Diagenetic traps in conglomerate in nearshore subaqueous fans in the steep slope zones of rift basins have been important exploration targets for subtle reservoirs in eastern China. However, the mechanism of how those...Diagenetic traps in conglomerate in nearshore subaqueous fans in the steep slope zones of rift basins have been important exploration targets for subtle reservoirs in eastern China. However, the mechanism of how those traps were formed is not clear, which inhibits further exploration for this type of reservoir. In order to solve the problem, we take as an example nearshore subaqueous fans in the upper part of the fourth member of the Shahejie Formation （Es4s） on the north slope of the Minfeng Subsag in the Dongying Sag. Combining different research methods, such as core observation, thin section examination, scanning electron microscope （SEM） observation, fluid-inclusion analysis, carbon and oxygen isotope analysis of carbonate cements, and analysis of core properties, we studied the genetic mechanisms of diagenetic traps on the basis of diagenetic environment evolution and diagenetic evolution sequence in different sub/micro-facies. Conglomerate in Es4s in the north Minfeng Subsag experienced several periods of transition between alkaline and acidic environments as ＂alkaline-acidic-alkaline-acidic-weak alkaline＂. As a result, dissolution and cementation are also very complex, and the sequence is ＂early pyrite cementation / siderite cementation / gypsum cementation / calcite and dolomite cementation- feldspar dissolution / quartz overgrowth quartz dissolution / ferroan calcite cementation / ankerite cementation / lime-mud matrix recrystallization / feldspar overgrowth carbonate dissolution / feldspar dissolution / quartz overgrowth / pyrite cementation＂. The difference in sedimentary characteristics between different sub/micro-facies of nearshore subaqueous fans controls diagenetic characteristics. Inner fan conglomerates mainly experienced compaction and lime-mud matrix recrystallization, with weak dissolution, which led to a reduction in the porosity and permeability crucial to reservoir formation. Lime-mud matrix recrystallization results in a rapid decrease in porosity and permeability in inner fan conglomerates in middle-to-deep layers. Because acid dissolution reworks reservoirs and hydrocarbon filling inhibits cementation, reservoirs far from mudstone layers in middle fan braided channels develop a great number of primary pores and secondary pores, and are good enough to be effective reservoirs of hydrocarbon. With the increase of burial depth, both the decrease of porosity and permeability of inner fan conglomerates and the increase of the physical property difference between inner fans and middle fans enhance the quality of seals in middle-to-deep layers. As a result, inner fan conglomerates can be sealing layers in middle-to-deep buried layers. Reservoirs adjacent to mudstones in middle fan braided channels and reservoirs in middle fan interdistributaries experienced extensive cementation, and tight cemented crusts formed at both the top and bottom of conglomerates, which can then act as cap rocks. In conclusion, diagenetic traps in conglomerates of nearshore subaqueous fans could be developed with inner fan conglomerates as lateral or vertical sealing layers, tight carbonate crusts near mudstone layers in middle fan braided channels as well as lacustrine mudstones as cap rocks, and conglomerates far from mudstone layers in middle fan braided channels as reservoirs. Lime-mud matrix recrystallization of inner fan conglomerates and carbonate cementation of conglomerates adjacent to mudstone layers in middle fan braided channels took place from 32 Ma B.R to 24.6 Ma B.P., thus the formation of diagenetic traps was from 32 Ma B.R to 24.6 Ma B.R and diagenetic traps have a better hydrocarbon sealing ability from 24 Ma B.P.. The sealing ability of inner fans gradually increases with the increase of burial depth and diagenetic traps buried more than 3,200 m have better seals.展开更多
Methodologies have been developed for calculating cutoffs of reservoir intervals with production capacity （RIPC） and reservoir intervals with accumulation capacity （RIAC） according to the types of pore throat stru...Methodologies have been developed for calculating cutoffs of reservoir intervals with production capacity （RIPC） and reservoir intervals with accumulation capacity （RIAC） according to the types of pore throat structures and dynamic force by using data from petrophysical analysis, production tests and mercury injection. The data are from clastic reservoirs in the third member （Es3） and the fourth member （Es4） of the Shahejie Formation in the Shengtuo area on the North Slope of the Dongying Sag, Jiyang Depression, China. The method of calculating cutoffs of RIPC is summarized as follows： 1） determination of permeability cutoffs of RIPC; 2） classification of types of pore-throat structures according to mercury injection data and then relating porosity to permeability and determining the relationship between porosity and permeability according to each type of pore-throat structure; and 3） calculating porosity cutoffs of RIPC using established correlation between porosity and permeability according to the type of pore throat structure. The method of calculating cutoffs of RIAC includes： 1） establishing a functional relationship between oil-water interracial tension and formation temperature; 2） calculating limiting values of maximum connected pore-throat radii according to formation temperature and dynamic forces of each reservoir interval; 3） correlating permeability with maximum connected pore-throat radius and then obtaining permeability cutoffs of RIAC; and 4） calculating porosity cutoffs on the basis of permeability cutoffs according to specific correlations, suitable for the type of porethroat structure. The results of this study show that porosity and permeability cutoffs of clastic reservoirs decrease with depth. For a fixed permeability cutoff, the porosity cutoff of R1PC varies because the type of pore throat is different. At a fixed temperature, porosity and permeability cutoffs of RIAC decrease as dynamic force increases. For a fixed permeability cutoff of effective hydrocarbon accumulation, the porosity cutoff also varies with different types of pore throat.展开更多
