2017 Vol. 35, No. 1
Display Method:
2017, 35(1): 1-9.
doi: 10.14027/j.cnki.cjxb.2017.01.001
Abstract:
Based on summing up the great variability and dynamic behavior of rivers, we analyzed the meander-braided transition characteristics and constructed its sedimentary sequence. According to the relationship between sinuosity and abandoned channels, we divided abandoned channels into three patterns which are avulsion, chute cutoff- and neck cutoff- abandoned channel, respectively, and each pattern has its particular sedimentary sequence model. Furthermore, by analyzing the sedimentary characteristics from a Permian fluvial outcrop located in Beijing western suburbs, we clarified the evolution of braided, meander-braided transition and meandering channels and distinguished three abandoned channels. We reached the conclusion that the main factor leading to meander-braided transition in the study area is the change of sediment supply caused by different climate conditions.
Based on summing up the great variability and dynamic behavior of rivers, we analyzed the meander-braided transition characteristics and constructed its sedimentary sequence. According to the relationship between sinuosity and abandoned channels, we divided abandoned channels into three patterns which are avulsion, chute cutoff- and neck cutoff- abandoned channel, respectively, and each pattern has its particular sedimentary sequence model. Furthermore, by analyzing the sedimentary characteristics from a Permian fluvial outcrop located in Beijing western suburbs, we clarified the evolution of braided, meander-braided transition and meandering channels and distinguished three abandoned channels. We reached the conclusion that the main factor leading to meander-braided transition in the study area is the change of sediment supply caused by different climate conditions.
2017, 35(1): 10-23.
doi: 10.14027/j.cnki.cjxb.2017.01.002
Abstract:
In recent years, it has widespread attention about study of the alluvial fan complex deposition mechanism and sedimentary process. Alluvial fan correlational research has made great improvement. This paper is summarized in details about alluvial fan classification and sedimentary characteristics, alluvial fan reservoir architecture characterization, and mechanism of alluvial fan formation. Later, the experimental simulation of the alluvial fan deposition process is carried out by using the experimental of flume sedimentation simulation. Research shows that:(1) Alluvial fan has a variety of classification schemes that reflects the diversity and complexity of alluvial fan. There are many methods to characterize alluvial fan, mainly includes modern sedimentation survey and outcrop, underground dense well pattern, and experimental of flume sedimentation simulation, and so on. Controlling alluvial fan development allogenetic cycle factors mainly include difference activity of tectonic, styles of contemporaneous fault combination, climate and provenance source change etc.; There are many kinds of autogenetic cycle factors under the control of alluvial fan depositional mechanism that determines the distribution of alluvial fan internal architecture unit.(2)Alluvial fan sedimentary process has following features. Firstly, after the initial prototype fan body formation, water laden sediments has trend to deposit on low lying position. Secondly, there are two important ways of channel migration, a single main channel migration and minor branched channel migration. A main reason of fan body to increase the length caused by a single main channel migration, while the important mechanism of the fan body width increases being minor branched channel migration. Multi-period fan body obvious retrograded overlying pattern of up-dipping towards the provenance with stable tectonic conditions. Finally, on the basis of the foregoing, the next research direction was discussed.
In recent years, it has widespread attention about study of the alluvial fan complex deposition mechanism and sedimentary process. Alluvial fan correlational research has made great improvement. This paper is summarized in details about alluvial fan classification and sedimentary characteristics, alluvial fan reservoir architecture characterization, and mechanism of alluvial fan formation. Later, the experimental simulation of the alluvial fan deposition process is carried out by using the experimental of flume sedimentation simulation. Research shows that:(1) Alluvial fan has a variety of classification schemes that reflects the diversity and complexity of alluvial fan. There are many methods to characterize alluvial fan, mainly includes modern sedimentation survey and outcrop, underground dense well pattern, and experimental of flume sedimentation simulation, and so on. Controlling alluvial fan development allogenetic cycle factors mainly include difference activity of tectonic, styles of contemporaneous fault combination, climate and provenance source change etc.; There are many kinds of autogenetic cycle factors under the control of alluvial fan depositional mechanism that determines the distribution of alluvial fan internal architecture unit.(2)Alluvial fan sedimentary process has following features. Firstly, after the initial prototype fan body formation, water laden sediments has trend to deposit on low lying position. Secondly, there are two important ways of channel migration, a single main channel migration and minor branched channel migration. A main reason of fan body to increase the length caused by a single main channel migration, while the important mechanism of the fan body width increases being minor branched channel migration. Multi-period fan body obvious retrograded overlying pattern of up-dipping towards the provenance with stable tectonic conditions. Finally, on the basis of the foregoing, the next research direction was discussed.
2017, 35(1): 24-34.
doi: 10.14027/j.cnki.cjxb.2017.01.003
Abstract:
Late Permian Changhsingian is an important period of reef buildup as well as a period of frequent volcanic activities in South China. Because reefs are peculiar ecosystems extremely sensitive to environment change, late Permian volcanism would surely have affected the reef growth and evolution of facies. However, most of the researches concerning the late Permian volcanism are concentrated near the Permo-Triassic boundary. Studies appear to be relatively lack on the relations between the evolution of reefs and volcanism throughout the whole late Permian Changhsingian. This paper documented the relationship between late Permian volcanism and the marginal platform reef evolution in Ziyun area of Guizhou province based on the analysis of magnetic susceptibility. The magnetic susceptibility data reflect two pulses of high magnetic susceptibility values respectively in the early Changhsingian and late Changhsingian, indicating two periods of intense volcanic activities. During both periods, reef-building organisms were clearly stressed and reef-building process was interrupted. The magnetic susceptibility values in the middle Changhsingian are generally lower, indicating weak volcanism activities that favored the rapid growth of reef-building organisms. However, the magnetic susceptibility values from the bioclastic limestone interbedded in reef limestone do not increase significantly, implying that turbulent water conditions prevented tiny volcano ash rich in ferromagnetic minerals from depositing in shallow reef environment although periodic volcanic eruption may have resulted in temporary breakdown of reef growth. This study shows that several beds of volcanic clay interbedded in the early Triassic mudstone possess abnormally high magnetic susceptibility values compared with the mudstone itself, indicating that much greater contributions of volcanic ash to the magnetic susceptibility values than the terrigenous material.
