigenous origin.Especially,the second member of Dengying Formation was formed in the deepest water environment and the paleosalinity of the fifth member of Dengying Formation is the highest;The geochemical characteristics and the sedimentary environment are closely correlated,and the geochemical characteristics can be regarded as an important mark in the sedimentary phase analyses.
Depositional environment analysis of Shanxi formation in Eastern Ordos Basin
The depositional environment was deeply analyzed in this paper by geochemical proxies,stratum and fossil distribution,and original sedimentary tectonics.The geochemical proxies in mudstone implied that tide and transgression reacted with the fresh water and mixed each other,concretely speaking,Element B implied the characteristics of long-term cycle of basic level,and Sr/Ba is a good proxy of super long-term cycle in the period of Shanxi in Eastern Ordos Basin.The sedimentary tectonics and fossil distribution indicated that the middle-level tide has directly influenced the delta in southeastern part of the area in the early period of Shanxi,gradually the delta progradation become strong and tide evacuated to the southeastern fringe of the basin in the middle and late period.What's more,the stratum distribution and lithofacies also illuminated that the stratum in Eastern Ordos Basin and West Shanxi province belongs to the same delta-sedimentary system,and the underwater dome in the east part of the area is not efficient to obstruct the flow to form the lake.Consequently,the delta in Shanxi Formation deposited in the shallow-sea continental shelf.
Application of some geolochemical indicators in determining of sedimentary environment of the Funing group (Paleogene), Jinhu depression, Jiangsu province
Departure curves for computing paleosalinity from boron in illites and shales
Calculation of Paleosalinites from boron and clay mineral data
Comparison of geochemical indices used for the interpretation of palaeoredox conditions in ancient mudstones
DOI:10.1016/0009-2541(94)90085-X
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ABSTRACT Eight geochemical indices used for the interpretation of bottom water palaeo-oxygen concentrations for argillaceous sedimentary rocks have been calculated for a suite of Upper Jurassic mudstones drawn from the Draupne and Heather Formations of the Norwegian North Sea, and the Kimmeridge Clay Formation from onshore England.The eight indices: DOP, C/S, U/Th, authigenic uranium, V/Cr, Ni/Co, Ni/V and have been examined by means of factor analysis, to identify the most reliable. DOP, U/Th, authigenic uranium, V/Cr and Ni/Co form an internally consistent set and are recommended as the most reliable of the indices. Ni/V, C/S and convey little palaeo-oxygenation information and are not regarded as reliable.The parameters identified as reliable (DOP, Ni/Co, V/Cr, U/Th and authigenic uranium), have been calibrated against DOP, for inter-comparison purposes, and to allow their interpretation against DOP-derived depositional conditions.
Analysis of late Palaeozoic glacial to postglacial sedimentary successions in South Africa by geochemical proxies-Response to climate evolution and sedimentary environment
DOI:10.1016/j.palaeo.2006.03.059
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Records of incisive climate changes during the late Palaeozoic Era are best documented in sedimentary basins of South Africa. Glacial (Upper Carboniferous鈥揈arly Permian Dwyka Group and equivalents) and postglacial deposits (Early to Middle Permian Ecca and Beaufort Groups) are known from the Main Karoo Basin in South Africa, from the Kalahari Basin (Namibia and Botswana) and other late Palaeozoic Basins of south Gondwana. Glacial deposits comprise diamictites and tillites followed by postglacial shales with variable organic matter contents and intercalated carbonate horizons. This study covers the final glaciation phase (Upper Pennsylvanian sselian/Sakmarian) and the transition into the postglacial stage of the Middle to Late Permian. For sedimentary successions from the Karoo and eastern Kalahari Basin element geochemical proxies (Chemical Index of Alteration, Zr/Ti, Rb/K, V/Cr and organic carbon content) supported by mineralogical data record pronounced variation in sedimentary conditions. These took place in the course of glacier retreat at the Dwyka/Ecca boundary where climate and weathering processes are known to have changed. The proxy signals indicate an increase in sea level and salinity, and a shift in redox conditions from oxygenated to temporary anoxic/euxinic conditions. Due to the synchronicity of these events, distant sampling localities in the Karoo Basin can be correlated. Despite facies differences between the Eastern Kalahari and Karoo Basin, the geochemical proxies point to a comparable climatic evolution during the postglacial phase. For the Early Permian, proxy signals indicate warm umid conditions in the Karoo and Kalahari Basin. During the Middle Permian increased aridity hampered chemical weathering processes as indicated by decreasing CIA values. Results show that: (i) proxies reflect provenance, synsedimentary redox regime and late diagenetic alteration, (ii) by combination of different independent geochemical proxies primary conditions of sedimentation and climate can be deduced, (iii) the geochemical proxies applied are independent of local environments and (iv) the data reveal supra-regional or even hemispherical changes in climate and sedimentary conditions. Thus geochemical proxies can be used as reliable markers for basin wide correlations. Knowledge about synchronicity on basin scale contributes to an improved understanding of SW Gondwana's interaction with and influence on global climatic evolution.
