Objective The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is one of the priority interval for shale gas exploration in the Sichuan Basin and its peripheral areas, and commercial shale gas has b...Objective The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is one of the priority interval for shale gas exploration in the Sichuan Basin and its peripheral areas, and commercial shale gas has been discovered from this interval in Jiaoshiba, Changning and Weiyuan shale gas fields in Sichuan Province. However, there is no significant discovery in other parts of the basin due to the different quality of black shale and the differences of tectonic evolution. Based on the progress of shale gas geological theory and exploration discoveries, as well as the theory of "source rock and cap rock controls on hydrocarbon accumulation", of the Upper Ordovician the main controlling factors Wufeng Formation-Lower Silurian Longmaxi Formation shale gas enrichment in the Sichuan Basin and its peripheral areas were analyzed, and the source rock and cap rock controls on the shale gas were also discussed. The results can provide new insights for the next shale gas exploration in this area.展开更多
Stable carbon isotope ratio (δ13Ccarb) analysis has been widely applied to the study of the inter-conti- nental or global marine carbonate correlation. Large-scale Cambrian-Ordovician carbonate platforms were devel...Stable carbon isotope ratio (δ13Ccarb) analysis has been widely applied to the study of the inter-conti- nental or global marine carbonate correlation. Large-scale Cambrian-Ordovician carbonate platforms were developed in the Tarim Basin. But research on fluctuation character- istics and global correlation of δ13Ccarb is still weak. Based on conodont biostratigraphy and whole-rock δ13Ccarb data in the Tahe oil-gas field of the northern Tarim Basin, the global correlation and genesis of positive carbon isotope excursions in the Darriwilian--Early Katian was exam- ined. Three positive excursions were identified in the Tahe oil-gas field including the middle Darriwilian carbon iso- tope excursion (MDICE), the Guttenberg carbon isotope excursion (GICE), and a positive excursion within the Pygodus anserinus conodont zone which is named the Early Sandbian carbon isotope excursion (ESICE) in this paper. Furthermore, these positive excursions had no direct relation with sea level fluctuations. MDICE and GICE could be globally correlated. The Middle-Upper Ordovi- cian Saergan Formation source rocks of the Kalpin outcrops were in accordance with the geological time of MDICE and ESICE. GICE had close relationship with the source rock of the Lianglitag Formation in the basin.Massive organic carbon burial was an important factor controlling the genesis of these positive excursions.展开更多
Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper Ordovician Wufeng For...Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Sichuan Basin and its surrounding areas and the relationship between these key interfaces with the deposition of organic-rich shale have been examined systematically. The Wufeng Formation-Longmaxi Formation has four types of marker beds with interface attributes, namely, the characteristic graptolite belt, Guanyinqiao Member shell layer, section with dense bentonite layers, and concretion section, which can be taken as key interfaces for stratigraphic division and correlation of the graptolite shale. The shell layer in Guanyinqiao Member is the most standard key interface in Wufeng Formation-Longmaxi Formation, and can also be regarded as an important indicator for judging the depositional scale of organic-rich shale in key areas. There are 8 dense bentonite sections of two types mainly occurring in 7 graptolite belts in these formations. They have similar interface characteristics with the shell layer in Guanyinqiao Member in thickness and natural gamma response, and belong to tectonic interfaces(i.e., event deposits). They have three kinds of distribution scales: whole region, large part of the region, and local part, and can be the third, fourth and fifth order sequence interfaces, and have a differential control effect on organic-rich shale deposits. The horizon the characteristic graptolite belt occurs first is the isochronous interface, which is not directly related to the deposition of organic-rich shale. Concretions only appear in local areas, and show poor stability in vertical and horizontal directions, and have no obvious relationship with the deposition of the organic-rich shale.展开更多
In order to reveal the relationship between the penecontemporaneous karstification and sedimentary microtopography in sequence stratigraphy,the sequence stratigraphic framework of Lianglitage formation in Upper Ordovi...In order to reveal the relationship between the penecontemporaneous karstification and sedimentary microtopography in sequence stratigraphy,the sequence stratigraphic framework of Lianglitage formation in Upper Ordovician is studied according to the well drilling,logging,geophysical data,detailed observations of core and the paleontology.The Lianglitage formation belongs to the sequence Ⅳ of Ordovician.The second member of Lianglitage formation is prograde sedimentation in highstand systems tract,and is favorable for developing reef flat.The development scale and thickness of reef flat are controlled by the variation of secondary sea level.The types and characteristics of karst in the highstand systems tract show that the late highstand systems tract is dissolved and cemented by the meteoric fresh water and mixed water.Penecontemporaneous karstification is developed at the top of parasequence and high place of geomorphology.Atmospheric diagenetic lens is formed.The developing regulations and controlling factors of penecontemporaneous karstification can provide new clues to the prediction and exploration of favorable reservoir in this area.展开更多
