Study On Lithofacies Paleographic Evolution Of Late Triassic-Neogene Sequence In Mid-Yangtze Region

Based on the theories and methods of plate tectonic theory and sedimentology,following the research idea of "the structure controlling the basin" and "the basin controlling the facies", this paper studies systematically the basin types,sequence filling,basin-mountain coupling relations,and the superimposition,reform,and juxtaposition relations between Meso-Cenozoic continental basins and underlying marine facies basins,analyzes the preservation condition units of marine facies hydrocarbon,dissects the constructive meaning and disruptive effect of later structural movement and sequence filling on the reservoir formation in marine facies basins,and points out the favorable prospect area in marine facies basins,The detailed study results are as follows..1.A systematic study of Meso-Cenozoic depositional systemsOn the basis of 5 measured outcrop profiles,combined with the comprehensive analysis of drilling and seismic data,four depositional systems,i.e.,the alluvial fan,fluvial,Delta,and lacustrine depositional systems are classified,the facies types or units which can reflect the essential characteristics and spatial distribution geometry of depositional systems are established.According to the depositional evolution features of Late Triassic-Neogene,it can be divided into five depositional modes.①the secondary fan depositional mode with the combination of fan delta and sublacustrine fan in the foreland basin;②the tertiary fan depositional mode with the combination of alluvial fan,delta and sublacustrine fan;③the alluvial fan-fluvial-lacustrine depositional mode of rifted lake basin;④the alluvial fanfan deltas - lacustrine depositional mode;⑤and the fluvial-delta-lacustrine depositional mode of depressed lake basin.2.Sequence division and their characteristicsAccording to the unconformities,erosion surfaces,and hiatus surfaces observed on surface outcrops,combined with the palaeontologic and depositional faices data,as well as the phenomena of onlap,downlap and truncation in seismic reflection wave groups,the space-time distribution range of unconformities is clarified as shown in the following①The Upper Triassic-Neogene of the middle Yangtze area can be divided into 3 primary sequences,10 secondary sequences,and 21 tertiary sequences.②The formation of sequence boundaries is closely related to the tectonic movement.According to the features of tectonic movement resulting in the formation of sequence boundaries,the sequence boundaries can be classified into the following genetic types:the uplift erosion unconformity,the syntectonic progressive unconformity,the orogenic uplift erosion unconformity,the subaqueous corrosion unconformity,and the stress field transformation or palaeoclimate alternation-incurred unconformity.③Ten identification marks of sequence boundary are identified in the Upper Triassic-Neogene of the work area:the paleocrust of weathering,the regional unconformity boundary,the cherty gravel bed,the erosion and scouring,the long-time exposure and kalkkruste,the discontinuous facies sequence or facies transform interface,and the ending phenomena on seismic profiles,such as the contact relations,e.g.,onlap and truncation,and palaeoclimate transform interface.④The factors controlling over the development of Meso-Cenozoic sequences in middle Yangtze area are analyzed.It is considered that the sequence development is the result of the synergic effect of four factors,i.e.,tectonic movement,lake level rise,deposition and climate3.Mapping and study of the lithofacies palaeogeography of structures and sequences for the systemTen system tracts(basin extension system tract and basin shrinkage system tract)of secondary sequences are selected to serve as the compilation units for the mapping of Meso-Cenozoic lithofacies palaeogeography.According to the idea of "the structure controlling the basin" and "the basin controlling the facies",the lithofacies paleographic map of structures and sequences of the whole area,which is more scientific,more isochronous, more continuous in origin,and more practical in exploration,is compiled for the first time(Innovation point).In the study area,the lithofacies paleography of the Late Triassic-Jurassic is associated with development and evolution of the basin.Available data indicate that the basin evolution can be divided into 4 stages,namely origination,early development,late development and abortion stages,which have different filling features from each other.①During the basin origination,fluvial filling was dominant.This stage is considered to correspond to Late Triassic when the Sequences TS2 and TS3 were deposited.During this stage,the basin consists of small isolated basins,which were small,fluvial and swamp-dominated,with undeveloped lacustrine deposits.②Early basin development stage—when lacustrine, deltaic and fluvial filling were deposited.It corresponds to the period when the Sequence TS4 was deposited.Since the episodeⅡof the Indosinian Movement,the Middle Yangtze Plate depressed entirely,with significantly widened sedimentary extent.The aborted provenance supplied relatively absent detrital material of terrigenous origin,resulting in much lacustrine deposits due to differentiation in paleogeography.③During the late basin development, fluvial sandstone-mudstone filling was dominant,which corresponds to the period when the Sequence TS5 was deposited.Since the EpisodeⅠof the Yansanian Movement,the Middle Yangtze Area has been in an intensive depression stage,with abundant detrital material of terrigenous origin supplied and deposited with great thicknesses.