Marine black shales: Depositional mechanisms and environments of ancient deposits
DOI:10.1146/annurev.ea.22.050194.002435
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Abstract Outlines the salient features and inferred depositional regimes for black shales in the stratigraphic record and provide guidelines for further study. Includes a discussion of OC preservation in modern marine environments as a prelude to interpretation of the ancient record, and of proposed biotic and geochemical indicators of degree of oxygenation in the water column. -from Authors
Relationship between inferred redox potential of the depositional environment and geochemistry of the Upper Pennsylvanian (Missourian) stark shale member of the dennis limestone, Wabaunsee County, Kansas, USA
DOI:10.1016/0009-2541(92)90031-Y
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Analyses of 21 samples collected from a core of the 52.8-cm-thick Stark Shale Member of the Dennis Limestone in Wabaunsee County, Kansas, demonstrate four cycles with two-orders-of-magnitude variations in contents of Cd, Mo, P, V and Zn, and order-of-magnitude variations in contents of organic carbon, Cr, Ni, Se and U. The observed variability in amounts and/or ratios of many metals and amounts and compositions of the organic matter appear related to the cause and degree of water-column stratification and the resulting absence/presence of dissolved O
Trace element characteristics and sedimentary environment of the Sinian system of the Fanjingshan area in Guizhou province
DOI:10.1007/s11442-008-0201-7
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In order to study characteristics of trace elements and evolution of sedimentary environment of Fanjingshan area in Sinian,the authors systematically collected samples from Yongyi section in upward direction.The analytical results of these samples reveal that REE display LREE enrichment and HREE depletion,Ce on the whole exhibits depletion and vertical decrease,and Eu anomaly shows three times of fluctuation,suggesting a general oxidation environment during Sinian deposition,but with local variation.Furthermore,the normalized REE distribution model of cap carbonate at the bottom of Doushantuo Formation is different from that of seep carbonate,implying that they lie in different sedimentary environments and have no genetic connection.The ratios of trace elements such as V/Cr,Ni/Co,U/Th and V/V+Ni indicate that the sedimentary environment during Sinian deposition was on the whole oxidization,but with vertical variation of both sedimentary environment and water column stratification.
REE characteristics and its geological significance of the Pemo-Carboniferous in Bohaiwan basin
The rare earth elements distribution in seawater
DOI:10.1038/296214a0
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The depth distributions of La, Ce, Nd, Sm, Eu, Gd, Dy, Er and Yb in the oceanic water column are used to evaluate the marine geochemical cycle of the rare earth elements and their application as water-mass tracers. (author)
Geochemical method for investigation of paleoredox conditions in sediments
Geochemical compositions and depositional conditions of Upper Jurassic and Lower Cretaceous Yorkshire clays, England
DOI:10.1017/S0016756800030028 URL [Cited within: 1]
Discussion on relationship between sedimentary evolution of the Triassic Yanchang Formation and the Early Indosinian Movement in Ordos Basin
The sedimentary characteristics of the lower part of the Yanchang Formation(Chang 10 hang 8 oil bearing intervals)are quite different from the middle and upper parts of the Yanchang Formation(Chang 7 hang 1 oil bearing intervals).During the deposition of the lower part of the Yanchang Formation,the western and southwestern areas of the basin were characterized by a gentle topography and the deposition was dominated by the braided fluvial,braided fluvial deltaic and coastal-shallow lacustrine facies.The content of chemically unstable components was relatively higher in clastic rocks.The depositional framework changed greatly during the deposition of Chang 7 interval of the Yanchang Formation.The sedimentary systems changed to alluvial fan and fan delta in the west and southwest.The size of the downwarping depression increased quickly and semi-deep and deep lakes covered an area of over 10 104 km2.Simultaneously,the depocenter of the Ordos Basin was shifted tens of kilometers southwestwards.The component maturity of the clastic rocks increased and the rock combination was obviously different in the west and east dispersal tracts of sediments.Furthermore,analyses of the distribution of the tuffs in the Yanchang Formation indicate that the tuffs are steadily distributed in the basal part of Chang 7 interval of the Yanchang Formation and they were derived from the syn-depositional volcanism.Their thickness decreased from the southwest to the northeast.These analyses indicate that the latest depositional period of Chang 8 interval of the Yanchang Formation is an important transitional stage in the basin's sedimentary evolution.A strong tectonic movement occurred in the western margin and southwestern margin of the basin and volcanic activities took place frequently.These events represent the first episode of the Indosinian Movement.