Based on the analysis of the morphology, order, cathodoluminescence and microelement of the calcite crystals in the pores of Upper Ordovician limestone in Tazhong oilfield of Tarim Basin, this paper suggest that the c...Based on the analysis of the morphology, order, cathodoluminescence and microelement of the calcite crystals in the pores of Upper Ordovician limestone in Tazhong oilfield of Tarim Basin, this paper suggest that the calcite crystals can divided 11 kinds of cements into three stages, and confirms their cementation sequence characteristics and formation environment. First stage of the cementation occurred on the bottom of the diagenetic environment, the product of which mainly are microcrystalline, fine shape, radiation fibrous, fasciculation, radiation axis shape calcites and a ball-like aragonite, whose filling pore is 0%–30%; the second one occurred in the atmospheric fresh water environment, with main cement types of crescent or pendulous shape, vadose silt, hyperplasia of coaxial, the filling pore of which is 5%–100%; the third one occurred in a burial environment, the crystals of which are bright and thick, mainly coarse sparry and poikilitic calcite, with filling pore of 5%–5%. In this paper, influence from each stage of cementation on porosity is analyzed, and pore evolution is established.展开更多
Based on field outcrop data,the effects of cyclic change of astronomical orbit and volcanic activity on organic carbon accumulation during the Late Ordovician-Early Silurian in the Upper Yangtze area were studied usin...Based on field outcrop data,the effects of cyclic change of astronomical orbit and volcanic activity on organic carbon accumulation during the Late Ordovician-Early Silurian in the Upper Yangtze area were studied using cyclostratigraphic and geochemical methods.d13 C and chemical index of alteration(CIA)were used to filter the astronomical orbit parameters recorded in sediments.It is found that the climate change driven by orbital cycle controls the fluctuations of sea level at different scales,obliquity forcing climate changes drive thermohaline circulation(THC)of the ocean,and THC-induced bottom currents transport nutrient-laden water from high latitude regions to the surface water of low-latitude area.Hence,THC is the main dynamic mechanism of organic-carbon supply.The marine productivity indexes of Ba/Al and Ni/Al indicate that volcanic activities had limited effect on marine productivity but had great influences on organic carbon preservation efficiency in late Hirnantian(E4).Paleo-ocean redox environmental indicators Th/U,V/Cr and V/(V+Ni)show that there is a significant correlation between volcanism and oxygen content in Paleo-ocean,so it is inferred that volcanisms controlled the organic carbon preservation efficiency by regulating oxygen content in Paleo-ocean,and the difference in volcanism intensity in different areas is an important factor for the differential preservation efficiency of organic carbon.The organic carbon input driven by orbital cycle and the preservation efficiency affected by volcanisms worked together to control the enrichment of organic carbon in the Middle–Upper Yangtze region.展开更多
Late Cambrian to Early Ordovician sedimentary rocks in the western Tarim Basin, Northwest China, are composed of shallow-marine platform carbonates. The Keping Uplift is located in the northwest region of this basin. ...Late Cambrian to Early Ordovician sedimentary rocks in the western Tarim Basin, Northwest China, are composed of shallow-marine platform carbonates. The Keping Uplift is located in the northwest region of this basin. On the basis of petrographic and geochemical features, four matrix replacement dolomites and one type of cement dolomite are identified. Matrix replacement dolomites include (1) micritic dolomites (MD1); (2) fine-coarse euhedral floating dolomites (MD2); (3) fine-coarse euhedral dolomites (MD3); and (4) medium-very coarse anhedral mosaic dolomites (MD4). Dolomite cement occurs in minor amounts as coarse saddle dolomite cement (CD1) that mostly fills vugs and fractures in the matrix dolomites. These matrix dolomites have δ18O values of ?9.7‰ to ?3.0‰ VPDB (Vienna Pee Dee Belemnite); δ13C values of ?0.8‰ to 3.5‰ VPDB; 87Sr/86Sr ratios of 0.708516 to 0.709643; Sr concentrations of 50 to 257 ppm; Fe contents of 425 to 16878 ppm; and Mn contents of 28 to 144 ppm. Petrographic and geochemical data suggest that the matrix replacement dolomites were likely formed by normal and evaporative seawater in early stages prior to chemical compaction at shallow burial depths. Compared with matrix dolomites, dolomite cement yields lower δ18O values (?12.9‰ to ?9.1‰ VPDB); slightly lower δ13C values (?1.6‰-0.6‰ VPDB); higher 87Sr/86Sr ratios (0.709165-0.709764); and high homogenization temperature (Th) values (98°C-225°C) and salinities (6 wt%-24 wt% NaCl equivalent). Limited data from dolomite cement shows a low Sr concentration (58.6 ppm) and high Fe and Mn contents (1233 and 1250 ppm, respectively). These data imply that the dolomite cement precipitated from higher temperature hydrothermal salinity fluids. These fluids could be related to widespread igneous activities in the Tarim Basin occurring during Permian time when the host dolostones were deeply buried. Faults likely acted as important conduits that channeled dolomitizing fluids from the underlying strata into the basal carbonates, leading to intense dolomitization. Therefore, dolomitization, in the Keping Uplift area is likely related to evaporated seawater via seepage reflux in addition to burial processes and hydrothermal fluids.展开更多