Late Permian Changhsingian is an important period of reef buildup as well as a period of frequent volcanic activities in South China. Because reefs are peculiar ecosystems extremely sensitive to environment change, late Permian volcanism would surely have affected the reef growth and evolution of facies. However, most of the researches concerning the late Permian volcanism are concentrated near the Permo-Triassic boundary. Studies appear to be relatively lack on the relations between the evolution of reefs and volcanism throughout the whole late Permian Changhsingian. This paper documented the relationship between late Permian volcanism and the marginal platform reef evolution in Ziyun area of Guizhou province based on the analysis of magnetic susceptibility. The magnetic susceptibility data reflect two pulses of high magnetic susceptibility values respectively in the early Changhsingian and late Changhsingian, indicating two periods of intense volcanic activities. During both periods, reef-building organisms were clearly stressed and reef-building process was interrupted. The magnetic susceptibility values in the middle Changhsingian are generally lower, indicating weak volcanism activities that favored the rapid growth of reef-building organisms. However, the magnetic susceptibility values from the bioclastic limestone interbedded in reef limestone do not increase significantly, implying that turbulent water conditions prevented tiny volcano ash rich in ferromagnetic minerals from depositing in shallow reef environment although periodic volcanic eruption may have resulted in temporary breakdown of reef growth. This study shows that several beds of volcanic clay interbedded in the early Triassic mudstone possess abnormally high magnetic susceptibility values compared with the mudstone itself, indicating that much greater contributions of volcanic ash to the magnetic susceptibility values than the terrigenous material.
2017, 35(1): 35-45.
doi: 10.14027/j.cnki.cjxb.2017.01.004
Abstract:
To further explore the geological significance of the tempestites of Paleozoic that exposed in the Olongbluk block, and investigate the effect of frequently strorm climate on the sedimentary characteristic of Paleozoic in northern Qaidam basin, a comprehensive field geology survey and laboratory analysis have been carried out in the study area, and several typical sedimentary structures have been distinguished in the area, e.g. scour, ripple mark, hummocky cross-stratification, and syngenetic deformation structure. According to the differernt lithology and other characteristics, each set of tempestite can be divided into different sections:activity period, off-load period, returning period, whirling period and rest period. Depending on the combination of the sections, tempestites can be divided into different kinds of storm sequences to represent different sedimentary environment. The early Paleozoic developed five storm sequences which represent in-situ-type sequence or near source sequence, and the late Paleozoic developed near source sequence, in-situ-agitational sequence and deep-slope sequence. Under the research of the lithological characteristics of Paleozoic, the Olongbluk area was in a shallow sea environment which was always effected by transgression in early Paleozoic,and changed into epicontinental sea environment in the late.
To further explore the geological significance of the tempestites of Paleozoic that exposed in the Olongbluk block, and investigate the effect of frequently strorm climate on the sedimentary characteristic of Paleozoic in northern Qaidam basin, a comprehensive field geology survey and laboratory analysis have been carried out in the study area, and several typical sedimentary structures have been distinguished in the area, e.g. scour, ripple mark, hummocky cross-stratification, and syngenetic deformation structure. According to the differernt lithology and other characteristics, each set of tempestite can be divided into different sections:activity period, off-load period, returning period, whirling period and rest period. Depending on the combination of the sections, tempestites can be divided into different kinds of storm sequences to represent different sedimentary environment. The early Paleozoic developed five storm sequences which represent in-situ-type sequence or near source sequence, and the late Paleozoic developed near source sequence, in-situ-agitational sequence and deep-slope sequence. Under the research of the lithological characteristics of Paleozoic, the Olongbluk area was in a shallow sea environment which was always effected by transgression in early Paleozoic,and changed into epicontinental sea environment in the late.
2017, 35(1): 46-56.
doi: 10.14027/j.cnki.cjxb.2017.01.005
Abstract:
In recent years, the growing breakthroughs in detrital-zircon U-Pb dating technology have fueled the development of provenance analysis of sedimentary basin, and this approach is further used to analyze paleogeographic environment, basin evolution and unroofing rate of provenance areas. Simultaneously, a number of potential problems that will lead to bias in applying detrital-zircon U-Pb geochronology to provenance analysis have increasingly attracted international attentions. On the basis of summarizing international relevant discussions, this paper concludes such considerations as sample intervals and hydrodynamic fractionation during sampling; reasonable choices of U-Pb dating methods, specific zircons and analytical numbers according to specific science issues during experiments; multi-recycling zircons, zircon-fertility ability, appropriate uplifting time and lag time during provenance comparison, and so on. The last of which is traditionally and simply conducted by matching detrital-zircon age spectrum of sediment units with crystallization age of source rocks in adjacent orogenic belts.
In recent years, the growing breakthroughs in detrital-zircon U-Pb dating technology have fueled the development of provenance analysis of sedimentary basin, and this approach is further used to analyze paleogeographic environment, basin evolution and unroofing rate of provenance areas. Simultaneously, a number of potential problems that will lead to bias in applying detrital-zircon U-Pb geochronology to provenance analysis have increasingly attracted international attentions. On the basis of summarizing international relevant discussions, this paper concludes such considerations as sample intervals and hydrodynamic fractionation during sampling; reasonable choices of U-Pb dating methods, specific zircons and analytical numbers according to specific science issues during experiments; multi-recycling zircons, zircon-fertility ability, appropriate uplifting time and lag time during provenance comparison, and so on. The last of which is traditionally and simply conducted by matching detrital-zircon age spectrum of sediment units with crystallization age of source rocks in adjacent orogenic belts.
2017, 35(1): 57-66.
doi: 10.14027/j.cnki.cjxb.2017.01.006
Abstract:
Well-preserved fossil specimens of ginkgoalean leaves were collected from the Middle Jurassic Yaojie Formation of the Baojishan Basin, Gansu Province. They were identified as Ginkgoites aganzhenensis Yang according to the macro-shape and micro-characteristics. The epidermal characteristics and stomatal parameters of these fossil specimens were obtained through a cuticular analysis. The paleo-CO2 of the early Middle Jurassic was quantitatively reconstructed using two methods, 974×10-6 by the stomatal index method which was out of the error range of CO2 confidence from GEOCARB Ⅲ, 1 823×10-6 by the stomatal ratio method which was plotted within the error range of CO2 confidence from GEOCARB Ⅲ based on the SI value of Ginkgoites aganzhenensis. It implied that the stomatal ratio method is more reliable in reconstructing paleo-CO2 concentration based on Ginkgoites aganzhenensis. Compared with the paleo-CO2 level reconstructed from Phoenicopsis and Solenites, we found that from Ginkgoites is more near to the optimum line of GEOCARB Ⅲ, suggesting that the paleo-CO2 reconstructed from Ginkgoites is more advisable. And such high CO2 concentration would have caused an increase in temperature of about 7.2℃, indicating that the paleoclimate in Baojishan Basin was warm and humid temperate climate in Aalenina-Bajocian of the Middle Jurassic.