Jurassic spora-pollen assemblages and paleoclamate in Innermongolia, Gansu, Qinghai, China
A larger part of Gansu,the west of Innermongolia and the northeast of Qinghai,divided into Alashan,Beiqilian-Hexizoulang,Zhongqilian and Qaidam stratigraphical divisions have been related.In this region there are widely well-developed Jurassic sediments,which include abundant coal and oil natural resources.Based on fully studing Jurassic spores and pollen grains in this area,with preceding achievement,seven spora-pollen assemblages have been established.The low Jurassic includes three assemblages: Protoconiferus-Taeniaesporites,Osmundacidites-Chasmatosporites-Disaccites and Classopollis-Cyathidites-Cycadopites.The Middle Jurassic includes three assemblages: Cyathidites-Quadraeculina-Cycadopites,Cyathidites-Callialasporites-Pinuspollenites and Classopollis-Cyathidites-Inaperturopllenites.The Upper Jurassic only establishes Classopollis assemblage.Using paleoecology of mother plant of spores and pollen,combing the range of special rocks implying paleoclimate in strata,Jurassic climate has been studied,which includes Jurassic climate structure and evolution rule.It is suggested that the Jurassic climate locates the subtropical-moderate climate zone.Under this background,there is a fluctuated temperature-rising and humidity-falling events occur in late early Jurassic(Toarcian).The late middle Jurassic climate is comparatively hot and arid.This hot and arid phenomenon is so further aggravate that the whole area is hot and arid in late Jurassic.The temperature is higher in Qaidam basin and Zhongqilian area than that of other place in the early-middle early Jurassic.The humidity in Yabulai basin is smaller than that of other place in the early middle Jurassic,but it is larger than that of other place in the late middle Jurassic in Qaidam basin.All of the research region are hot and arid in late Jurassic.The early-middle Jurassic is major period of assembling and generating coal and hydrocarbon with flourishing fauna and flora,coal bed and dark mudstone in Alashan,Zhongqilian and north of Qaidam basin.It is advantage for forming oil and gas-bearing depression,which is advantaged explorating area.
Paleoclimatic characteristics during sedimentary periond of main source rocks of Yanchang Formation (Triassic) in eastern Gansu
DOI:10.1007/s11442-006-0415-5
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A great number of sporopollen fossils have been discovered in Chang 8 and Chang 7 intervals of Yanchang Formation from drilling cores of Longdong area in the southwestern Ordos Basin,and they were named as Aratisporites-Punctatisporites assemblage and Asseretospora-Walchiites assemblage,respectively.Their characteristics are similar to the sporopollen assemblages of Tongchuan Formation and Yanchang Formation discovered in the southeast of thebasin,and the geologic times of the assemblages are Ladinian of the late Middle Triassic and Carnian of the early Late Triassic.Based on the environmental distribution of main plant types at present and the analysis of the content of types to recognize the relationship between palynological flora and the characteristics of stratagraphic distribution,we can infer that the climate was warm and moist then,rainfall was plentiful,and the vegetation was luxuriantduring Middle Triassic and Late Triassic in Longdong area.The paleoclimate reflected by palynological flora was warm-wet or hot-wet in temperateto subtropical.The ecological types of palynological flora and the diversity curves of palynoflora indicate that the Chang 8 and Chang 7 phases were suitable period of continuously warm and humid in the area,and the extensive lacustrine transgression occurredin Chang 8 phrase during the development of lake as well as sustaining maximum flooding in Chang 7 phase.It was semi-humidity temperate to subtropical climate during Middle Triassic and Late Triassic in northern China.The palynological flora discovered from the Chang 8 and Chang 7 intervals in Longdong area is obviously characteristic of the northern China flora. However,becausethearea was close to the large-scale lake at that time,and the Chang 7 and Chang 8 phases werethe great prosperous period that lake wasbeing expanded,so the characteristics of moister paleoenvironment was indicated.It was just because of long-term stable warm and wet climate and the extended deep lake environmentthat created conditions for large-scale growth of hydrophilousalga,consequently they provided organic matter for the high-quality source rocks of Chang 7 section in thearea.
