2008 Vol. 33, No. 4
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2008, 33(4): 431-442.
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Abstract:
The South China Sea (SCS) has relatively complete physiographical units and ecologic environments, where there are a lot of preservative radiolarian species and specimens in the sediments.Methodsof quantitative statistics and clustering were used to analyze the radiolarian assemblages in surface sediments of the SCS, to understand their compositions and distributions.Resultsof clustering analyses show that the radiolarian assemblages in the SCS surface sediments may mainly be grouped as five types, including assemblages of shallow shelf, coral slope, central basin, southern rich nutrition and summer upwelling, corresponding to different special marine environments with certain radiolarian communities of dominant species.It is also indicated that the north, central and south of the SCS present as three areas of different ecologic conditions, such as the northern sea area affected mostly by terrigenous input, central sea area controlled mainly by the central basin waters, submarine volcanic and summer upwelling activities and the southern sea area falls obviously into a typical coral environment, where there is a better ecologic and sedimental environment for radiolarian existing than that in the northern sea area.The radiolarian assemblages and distributions in the SCS surface sediments well reflect different marine ecologic conditions and sedimental environments, which may be used as reliable evidences for paleoceanographic explanation.
The South China Sea (SCS) has relatively complete physiographical units and ecologic environments, where there are a lot of preservative radiolarian species and specimens in the sediments.Methodsof quantitative statistics and clustering were used to analyze the radiolarian assemblages in surface sediments of the SCS, to understand their compositions and distributions.Resultsof clustering analyses show that the radiolarian assemblages in the SCS surface sediments may mainly be grouped as five types, including assemblages of shallow shelf, coral slope, central basin, southern rich nutrition and summer upwelling, corresponding to different special marine environments with certain radiolarian communities of dominant species.It is also indicated that the north, central and south of the SCS present as three areas of different ecologic conditions, such as the northern sea area affected mostly by terrigenous input, central sea area controlled mainly by the central basin waters, submarine volcanic and summer upwelling activities and the southern sea area falls obviously into a typical coral environment, where there is a better ecologic and sedimental environment for radiolarian existing than that in the northern sea area.The radiolarian assemblages and distributions in the SCS surface sediments well reflect different marine ecologic conditions and sedimental environments, which may be used as reliable evidences for paleoceanographic explanation.
2008, 33(4): 443-457.
Abstract:
Focusing on understanding geological time, stratigraphy has made three milestone achievements characterized by lithostratigraphy, biostratigraphy and cyclostratigraphy.A geological time scale based mainly on both biostratigraphy and radiometric dating created stratigraphic refulgence, but at the cost of lowered accuracy and preciseness and weakened function of serving the human society.The paper briefly reviews the long history of stratigraphy in understanding geological time, expatiating on the origin, formation and development of cyclostratigraphical concepts, and on similarities and differences between cyclostratigraphy and sequence stratigraphy in the scientific objective, research content and method.Taking the Late Devonian Frasnian-Famennian transitional marine carbonate successions from Guangxi, South China as an example, the paper discusses the research method of cyclostratigraphy, and the prospect and scientific significance of cyclostratigraphy in establishing the high-resolution geological time scale which bridges the gap between the existing geological time coordinate with time unit of million-year scale and anthropological time units such as millennium, century and year, etc.
Focusing on understanding geological time, stratigraphy has made three milestone achievements characterized by lithostratigraphy, biostratigraphy and cyclostratigraphy.A geological time scale based mainly on both biostratigraphy and radiometric dating created stratigraphic refulgence, but at the cost of lowered accuracy and preciseness and weakened function of serving the human society.The paper briefly reviews the long history of stratigraphy in understanding geological time, expatiating on the origin, formation and development of cyclostratigraphical concepts, and on similarities and differences between cyclostratigraphy and sequence stratigraphy in the scientific objective, research content and method.Taking the Late Devonian Frasnian-Famennian transitional marine carbonate successions from Guangxi, South China as an example, the paper discusses the research method of cyclostratigraphy, and the prospect and scientific significance of cyclostratigraphy in establishing the high-resolution geological time scale which bridges the gap between the existing geological time coordinate with time unit of million-year scale and anthropological time units such as millennium, century and year, etc.
2008, 33(4): 458-464.