Well-preserved fossil specimens of ginkgoalean leaves were collected from the Middle Jurassic Yaojie Formation of the Baojishan Basin, Gansu Province. They were identified as Ginkgoites aganzhenensis Yang according to the macro-shape and micro-characteristics. The epidermal characteristics and stomatal parameters of these fossil specimens were obtained through a cuticular analysis. The paleo-CO2 of the early Middle Jurassic was quantitatively reconstructed using two methods, 974×10-6 by the stomatal index method which was out of the error range of CO2 confidence from GEOCARB Ⅲ, 1 823×10-6 by the stomatal ratio method which was plotted within the error range of CO2 confidence from GEOCARB Ⅲ based on the SI value of Ginkgoites aganzhenensis. It implied that the stomatal ratio method is more reliable in reconstructing paleo-CO2 concentration based on Ginkgoites aganzhenensis. Compared with the paleo-CO2 level reconstructed from Phoenicopsis and Solenites, we found that from Ginkgoites is more near to the optimum line of GEOCARB Ⅲ, suggesting that the paleo-CO2 reconstructed from Ginkgoites is more advisable. And such high CO2 concentration would have caused an increase in temperature of about 7.2℃, indicating that the paleoclimate in Baojishan Basin was warm and humid temperate climate in Aalenina-Bajocian of the Middle Jurassic.
2017, 35(1): 67-74.
doi: 10.14027/j.cnki.cjxb.2017.01.007
Abstract:
Badain Jaran Desert is the second largest area of shifting sands in China and features the tallest dunes on Earth. Identifying provenance of aeolian deposits in the Badain Jaran Desert is critical to understand the formation and development of mega-dunes. Quartz is the most common mineral of Chinese desert sediments, and oxygen isotopic composition of quartz has been regarded as a useful tracer of provenance. Oxygen isotopic compositions were measured by isotopic techniques in various size fractions of quartz, which were extracted from aeolian and lacustrine sediments collected on northwestern, eastern, and southeastern part in the Badain Jaran Desert. The results indicate that the δ18O values increase with decreasing grain size in quartz from aeolian and lacustrine sediments, and the δ18O composition of quartz in various size fractions has notable variations ranging from 3.1‰ to 9.9‰. The δ18O values in quartz in all size fractions are between 9.4‰ and 19.3‰, in which aeolian sands range from 9.5‰ to 16.6‰, and lacustrine sediments vary from 9.4‰ to 19.3‰. In the study area, the δ18O values of fine-grained (<16 μm) quartz show significant differences between different size fractions of the grains (i.e., 16~64 μm,125~154 μm, 200~250 μm, >300 μm), whereas two coarser fractions (200~250 μm and >300 μm) exhibit no significantly different in the δ18O values. Furthermore, when isotopic results are compared with available findings at regional scale, the δ18O values of fine-grained (<16 μm) quartz have no significant difference between Badain Jaran Desert, Qaidam Basin Desert, and Mongolian Gobi Desert, but that of silt-grained (16~64 μm) quartz exist notable difference between the Badain Jaran Desert and Mongolian Gobi Desert. From these observations, it appears likely that the fine-grained components in each sample probably have a more distal origin. A large range of δ18O values, from 9.4‰ to 19.3‰ is present within the aeolian dune and lacustrine sediments, fall within the range of values considered to be indicative of igneous, metamorphic, and sedimentary rocks. The variable δ18O values seem to result from the mixing ratio of igneous, sedimentary and metamorphic rocks. These differences in δ18O values can be related to regional geological setting, mixing ratio of various detrital components, and grain size dependence so on. In the dominant size fraction (125~154 μm, 200~250 μm) of sediments in Badain Jaran Desert, δ18O values are mostly between 11.0 ‰ and 14.9‰, but extend down to 9.4‰. It suggests that quartz detritus is mostly derived from sedimentary and metamorphic rocks, which is considered as mostly originating from Qilian Shan mountainous areas although the local source cannot be ruled out.
Badain Jaran Desert is the second largest area of shifting sands in China and features the tallest dunes on Earth. Identifying provenance of aeolian deposits in the Badain Jaran Desert is critical to understand the formation and development of mega-dunes. Quartz is the most common mineral of Chinese desert sediments, and oxygen isotopic composition of quartz has been regarded as a useful tracer of provenance. Oxygen isotopic compositions were measured by isotopic techniques in various size fractions of quartz, which were extracted from aeolian and lacustrine sediments collected on northwestern, eastern, and southeastern part in the Badain Jaran Desert. The results indicate that the δ18O values increase with decreasing grain size in quartz from aeolian and lacustrine sediments, and the δ18O composition of quartz in various size fractions has notable variations ranging from 3.1‰ to 9.9‰. The δ18O values in quartz in all size fractions are between 9.4‰ and 19.3‰, in which aeolian sands range from 9.5‰ to 16.6‰, and lacustrine sediments vary from 9.4‰ to 19.3‰. In the study area, the δ18O values of fine-grained (<16 μm) quartz show significant differences between different size fractions of the grains (i.e., 16~64 μm,125~154 μm, 200~250 μm, >300 μm), whereas two coarser fractions (200~250 μm and >300 μm) exhibit no significantly different in the δ18O values. Furthermore, when isotopic results are compared with available findings at regional scale, the δ18O values of fine-grained (<16 μm) quartz have no significant difference between Badain Jaran Desert, Qaidam Basin Desert, and Mongolian Gobi Desert, but that of silt-grained (16~64 μm) quartz exist notable difference between the Badain Jaran Desert and Mongolian Gobi Desert. From these observations, it appears likely that the fine-grained components in each sample probably have a more distal origin. A large range of δ18O values, from 9.4‰ to 19.3‰ is present within the aeolian dune and lacustrine sediments, fall within the range of values considered to be indicative of igneous, metamorphic, and sedimentary rocks. The variable δ18O values seem to result from the mixing ratio of igneous, sedimentary and metamorphic rocks. These differences in δ18O values can be related to regional geological setting, mixing ratio of various detrital components, and grain size dependence so on. In the dominant size fraction (125~154 μm, 200~250 μm) of sediments in Badain Jaran Desert, δ18O values are mostly between 11.0 ‰ and 14.9‰, but extend down to 9.4‰. It suggests that quartz detritus is mostly derived from sedimentary and metamorphic rocks, which is considered as mostly originating from Qilian Shan mountainous areas although the local source cannot be ruled out.