Acritarch assemblage in Yanchang Formation in eastern Gansu province and its environmental implications
DOI:10.1007/s11442-006-0415-5
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In this paper, the microfossil assemblage from drilling core samples of Xifeng oilfield is discussed with palynologic analystic method, to clarify terrestrial sediment environment of Yanchang Formation in Ordos basin during the Middle and Late Triassic. The research shows abundant acritarchs in the main oil-source rocks, namely in Chang 7 Section of Yanchang Formation. All these acritarchs, highly abundant in fossils, but monotone in species, are dominated absolutely by Leiophaerids acritarchs Leiosphaeridia, often associated with acanthomorphic acritarchs Micrhystridium. The contrast in the ecological distribution and assemblage between contemporaneous acritarchs, especially acanthomorphic acritarchs, illustrates that the sedimentary surroundings were fresh lacustrine. These acritarchs, not only monotone in species but also conspicuous in echinulate process, suggest that some acanthomorphic marine acritarchs survived long-time evolution in fresh water. The sedimentary environment of Chang 7 Section, the main source rock in the area, turned into the climax of lake transgression, indicating the supply of the large-scale lake water body from rivers instead of from the rise in sea level. These evidences prove that the Early and Middle Triassic paralic and lacustrine features had already been inactive in the Middle and Late Triassic Ordos basin.
Study of character on sedimentary water and palaeoclimate for Chang7 oil layer in Ordos Basin
Based on Sr concentration,the ratios of Sr/Br and Th/U,and biogliph fossil,we discuss the water condition and paleoclimate for the deposition of Chang7 oil layer in the Ordos basin.The results show that the ratio of Sr/Br and Th/U is from 0.19 to 0.65,and 0.36 to 5.03,respectively,indicating the water during the Chang7 Period was continental freshwater.The V/Ni ratio of 1.75-5.53 is fallen into the score of the reducing environment.The element assemblage,biogliph fossil,and Co quantitative match indicate that the water depth at the Chang7 Period was half-deep to deep lake,with the maximum depth of 50-120m.The paleoclimate was confirmed as warm and humid climate by sporopollen assemblage.
Rb and Sr geochemical characterization of the Chinese loess and its implications for palaeomonsoon climate
DOI:10.1111/j.1755-6724.2000.tb00462.x
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Concentrations and distributions of Rb and Sr in loess and palaeosol samples from the Luochuan profile have been studied, in grain-size fractions and chemical forms. Results show that Rb is quite rich in the fraction of less than 2 m, and exits only in the residual phase, while Sr is distributed in sand and silt fractions, occurring in the residual and carbonate phases. Therefore, variations of the concentrations of Rb and Sr in loess and palaeosols are closely related not only to dustfall compositions depending on winter monsoon wind strength but also to summer monsoon-induced pedogenic intensity. This indicates that variations of Rb/Sr ratio in Chinese loess sequences represent cyclic alternations of East Asian summer and winter monsoon climates on the orbital time-scale. Variations in Sr concentrations along the sequence bear a high similarity to the oxygen isotope record of the deep sea sediments over the past 2.5 Ma, whereas variations in Rb concentrations are in good agreement with the record of magnetic susceptibility determined with the duplicated samples from the same profile. This suggests that (1) the high-latitude ice volume is responsible for East Asian monsoon variations and (2) the signal of magnetic susceptibility depends dominantly on pedogenesis in Chinese loess sequences.