④During the basin abortion, fluvial filling was dominant.This stage corresponds to the period when the Sequence TS6 was deposited.During this stage,the Jinmen-Dangyang and Qianjiang-Eastern Hubei trend was in an uplift area,except for the Western Hubei- Eastern Chongqin-Zhigui Basin trend in a depression area.During this stage,the deposition area is significantly reduced,with the basement intensively depressed,and very thick deposits dominated by fluvial sandstone-mudstones.In the study area,evolution of the lithofacies paleography of the Cretaceous-Neogene sequence is characterized by NW-trending fault-depression in the basin during the episodesⅢandⅣof the Yanshanian Movement when the Sequences Ts7 and Ts8 were deposited, as well as by the arid paleoclimate of the subtropical zone,with wide,shallow,oxidized lakes, where deposits in oxidized colors were dominant,with undeveloped dark mudstones,less potential for hydrocarbon generation.The deposits consist of polyenic(mostly in the west) and proximal detrital material,including alluvial-fluvial-deltaic-oxidized shore neritic deposits from margin to center of the basin.Several NW-trending fault fronts represent great subsidence zones in absence of a common depocentre.During the episodeⅤof the Yanshan Movement when the Sequences Ts9 and Ts10 were deposited,multi-period fault-depression cycles occurred,which can be divided into the early cycle dominated by fault depression,and mid- late cycle was dominated by depression, when the entire basin was in hemi-arid,arid alternative with humid climate in the mid subtropical zone,with neritic-(hemi)abyssal deposits,including well-developed dark mudstones.This stage represents a main period when the two main source rocks were developed.Himalayan when the Sequence Ts11 was deposited.During the Himalayan Movement of the latest Oligocene,the basin was entirely uplifted and eroded,representing the end of the rift-depression basin development.During the Miocene of the Neogene,the basin was in a slow depression and subsidence period,with the subsidence center in the North Qianjiang sag, where fluvial deposits were deposited. 4.Study of the basin - mountain coupling and material response relations of Meso-CenozoicBased on the analyses of sequence stratigraphy framework,sequence lithofacies palaeogeography,sediment combination,and subsidence history of Late Triassic Late Triassic -Neogene,this paper explains the tectonic action process of basin,the tectonic action process of the orogenic belt of basinal margin,and the filling process of basinal sediments. and reveals the evolutionary process of basins and mountains in the middle Yangtze region since Indo Chinese for the first time;①Early Triassic(including Late Permian)to Middle Triassic recorded the transformation of the northern margin of Yangtze from a continental margin basin to a residual marine basin,reflected the soft collision between North China(Qin Ling)plate and Yangtze plate in Indo-Chinese epoch,and showed the process of collision in the study area without orogenesis;②Late Triassic to Early Cretaceous(partly nonsequential in the study area)recorded the three episodic evolutionary phases in the foreland(Molasse) basin,reflected the oblique land - land collision between North China plate(Qin Ling)and Yangtze plate in Early Yanshanian,and the episodic(pulse)process of orogenesis by advance thrust uplifting;The study indicates that there is a good coupling relation between the orogenic belt and the foreland basin,which is characterized mainly by the coincidence relations in the following points:the uplifting process of the orogenic belt and the subsidence history of the basin,the stratigraphic denudation and the sedimentary fills,the stratigraphic unconformity distribution and the shifting regularity of the orogenic belt,the tectonic subsidence rate and the accumulation style of formations.In summary,the sedimentary filling characteristics of the foreland basin are resulted form the formation and evolution of the orogenic belt and the subsidence of the foreland basin.③In the stage of foreland basin reform and rifted basin development from Late Cretaceous to Paleogene,the structural setting underwent significant changes.Affected by a lot of factors such as the intense activity of Yan-Lu fault,the material upwelling of asthenosphere in eastern China,and the instability of the thrust uplifting-induced thickened lithosphere because of gravity,there formed some rapidly and evenly uplifted and strike-slip uplifted or depressed structures,as well as a series of reversed extension downfaulted basins(Innovation point).5.The relationships of superposition,reform,and juxtaposition between Meso-Cenozoic basins and underlying marine facies basins in middle Yangtze region provides us with foundations for the analysis of the regularities of marine facies reservoir formation,the analysis of oil and gas exploration prospects in marine facies basins,and the optimization of marine facies oil and gas exploration districts.The analysis follows the idea of "the structure controlling the basin" and "the basin controlling the facies",and applies the dynamic analysis process of sequence filling into the analysis of the preservation conditions and exploration prospects for oil and gas exploration The superposition relations for Meso-Cenozoic basins and Palaeozoic basins can be divided into 5 types:①the successive coverage;②the earlier exposed coverage;③the later exposed coverage;④the discontinuously exposed coverage; and⑤the continuously exposed coverage.According to the superposition relations,four oil/gas preservation condition units are classified for Middle Yangtze region,i.e.,the successive type,the reconstruction(sedimentary reconstruction)type,the superposition type, and the post-eroded relic type.The region of Western Hubei and Eastern Chongqing and the region of Zigui Basin belong in the type of successive oil/gas preservation condition unit;The southern part of Jianghan Plain,the successive type - the reconstruction type of oil/gas preservation condition unit;The arc structure belt of Dahongshan Mountain,the superposition type of oil/gas preservation condition unit;The area between the southern part of Jianghan Plain and the arc structure belt of Dahongshan Mountain,the sucessive typepost-eroded relic type of oil/gas preservation condition unit;And the Western Hunan and Hubei,the post-eroded preservation condition type of superposed oil/gas units(Innovation point).6.Through the in depth study,it is considered that in the cases of complex structural features and not fully matched conditions for the marine facies exploration structural traps and oil/gas formation time,the main oil/gas-bearing areas must be the trap structures in the large sized epeirocratic craton,the periphery area of the palaeohigh,the thrusting in-place geologic body in the structural belt of thrusting and overriding,and the buried structural traps formed due to severe inconsistency between upper and lower structures.Four levels of areas are selected as the marine facies oil and gas exploration prospects,i.e.,the folded belt of Western Hubei and Eastern Chongqing,the basinal area of Zigui,the periphery area of the palaeohigh(including the northern part of Yidu-Hefeng anticlinorium,and the southern part of Dangyang synclinorium),and the southern part of Jianghan Basin.