2017, 35(1): 75-84.
doi: 10.14027/j.cnki.cjxb.2017.01.008
Abstract:
The definition and classification of calcareous root tubes which have been used in different studies are still unified. However, it remains uncertain that whether different definitions of calcareous root tubes indicated the same morphology or not. At the same time, there were disagreements over the formation mechanism and paleoenvironment significance of calcareous root tubes. Some research results suggested that the formation of calcareous root tubes in this area indicated relatively humid climate conditions, while others argued that they were formed by recrystallization of calcium carbonate required the presence of underground water. In this study, 26 calcareous root tubes samples collected from the Alashan Desert are classified using morphological method, which was based on previous studies about the definition and formation mechanism of calcareous root tubes. Then the paleoenvironment significance for different types of calcareous root tubes was discussed. The results showed that calcareous root tubes were products of terricolous plants and were formed by encrustation of plant roots by secondary carbonates, including five subclasses, namely calcareous sheath, rhizocretion, root mould, root tubule, and root cast. Calcareous sheath and root tubule have hollow tubes in morphology, and then rhizocretion and root cast have structure of double layer cemented by calcium carbonate or solid construction filled with coarse gravel or sediment. Root mould is simply tubular voids left after roots decayed. According to whether the formation process is under the influence of plant growth and rhizosphere microorganism activity or not, the formation process can be divided into two patterns. Calcareous sheath and rhizocretion were formed by the influence of plant growth and rhizosphere microorganism activity, while the formation of root mould, root tubule, and root cast were not influenced by them. Thus, the classification of the above five subclasses can be distinguished by morphology character clearly. Results from n-alkanes indicated that abundant short chain alkanes which indicate the influence of plant growth and rhizosphere microorganism activity occurred in calcareous root tubes. Therefore, calcareous root tubes can be divided into calcareous sheath and rhizocretion in the Alashan Desert. Both of them occurred at the same sampling point. However, the quantity percentage of rhizocretion was variates in the Holocene. During the middle-Holocene, the quantity percentage of rhizocretion is significantly higher than that in the late Holocene. The formation of rhizocretion were jointly controlled by calcium solution leaching, CO2 produced by the rot of root, and rhizosphere becoming alkaline. Calcium solution leaching could be resulted in higher effective humidity. Therefore, changes in the quantity percentage of rhizocretion could indicate levels of effective humidity, higher quantity percentage of rhizocretion could be interpreted as a humid environment, and vice versa.
The definition and classification of calcareous root tubes which have been used in different studies are still unified. However, it remains uncertain that whether different definitions of calcareous root tubes indicated the same morphology or not. At the same time, there were disagreements over the formation mechanism and paleoenvironment significance of calcareous root tubes. Some research results suggested that the formation of calcareous root tubes in this area indicated relatively humid climate conditions, while others argued that they were formed by recrystallization of calcium carbonate required the presence of underground water. In this study, 26 calcareous root tubes samples collected from the Alashan Desert are classified using morphological method, which was based on previous studies about the definition and formation mechanism of calcareous root tubes. Then the paleoenvironment significance for different types of calcareous root tubes was discussed. The results showed that calcareous root tubes were products of terricolous plants and were formed by encrustation of plant roots by secondary carbonates, including five subclasses, namely calcareous sheath, rhizocretion, root mould, root tubule, and root cast. Calcareous sheath and root tubule have hollow tubes in morphology, and then rhizocretion and root cast have structure of double layer cemented by calcium carbonate or solid construction filled with coarse gravel or sediment. Root mould is simply tubular voids left after roots decayed. According to whether the formation process is under the influence of plant growth and rhizosphere microorganism activity or not, the formation process can be divided into two patterns. Calcareous sheath and rhizocretion were formed by the influence of plant growth and rhizosphere microorganism activity, while the formation of root mould, root tubule, and root cast were not influenced by them. Thus, the classification of the above five subclasses can be distinguished by morphology character clearly. Results from n-alkanes indicated that abundant short chain alkanes which indicate the influence of plant growth and rhizosphere microorganism activity occurred in calcareous root tubes. Therefore, calcareous root tubes can be divided into calcareous sheath and rhizocretion in the Alashan Desert. Both of them occurred at the same sampling point. However, the quantity percentage of rhizocretion was variates in the Holocene. During the middle-Holocene, the quantity percentage of rhizocretion is significantly higher than that in the late Holocene. The formation of rhizocretion were jointly controlled by calcium solution leaching, CO2 produced by the rot of root, and rhizosphere becoming alkaline. Calcium solution leaching could be resulted in higher effective humidity. Therefore, changes in the quantity percentage of rhizocretion could indicate levels of effective humidity, higher quantity percentage of rhizocretion could be interpreted as a humid environment, and vice versa.
2017, 35(1): 85-92.
doi: 10.14027/j.cnki.cjxb.2017.01.009
Abstract:
Fujiang river alluvial fan is located in the Wudu Basin, the North-Eastern part of Longmen mountains, which is a foreland basin formed near the eastern margin of Tibetan Plateau, and the alluvial fan covers an area of about 25 km2. The alluvial fan has developed in a right-lateral strike-slip domain settings, it belongs to the Jiangyou Fault and Xiangshui-Rangshui Fault. Through the field survey, sedimentary sequence and cosmogenic nuclide burial ages data of alluvial fan, and combined with the previous research results and geological survey data, three alluvial fan's formation, distribution and tectonic activities along the Fujiang river are determined respectively. Early Pleistocene alluvial fan(1.84 Ma), Middle Pleistocene alluvial fan(0.54 Ma) and Holocene alluvial fan distributed near the northeast-southwest directions in foreland of Longmen mountains. In the middle Pleistocene, Jiangyou rupture is right-lateral strike-slip thrust fault, causing the early Pleistocene Fujiang river alluvial fan head right-lateral slip about 3.2 km, and the fan uplifted about 50 m. As Jiangyou rupture continued right-lateral strike-slip and thrust, the middle Pleistocene alluvial fan's head was right-lateral displaced about 0.8 km. During the Quaternary period, the early and middle Pleistocene alluvial fan total was right-lateral displaced about 4 km, 0.8 km, respectively. The middle Pleistocene alluvial fan uplifted about 50 m, The early Pleistocene alluvial fan total uplifted about 100 m. Sedimentary center of Fujiang river alluvial fan moved to the right downstream. Characterized by coarsening upward sedimentary sequence, gravel layer of alluvial fans asymmetrically distributed. The results show the Longmen mountains has been uplifted and the rupture right-lateral strike-slip and thrust, the Fujiang river alluvial fans expanded as a correspondence to the tectonic activities.
Fujiang river alluvial fan is located in the Wudu Basin, the North-Eastern part of Longmen mountains, which is a foreland basin formed near the eastern margin of Tibetan Plateau, and the alluvial fan covers an area of about 25 km2. The alluvial fan has developed in a right-lateral strike-slip domain settings, it belongs to the Jiangyou Fault and Xiangshui-Rangshui Fault. Through the field survey, sedimentary sequence and cosmogenic nuclide burial ages data of alluvial fan, and combined with the previous research results and geological survey data, three alluvial fan's formation, distribution and tectonic activities along the Fujiang river are determined respectively. Early Pleistocene alluvial fan(1.84 Ma), Middle Pleistocene alluvial fan(0.54 Ma) and Holocene alluvial fan distributed near the northeast-southwest directions in foreland of Longmen mountains. In the middle Pleistocene, Jiangyou rupture is right-lateral strike-slip thrust fault, causing the early Pleistocene Fujiang river alluvial fan head right-lateral slip about 3.2 km, and the fan uplifted about 50 m. As Jiangyou rupture continued right-lateral strike-slip and thrust, the middle Pleistocene alluvial fan's head was right-lateral displaced about 0.8 km. During the Quaternary period, the early and middle Pleistocene alluvial fan total was right-lateral displaced about 4 km, 0.8 km, respectively. The middle Pleistocene alluvial fan uplifted about 50 m, The early Pleistocene alluvial fan total uplifted about 100 m. Sedimentary center of Fujiang river alluvial fan moved to the right downstream. Characterized by coarsening upward sedimentary sequence, gravel layer of alluvial fans asymmetrically distributed. The results show the Longmen mountains has been uplifted and the rupture right-lateral strike-slip and thrust, the Fujiang river alluvial fans expanded as a correspondence to the tectonic activities.
