The Xisha Block is a minor one in the South China Sea and an important tectonic unit in the northwestern part of the region.Zircon SHRIMP U-Pb ages for three volcanic intrusive core samples from Xike-1,an exploratory ...The Xisha Block is a minor one in the South China Sea and an important tectonic unit in the northwestern part of the region.Zircon SHRIMP U-Pb ages for three volcanic intrusive core samples from Xike-1,an exploratory well penetrating the bioherms of the Xisha Islands.The core samples are from the Miocene reef carbonate bedrock and are recognized as dark-gray biotite-hornblende gabbro,gray fne-grained biotite diorite,and gray fine-grained granite,respectively.Zircon cathodoluminescence (CL)images and trace Th,U and Pb compositions of the zircons show that these rocks are of volcanic intrusive origin.Zircon SHRIMP U-Pb dating yielded six groups of ages,ranging from 2451-1 857 Ma to early Cretaceous, which indicate that the formation and evolution of the Xisha Block was affected by the evolution and closure of Neotethys Ocean,probably within its eastern extension into South China Sea.Both old,deepsourced material,including fragments from Rodina supercontinent,and recent mantle-derived magma products contributed to the emergence and formation of the Xisha block.The SHRIMP U-Pb results also proved that this process differed from that of the Kontum massif,the Hainan Block,and the South China Block,but is similar to that of the Nansha and Zhongsha blocks.The process was associated with the effects of Yanshanian magmatism induced by subduction mechanisms of the Paleo-Pacific Plate or the reworking of the multiple magmatisms since the Early to mid-Yanshanian,possibly jointly experienced by the Xisha-Zhongsha-Nansha Block.展开更多
A recent study suggests that the deep dolomites in well Xike-1 located more than 1000 m deep have a very low porosity,and no permeability for most of the samples.Given that the largest oil and gas fields in the South ...A recent study suggests that the deep dolomites in well Xike-1 located more than 1000 m deep have a very low porosity,and no permeability for most of the samples.Given that the largest oil and gas fields in the South China Sea are all composed of biohermal dolomites while the deep dolomites of Xike-1 serve as the caprock formations rather than the reservoirs,this represents the first discovery and is of great theoretical importance.In this paper,core photos of the tight dolomites are presented.Hand specimens description and systematic porosity-permeability reveal a very low porosity,and no permeability for most of the samples.PLM,SEM and XRPD analysis discovered that the dolostones in the deep tight dolomites are quite large in size and euhedral or subhedral in shape.Alizarin red-stained sections are mostly pink in color and have a full coverage,which are considered as evidence for multiple calcite intrusions after the formation of ankerite dolomites.Biological relict textures are developed.The order degree is medium to low.The dolomites could be the product of penecontemporaneous dolomitization.展开更多
文摘The Xisha Block is a minor one in the South China Sea and an important tectonic unit in the northwestern part of the region.Zircon SHRIMP U-Pb ages for three volcanic intrusive core samples from Xike-1,an exploratory well penetrating the bioherms of the Xisha Islands.The core samples are from the Miocene reef carbonate bedrock and are recognized as dark-gray biotite-hornblende gabbro,gray fne-grained biotite diorite,and gray fine-grained granite,respectively.Zircon cathodoluminescence (CL)images and trace Th,U and Pb compositions of the zircons show that these rocks are of volcanic intrusive origin.Zircon SHRIMP U-Pb dating yielded six groups of ages,ranging from 2451-1 857 Ma to early Cretaceous, which indicate that the formation and evolution of the Xisha Block was affected by the evolution and closure of Neotethys Ocean,probably within its eastern extension into South China Sea.Both old,deepsourced material,including fragments from Rodina supercontinent,and recent mantle-derived magma products contributed to the emergence and formation of the Xisha block.The SHRIMP U-Pb results also proved that this process differed from that of the Kontum massif,the Hainan Block,and the South China Block,but is similar to that of the Nansha and Zhongsha blocks.The process was associated with the effects of Yanshanian magmatism induced by subduction mechanisms of the Paleo-Pacific Plate or the reworking of the multiple magmatisms since the Early to mid-Yanshanian,possibly jointly experienced by the Xisha-Zhongsha-Nansha Block.
基金This work was supported by the National Natural Science Foundation of China(41106064)the Science and Technology Basic Resources Investigation Program of China(2017FY201407).
文摘A recent study suggests that the deep dolomites in well Xike-1 located more than 1000 m deep have a very low porosity,and no permeability for most of the samples.Given that the largest oil and gas fields in the South China Sea are all composed of biohermal dolomites while the deep dolomites of Xike-1 serve as the caprock formations rather than the reservoirs,this represents the first discovery and is of great theoretical importance.In this paper,core photos of the tight dolomites are presented.Hand specimens description and systematic porosity-permeability reveal a very low porosity,and no permeability for most of the samples.PLM,SEM and XRPD analysis discovered that the dolostones in the deep tight dolomites are quite large in size and euhedral or subhedral in shape.Alizarin red-stained sections are mostly pink in color and have a full coverage,which are considered as evidence for multiple calcite intrusions after the formation of ankerite dolomites.Biological relict textures are developed.The order degree is medium to low.The dolomites could be the product of penecontemporaneous dolomitization.
