| In order to study the occurrence regularity of coal series mineral resources in Ordos Basin,the controlling effect of tectonic-thermal evolution and tectonic pattern of the basin on the occurrence regularity of coal series mineral resources were studied based on the idea of structural control mineralization.By using the knowledge of tectonics,sedimentology,coal geology,unconventional oil and gas geology and geochemistry,combined with information collection,field geological survey,sample tests as well as the thematic map preparation,the paper analyzed the tectonic-thermal evolution history and structural features of coal belt of Ordos Basin.The types and temporal and spatial distribution of coal series mineral resources were determined and the controlling effect of tectonic-thermal evolution and tectonic framework of the basin on the formation,enrichment,transformation and distribution of coal series mineral resources were studied.Finally,the coupling metallogenic model of coal series mineral resources under the constraint of structure were summarized and proposed.The Ordos Baisn that located in the structural domain conjunction of eastern and western China is a compound cratonic basin,which has experienced multi-tectonic systems,multi-evolution stages,multi-sedimentary systems,multi-prototype basin superimposed.The final tectonic framework of the basin has experienced three stages of tectonic evolution: basement formation,basin development and basin modification.The stage of the north China epicontinental sedimentation in the early Paleozoic and the inland lake basin evolution in the Mesozoic are important period for the formation of coal measures in the Carboniferous-Permian and the Jurassic.It provides the basic geological conditions for the enrichment of coal series mineral resources.Since the Cenozoic,the basin modification has made the coal series mineral resources readjustment and relocation.Since the late Paleozoic,the burial history of the basin has gone through five stages: slow deposition phase(I)during the late Carboniferous to the early Triassic;rapid deposition phase(II)during the early Triassic to the late Triassic;volatility deposition phase(III)during late Triassic to middle Jurassic;structure transition phase(IV)during middle Jurassic to late Cretaceous;strong uplift phase(V)during the late Cretaceous to the Quaternary.In addition,different tectonic units showed some diversity in basin.The Ordos basin belongs to medium temperature type basin for the higher basin's paleo-geothermal gradient in various evolution stages than the present geothermal gradient.In the late Mesozoic(late Jurassic-early Cretaceous)geothermal gradient was highest,up to 3.3-4.52?/100 m.It was caused by early Cretaceous tectonics thermal events(about the 140-140Ma)which enhanced the deep lithosphere thermal activity.Combined with the buried history,thermal evolution history and hydrocarbon generation history of the basin,the formation of unconventional gas can be divided into three phase: The early hydrocarbon generation was controlled by plutonic metamorphism(late Triassic-middle Jurassic),the peak period of hydrocarbon generation was controlled by plutonic metamorphism and regional thermal metamorphism(middle Jurassic-late Cretaceous),and the adjustment and transformation period,which unconventional gas occurred secondary migration and accumulation(after the Cretaceous).In general,the hydrocarbon generation of coal-bearing source rocks of the Ordos Basin are later,in addition to the edge of the basin,the late-stage adjustment and reconstruction of the basin is weak,and the faults are not developed,which is beneficial to the preservation and accumulation of unconventional gas.The formation,migration and accumulation of coal series mineral resources were controlled by complicated tectonic-thermal evolution.The integrity and continuity of coal series were damaged by tectonic stress which rendering them into faults or folds in the process of tectonic evolution.The region-divided and zonation of structural belt can be developed with the changing of orientation and intensity of the tectonic stress field,which control the spatial distribution of coal series mineral resources.During tectonic evolution,tectonic uplift and sedimentation control the preservation of unconventional gas,and the formation of metal elements and sandstone-type uranium in coal series.The thermal events during the period of structural change can not only improve the degree of metamorphism of coal-bearing source rocks,but also provide the source and driving force for the formation and migration of metal elements and sandstone-type uranium deposits in the coal series.The final location of coal series mineral resources were controlled by the present structural framework of Ordos Basin.The deformation characteristics of Ordos Basin are weak deformation in internal but strong deformation in edge.The depth of coal series burial in internal basin is large,and the tectonic deformation is weak,which is favorable to the formation and preservation of large continuous unconventional gas reservoir.The strong deformation,magma hydrothermal and groundwater flow activity in edge of the basin,coupled with the adjacent areas of the source area,were conducive to metal elements,uranium element migration and enrichment.According to the present structural framework and current occurrence of coal series,the Ordos Basin were divided into six coal series belts: the fold-thrust coal series belt in western edge,the flexure coal series belt in eastern edge,the Yimeng uplift coal series belt,the Tianhuan depression coal series belt,the Shanbei monocline coal series belt and the Weibei break-uplift coal series belt.The different structural styles in coal series belt determined the occurrence regularities of coal series mineral resources.The thrust-nappe structure is well developed in the fold-thrust coal series belt in western edge.The thrust-nappe structure is characterized by plane zoning which made the coal series mineral resources changing along structure