Figuring out whether the sedimentary provenance regions of the thick deep-water turbidite systems deposited during Middle–Upper Ordovician in South Quruqtagh are the intracontinental uplifts or the peripheral orogeni...Figuring out whether the sedimentary provenance regions of the thick deep-water turbidite systems deposited during Middle–Upper Ordovician in South Quruqtagh are the intracontinental uplifts or the peripheral orogenic belts is of great significance for us to understand the tectono-sedimentary nature of the northeastern Tarim Basin and basin-range coupling processes in the middle Paleozoic.This paper reports the in situ LA-ICP-MS U–Pb ages and Hf isotope data on detrital zircons from two Middle–Upper Ordovician sandstone samples which were collected from the Charchag Formation and the Zatupo Formation in South Quruqtagh,respectively.The results show that the studied two samples have extremely similar U–Pb age patterns and Hf isotopic compositions,reflecting multiphase tectonothermal events with age groups of 527–694,713–870 Ma(peaking at 760 Ma),904–1,090,1,787–2,094 Ma(peaking at 1,975 Ma)and 2,419–2,517 Ma.Combining previous studies,the presence of age groups of 713–1,090 and1,787–2,094 Ma,respectively,demonstrates that Tarim had ever been a part of Rodinia and Columbia supercontinent.Moreover,98%of 713–870 Ma detrital zircons are characterized by negative e Hf(t)values ranging from-38.07 to-0.61,which are highly consistent with those of Neoproterozoic granites from the Quruqtagh area.No Early Paleozoic ages(*470–500 Ma)signifying subduction or collision events in Altyn Tagh were detected in the two samples,indicating that the Middle–Late Ordovician sediments in South Quruqtagh and northern Mangar depression were mainly derived from intracontinental uplifts,i.e.,the North Quruqtagh uplift or the Tabei paleo-uplift,rather than the Altyn Tagh.In conjunction with regional sedimentary-tectonic background and previous studies,we proposed preliminarily that the northeastern Tarim remained as a passive continental margin in Late Ordovician and changed into an active-continental margin in Silurian due to the southward subduction of the South-Tianshan Ocean.展开更多
Patterns of septal insertion were analyzed in three species of Agetolites from the Upper Ordovician in Yushan County,Jiangxi Province of China.Agetolites is a peculiar coral possessing features of both tabulate(e.g.,m...Patterns of septal insertion were analyzed in three species of Agetolites from the Upper Ordovician in Yushan County,Jiangxi Province of China.Agetolites is a peculiar coral possessing features of both tabulate(e.g.,mural pores)and rugose corals(e.g.,well-developed septa).Septal patterns have long been considered to be indicative of the origin of corals,and the pattern of septal insertion in Agetolites has been known to be typical of rugosans.This study,however,reveals that the patterns in Agetolites are rather random and cannot be an indication of its rugosan affinity.It is suggested that Agetolites probably has a closer phylogenetic relation with tabulates than with either rugosans or scleractinians.Two insertion patterns of the first six septa were detected(Types Ⅰ and Ⅱ).Type I is interpreted as scleractinian pattern whereas Type II as typical pattern of rugosans.Types 1 and 2 are septal patterns of the second stage recognized in this study.Type 1 insertion occurs in six sectors located in the center of each interseptal space,suggesting scleractinian pattern of insertion.Type 2 insertion occurs in four sectors that are comparable with alar and counter sectors in rugose corals.Four patterns of the third(Types A–D)and fourth stages(Types a–d)of septal insertion have also been recognized in Agetolites.Type A only occurs in two adjacent sectors possibly due to the incompleteness of septal development.Type B presents rugosan pattern.Types C and D resemble scleractinian pattern in most aspects.Types a,b,and c present no clear attribution to rugosan or scleractinian pattern probably due to the incompleteness of septal development whereas Type d is observable in rugose corals.展开更多
Since 1985, samples with a total weight of more than 14,000 kg, mainly from three key sections in western and northwestern Hunan, South China, have been processed for conodonts. In strata older than the late Late Camb...Since 1985, samples with a total weight of more than 14,000 kg, mainly from three key sections in western and northwestern Hunan, South China, have been processed for conodonts. In strata older than the late Late Cambrian paraconodonts have proved useful for stratigraphic subdivision and correlation. Thirteen conodont zones are proposed in the Middle Cambrian through lowermost Ordovician. The correlation between these zones and those of North China, western U. S.A., western Newfoundland, Canada, and Iran is discussed. In ascending order, these 13 zones are as follows: The Gapparodus bisulcatus-Westergaardodina brevidens Zone, Shandongodus priscus-Hunanognathus tricuspidatus Zone, Westergaardodina quadrata Zone, Westergaardodina matsushitai-W. grandidens Zone, Westergaardodina lui-W. am Zone, Westergaardodina cf. calix-Prooneotodus rotundatus Zone, Proconodontus tenuiserratus Zone, Proconodontus Zone, Eoconodontus Zone, Cordylodus proavus Zone, Cordylodus intermedius Zone, Cordylodus lindstromi Zone, and Cordylodus angulatus Zone (lower part). The Westergaardodina lui-W. ani and Westergaardodina cf. calix-Prooneotodus rotundatus Zones replace the Westergaardodina proligula and Westergaardodina cf. behrae-Prooneotodus rotundatus Zones, respectively, in the lowermost Upper Cambrian. Two new species (Westergaardodina Iui and Westergaardodina ani) and one conditionally identified species (Westergaardodina cf. calix) are described.展开更多