Abstract:
The debate of the definition of lithostratigraphic units in formation scale in eastern Tianshan was presented long before.During 1∶50000 regional geology and mineral survey, some conodonts fossils such as Streptognathodus suberectus, Idiognathoides sinuate etc.and coral fossils such as Pelalaxis intermedia, Lithostrotionella rarivesicula, Fomichevella kiaeri were discovered in the two limestones of the Qi'eshan Group from Kumutag sand-rigde.The age of conodont fossils within the stratum was determined as the Upper Carboniferous Luosu Stage-Dalaan Stage, and the age of coral fossils within the stratum was confirmed as the Dalaan Stage-Xiaodushan Stage.Therefore, they belong to Dikan'er Formation and Qishan Formation.The two volcanic rocks separated by two sedimentary rocks have some characteristics such as regular distribution in space, diagnostic rock association, distinguishable in field geolgy and comparable in study area.And their ages are Early Carboniferous and Late Carboniferous respectively.Therefore, the Qi'eshan Group can be further divided into four lithostratigraphic units according to formation scale from early to late, which are Xiaorequanzi Formation, Dikan'er Formation, Qi'eshan Formation and Qishan Formation.
The debate of the definition of lithostratigraphic units in formation scale in eastern Tianshan was presented long before.During 1∶50000 regional geology and mineral survey, some conodonts fossils such as Streptognathodus suberectus, Idiognathoides sinuate etc.and coral fossils such as Pelalaxis intermedia, Lithostrotionella rarivesicula, Fomichevella kiaeri were discovered in the two limestones of the Qi'eshan Group from Kumutag sand-rigde.The age of conodont fossils within the stratum was determined as the Upper Carboniferous Luosu Stage-Dalaan Stage, and the age of coral fossils within the stratum was confirmed as the Dalaan Stage-Xiaodushan Stage.Therefore, they belong to Dikan'er Formation and Qishan Formation.The two volcanic rocks separated by two sedimentary rocks have some characteristics such as regular distribution in space, diagnostic rock association, distinguishable in field geolgy and comparable in study area.And their ages are Early Carboniferous and Late Carboniferous respectively.Therefore, the Qi'eshan Group can be further divided into four lithostratigraphic units according to formation scale from early to late, which are Xiaorequanzi Formation, Dikan'er Formation, Qi'eshan Formation and Qishan Formation.
2008, 33(4): 465-473.
Abstract:
In the present study, we describe the formation of iron ores collected in the intertidal zone of the Zhoushan Island in Zhejiang Province in the East China Sea, where ancient-wood layers were buried.Morphological, mineralogical and geochemical analyses were performed on the iron ores and the surrounding geological material.The results show that the iron ores are not only composed of spherical and fibre-like aggregates of goethite, but also contain the dead bacterial sheaths, which present morphological characteristics reminiscent of bacterial activity.Similar present-day biomineralization characteristics were also observed in an iron seepage system near the studied intertidal zone.The presence of Leptothrix-like sheaths and Gallionella-like stalks in the present-day environment promoted the oxidization of Fe2+ to Fe3+ and the rapid precipitation of biogenic iron oxides around the bacteria.The role of bacteria in mineral formation in the seepage area is believed to represent an analogue mechanism for the formation of the iron ores.It is hypothesized that the degradation of the ancient wood provided humic substances which accelerated the leaching process of iron from the surrounding bedrock and soils, and then created local biogeochemical conditions which led to the biomineralization of the iron ores.The present findings help elucidate the role of bacteria and humic substances in the formation of iron ores in the history time.
In the present study, we describe the formation of iron ores collected in the intertidal zone of the Zhoushan Island in Zhejiang Province in the East China Sea, where ancient-wood layers were buried.Morphological, mineralogical and geochemical analyses were performed on the iron ores and the surrounding geological material.The results show that the iron ores are not only composed of spherical and fibre-like aggregates of goethite, but also contain the dead bacterial sheaths, which present morphological characteristics reminiscent of bacterial activity.Similar present-day biomineralization characteristics were also observed in an iron seepage system near the studied intertidal zone.The presence of Leptothrix-like sheaths and Gallionella-like stalks in the present-day environment promoted the oxidization of Fe2+ to Fe3+ and the rapid precipitation of biogenic iron oxides around the bacteria.The role of bacteria in mineral formation in the seepage area is believed to represent an analogue mechanism for the formation of the iron ores.It is hypothesized that the degradation of the ancient wood provided humic substances which accelerated the leaching process of iron from the surrounding bedrock and soils, and then created local biogeochemical conditions which led to the biomineralization of the iron ores.The present findings help elucidate the role of bacteria and humic substances in the formation of iron ores in the history time.