distribution.The coal series at root of thrust-nappe structure system were erosion exhausted where the exposure of the metamorphosed basement can be the provenance of metal elements.The middle of thrust-nappe structure system is conducive to unconventional gas enrichment and preservation for the broad and gentle synclines.The striker belt is conducive to the formation of metal minerals and sandstone-type uranium deposits for the intensive folds and faults.The leading edge recoil zone structure is characterized by gentle synclines and sparse reverse faults,which is favorabale for unconventional gas enrichment and preservation.The large uniclinal structures in eastern edge of Ordos basin were beneficial to the formation of unconventional gas,anticlines and normal faults contributed to the effusion of unconventional gas but synclines and reverse faults were beneficial to the preservation of unconventional gas.Under the controlling of tectonic-thermal evolution and tectonic framework,the coal series mineral resources are abundant in Ordos Basin.They can be divided into energy minerals and metallic minerals.The former includes coal,coal bed methane,shale gas,tight sandstone gas,oil shale and sandstone-type uranium deposit minerals,and the latter includes lithium mineral,gallium mineral and Al2O3.In the plane,the coal and unconventional gas were widely distributed while the oil shale,sandstone-type uranium deposits and metal elements in coal were rich in the margin of Ordos Basin.Vertically,energy resources developed in the Carboniferous Permian and Jurassic coal measures,and metallic minerals were mainly developed in the Carboniferous Permian coal measures.According to the distribution characteristics of coal series mineral resources from Ordos Basin,aiming at the two main coal bearing strata of Carboniferous Permian and Jurassic,the combination types of coal series mineral resources in different ages are divided.The mineral resources in Carboniferous-Permian coal series is divided into four types,including the Combination of coal-unconventional gas-oil shale-metal mineral in coal series in north Ordos Basin,Combination of coal-coal gas-metal mineral in coal series in south Ordos Basin,Combination of coal-unconventional gas-metal mineral in coal series in west Ordos Basin and Combination of coal and unconventional gas in coal series in east Ordos Basin,while the mineral resources in Jurassic coal series can be divided into three types,which include Combination of coal-coal gas-sandstone-type uranium deposit in north Ordos Basin,Combination of coal-coal gas-shale gas-oil shale-metal mineral-sandstone-type uranium deposit in coal series in south Ordos Basin and Combination of coal-unconventional gas-oil shale-sandstone-type uranium deposit in coal series in west Ordos Basin.Based on the data from testing by quantitative identification of coal macerals,vitrinite reflectance,rock pyrolysis and pressure mercury injection,coal macerals of Carboniferous-Permian coal measures in the northeast of the Ordos Basin are dominated by inertinite and vitrinite,and the coal stayed at low mature-mature stage with vitrinite reflectance ranging from 0.44-0.81%.The TOC content of the shale samples varies from 0.36% to 31.58%(avg.5.46%),the value of Ro ranges from 0.45-0.72%(average 0.58%).The Rock-Eval Tmax of the studied samples ranges from 408°C to 453°C,averaging 432°C.The hydrocarbon generative potential(G.P.= S1 + S2)of all the samples ranges from 3.25 to 82.82 mg/g rock,averaging 23.7 ?/g.The type of organic matter comprised type III(humic type)and type II2(sapropelic-humic type),which the type II2 is dominated.Different rocks show different porosities and pore structures.The coal samples show the highest porosity(10.4–18%,avg.14.3%)and permeability(2.17–2.93 mD,avg.2.6 mD),followed by sandstones with porosity ranging from 10.1% to 13.0%(avg.11.55%)and permeability varying between 0.51 and 1.16 mD(avg.0.84 mD).The shale samples have the lowest porosity(1.1–4.7%,avg.2.48%)and permeability(0.003–0.37 mD,avg.0.1 mD).Therefore,the development of hydrocarbon source rocks,the superimposition vertically of reservoir space and the differences of pore structure combined to form serval source-reservoir-cap assemblages,which is conducive to the generation,preservation and enrichment of hydrocarbon gas.This feature meets the commonly seen accumulation model of unconventional gas development,referred to as the “Short-distance transport,nearest-reservoir enrichment” model.The northeast of the Ordos Basin is the potential area of unconventional gas coupling.The tectonic-thermal evolution process of the basin was the key for the coupling metallogenic of coal series mineral resources,which is mainly reflected at the coupling of metallogenic time and spatial position.The steady subsidence stage of the basin was an important formative period of organic minerals in coal series,while the inorganic minerals mainly formed in the tectonic activity stage of the basin.Under the geological effects of tectonic-thermal evolution in the basin,both the organic minerals and inorganic minerals represent the correlation and coupling relationship in the aspects of metallogenic parent material,metallogenic environment,metallogenic epoch and occurrence horizon,especially in the key period of the multi-cycle evolution and reformation of the basin,which was also the essential period of the coupling metallogenic of coal series mineral resources.The tectonic-thermal events during this period can not only improve the metamorphic degree of coal-bearing source rocks,but also promote the hydrocarbon generation and expulsion.It can also provide the source for the formation and migration of metal minerals.The tectonic stress,thermal stress and deep source fluid pressure in the process can provide the driving force for the migration of unconventional gas and metal elements,and will cause a large number of cracks in the rock,which can be acted as a favorable migration channel of coal series mineral resources. |
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