As the hydrocarbon generation and storage mechanisms of high quality shales of Upper Ordovician Wufeng Formation– Lower Silurian Longmaxi Formation remain unclear, based on geological conditions and experimental mode...As the hydrocarbon generation and storage mechanisms of high quality shales of Upper Ordovician Wufeng Formation– Lower Silurian Longmaxi Formation remain unclear, based on geological conditions and experimental modelling of shale gas formation, the shale gas generation and accumulation mechanisms as well as their coupling relationships of deep-water shelf shales in Wufeng–Longmaxi Formation of Sichuan Basin were analyzed from petrology, mineralogy, and geochemistry. The high quality shales of Wufeng–Longmaxi Formation in Sichuan Basin are characterized by high thermal evolution, high hydrocarbon generation intensity, good material base, and good roof and floor conditions;the high quality deep-water shelf shale not only has high biogenic silicon content and organic carbon content, but also high porosity coupling. It is concluded that:(1) The shales had good preservation conditions and high retainment of crude oil in the early times, and the shale gas was mainly from cracking of crude oil.(2) The biogenic silicon(opal A) turned into crystal quartz in early times of burial diagenesis, lots of micro-size intergranular pores were produced in the same time;moreover, the biogenic silicon frame had high resistance to compaction, thus it provided the conditions not only for oil charge in the early stage, but also for formation and preservation of nanometer cellular-like pores, and was the key factor enabling the preservation of organic pores.(3) The high quality shale of Wufeng–Longmaxi Formation had high brittleness, strong homogeneity, siliceous intergranular micro-pores and nanometer organic pores, which were conducive to the formation of complicated fissure network connecting the siliceous intergranular nano-pores, and thus high and stable production of shale gas.展开更多
Acoustic wave velocity has been commonly utilized to predict subsurface geopressure using empirical relations.Acoustic wave velocity is, however, affected by many factors. To estimate pore pressure accurately, we here...Acoustic wave velocity has been commonly utilized to predict subsurface geopressure using empirical relations.Acoustic wave velocity is, however, affected by many factors. To estimate pore pressure accurately, we here propose to use elastic rock physics models to understand and analyze quantitatively the various contributions from these different factors affecting wave velocity. We report a closed-form relationship between the frame flexibility factor(γ) in a rock physics model and differential pressure, which presents the major control of pressure on elastic properties such as bulk modulus and compressional wave velocity. For a gas-bearing shale with abundant micro-cracks and fractures, its bulk modulus is much lower at abnormally high pore pressure(high γ values) where thin cracks and flat pores are open than that at normal hydrostatic pressure(low γ values) where pores are more rounded on average. The developed relations between bulk modulus and differential pressure have been successfully applied to the Upper Ordovician Wufeng and Lower Silurian Longmaxi formations in the Dingshan area of the Sichuan Basin to map the three-dimensional spatial distribution of pore pressure in the shale, integrating core, log and seismic data. The estimated results agree well with field measurements. Pressure coefficient is positively correlated to gas content. The relations and methods reported here could be useful for hydrocarbon exploration, production, and drilling safety in both unconventional and conventional fields.展开更多
基金supported by the National Natural Science Foundation of China(grant No.41202103)
文摘Objective The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is one of the priority interval for shale gas exploration in the Sichuan Basin and its peripheral areas, and commercial shale gas has been discovered from this interval in Jiaoshiba, Changning and Weiyuan shale gas fields in Sichuan Province. However, there is no significant discovery in other parts of the basin due to the different quality of black shale and the differences of tectonic evolution. Based on the progress of shale gas geological theory and exploration discoveries, as well as the theory of "source rock and cap rock controls on hydrocarbon accumulation", of the Upper Ordovician the main controlling factors Wufeng Formation-Lower Silurian Longmaxi Formation shale gas enrichment in the Sichuan Basin and its peripheral areas were analyzed, and the source rock and cap rock controls on the shale gas were also discussed. The results can provide new insights for the next shale gas exploration in this area.