2017, 35(1): 93-101.
doi: 10.14027/j.cnki.cjxb.2017.01.010
Abstract:
Stalagmite has increasingly become an important part of global change research due to its exclusive advantages of precise dating and high-resolution records. The study of carbonate sedimentary mechanism is significant of understanding the stalagmite sedimentation rate, crystal morphology and precisely interpreting climate proxies, and reasonable protection of modern cave. Jiguan Cave in Henan Province is selected as the example in Asian monsoon-sensitive area in the north-south China intersection, we have monitored the drips and carbonate deposition for 6 consecutive hydrological years from 2010 to 2015, and found:(1)The hydrochemical indices of drips have the similar pattern compared with deposition rate. The biological activity and soil CO2 are strengthened in wet season due to the increase of precipitation and air temperature, as a result, the dripping rate, Ca2+, HCO3-, EC and deposition rate of drips also increase, dry season shows opposite pattern. The inhibition of cave CO2 to calcite deposition is masked by drip saturation. (2)The water-rock effect goes weak for the impact of precipitation markedly decreasing from 2012-2013, and saturation of drips became minimum in 2014 when calcite hiatus also occurred. The deposition was collected again in Nov.2014 becausing increasing rain in the latter half of the year. (3)The cave was washed by artificial leaching and NO3- of drips rised more than 10 times. Simulation result shows that~60% CaCO3 can be more dissolved than before, which suggests the inhibition of NO3- to calcite deposition. This may be the reason why the calcite hiatus happened during the drip was with high degree of saturation in some months. It is suggested that the water quality should be taken into consideration in the process of water supply.
Stalagmite has increasingly become an important part of global change research due to its exclusive advantages of precise dating and high-resolution records. The study of carbonate sedimentary mechanism is significant of understanding the stalagmite sedimentation rate, crystal morphology and precisely interpreting climate proxies, and reasonable protection of modern cave. Jiguan Cave in Henan Province is selected as the example in Asian monsoon-sensitive area in the north-south China intersection, we have monitored the drips and carbonate deposition for 6 consecutive hydrological years from 2010 to 2015, and found:(1)The hydrochemical indices of drips have the similar pattern compared with deposition rate. The biological activity and soil CO2 are strengthened in wet season due to the increase of precipitation and air temperature, as a result, the dripping rate, Ca2+, HCO3-, EC and deposition rate of drips also increase, dry season shows opposite pattern. The inhibition of cave CO2 to calcite deposition is masked by drip saturation. (2)The water-rock effect goes weak for the impact of precipitation markedly decreasing from 2012-2013, and saturation of drips became minimum in 2014 when calcite hiatus also occurred. The deposition was collected again in Nov.2014 becausing increasing rain in the latter half of the year. (3)The cave was washed by artificial leaching and NO3- of drips rised more than 10 times. Simulation result shows that~60% CaCO3 can be more dissolved than before, which suggests the inhibition of NO3- to calcite deposition. This may be the reason why the calcite hiatus happened during the drip was with high degree of saturation in some months. It is suggested that the water quality should be taken into consideration in the process of water supply.
2017, 35(1): 117-123.
doi: 10.14027/j.cnki.cjxb.2017.01.012
Abstract:
The numerical sedimentary forward simulation of Lower-Middle Ordovician of Shunnan area in Tarim Basin was presented by means of software Dionisos, which are controlled by primary model, tectonic subsidence, fluctuation of sea level, sedimentation rate and wave energy. Through the validation, the appropriate model has been chosen and then the sedimentary process is clear. The evolution of this area has four deposititional cycles. The distribution of carbonate reef, beach and short-time exposed surfaces has been analyzed. During Early-Middle Ordovician, there developed four frequent short-time exposure surfaces which occurred at the top of the depositional cycle in Shunnan area and resulted in the formation of vugular-pore in carbonate deposits subjected to leaching and dissolution of meteoric fresh water,leading to which makes a change that only the fracture dominates the development of the carbonate reservoir.
The numerical sedimentary forward simulation of Lower-Middle Ordovician of Shunnan area in Tarim Basin was presented by means of software Dionisos, which are controlled by primary model, tectonic subsidence, fluctuation of sea level, sedimentation rate and wave energy. Through the validation, the appropriate model has been chosen and then the sedimentary process is clear. The evolution of this area has four deposititional cycles. The distribution of carbonate reef, beach and short-time exposed surfaces has been analyzed. During Early-Middle Ordovician, there developed four frequent short-time exposure surfaces which occurred at the top of the depositional cycle in Shunnan area and resulted in the formation of vugular-pore in carbonate deposits subjected to leaching and dissolution of meteoric fresh water,leading to which makes a change that only the fracture dominates the development of the carbonate reservoir.
2017, 35(1): 124-138.
doi: 10.14027/j.cnki.cjxb.2017.01.013
Abstract:
Based on the studies of the progradation reflections on 3-D seismic data, the matrix mineral assemblages and the heavy mineral assemblages in sandstone cores and the shape of the gamma-ray curves of sandstone, we concluded that the provenance of Chang 7-Chang 8 members in Jinghe oilfield mainly came from the magmatic rocks and metamorphic rocks area which are located in the southwest of the Ordos basin, and the provenance of Zhengning area might come from the south. Cores and logging analysis show that the sandstone of Chang 72 member is turbidite sandstone reservoir, which developed nine turbidite water channels or lobate turbidite fans in NE direction. The Chang 81 member is a water-transgression sedimentary process from fluvial facies to semi-deep lake facies, which developed three water channels in NE direction and one in NS. The Chang 812 member is a shore-shallow lake facial-fluvial facial deposition, which developed underwater distributary channel sandstone; the Chang811 member developed the water-transgression delta distributary channel sandstone under the background of shallow lake to the semi-deep lake facial deposition.
Based on the studies of the progradation reflections on 3-D seismic data, the matrix mineral assemblages and the heavy mineral assemblages in sandstone cores and the shape of the gamma-ray curves of sandstone, we concluded that the provenance of Chang 7-Chang 8 members in Jinghe oilfield mainly came from the magmatic rocks and metamorphic rocks area which are located in the southwest of the Ordos basin, and the provenance of Zhengning area might come from the south. Cores and logging analysis show that the sandstone of Chang 72 member is turbidite sandstone reservoir, which developed nine turbidite water channels or lobate turbidite fans in NE direction. The Chang 81 member is a water-transgression sedimentary process from fluvial facies to semi-deep lake facies, which developed three water channels in NE direction and one in NS. The Chang 812 member is a shore-shallow lake facial-fluvial facial deposition, which developed underwater distributary channel sandstone; the Chang811 member developed the water-transgression delta distributary channel sandstone under the background of shallow lake to the semi-deep lake facial deposition.