2008, 33(4): 474-486.
Abstract:
This paper carries out a study of U-Pb zircon dating using LA-ICP-MS method, geochemical and Sr-Nd-Pb isotopic compositions of the Dangchuan and Shimen granites in Dangchuan area of the West Qinling orogenic belt.The results show that the Dangchuan and Shimen granites have magma crystallization ages of 438±3Ma and 220±2Ma, respectively.Geochemical signatures indicate that the Dangchuan granite is similar to C-type adakitic rock that resulted from partial melting of thickened crustal material and the Shimen granite is similar to common anatexis-type granite.The Dangchuan granite has initial Sr isotopic ratios (ISr) of 0.70660-0.70929 and εNd (t) values of-2.24 to-4.48, and the Shimen granite has ISr of 0.70581-0.70804 and εNd (t) values of-3.73 to-4.72.The Sr-Nd isotopic composition of the Dangchuan and Shimen granites provides an additional eveidence that both the granites were derived from crustal source.However, Pb isotopic composition of the Dangchuan granite is distinct from that of the Shimen granite.The Dangchuan granite is characterized by more radiogenic Pb isotopic composition with initial Pb isotopic ratios of 206Pb/204Pb=18.288-18.484, 207Pb/204Pb=15.677-15.693 and 208Pb/204Pb=38.182-38.283, whereas the Shimen granite is characterized by relatively less radiogenic Pb isotopic composition with initial Pb isotopic ratios of 206Pb/204Pb=17.989-18.189, 207Pb/204Pb=15.560-15.567 and 208Pb/204Pb=37.982-38.000.It is strongly suggested that the magmas of the Dangchuan and Shimen granites were derived from different crustal sources.The Early Paleozoic and Early Mesozoic tectono-magma events, their petrogenesis and magma sources of Dangchuan area in the West Qinling orogenic belt can be comparable with those of the North Qinling tectonic unit in the East Qinling orogenic belt, indicating that the North Qinling tectonic unit can be extended to Dangchuan area in the West Qinling orogenic belt.In the East Qinling orogenic belt, the South Qinling continental crust was subducted beneath the North Qinling crust due to Early Mesozoic continental collision between the Yangtze and North China blocks.This continental subduction also occurred in the West Qinling orogenic belt.
This paper carries out a study of U-Pb zircon dating using LA-ICP-MS method, geochemical and Sr-Nd-Pb isotopic compositions of the Dangchuan and Shimen granites in Dangchuan area of the West Qinling orogenic belt.The results show that the Dangchuan and Shimen granites have magma crystallization ages of 438±3Ma and 220±2Ma, respectively.Geochemical signatures indicate that the Dangchuan granite is similar to C-type adakitic rock that resulted from partial melting of thickened crustal material and the Shimen granite is similar to common anatexis-type granite.The Dangchuan granite has initial Sr isotopic ratios (ISr) of 0.70660-0.70929 and εNd (t) values of-2.24 to-4.48, and the Shimen granite has ISr of 0.70581-0.70804 and εNd (t) values of-3.73 to-4.72.The Sr-Nd isotopic composition of the Dangchuan and Shimen granites provides an additional eveidence that both the granites were derived from crustal source.However, Pb isotopic composition of the Dangchuan granite is distinct from that of the Shimen granite.The Dangchuan granite is characterized by more radiogenic Pb isotopic composition with initial Pb isotopic ratios of 206Pb/204Pb=18.288-18.484, 207Pb/204Pb=15.677-15.693 and 208Pb/204Pb=38.182-38.283, whereas the Shimen granite is characterized by relatively less radiogenic Pb isotopic composition with initial Pb isotopic ratios of 206Pb/204Pb=17.989-18.189, 207Pb/204Pb=15.560-15.567 and 208Pb/204Pb=37.982-38.000.It is strongly suggested that the magmas of the Dangchuan and Shimen granites were derived from different crustal sources.The Early Paleozoic and Early Mesozoic tectono-magma events, their petrogenesis and magma sources of Dangchuan area in the West Qinling orogenic belt can be comparable with those of the North Qinling tectonic unit in the East Qinling orogenic belt, indicating that the North Qinling tectonic unit can be extended to Dangchuan area in the West Qinling orogenic belt.In the East Qinling orogenic belt, the South Qinling continental crust was subducted beneath the North Qinling crust due to Early Mesozoic continental collision between the Yangtze and North China blocks.This continental subduction also occurred in the West Qinling orogenic belt.