基金supported by the National Key Scientific Project of China(No.2011ZX05005-0042016ZX05005-002)the National Basic Research Program of China(973 Program)(No.2012CB214806)
文摘Stable carbon isotope ratio (δ13Ccarb) analysis has been widely applied to the study of the inter-conti- nental or global marine carbonate correlation. Large-scale Cambrian-Ordovician carbonate platforms were developed in the Tarim Basin. But research on fluctuation character- istics and global correlation of δ13Ccarb is still weak. Based on conodont biostratigraphy and whole-rock δ13Ccarb data in the Tahe oil-gas field of the northern Tarim Basin, the global correlation and genesis of positive carbon isotope excursions in the Darriwilian--Early Katian was exam- ined. Three positive excursions were identified in the Tahe oil-gas field including the middle Darriwilian carbon iso- tope excursion (MDICE), the Guttenberg carbon isotope excursion (GICE), and a positive excursion within the Pygodus anserinus conodont zone which is named the Early Sandbian carbon isotope excursion (ESICE) in this paper. Furthermore, these positive excursions had no direct relation with sea level fluctuations. MDICE and GICE could be globally correlated. The Middle-Upper Ordovi- cian Saergan Formation source rocks of the Kalpin outcrops were in accordance with the geological time of MDICE and ESICE. GICE had close relationship with the source rock of the Lianglitag Formation in the basin.Massive organic carbon burial was an important factor controlling the genesis of these positive excursions.
基金Supported by the PetroChina Science and Technology Project(2021DJ1904)PetroChina Exploration and Production Company Marine Shale Gas Selection Project(kt2018-01-06)。
文摘Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Sichuan Basin and its surrounding areas and the relationship between these key interfaces with the deposition of organic-rich shale have been examined systematically. The Wufeng Formation-Longmaxi Formation has four types of marker beds with interface attributes, namely, the characteristic graptolite belt, Guanyinqiao Member shell layer, section with dense bentonite layers, and concretion section, which can be taken as key interfaces for stratigraphic division and correlation of the graptolite shale. The shell layer in Guanyinqiao Member is the most standard key interface in Wufeng Formation-Longmaxi Formation, and can also be regarded as an important indicator for judging the depositional scale of organic-rich shale in key areas. There are 8 dense bentonite sections of two types mainly occurring in 7 graptolite belts in these formations. They have similar interface characteristics with the shell layer in Guanyinqiao Member in thickness and natural gamma response, and belong to tectonic interfaces(i.e., event deposits). They have three kinds of distribution scales: whole region, large part of the region, and local part, and can be the third, fourth and fifth order sequence interfaces, and have a differential control effect on organic-rich shale deposits. The horizon the characteristic graptolite belt occurs first is the isochronous interface, which is not directly related to the deposition of organic-rich shale. Concretions only appear in local areas, and show poor stability in vertical and horizontal directions, and have no obvious relationship with the deposition of the organic-rich shale.
基金Project(2008ZX05004-004)supported by the State Key Scientific Research Programs,ChinaProject(SZD0414)supported by the Sichuan Province Key Discipline Construction Project,ChinaProject(KZCX2-YW-Q05-01)supported by the Chinese Academy of Sciences Innovation Engineering Directional Project
文摘In order to reveal the relationship between the penecontemporaneous karstification and sedimentary microtopography in sequence stratigraphy,the sequence stratigraphic framework of Lianglitage formation in Upper Ordovician is studied according to the well drilling,logging,geophysical data,detailed observations of core and the paleontology.The Lianglitage formation belongs to the sequence Ⅳ of Ordovician.The second member of Lianglitage formation is prograde sedimentation in highstand systems tract,and is favorable for developing reef flat.The development scale and thickness of reef flat are controlled by the variation of secondary sea level.The types and characteristics of karst in the highstand systems tract show that the late highstand systems tract is dissolved and cemented by the meteoric fresh water and mixed water.Penecontemporaneous karstification is developed at the top of parasequence and high place of geomorphology.Atmospheric diagenetic lens is formed.The developing regulations and controlling factors of penecontemporaneous karstification can provide new clues to the prediction and exploration of favorable reservoir in this area.
