| Xihu sag is the largest subunit of the East China Sea basin,in which fruitful results have been achieved after oil-gas exploration and development in the past few years.In recent years,a breakthrough in the formation and development conditions of large reservoir groups in central-north Xihu Depression Huagang Formation has been made through sedimentary reservoir study.However,in addition to Huagang Formation of Xihu Depression,Pinghu Formation is also a target stratum being focused on.However,it is not clear enough to understand the sequence stratigraphic division and sedimentary environment of Pinghu Formation etc,due to the fact that East China Sea basin stands on the tectonic intersection area,along with a complex tectonic background,especially for the western slope belt in Xihu sag.In this paper,a high precision sequence stratigraphy framework for Pinghu Formation in Pinghu Slope on Xihu Depression has been set up through the drilling,logging,seismic and analytical testing data,and by using the theoretical and technical methods of sequence stratigraphy and seismic sedimentology etc,.The characteristics of the source system have been analyzed,with the type of Pinghu Formation sedimentary system been clarified,sand body time-space distribution been predicted,and a depositional model during Pinghu period been established to explore the technical methods for multidisciplinary comprehensive research on the sedimentary system.Through comprehensive research,the following achievements and understandings are mainly achieved:1.The main sequence interface of Eocene Pinghu formation in Xihu Sag has been redefined,and a stratigraphic framework of Pinghu formation is constructed as well as four prediction models of sand bodies combined.By comprehensive utilization of Pinghu slope drilling,logging and seismic data,a total of four sequence interfaces are identified in Pinghu Formation,namely T40,T34,T32 and T30,along with a regional marine surface,T33,on the basis of the well-shock fine calibration,seismic reflection structure,logging curve,core observation and biomarker analysis etc.,Among them,T40 is a regional unconformity interface,T34 interface is a phase transition interface,T32 interface is the later uplift transition interface,and T30 is the fault depression transition interface.According to this,Pinghu Formation can be divided into a two-level sequence and three three-level sequences: SQ1,SQ2 and SQ3.As a whole,the three sequences of Pinghu formation are completely developed,and each sequence system is divided into two parts,that is,Transgression systems tract and Highstand systems tract,of which the sand content of SQ1 and SQ3 sequences is relatively high,while high shale content in SQ2.In particular,the transgression system of SQ2 sequence contains large mudstone thickness involved in stable development,which is a good contrast marker bed in the work area.Combining the research content of sequence,source-deposition system and sedimentary system,a geological prediction model for guiding the subsequent meticulous depiction of geophysical sand bodies can be extracted,including multi-slope fold slope type,transition zone type,single fracture type and graben-horst type.The multi-slope fold slope type develops in Pingbei area,with distribution of sand bodies controlled by the late uplift slow slope.The pushing distance of sand bodies is far away,and the main sand body type is delta front sand.The transition zone type develops in Pingzhong and Pingnan areas,and multiple transform faults forms multiple sand body conveying channels.The multi-stage faults forms multiple unloading zones of thick sand bodies in vertical.While,single fracture type develops in Pingnan area,with Pingxi fault becoming the main controlling factor of the study area in geomorphology and sedimentation.The graben-horst type developd in Huazhong zone,is mainly graben-horst landscape controlled by several different faults with dudflat facies-mixed flat developed in interval,and vertically some coarse-fine sedimentary fillings.2.According to the core and logging data,three sedimentary types are divided,planar distribution characteristics of the sedimentary system in the Pinghu period are determined.From the aspects of lithologic association,sedimentary structure,paleontology and paleosalinity,etc.,it has been proved that Pinghu Formation in the study area generally belongs to a semi-closed gulf environment with obvious tidal action,and 3 types of sedimentary systems namely,fluvial-dominated delta,tide-dominated delta and tidal flat system are identified.SQ3 in the fluvial-dominated delta vertically is the most developed,and the sedimentary composition is divided into delta plain subfacies,delta front subfacies and prodelta subfacies.The tide-dominated delta is the most developed on SQ2 sequence,and the sedimentary formation has relatively thin clastic particles,with a low deposition rate,generally small sandstone thickness,and obvious water-transgression – water-subsidence features,coupled with lenticular bedding,flaser bedding,wavy bedding and pinnate cross-bedding;The tidal flat system mainly develops in the area of river and sea interaction where the terrain is smooth and it is apparently influenced by tide.In combination with the characteristics of Yuanhui system,the planar distribution characteristics of the sedimentary system within different systems in Pinghu formation of Pinghu slope zone are determined.In general,the delta development of Pingbei area is completedly developed in Pinghu formation during the sedimentary period,and sand bodies are large in scale with a wide range of distribution and continuous development.The sediments in Pingzhong Baoyunting area have entered the slope zone through several main conversion belt channels with great thickness of sand bodies in vertical.While,sand bodies in Pingnan area is in small