2008, 33(4): 487-503.
Abstract:
Trace element compositions of rutiles in eclogites from the Chinese Continental Scientific Drilling (CCSD) main hole were analyzed using LA-ICP-MS.The results indicate that Nb and Ta contents of rutiles are significantly controlled by whole rock compositions, while Zr and Hf show no obvious dependence on the whole rock compositions.Coupled with enrichments of Pb and Sr at the rim of the interstitial rutiles, Zr contents decrease from the core to the rim.Nb/Ta ratios of rutiles are much higher than those of the parent eclogites.Moreover, Nb/Ta ratios of rutile show no correlation with Nb and Ta concentrations and Nb/Ta ratios of the parent eclogites, while Nb/Ta ratios of eclogites correlate negatively with the TiO2 contents.These observations suggest that rutiles were metamorphic products of the Triassic ultrahigh pressure (UHP) metamorphism, rather than formed by crystallization in equilibrium with melt.The partially decoupling between rutiles and parent eclogites in terms of Nb/Ta ratios implies that although rutiles are the dominant carriers of Nb and Ta in eclogites, they might not completely reflect the Nb/Ta ratios of the parent eclogites, especially for low-Ti eclogites (TiO2 < 0.5%).Nb/Ta ratios of rutiles are dominated by the combination of Nb, Ta and TiO2 contents of the parent eclogites.The positive correlations between temperatures recorded by Zr contents and the size of rutile indicate that Zr in rutile was not immediately closed after rutile formation.On the other hand, the enrichments of Pb and Sr at the rim of the interstitial rutiles suggest influence of subsequent fluids on the compositions of rutiles, which could have affected Zr of the rutiles as well.This study provides the evidence for the Zr diffusion at a high temperature and fluid activity that could have contributed to the lower temperatures (598-827℃) recorded by Zr-in-rutile for the eclogites from the Dabie-Sulu UHP terrane.
Trace element compositions of rutiles in eclogites from the Chinese Continental Scientific Drilling (CCSD) main hole were analyzed using LA-ICP-MS.The results indicate that Nb and Ta contents of rutiles are significantly controlled by whole rock compositions, while Zr and Hf show no obvious dependence on the whole rock compositions.Coupled with enrichments of Pb and Sr at the rim of the interstitial rutiles, Zr contents decrease from the core to the rim.Nb/Ta ratios of rutiles are much higher than those of the parent eclogites.Moreover, Nb/Ta ratios of rutile show no correlation with Nb and Ta concentrations and Nb/Ta ratios of the parent eclogites, while Nb/Ta ratios of eclogites correlate negatively with the TiO2 contents.These observations suggest that rutiles were metamorphic products of the Triassic ultrahigh pressure (UHP) metamorphism, rather than formed by crystallization in equilibrium with melt.The partially decoupling between rutiles and parent eclogites in terms of Nb/Ta ratios implies that although rutiles are the dominant carriers of Nb and Ta in eclogites, they might not completely reflect the Nb/Ta ratios of the parent eclogites, especially for low-Ti eclogites (TiO2 < 0.5%).Nb/Ta ratios of rutiles are dominated by the combination of Nb, Ta and TiO2 contents of the parent eclogites.The positive correlations between temperatures recorded by Zr contents and the size of rutile indicate that Zr in rutile was not immediately closed after rutile formation.On the other hand, the enrichments of Pb and Sr at the rim of the interstitial rutiles suggest influence of subsequent fluids on the compositions of rutiles, which could have affected Zr of the rutiles as well.This study provides the evidence for the Zr diffusion at a high temperature and fluid activity that could have contributed to the lower temperatures (598-827℃) recorded by Zr-in-rutile for the eclogites from the Dabie-Sulu UHP terrane.
2008, 33(4): 504-514.