In recent years, great attention has been paid to oil and gas exploration in the Carboniferous-Permian strata of the Bohaiwan basin, especially the Carboniferous-Permian marine transgression, using data from drilling,...In recent years, great attention has been paid to oil and gas exploration in the Carboniferous-Permian strata of the Bohaiwan basin, especially the Carboniferous-Permian marine transgression, using data from drilling, outcrops and carbonate acid-insoluble residue experiments together with the tectonic evo- lutionary history of the peripheral orogenic zones of the North China plate and the Tan-Lu fault zones. The .＇;tudy concludes that marine transgressions took place on six occasions during Carboniferous-Perm- ian time in the Jiyang Depression. The marine transgressions were concentrated in the Late Carbonifer- ous： two marine transgressions occurred in the early Late Carboniferous, and the scale of the first was smaller and the time was shorter than those of the second. The other four marine transgressions hap- pened in the late Late Carboniferous, the first and the fourth of which were larger in scale and longer in time than the second and the third. The seawater came from the Jiaobei area, the eastward part of the Qinling-Dabie residual sea basin, and invaded progressively as a planar flow from south to north and from east to west. These findings have great significance for thorough analysis of the sedimentary characteristics and evolution of the Carboniferous-Permian strata in the livang Denression.展开更多
基金co-funded by the National Natural Science Foundation of China (41102058)the National Science and Technology Special Grant (2011ZX05006-003)the Fundamental Research Funds for the Central Universities (12CX04001A)
文摘Diagenetic traps in conglomerate in nearshore subaqueous fans in the steep slope zones of rift basins have been important exploration targets for subtle reservoirs in eastern China. However, the mechanism of how those traps were formed is not clear, which inhibits further exploration for this type of reservoir. In order to solve the problem, we take as an example nearshore subaqueous fans in the upper part of the fourth member of the Shahejie Formation （Es4s） on the north slope of the Minfeng Subsag in the Dongying Sag. Combining different research methods, such as core observation, thin section examination, scanning electron microscope （SEM） observation, fluid-inclusion analysis, carbon and oxygen isotope analysis of carbonate cements, and analysis of core properties, we studied the genetic mechanisms of diagenetic traps on the basis of diagenetic environment evolution and diagenetic evolution sequence in different sub/micro-facies. Conglomerate in Es4s in the north Minfeng Subsag experienced several periods of transition between alkaline and acidic environments as ＂alkaline-acidic-alkaline-acidic-weak alkaline＂. As a result, dissolution and cementation are also very complex, and the sequence is ＂early pyrite cementation / siderite cementation / gypsum cementation / calcite and dolomite cementation- feldspar dissolution / quartz overgrowth quartz dissolution / ferroan calcite cementation / ankerite cementation / lime-mud matrix recrystallization / feldspar overgrowth carbonate dissolution / feldspar dissolution / quartz overgrowth / pyrite cementation＂. The difference in sedimentary characteristics between different sub/micro-facies of nearshore subaqueous fans controls diagenetic characteristics. Inner fan conglomerates mainly experienced compaction and lime-mud matrix recrystallization, with weak dissolution, which led to a reduction in the porosity and permeability crucial to reservoir formation. Lime-mud matrix recrystallization results in a rapid decrease in porosity and permeability in inner fan conglomerates in middle-to-deep layers. Because acid dissolution reworks reservoirs and hydrocarbon filling inhibits cementation, reservoirs far from mudstone layers in middle fan braided channels develop a great number of primary pores and secondary pores, and are good enough to be effective reservoirs of hydrocarbon. With the increase of burial depth, both the decrease of porosity and permeability of inner fan conglomerates and the increase of the physical property difference between inner fans and middle fans enhance the quality of seals in middle-to-deep layers. As a result, inner fan conglomerates can be sealing layers in middle-to-deep buried layers. Reservoirs adjacent to mudstones in middle fan braided channels and reservoirs in middle fan interdistributaries experienced extensive cementation, and tight cemented crusts formed at both the top and bottom of conglomerates, which can then act as cap rocks. In conclusion, diagenetic traps in conglomerates of nearshore subaqueous fans could be developed with inner fan conglomerates as lateral or vertical sealing layers, tight carbonate crusts near mudstone layers in middle fan braided channels as well as lacustrine mudstones as cap rocks, and conglomerates far from mudstone layers in middle fan braided channels as reservoirs. Lime-mud matrix recrystallization of inner fan conglomerates and carbonate cementation of conglomerates adjacent to mudstone layers in middle fan braided channels took place from 32 Ma B.R to 24.6 Ma B.P., thus the formation of diagenetic traps was from 32 Ma B.R to 24.6 Ma B.R and diagenetic traps have a better hydrocarbon sealing ability from 24 Ma B.P.. The sealing ability of inner fans gradually increases with the increase of burial depth and diagenetic traps buried more than 3,200 m have better seals.