基金Supported by Twelfth Five-Year Major National Science and Technology Projects of China (No.011ZX05004-004)
文摘Based on the analysis of the morphology, order, cathodoluminescence and microelement of the calcite crystals in the pores of Upper Ordovician limestone in Tazhong oilfield of Tarim Basin, this paper suggest that the calcite crystals can divided 11 kinds of cements into three stages, and confirms their cementation sequence characteristics and formation environment. First stage of the cementation occurred on the bottom of the diagenetic environment, the product of which mainly are microcrystalline, fine shape, radiation fibrous, fasciculation, radiation axis shape calcites and a ball-like aragonite, whose filling pore is 0%–30%; the second one occurred in the atmospheric fresh water environment, with main cement types of crescent or pendulous shape, vadose silt, hyperplasia of coaxial, the filling pore of which is 5%–100%; the third one occurred in a burial environment, the crystals of which are bright and thick, mainly coarse sparry and poikilitic calcite, with filling pore of 5%–5%. In this paper, influence from each stage of cementation on porosity is analyzed, and pore evolution is established.
基金Supported by the China National Science and Technology Major Project(2017ZX05063002-009)National Natural Science Foundation of China(4177021173,41972120)CNPC-Southwest Petroleum University Innovation Consortium Science and Technology Cooperation Project(2020CX020000)。
文摘Based on field outcrop data,the effects of cyclic change of astronomical orbit and volcanic activity on organic carbon accumulation during the Late Ordovician-Early Silurian in the Upper Yangtze area were studied using cyclostratigraphic and geochemical methods.d13 C and chemical index of alteration(CIA)were used to filter the astronomical orbit parameters recorded in sediments.It is found that the climate change driven by orbital cycle controls the fluctuations of sea level at different scales,obliquity forcing climate changes drive thermohaline circulation(THC)of the ocean,and THC-induced bottom currents transport nutrient-laden water from high latitude regions to the surface water of low-latitude area.Hence,THC is the main dynamic mechanism of organic-carbon supply.The marine productivity indexes of Ba/Al and Ni/Al indicate that volcanic activities had limited effect on marine productivity but had great influences on organic carbon preservation efficiency in late Hirnantian(E4).Paleo-ocean redox environmental indicators Th/U,V/Cr and V/(V+Ni)show that there is a significant correlation between volcanism and oxygen content in Paleo-ocean,so it is inferred that volcanisms controlled the organic carbon preservation efficiency by regulating oxygen content in Paleo-ocean,and the difference in volcanism intensity in different areas is an important factor for the differential preservation efficiency of organic carbon.The organic carbon input driven by orbital cycle and the preservation efficiency affected by volcanisms worked together to control the enrichment of organic carbon in the Middle–Upper Yangtze region.
基金in part provided by Petro China(Grant No.09HT10500000068)Sinopec(Grant No.G5800-07-ZS-WX032)the State Key Lab of Oil/Gas Reservoir Geology and Exploitation at CDUT(Grant No.PLC200801)to H.Qing
文摘Late Cambrian to Early Ordovician sedimentary rocks in the western Tarim Basin, Northwest China, are composed of shallow-marine platform carbonates. The Keping Uplift is located in the northwest region of this basin. On the basis of petrographic and geochemical features, four matrix replacement dolomites and one type of cement dolomite are identified. Matrix replacement dolomites include (1) micritic dolomites (MD1); (2) fine-coarse euhedral floating dolomites (MD2); (3) fine-coarse euhedral dolomites (MD3); and (4) medium-very coarse anhedral mosaic dolomites (MD4). Dolomite cement occurs in minor amounts as coarse saddle dolomite cement (CD1) that mostly fills vugs and fractures in the matrix dolomites. These matrix dolomites have δ18O values of ?9.7‰ to ?3.0‰ VPDB (Vienna Pee Dee Belemnite); δ13C values of ?0.8‰ to 3.5‰ VPDB; 87Sr/86Sr ratios of 0.708516 to 0.709643; Sr concentrations of 50 to 257 ppm; Fe contents of 425 to 16878 ppm; and Mn contents of 28 to 144 ppm. Petrographic and geochemical data suggest that the matrix replacement dolomites were likely formed by normal and evaporative seawater in early stages prior to chemical compaction at shallow burial depths. Compared with matrix dolomites, dolomite cement yields lower δ18O values (?12.9‰ to ?9.1‰ VPDB); slightly lower δ13C values (?1.6‰-0.6‰ VPDB); higher 87Sr/86Sr ratios (0.709165-0.709764); and high homogenization temperature (Th) values (98°C-225°C) and salinities (6 wt%-24 wt% NaCl equivalent). Limited data from dolomite cement shows a low Sr concentration (58.6 ppm) and high Fe and Mn contents (1233 and 1250 ppm, respectively). These data imply that the dolomite cement precipitated from higher temperature hydrothermal salinity fluids. These fluids could be related to widespread igneous activities in the Tarim Basin occurring during Permian time when the host dolostones were deeply buried. Faults likely acted as important conduits that channeled dolomitizing fluids from the underlying strata into the basal carbonates, leading to intense dolomitization. Therefore, dolomitization, in the Keping Uplift area is likely related to evaporated seawater via seepage reflux in addition to burial processes and hydrothermal fluids.