Abstract:
A wide range of well-preserved Jehol biota fossils have been found in the sedimentary rocks of the Yixian Formation from the Sihetun area in West Liaoning Province.We have analyzed major and trace element compositions of fifty samples of clastic sedimentary rocks from this section.Combined with data previously publicized by others, the conclusions are as follows: firstly, this set of sedimentary rocks share similar chemical characteristics to the associated volcanic rocks, with LREE enrichment, flat HREE, high-Mg#, high contents of large ion lithophile element (LILE) and variable REE anomalies.Their spider diagrams show significant negative Nb and positive Pb anomalies.It is deducted that the sedimentary rocks were mainly derived from the high-Mg# volcanic rocks from the Yixian Formation.Secondly, these sedimentary rocks show variable Eu anomalies (Eu/Eu=0.48-1.81) with the average value being 0.75.The positive anomaly is due to presence of plagioclase.Moreover, tectonic discrimination diagram of Th-Co-Zr/10 suggests that these sedimentary rocks were formed in a tectonic setting similar to the continental island arc and active continental margin.
A wide range of well-preserved Jehol biota fossils have been found in the sedimentary rocks of the Yixian Formation from the Sihetun area in West Liaoning Province.We have analyzed major and trace element compositions of fifty samples of clastic sedimentary rocks from this section.Combined with data previously publicized by others, the conclusions are as follows: firstly, this set of sedimentary rocks share similar chemical characteristics to the associated volcanic rocks, with LREE enrichment, flat HREE, high-Mg#, high contents of large ion lithophile element (LILE) and variable REE anomalies.Their spider diagrams show significant negative Nb and positive Pb anomalies.It is deducted that the sedimentary rocks were mainly derived from the high-Mg# volcanic rocks from the Yixian Formation.Secondly, these sedimentary rocks show variable Eu anomalies (Eu/Eu=0.48-1.81) with the average value being 0.75.The positive anomaly is due to presence of plagioclase.Moreover, tectonic discrimination diagram of Th-Co-Zr/10 suggests that these sedimentary rocks were formed in a tectonic setting similar to the continental island arc and active continental margin.
2008, 33(4): 515-522.
Abstract:
The study of burnt rocks faciliates the discussion on the tectonic movement, paleoclimate and paleogeography that coal seams were subject to after they were formed.In order to obtain the basic data on the features of the burnt rocks, a systematic study of petrology and REE geochemistry on burnt rocks in Shenmu, North Shaanxi Province, has been done, using the methods of SEM, EDS, susceptibility measurements and ICP-MS.The burnt rocks are divided into two series in the section: the melted rocks and the baked rocks.SEM and EDS analyses reveal that all the minerals show burnt and melted traces, and there are no clay minerals except illite found in the burnt rocks.Susceptibility measurements reveal that the burnt rocks have abnormally high susceptibility values, whereas a geochemical analysis shows that the REE distribution pattern of burnt rocks is similar to that of sedimentary rocks (initial rocks).In the longitudinal section, the REE gradually decreases as the burning degree increases (from baked rocks to melted rocks), and the total REE of melted rocks is obviously lower than that of baked rocks.Besides, the melted rocks show apparent negative Ce anomalies, while the baked rocks show no anomaly of Ce, and sometimes even show positive anomalies.
The study of burnt rocks faciliates the discussion on the tectonic movement, paleoclimate and paleogeography that coal seams were subject to after they were formed.In order to obtain the basic data on the features of the burnt rocks, a systematic study of petrology and REE geochemistry on burnt rocks in Shenmu, North Shaanxi Province, has been done, using the methods of SEM, EDS, susceptibility measurements and ICP-MS.The burnt rocks are divided into two series in the section: the melted rocks and the baked rocks.SEM and EDS analyses reveal that all the minerals show burnt and melted traces, and there are no clay minerals except illite found in the burnt rocks.Susceptibility measurements reveal that the burnt rocks have abnormally high susceptibility values, whereas a geochemical analysis shows that the REE distribution pattern of burnt rocks is similar to that of sedimentary rocks (initial rocks).In the longitudinal section, the REE gradually decreases as the burning degree increases (from baked rocks to melted rocks), and the total REE of melted rocks is obviously lower than that of baked rocks.Besides, the melted rocks show apparent negative Ce anomalies, while the baked rocks show no anomaly of Ce, and sometimes even show positive anomalies.