2017, 35(1): 139-150.
doi: 10.14027/j.cnki.cjxb.2017.01.014
Abstract:
In order to solve the problem of reservoir heterogeneity in Carboniferous KT-II carbonates in Zanazor on the eastern margin of the Pre-Caspian basin, the internal sedimentary models of sedimentary microfacies are characterized more detailedly and meticulously. The results show that:①The carbonate rocks of the Carboniferous KT-II in the study area are dominated by sparry grainstones, followed by packstones and mudstones. ②According to the different sedimentary characteristics and depositional environment, carbonate sediments can be divided into tidal flat, shoal and depression. Based on the characteristics of water depth, hydrodynamic condition and salinity, the carbonate shoal is divided into two kinds of microfacies:the break-wave shoal and up-wave shoal. According to their composition, the carbonate shoal is subdivided into four types:fusulinids shoal, foraminifera shoal, algal shoal and oolitic shoal. ③According to Wilson's idealized carbonate model, the study area belongs to the interior sedimentary area of the open platform, and corresponds to the Irwin's epicontinental sea energy model. The tidal flat corresponds to the Z-zone close to the coast; the shoal corresponds to the the Y-zone near the coast; and depression corresponds to the X-zone far from the coast.
In order to solve the problem of reservoir heterogeneity in Carboniferous KT-II carbonates in Zanazor on the eastern margin of the Pre-Caspian basin, the internal sedimentary models of sedimentary microfacies are characterized more detailedly and meticulously. The results show that:①The carbonate rocks of the Carboniferous KT-II in the study area are dominated by sparry grainstones, followed by packstones and mudstones. ②According to the different sedimentary characteristics and depositional environment, carbonate sediments can be divided into tidal flat, shoal and depression. Based on the characteristics of water depth, hydrodynamic condition and salinity, the carbonate shoal is divided into two kinds of microfacies:the break-wave shoal and up-wave shoal. According to their composition, the carbonate shoal is subdivided into four types:fusulinids shoal, foraminifera shoal, algal shoal and oolitic shoal. ③According to Wilson's idealized carbonate model, the study area belongs to the interior sedimentary area of the open platform, and corresponds to the Irwin's epicontinental sea energy model. The tidal flat corresponds to the Z-zone close to the coast; the shoal corresponds to the the Y-zone near the coast; and depression corresponds to the X-zone far from the coast.
2017, 35(1): 151-162.
doi: 10.14027/j.cnki.cjxb.2017.01.015
Abstract:
Constant-rate mercury intrusion (CRMI) was performed on 16 samples from the He8 Member tight sandstone of the Permian Xiashihezi Formation in Ordos Basin, and combined with the NMR experiment, the pore (pore and throat) structure features were investigated finely. Based on the research of the pore structure features, fractal characteristics of pore (pore and throat) in tight sandstone reservoir were carried out, and the relationship between pore fractal and percolation characteristics and pore structure parameters were quantitatively characterized. The results show that the effective pores and throats of tight gas reservoir are controlled by submicron and micron scale pore-throats. The pores are mainly composed of macrospores and mesopores, and the throats consist of micro throats, micro-fine throats and fine throats. In the light of effective pores and throats recognized by CRMI and NMR, it is suggested that both pore-throat and throat distribution conform to the fractural structure with dimension value D1 and D2, respectively, while pore distribution does not. Based on the characteristics of pore fractal structure and percolation, pore fractal structures of He 8 Member tight gas reservoir are divided into two categories:typeⅠhas the staged fractal characteristic, i.e., there is a clear inflection point with about 1 MPa pressure of mercury injection (MIP). When the MIP is greater than 1 MPa, the reservoir pore-throat has fractal characteristic and the throats contribute to mercury saturation increment (MSI). On the contrary, the MSI is almost entirely contributed by the pores. Type Ⅱ is the integral fractal, and the MSI is almost all contributed by the throats. The fractal dimensions of tight gas reservoir pore which has a certain correlation with pore micro geometrical parameters where as its relationship with permeability, average throat radius and mainstream throat radius are obvious negative correlation. The fractal dimensions show a positive linear correlation with micro heterogeneity coefficient, while no direct or weak relationships with porosity, average pore radius and average pore-throat radius ratio.
Constant-rate mercury intrusion (CRMI) was performed on 16 samples from the He8 Member tight sandstone of the Permian Xiashihezi Formation in Ordos Basin, and combined with the NMR experiment, the pore (pore and throat) structure features were investigated finely. Based on the research of the pore structure features, fractal characteristics of pore (pore and throat) in tight sandstone reservoir were carried out, and the relationship between pore fractal and percolation characteristics and pore structure parameters were quantitatively characterized. The results show that the effective pores and throats of tight gas reservoir are controlled by submicron and micron scale pore-throats. The pores are mainly composed of macrospores and mesopores, and the throats consist of micro throats, micro-fine throats and fine throats. In the light of effective pores and throats recognized by CRMI and NMR, it is suggested that both pore-throat and throat distribution conform to the fractural structure with dimension value D1 and D2, respectively, while pore distribution does not. Based on the characteristics of pore fractal structure and percolation, pore fractal structures of He 8 Member tight gas reservoir are divided into two categories:typeⅠhas the staged fractal characteristic, i.e., there is a clear inflection point with about 1 MPa pressure of mercury injection (MIP). When the MIP is greater than 1 MPa, the reservoir pore-throat has fractal characteristic and the throats contribute to mercury saturation increment (MSI). On the contrary, the MSI is almost entirely contributed by the pores. Type Ⅱ is the integral fractal, and the MSI is almost all contributed by the throats. The fractal dimensions of tight gas reservoir pore which has a certain correlation with pore micro geometrical parameters where as its relationship with permeability, average throat radius and mainstream throat radius are obvious negative correlation. The fractal dimensions show a positive linear correlation with micro heterogeneity coefficient, while no direct or weak relationships with porosity, average pore radius and average pore-throat radius ratio.