基金supported by the National Natural Science Foundation of China(41172096)the National Science and Technology Major Project of China(2011ZX05008-003)
文摘Figuring out whether the sedimentary provenance regions of the thick deep-water turbidite systems deposited during Middle–Upper Ordovician in South Quruqtagh are the intracontinental uplifts or the peripheral orogenic belts is of great significance for us to understand the tectono-sedimentary nature of the northeastern Tarim Basin and basin-range coupling processes in the middle Paleozoic.This paper reports the in situ LA-ICP-MS U–Pb ages and Hf isotope data on detrital zircons from two Middle–Upper Ordovician sandstone samples which were collected from the Charchag Formation and the Zatupo Formation in South Quruqtagh,respectively.The results show that the studied two samples have extremely similar U–Pb age patterns and Hf isotopic compositions,reflecting multiphase tectonothermal events with age groups of 527–694,713–870 Ma(peaking at 760 Ma),904–1,090,1,787–2,094 Ma(peaking at 1,975 Ma)and 2,419–2,517 Ma.Combining previous studies,the presence of age groups of 713–1,090 and1,787–2,094 Ma,respectively,demonstrates that Tarim had ever been a part of Rodinia and Columbia supercontinent.Moreover,98%of 713–870 Ma detrital zircons are characterized by negative e Hf(t)values ranging from-38.07 to-0.61,which are highly consistent with those of Neoproterozoic granites from the Quruqtagh area.No Early Paleozoic ages(*470–500 Ma)signifying subduction or collision events in Altyn Tagh were detected in the two samples,indicating that the Middle–Late Ordovician sediments in South Quruqtagh and northern Mangar depression were mainly derived from intracontinental uplifts,i.e.,the North Quruqtagh uplift or the Tabei paleo-uplift,rather than the Altyn Tagh.In conjunction with regional sedimentary-tectonic background and previous studies,we proposed preliminarily that the northeastern Tarim remained as a passive continental margin in Late Ordovician and changed into an active-continental margin in Silurian due to the southward subduction of the South-Tianshan Ocean.
基金supported by the Invitation and Training of Outstanding Overseas Student Researcher Program by Korea Science and Engineering Foundation(Grant No.KOSEF 211-2007-1-C00047)Korea Research Foundation funded by the Korean Government(MOEHRD)to DJL(Grant No.KRF-2006-312-C00690)
文摘Patterns of septal insertion were analyzed in three species of Agetolites from the Upper Ordovician in Yushan County,Jiangxi Province of China.Agetolites is a peculiar coral possessing features of both tabulate(e.g.,mural pores)and rugose corals(e.g.,well-developed septa).Septal patterns have long been considered to be indicative of the origin of corals,and the pattern of septal insertion in Agetolites has been known to be typical of rugosans.This study,however,reveals that the patterns in Agetolites are rather random and cannot be an indication of its rugosan affinity.It is suggested that Agetolites probably has a closer phylogenetic relation with tabulates than with either rugosans or scleractinians.Two insertion patterns of the first six septa were detected(Types Ⅰ and Ⅱ).Type I is interpreted as scleractinian pattern whereas Type II as typical pattern of rugosans.Types 1 and 2 are septal patterns of the second stage recognized in this study.Type 1 insertion occurs in six sectors located in the center of each interseptal space,suggesting scleractinian pattern of insertion.Type 2 insertion occurs in four sectors that are comparable with alar and counter sectors in rugose corals.Four patterns of the third(Types A–D)and fourth stages(Types a–d)of septal insertion have also been recognized in Agetolites.Type A only occurs in two adjacent sectors possibly due to the incompleteness of septal development.Type B presents rugosan pattern.Types C and D resemble scleractinian pattern in most aspects.Types a,b,and c present no clear attribution to rugosan or scleractinian pattern probably due to the incompleteness of septal development whereas Type d is observable in rugose corals.