2008, 33(4): 523-530.
Abstract:
Sinian Dongpo Formation overlying tillite of Luoquan Formation consists mainly of shales, silty shales and siltstones.Lenticular dolomites, dolomitic siltstones and sandstones are unexpectedly found in Dongpo Formation.Soft-sediment reformation occurs in dolomitic siltstones which are inconsistent with the shales of Dongpo Formation.Negative anomaly of δ13C is found in lenticular dolomites and dolomitic siltstones.δ13C of lenticular dolomites is-4.19‰ to-6.18‰ and δ13C of dolomitic siltstones mostly between-2‰ to-4‰.So it is considered that lenticular dolomites, dolomitic siltstones and sandstones in Dongpo Formation, similar to cap dolomite of Sinian and seep carbonates overlying tillite of Sturtian ice-age in South China, have been formed by interaction between CO2 seeped from gas hydrate and Mg2+ in sea water, that is, lenticular dolomites, dolomitic siltstones and sandstones in Dongpo Formation are the cause of formation of cold spring.
Sinian Dongpo Formation overlying tillite of Luoquan Formation consists mainly of shales, silty shales and siltstones.Lenticular dolomites, dolomitic siltstones and sandstones are unexpectedly found in Dongpo Formation.Soft-sediment reformation occurs in dolomitic siltstones which are inconsistent with the shales of Dongpo Formation.Negative anomaly of δ13C is found in lenticular dolomites and dolomitic siltstones.δ13C of lenticular dolomites is-4.19‰ to-6.18‰ and δ13C of dolomitic siltstones mostly between-2‰ to-4‰.So it is considered that lenticular dolomites, dolomitic siltstones and sandstones in Dongpo Formation, similar to cap dolomite of Sinian and seep carbonates overlying tillite of Sturtian ice-age in South China, have been formed by interaction between CO2 seeped from gas hydrate and Mg2+ in sea water, that is, lenticular dolomites, dolomitic siltstones and sandstones in Dongpo Formation are the cause of formation of cold spring.
2008, 33(4): 531-537.
Abstract:
Polynomial and power functions have been used to fit the time-depth relation based on data from wells in the Baiyun sag, Pearl River Mouth basin, northern South China Sea.For the time-depth relation of the strata deeper than the termination depth of the wells, the 3rd-order polynomial function may be applicable only if the coefficient of its 3rd-oder term is negative and the coefficient of its 2nd-order term is positive.This function, however, is not applicable to the depth because it inevitably leads to the velocity inversion at depth.The fitting of 2nd-order polynomial function does not cause the velocity inversion, but the velocity increases linearly towards the depth, violating the natural law that the velocity increment decreases downwards in the Earth's strata and crust.The power function D=atb+c with 1 < b < 2 may approximate the time-depth relation for both shallow and deeper strata and crust without the problems encountered by using 3rd-and 2nd-order polynomial functions.It is not guaranteed, however, that for each well the fitted power function is suitable for the entire region and all the depths.For a regional time-depth conversion, the best practice is to divide the region and fit the power function for multiple wells, and then select the best functions for each sub-regions.
Polynomial and power functions have been used to fit the time-depth relation based on data from wells in the Baiyun sag, Pearl River Mouth basin, northern South China Sea.For the time-depth relation of the strata deeper than the termination depth of the wells, the 3rd-order polynomial function may be applicable only if the coefficient of its 3rd-oder term is negative and the coefficient of its 2nd-order term is positive.This function, however, is not applicable to the depth because it inevitably leads to the velocity inversion at depth.The fitting of 2nd-order polynomial function does not cause the velocity inversion, but the velocity increases linearly towards the depth, violating the natural law that the velocity increment decreases downwards in the Earth's strata and crust.The power function D=atb+c with 1 < b < 2 may approximate the time-depth relation for both shallow and deeper strata and crust without the problems encountered by using 3rd-and 2nd-order polynomial functions.It is not guaranteed, however, that for each well the fitted power function is suitable for the entire region and all the depths.For a regional time-depth conversion, the best practice is to divide the region and fit the power function for multiple wells, and then select the best functions for each sub-regions.
2008, 33(4): 538-546.