2017, 35(1): 163-172.
doi: 10.14027/j.cnki.cjxb.2017.01.016
Abstract:
Based on analysis on casting thin sections, SEM, cathodeluminescence, inclusion, and geochemical data, and combining with the method of statistics and calculation, we have discussed the petrologic feature, diagenetic sequence, and quantitative evolution of porosity from the different grain-sized sandstone of the Lower Shihezi Formation in the study area. This research shows that the coarse sandstone has the characteristic with high rigidity particle and the plastic debris is enriched in fine-grained sandstone. silicic cementation and carbonate cementation are enriched in coarse sandstone, medium and fine sandstone has the high content of kaolinite. On the diagenetic evolution process, the experience of diagenesis is more completely in coarse sandstone, but the fine sandstone had already become densification in period B of early diagenetic stage. The medium sandstone is the transition between the two types. The period of Permian to middle Triassic is penecontemporaneous to period B of early diagenetic stage, the dominant diagenesis is compaction, which has the greatest influence on the porosity of fine sandstone. In the late Triassic to Jurassic is the period A of middle diagenetic stage, the dominant diagenesis are corrosion and cementation. The coarse sandstone dissolution is strong, so has a higher porosity. Fine sandstone is not developed in dissolution and cementation by and large. The period B of middle diagenetic stage is developed after middle Jurassic, the dominant diagenesis is ferrous carbonate cementation. Coarse sandstone has strong cementation to reduce the porosity, but the property is still good. The final porosity is 11.07%, 6.08% and 2.60%. In conclusion, coarse sandstone has better properties because lower compaction and higher corrosion, medium sandstone is secondary formation because compaction and cementation, fine sandstone is the worst formation because of higher compaction.
Based on analysis on casting thin sections, SEM, cathodeluminescence, inclusion, and geochemical data, and combining with the method of statistics and calculation, we have discussed the petrologic feature, diagenetic sequence, and quantitative evolution of porosity from the different grain-sized sandstone of the Lower Shihezi Formation in the study area. This research shows that the coarse sandstone has the characteristic with high rigidity particle and the plastic debris is enriched in fine-grained sandstone. silicic cementation and carbonate cementation are enriched in coarse sandstone, medium and fine sandstone has the high content of kaolinite. On the diagenetic evolution process, the experience of diagenesis is more completely in coarse sandstone, but the fine sandstone had already become densification in period B of early diagenetic stage. The medium sandstone is the transition between the two types. The period of Permian to middle Triassic is penecontemporaneous to period B of early diagenetic stage, the dominant diagenesis is compaction, which has the greatest influence on the porosity of fine sandstone. In the late Triassic to Jurassic is the period A of middle diagenetic stage, the dominant diagenesis are corrosion and cementation. The coarse sandstone dissolution is strong, so has a higher porosity. Fine sandstone is not developed in dissolution and cementation by and large. The period B of middle diagenetic stage is developed after middle Jurassic, the dominant diagenesis is ferrous carbonate cementation. Coarse sandstone has strong cementation to reduce the porosity, but the property is still good. The final porosity is 11.07%, 6.08% and 2.60%. In conclusion, coarse sandstone has better properties because lower compaction and higher corrosion, medium sandstone is secondary formation because compaction and cementation, fine sandstone is the worst formation because of higher compaction.
2017, 35(1): 173-181.
doi: 10.14027/j.cnki.cjxb.2017.01.017
Abstract:
It was the traditional view that Paleogene of Bonan Sag had three hydrocarbon accumulation periods and the main period was later. With the proposed new theory that source rock in saline lacustrine basin was generated and expulsed early and the quantity of generation and expulsion is various. Early reservoir forming process of Bonan Paleogene was re-evaluated using the analysis and testing data of drilling and seismic exploration. The research shows that source rock formed under saline environment is well developed in the upper part of 4th member of Shahejie Formation of Bonan sag, and geochemical parameters indicate that source rocks can generate a great number of oil and gas and that is the material basis of early reservoir forming. Fluorescent color and homogenization temperature of fluid inclusion indicate that early reservoir forming is developed in Luojia area of Bonan sag and Sanhecun sag. Oil and gas of Paleonege at Sanhecun sag was transferred from Luojia area at early sedimentary stage of Dongying group when Gunan fault throw was small. Re-recognition on early reservoir forming not only made breakthroughs in oil and gas exploration of Paleonege in Sanhecun sag, but also built complete reservoir forming period sequence. Rock wettability alternation caused by early reservoir forming helped successive development of dominant migrating pathway, organic acid filling through the process of reservoir forming helped development of the reservoir space, and, more importantly, re-recognition of early reservoir forming will help transforming the exploration strategy and expand the exploration area, which will bring significant geological and exploration implication.
It was the traditional view that Paleogene of Bonan Sag had three hydrocarbon accumulation periods and the main period was later. With the proposed new theory that source rock in saline lacustrine basin was generated and expulsed early and the quantity of generation and expulsion is various. Early reservoir forming process of Bonan Paleogene was re-evaluated using the analysis and testing data of drilling and seismic exploration. The research shows that source rock formed under saline environment is well developed in the upper part of 4th member of Shahejie Formation of Bonan sag, and geochemical parameters indicate that source rocks can generate a great number of oil and gas and that is the material basis of early reservoir forming. Fluorescent color and homogenization temperature of fluid inclusion indicate that early reservoir forming is developed in Luojia area of Bonan sag and Sanhecun sag. Oil and gas of Paleonege at Sanhecun sag was transferred from Luojia area at early sedimentary stage of Dongying group when Gunan fault throw was small. Re-recognition on early reservoir forming not only made breakthroughs in oil and gas exploration of Paleonege in Sanhecun sag, but also built complete reservoir forming period sequence. Rock wettability alternation caused by early reservoir forming helped successive development of dominant migrating pathway, organic acid filling through the process of reservoir forming helped development of the reservoir space, and, more importantly, re-recognition of early reservoir forming will help transforming the exploration strategy and expand the exploration area, which will bring significant geological and exploration implication.
2017, 35(1): 182-192.
doi: 10.14027/j.cnki.cjxb.2017.01.018
Abstract:
Aromatic geochemical characteristics of thirteen light oil samples from nine Wells have been studied systematically by using GC-MS experimental technique. The results show that the distribution and relative content of aromatic compounds in the light oils from Well NB14-1, Well NB14-2 and well NB14-3 in the north of Pinghu slope belt are different from those of Well NB25-1 and Well NB25-2 in the middle of Pinghu slope belt, Well HY1-1 in the Xiciao, and Well HY2-1 and Well HY2-2 in Huangyan tectonic belt. The major components of aromatic fraction in light oils are bicyclic and tricyclic compounds. The light oil samples from the former wells contain more phenanthrene hydrocarbons and less naphthalene and biphenyl hydrocarbons. Meanwhile, the latter wells contain more naphthalene and biphenyl hydrocarbons and less phenanthrene hydrocarbons. The study on the source and depositional environment shows that the source material of light oils is mainly terrestrial higher plants input, and the depositional environment is oxidizing environment, but the latter wells has a higher 1,2,5-TMN and 1,2,5,6-TeMN relative content, and has a greater relative content of dibenzofuran and Pr/Ph value, reflecting that which has a greater degree of higher terrestrial plants input, with a stronger oxidative deposition environment. Alkyl-naphthalene, alkyl-phenanthrene and alkyl-dibenzothiophen are found to be optimal parameters for assessing thermal maturity characteristics, and the study shows that the light oils have reached mature-high mature oil characteristics, and the light oils in Well NB14-1, Well NB14-2 and NB14-3 have a higher maturity relatively.