基金This study was supported by the National Natural Science Foundation of China(Grants 4037200140072007+3 种基金49772083 to Dong Xiping)by the Laboratory of Paleobiology and Stratigraphy,Nanjing Institute of Geology and Palaeontology,Chinese Academy of Sciences(Grant 023106 to Dong Xiping)by the Research Fund for Doctoral Program of High Education(Grant 2000000127 to Dong Xiping)by a travel grant from the Ohio State University(to Stig M.Bergstrom).
文摘Since 1985, samples with a total weight of more than 14,000 kg, mainly from three key sections in western and northwestern Hunan, South China, have been processed for conodonts. In strata older than the late Late Cambrian paraconodonts have proved useful for stratigraphic subdivision and correlation. Thirteen conodont zones are proposed in the Middle Cambrian through lowermost Ordovician. The correlation between these zones and those of North China, western U. S.A., western Newfoundland, Canada, and Iran is discussed. In ascending order, these 13 zones are as follows: The Gapparodus bisulcatus-Westergaardodina brevidens Zone, Shandongodus priscus-Hunanognathus tricuspidatus Zone, Westergaardodina quadrata Zone, Westergaardodina matsushitai-W. grandidens Zone, Westergaardodina lui-W. am Zone, Westergaardodina cf. calix-Prooneotodus rotundatus Zone, Proconodontus tenuiserratus Zone, Proconodontus Zone, Eoconodontus Zone, Cordylodus proavus Zone, Cordylodus intermedius Zone, Cordylodus lindstromi Zone, and Cordylodus angulatus Zone (lower part). The Westergaardodina lui-W. ani and Westergaardodina cf. calix-Prooneotodus rotundatus Zones replace the Westergaardodina proligula and Westergaardodina cf. behrae-Prooneotodus rotundatus Zones, respectively, in the lowermost Upper Cambrian. Two new species (Westergaardodina Iui and Westergaardodina ani) and one conditionally identified species (Westergaardodina cf. calix) are described.
基金Supported by the China National Science and Technology Major Project(2017ZX05036,2017ZX05036001).
文摘As the hydrocarbon generation and storage mechanisms of high quality shales of Upper Ordovician Wufeng Formation– Lower Silurian Longmaxi Formation remain unclear, based on geological conditions and experimental modelling of shale gas formation, the shale gas generation and accumulation mechanisms as well as their coupling relationships of deep-water shelf shales in Wufeng–Longmaxi Formation of Sichuan Basin were analyzed from petrology, mineralogy, and geochemistry. The high quality shales of Wufeng–Longmaxi Formation in Sichuan Basin are characterized by high thermal evolution, high hydrocarbon generation intensity, good material base, and good roof and floor conditions;the high quality deep-water shelf shale not only has high biogenic silicon content and organic carbon content, but also high porosity coupling. It is concluded that:(1) The shales had good preservation conditions and high retainment of crude oil in the early times, and the shale gas was mainly from cracking of crude oil.(2) The biogenic silicon(opal A) turned into crystal quartz in early times of burial diagenesis, lots of micro-size intergranular pores were produced in the same time;moreover, the biogenic silicon frame had high resistance to compaction, thus it provided the conditions not only for oil charge in the early stage, but also for formation and preservation of nanometer cellular-like pores, and was the key factor enabling the preservation of organic pores.(3) The high quality shale of Wufeng–Longmaxi Formation had high brittleness, strong homogeneity, siliceous intergranular micro-pores and nanometer organic pores, which were conducive to the formation of complicated fissure network connecting the siliceous intergranular nano-pores, and thus high and stable production of shale gas.
文摘Acoustic wave velocity has been commonly utilized to predict subsurface geopressure using empirical relations.Acoustic wave velocity is, however, affected by many factors. To estimate pore pressure accurately, we here propose to use elastic rock physics models to understand and analyze quantitatively the various contributions from these different factors affecting wave velocity. We report a closed-form relationship between the frame flexibility factor(γ) in a rock physics model and differential pressure, which presents the major control of pressure on elastic properties such as bulk modulus and compressional wave velocity. For a gas-bearing shale with abundant micro-cracks and fractures, its bulk modulus is much lower at abnormally high pore pressure(high γ values) where thin cracks and flat pores are open than that at normal hydrostatic pressure(low γ values) where pores are more rounded on average. The developed relations between bulk modulus and differential pressure have been successfully applied to the Upper Ordovician Wufeng and Lower Silurian Longmaxi formations in the Dingshan area of the Sichuan Basin to map the three-dimensional spatial distribution of pore pressure in the shale, integrating core, log and seismic data. The estimated results agree well with field measurements. Pressure coefficient is positively correlated to gas content. The relations and methods reported here could be useful for hydrocarbon exploration, production, and drilling safety in both unconventional and conventional fields.