Abstract:
Based on the three-layer model of seafloor hydrothermal sulfide deposit, the deposit models with diverse topography have been built.With numerical approach, the distribution of inner thermal and flow fields of the large hydrothermal sulfide deposit model featuring sloping crust and relief top is simulated to discuss the effect of the different relief on the formation processes of the hydrothermal sulfide deposit.The results indicate: (1) the sloping seafloor has a limited influence on the distribution of thermal and flow fields within the hydrothermal sulfide deposit. (2) The relief top is one of the dominating factors that control distribution of the thermal and flow fields within the hydrothermal sulfide deposit. (3) Hydrothermal sulfides precipitate and accumulate rapidly near the region of focused venting, thus easily forming high relief.As the continuous rising of this high relief, the distribution mode of the thermal and flow fields gradually changes, which causes the hydrothermal venting to gradually cease or redirect.
Based on the three-layer model of seafloor hydrothermal sulfide deposit, the deposit models with diverse topography have been built.With numerical approach, the distribution of inner thermal and flow fields of the large hydrothermal sulfide deposit model featuring sloping crust and relief top is simulated to discuss the effect of the different relief on the formation processes of the hydrothermal sulfide deposit.The results indicate: (1) the sloping seafloor has a limited influence on the distribution of thermal and flow fields within the hydrothermal sulfide deposit. (2) The relief top is one of the dominating factors that control distribution of the thermal and flow fields within the hydrothermal sulfide deposit. (3) Hydrothermal sulfides precipitate and accumulate rapidly near the region of focused venting, thus easily forming high relief.As the continuous rising of this high relief, the distribution mode of the thermal and flow fields gradually changes, which causes the hydrothermal venting to gradually cease or redirect.
2008, 33(4): 547-554.
Abstract:
In the Songliao basin, a network of normal faults, characterized by relative variability in strike and short length, was widely produced in sediments accumulated during the early post rifting.Based upon 3-D high-resolution seismic data from the Taipingtun block in the Daqing oilfield, a systematic study of faults in the block was made in this paper.Calculated extensional strain is 0.0594±0.0209 in these rocks, and turns markedly smaller in the above or the below layers.This is inconsistent with the generally monotonous increase of extensional strain in rift basin with buried depth, implying that the simple regional extension could not account for this abnormal phenomenon, in turn.Volumetric contraction in the process of compaction, in our opinion, is the main reason for this phenomenon.Three main possible mechanisms for such layer-parallel shortening are hydrocarbon generation, transformation from smectite to illite, and lateral compaction, among which the last is considered the most important mechanism in this case.In addition, the preferred orientation of these normal faults indicates the influence of tectonic stress, although relatively slight, on their propagation.
In the Songliao basin, a network of normal faults, characterized by relative variability in strike and short length, was widely produced in sediments accumulated during the early post rifting.Based upon 3-D high-resolution seismic data from the Taipingtun block in the Daqing oilfield, a systematic study of faults in the block was made in this paper.Calculated extensional strain is 0.0594±0.0209 in these rocks, and turns markedly smaller in the above or the below layers.This is inconsistent with the generally monotonous increase of extensional strain in rift basin with buried depth, implying that the simple regional extension could not account for this abnormal phenomenon, in turn.Volumetric contraction in the process of compaction, in our opinion, is the main reason for this phenomenon.Three main possible mechanisms for such layer-parallel shortening are hydrocarbon generation, transformation from smectite to illite, and lateral compaction, among which the last is considered the most important mechanism in this case.In addition, the preferred orientation of these normal faults indicates the influence of tectonic stress, although relatively slight, on their propagation.
2008, 33(4): 555-563.
Abstract:
On the basis of the integrated analysis of the regional geology, tectonic and deposition data, combined with the achievements of the predecessor and the tectonic setting of Northwest China in the Mesozoic, and the research on the distribution feature of igneous rocks, balanced cross section, geothermal history, clastic mineral coefficients, this paper proves that in the Early-Middle Jurassic, the Northwest China formd in a comparatively weak outspread environments between the intensive collision and extrusion of two tectonic movements (Indosinian movement and Yanshan movement).The formation and transformation mechanism of the Yanqi basin in the Jurassic is also discussed.The structure framework was determined as forland basin type and the filled sequence was built.