Aromatic geochemical characteristics of thirteen light oil samples from nine Wells have been studied systematically by using GC-MS experimental technique. The results show that the distribution and relative content of aromatic compounds in the light oils from Well NB14-1, Well NB14-2 and well NB14-3 in the north of Pinghu slope belt are different from those of Well NB25-1 and Well NB25-2 in the middle of Pinghu slope belt, Well HY1-1 in the Xiciao, and Well HY2-1 and Well HY2-2 in Huangyan tectonic belt. The major components of aromatic fraction in light oils are bicyclic and tricyclic compounds. The light oil samples from the former wells contain more phenanthrene hydrocarbons and less naphthalene and biphenyl hydrocarbons. Meanwhile, the latter wells contain more naphthalene and biphenyl hydrocarbons and less phenanthrene hydrocarbons. The study on the source and depositional environment shows that the source material of light oils is mainly terrestrial higher plants input, and the depositional environment is oxidizing environment, but the latter wells has a higher 1,2,5-TMN and 1,2,5,6-TeMN relative content, and has a greater relative content of dibenzofuran and Pr/Ph value, reflecting that which has a greater degree of higher terrestrial plants input, with a stronger oxidative deposition environment. Alkyl-naphthalene, alkyl-phenanthrene and alkyl-dibenzothiophen are found to be optimal parameters for assessing thermal maturity characteristics, and the study shows that the light oils have reached mature-high mature oil characteristics, and the light oils in Well NB14-1, Well NB14-2 and NB14-3 have a higher maturity relatively.
2017, 35(1): 193-202.
doi: 10.14027/j.cnki.cjxb.2017.01.019
Abstract:
A series of biodegraded crude oils collected from Bohai Bay Basin, China, has been characterized by gas chromatography and gas chromatography-mass spectrometry. The results show that the distributions of tricyclic terpanes are abnormal in four severely biodegraded crude oil samples collected from Well PL15-8D and PL9-4. The four crude oils subjected to severe biodegradation which removed all regular steranes and most of the pentacyclic triterpanes, leaving the 25-norhopane as a dominant component in the m/z191 mass chromatograms of the saturated hydrocarbon fraction. However, the pregnanes, diasteranes and triaromatic steroid hydrocarbons appear to be relatively unaffected. The tricyclic terpanes show three abnormal distribution patterns, namely, predominant by C23, C20 and C23, C20 and C24 components, respectively, which are totally different from the source rocks and other crude oils from the Bohai Bay Basin. These abnormal distribution patterns of tricyclic terpanes are due to the preferential removal of C19~C23 over C24+ tricyclic terpanes. The ratios of the C19~C26 tricyclic terpanes to pregnane and the C19~C26 tricyclic terpanes to C27 diasteranes decrease with increasing biodegradation, but with different rates, indicating that alteration of tricyclic terpanes with different carbon numbers occurs concurrently and the C24+ tricyclic terpanes are much more resistant to microbial degradation than the C19~C23 compounds. The changes of the ratios of C19~C25 tricyclic terpanes to C26 tricyclic terpane also suggest that the C19~C23 tricyclic terpanes are more susceptible to biodegradation than C24~C26 tricyclic terpanes and C24~C25 tricyclic terpanes are more susceptible to biodegradation than C26 tricyclic terpanes. The absolute concentration of C19 to C26 tricyclic terpanes and the relative depletion of C19~C23 tricyclic terpanes show that the susceptibility of C19 to C26 tricyclic terpanes to biodegradation decreases with increasing carbon number of the components. In general, the resistance to biodegradation of tricyclic terpane series is found to increase with increasing of the carbon number, except for the C20 tricyclic terpane. No demethylated tricyclic terpanes were detected in these severely biodegraded oils, suggesting that the degradation pathway of tricyclic terpanes occurred in reservoirs might be similar to hopanes, namely, they could be degraded without forming demethylated counterparts. A putative biodegradation pathway is that the methyl on the side chain of the C cycle was oxidized to carboxyl. The prerequisite of taking tricyclic terpanes as indicators for correlating degraded oil is that they are unaffected by microbe. The regular steranes, hopanes and tricyclics which have been degraded can not be used for oil-source correlation research, while, the triaromatic steranes are non-degraded and can be used for such purpose.
A series of biodegraded crude oils collected from Bohai Bay Basin, China, has been characterized by gas chromatography and gas chromatography-mass spectrometry. The results show that the distributions of tricyclic terpanes are abnormal in four severely biodegraded crude oil samples collected from Well PL15-8D and PL9-4. The four crude oils subjected to severe biodegradation which removed all regular steranes and most of the pentacyclic triterpanes, leaving the 25-norhopane as a dominant component in the m/z191 mass chromatograms of the saturated hydrocarbon fraction. However, the pregnanes, diasteranes and triaromatic steroid hydrocarbons appear to be relatively unaffected. The tricyclic terpanes show three abnormal distribution patterns, namely, predominant by C23, C20 and C23, C20 and C24 components, respectively, which are totally different from the source rocks and other crude oils from the Bohai Bay Basin. These abnormal distribution patterns of tricyclic terpanes are due to the preferential removal of C19~C23 over C24+ tricyclic terpanes. The ratios of the C19~C26 tricyclic terpanes to pregnane and the C19~C26 tricyclic terpanes to C27 diasteranes decrease with increasing biodegradation, but with different rates, indicating that alteration of tricyclic terpanes with different carbon numbers occurs concurrently and the C24+ tricyclic terpanes are much more resistant to microbial degradation than the C19~C23 compounds. The changes of the ratios of C19~C25 tricyclic terpanes to C26 tricyclic terpane also suggest that the C19~C23 tricyclic terpanes are more susceptible to biodegradation than C24~C26 tricyclic terpanes and C24~C25 tricyclic terpanes are more susceptible to biodegradation than C26 tricyclic terpanes. The absolute concentration of C19 to C26 tricyclic terpanes and the relative depletion of C19~C23 tricyclic terpanes show that the susceptibility of C19 to C26 tricyclic terpanes to biodegradation decreases with increasing carbon number of the components. In general, the resistance to biodegradation of tricyclic terpane series is found to increase with increasing of the carbon number, except for the C20 tricyclic terpane. No demethylated tricyclic terpanes were detected in these severely biodegraded oils, suggesting that the degradation pathway of tricyclic terpanes occurred in reservoirs might be similar to hopanes, namely, they could be degraded without forming demethylated counterparts. A putative biodegradation pathway is that the methyl on the side chain of the C cycle was oxidized to carboxyl. The prerequisite of taking tricyclic terpanes as indicators for correlating degraded oil is that they are unaffected by microbe. The regular steranes, hopanes and tricyclics which have been degraded can not be used for oil-source correlation research, while, the triaromatic steranes are non-degraded and can be used for such purpose.