Effect of sedimentary environment on fine grained sediments in Chang 7 Fm, southwestern Ordos Basin
Study on palaeosalinity of Chang6 oil reservoir set in Ordos Basin
Take the geochemical indicators of boron,equivalent boron,adsorptive K++Na+,Sr and the ratio of Sr/Br etc.as palaeosalinity indicators,the palaeosalinity of Chang6 oil reservoir set in Ordos Basin is analysed and calculated by using the formula advanced by T.D.Adams and E.L.Couch.The results indicate that the Late Triassic Ordos Basin is not an inland fresh water lake as recognized for a long time,but a brackish water to semisalt water marginal lake caused by intermittent seawater ingression.The paleosalinity of water body of the reservoir set was 0.9400/01.016% during its deposition.The contents of K++Na+ and the ratio of K/Na in the above mentioned paleosalinity do not correspond with that of Chang6 reservoir set;the contents of boron,equivalent borom, adsorptive K++Na+,Sr and Sr/Ba ratio are reliable identifying indicators of the paleosalinity.After the regression analysis of the identifying results of various indicators,it is suggested that the results calculated by E.L.Couch formula are m
Reconstructionand analysts of paleosalanity and paleoenvironment of the Chang6 Member in the Gengwan region, Ordos Basin
DOI:10.1002/clen.200700058
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B,Sr,Rb,Sr/Ba ratio,Rb/K ratio,K+Na mass fraction and clay mineral X-diffraction analysis were used to reconstruct the paleosalinity of the Chang 6 reservoir in the Gengwan region,Ordos Basin.The results showed that the paleosalinity of the Chang ranged from 0.48‰ to 4.43‰,with an average value of 1.87‰,indicating a fresh-mildly brackish environment,partly brackish environment.The water became salter gradually from the period of Chang 63 to Chang 61.The mildly brackish and close environment was favorable for the development of high quality source rocks and for the formation of early rim chlorite cementation,which resisted the compaction and dissolution,preserved primary inter-granular pores and provided condition for the development of reservoir.The paleosalinity was contoured and three paleosalinity distribution areas,the fresh water area,mildly brackish water area and brackish water area,were classified.It was considered that the 0.5‰ contour line was the location of paleo lakeshore which was the boundary of fresh water rivers and mildly brackish lake.It was an important evidence for predicting favorable stratigraphic reservoir.
Summary of quantified research mothod on paleosalinity
Recovery paleosalinity in sedimentary environment: An example of mudstone in Shuixigou Group, southwestern margin of Turpan Hami Basin
Petrology and element geochemistry and development environment of Yanchang Formation Chang-7 high quality source rocks in Ordos Basin
The core of Yanchang Formation Chang-7 high quality source rock (oil shale) is black,hard,light,and it looks slip-shape after demorphism. Rock slice observation and SEM-energy spectrum test show that the Chang-7 high quality source rocks are characterized by the extraordinary development of organic matter laminas,being rich in framboidal pyrite and abundant cellophane,and crystal fragment and tuff laminae are often observed. Comprehensive test results reveal that the chemical compositions of Chang-7 high quality source rock are characterized by high S2-,Fe,P2O5 contents and relatively low Al2O3,SiO2,CaO and MgO contents. In trace elements,Mo,Cu and U are obviously positive anomalies,V and Pb are positive anomalies,and the others are no or negative anomalies. REE is low. Low Sr/Ba ratios and B content reflect nonmarine lake-basin deposition feature with little salinity. Some geochemical parameters,such as high V/(V+ Ni) and very high U/Th ratios,indicate anoxic sedimentary environment.
Leading effect of high class source rock of Chang 7 in Ordos Basin on enrichment of low permeability oil-gas accumulation: Hydrocarbon generation and expulsion mechanism
Petroleum geological significance of microelements V and Ni
Microelements V and Ni are the main micro metallic element components in the petroleum.Because the material,energy and power foundations of upward migration exist,microelements V and Ni in oil gas pool source migrate vertically above and are distributed in the above environment.Using different methods to extract and analyze the V and the Ni content in soil,rock debris and formation water,we can know the V and Ni distribution characteristics and therefore may understand such information as oiliness in strata or the lower beds,the environment of deposition,the marine or continental deposit,the origin,and the oil source rock quality.