基金co-funded by National Natura Science Foundation of China (Grant No. 41102058 Gran No. U1262203)+4 种基金the National Science and Technology Special Grant (No. 2011ZX05006-003)Shandong Natura Science Foundation (Grant No. ZR2011DQ017)the Fundamental Research Funds for the Central Universities (No. 12CX04001A No. 13CX02035A No. 13CX02036A)
文摘Methodologies have been developed for calculating cutoffs of reservoir intervals with production capacity （RIPC） and reservoir intervals with accumulation capacity （RIAC） according to the types of pore throat structures and dynamic force by using data from petrophysical analysis, production tests and mercury injection. The data are from clastic reservoirs in the third member （Es3） and the fourth member （Es4） of the Shahejie Formation in the Shengtuo area on the North Slope of the Dongying Sag, Jiyang Depression, China. The method of calculating cutoffs of RIPC is summarized as follows： 1） determination of permeability cutoffs of RIPC; 2） classification of types of pore-throat structures according to mercury injection data and then relating porosity to permeability and determining the relationship between porosity and permeability according to each type of pore-throat structure; and 3） calculating porosity cutoffs of RIPC using established correlation between porosity and permeability according to the type of pore throat structure. The method of calculating cutoffs of RIAC includes： 1） establishing a functional relationship between oil-water interracial tension and formation temperature; 2） calculating limiting values of maximum connected pore-throat radii according to formation temperature and dynamic forces of each reservoir interval; 3） correlating permeability with maximum connected pore-throat radius and then obtaining permeability cutoffs of RIAC; and 4） calculating porosity cutoffs on the basis of permeability cutoffs according to specific correlations, suitable for the type of porethroat structure. The results of this study show that porosity and permeability cutoffs of clastic reservoirs decrease with depth. For a fixed permeability cutoff, the porosity cutoff of R1PC varies because the type of pore throat is different. At a fixed temperature, porosity and permeability cutoffs of RIAC decrease as dynamic force increases. For a fixed permeability cutoff of effective hydrocarbon accumulation, the porosity cutoff also varies with different types of pore throat.
基金sponsored by the National Science and Technology Major Project of China (No.2011ZX05008)
文摘In recent years, great attention has been paid to oil and gas exploration in the Carboniferous-Permian strata of the Bohaiwan basin, especially the Carboniferous-Permian marine transgression, using data from drilling, outcrops and carbonate acid-insoluble residue experiments together with the tectonic evo- lutionary history of the peripheral orogenic zones of the North China plate and the Tan-Lu fault zones. The .＇;tudy concludes that marine transgressions took place on six occasions during Carboniferous-Perm- ian time in the Jiyang Depression. The marine transgressions were concentrated in the Late Carbonifer- ous： two marine transgressions occurred in the early Late Carboniferous, and the scale of the first was smaller and the time was shorter than those of the second. The other four marine transgressions hap- pened in the late Late Carboniferous, the first and the fourth of which were larger in scale and longer in time than the second and the third. The seawater came from the Jiaobei area, the eastward part of the Qinling-Dabie residual sea basin, and invaded progressively as a planar flow from south to north and from east to west. These findings have great significance for thorough analysis of the sedimentary characteristics and evolution of the Carboniferous-Permian strata in the livang Denression.