On the basis of the integrated analysis of the regional geology, tectonic and deposition data, combined with the achievements of the predecessor and the tectonic setting of Northwest China in the Mesozoic, and the research on the distribution feature of igneous rocks, balanced cross section, geothermal history, clastic mineral coefficients, this paper proves that in the Early-Middle Jurassic, the Northwest China formd in a comparatively weak outspread environments between the intensive collision and extrusion of two tectonic movements (Indosinian movement and Yanshan movement).The formation and transformation mechanism of the Yanqi basin in the Jurassic is also discussed.The structure framework was determined as forland basin type and the filled sequence was built.
2008, 33(4): 565-571.
Abstract:
An oil cracking experiment in a closed system was designed and carried out.Based on this, kinetic models for describing gas generation from oil cracking and carbon isotope fractionation during this process were established, calibrated and then applied to Luojia 7 well located in the Luojiazhai gas field respectively.From the hydrocarbon generation kinetics research, it is found that Feixianguan Formation ancient oil reservoirs are "high efficiency gas sources".Their oil almost cracked completely in only about 20Ma, the period of 172-151Ma in the Middle-Late Jurassic.And along vertical faults, the oil-cracking gas migrated fast into the Feixianguan Formation oolitic reservoir with a high porosity and permeability and sealed perfectly by gypsum caprocks in the fourth member of Feixianguan Formation, Jialingjiang Formation and Leikoupo Formation.Consequently, a highly efficienct migration and accumulation of natural gas could be formed.Carbon isotope fractionation kinetics research proves that the accumulation participation rate of methane amounts to 87%.It is a new and effective method to evaluate the migration and accumulation efficiency of natural gas with a combination of hydrocarbon generation kinetics and carbon isotope fractionation kinetics.
An oil cracking experiment in a closed system was designed and carried out.Based on this, kinetic models for describing gas generation from oil cracking and carbon isotope fractionation during this process were established, calibrated and then applied to Luojia 7 well located in the Luojiazhai gas field respectively.From the hydrocarbon generation kinetics research, it is found that Feixianguan Formation ancient oil reservoirs are "high efficiency gas sources".Their oil almost cracked completely in only about 20Ma, the period of 172-151Ma in the Middle-Late Jurassic.And along vertical faults, the oil-cracking gas migrated fast into the Feixianguan Formation oolitic reservoir with a high porosity and permeability and sealed perfectly by gypsum caprocks in the fourth member of Feixianguan Formation, Jialingjiang Formation and Leikoupo Formation.Consequently, a highly efficienct migration and accumulation of natural gas could be formed.Carbon isotope fractionation kinetics research proves that the accumulation participation rate of methane amounts to 87%.It is a new and effective method to evaluate the migration and accumulation efficiency of natural gas with a combination of hydrocarbon generation kinetics and carbon isotope fractionation kinetics.
2008, 33(4): 572-576.
Abstract:
The research on approaches to calculate shaliness accurately and efficiently is of great significance for reservoir capacity estimation, as shaliness is an important parameter to evaluate the reservoir qualitatively and quantitatively.Considering logging data GR (natural gamma ray) is the measured output from its real value which passes through a convolution filter, this paper proposes a new approach to calculate the value of shaliness according to GR values directly.The inversion filter factors can be acquired from a certain sequence of the measured logging data GR and the real value of GR, by taking advantage of Wiener filtering.Then, shaliness can be expressed as a convolution between GR sequence and the inversion filter factors.Experiments on core-analysis-data supported oil well K13, Northwest China show that shaliness values obtained with the above method are much more accurate than those obtained with conventional methods.Its trial application in oil well K11, S44, S4 of the same exploration area has proved to be very effective.
The research on approaches to calculate shaliness accurately and efficiently is of great significance for reservoir capacity estimation, as shaliness is an important parameter to evaluate the reservoir qualitatively and quantitatively.Considering logging data GR (natural gamma ray) is the measured output from its real value which passes through a convolution filter, this paper proposes a new approach to calculate the value of shaliness according to GR values directly.The inversion filter factors can be acquired from a certain sequence of the measured logging data GR and the real value of GR, by taking advantage of Wiener filtering.Then, shaliness can be expressed as a convolution between GR sequence and the inversion filter factors.Experiments on core-analysis-data supported oil well K13, Northwest China show that shaliness values obtained with the above method are much more accurate than those obtained with conventional methods.Its trial application in oil well K11, S44, S4 of the same exploration area has proved to be very effective.