scale with mostly distribute in isolation.3.Rock facies were predicted by using a variety of geophysical methods,and the space distribution characteristics of sand bodies were meticulously depicted.Under the guidance of a sand body prediction model,using seismic attributes,sparse pulse inversion and geostatistical inversion combined with geological methods,the time and space distribution law of the sedimentary system and the distribution law of sand bodies were depicted from the plane and vertical aspects.Through the seismic attributes,the plane distribution characteristics of the sedimentary system could be qualitatively identified,and the results show that Xihu slope zone contains stable provenances,there are multiple provenances existing on Laiheting provenance area of Pingbei,the northern concave sedimentary filling is mainly controlled,the NB19-6-1provenance area is also controlled in part,and the flower leaf forms of delta sand bodies are clear;sand bodies were transported through the fault transformation zone channel in Baoyunting provenance area of Pingzhong,and the provenance channels have finally developed into two directions,that is,one into deep depression area,and the other into NB19-6-1 provenance area,where the transport distance of provenances is long and the range involved is wide;The development scale of sand bodies isolate in Pingnan area is relatively small,and the transport distance of provenances is relatively close.However,geostatistical inversion can quantitatively identify the space and time distribution of different rock facies.Based on the analysis of rock physical parameters,the density of sandstone,natural gamma and acoustic time difference were lower than those of mudstone,and the rock facies were corrected by logging curve on the basis of this.The facies identification results reveal that the sand mudstone has great difference in wave impedance,which also provides a precondition for the prediction of geostatistical inversion for rock facies.The rock facies prediction section shows that the vertical sand bodies overlap and develop in Pinghu Formation,the lower sand bodies have a good horizontal continuity,and the lens reflex structure is clear in the direction of cross-cutting provenances.In the direction along provenances,the sand bodies have an obvious progradational reflection configuration.The planar graph shows that the sand bodies in Pingbei area distribute like leaf with the characteristic of multiple branches.The sand bodies in Pingzhong area distribute in the north-east direction,and mainly deposit in the descending fault troughs or fault depressions.The traditional wave impedance bodies can be transformed to rock facies by geostatistical inversion,with the advantages of high lateral resolution in seismic data and high vertical resolution in well logging data,and a vertical resolution of about 2m.4.A "three-partition" model of the sedimentary system of the study area is proposed.Based on the analysis of provenance system,logging image,sediment composition and particle size of sediment and so on,there are significant differences in the north,middle and south sections of Pinghu slope.At the same time,combined with the plane distribution characteristics of the sedimentary system,the "three-partition" model of Pinghu formation sedimentary system has been established,and Pinghu slope is divided into three large sedimentary areas,wherein Pingbei fluvial-dominated delta deposition zone,Pingzhong tide-dominated delta deposition zone,and Pingnan tidal flat system deposition zone.Pingbei Laiheting zone is under obvious dynamic control of river water,and the influence of marine hydrodynamics is relatively weak,showing a leaf form of moving from west to east on a large scale;Pingzhong Baoyunting zone is adjacent to the bay mouth with significantly marine hydrodynamic effect and obvious tidal action,which has a strong reform to the detrital materials from rivers,showing the characteristics of tidal control delta,reduced lateral connectivity of sand bodies,and less scale;Pingnan area is located at the mouth of the semi-closed bay with marine hydrodynamic forces basically controlling the spatial distribution and scale of sand bodies,which is mainly characterized by a small range of sand body propulsion and tidal flat deposition.5.The time and space evolution law of the sedimentary system of Pinghu formation was summarized,and the main controlling factors for sedimentary system the development were clarified.During the sedimentary period of Pinghu formation,influenced by the interaction between river water and ocean current,the sedimentary system of Pinghu slope was characterized by "two changes and two un-changes".The "two un-changes" means that the provenance direction is unchanged,and the transport channel of sand bodies is unchanged.The provenances on the western slope are stable,and the main channels are lower cut valley,fault and transform fault.The "two changes" means that deltas are involved in continuous progradation from the early to the late,and sand bodies become larger and wider.At the same time,the transformation action of ocean current is also changing,and the beach bars of Pingzhong and Pingbei change gradually from development to non-development,and the beach bar of Pingnan changes from non-development to development.This evolution law is jointly influenced by regional tectonic movement,paleogeomorphology,fault transformation zone and sea level change and other factors.The change background for the sea level during Pinghu period is regressive as a whole,Xihu basin is a sea-limited basin prototype and is open to the south and connected to the wide sea,and the west slope zone has slow regional slope and obvious tidal action,so the tidal flat deposition developed,and at the time of delta deposit,also being affected by different degrees of tidal action,front deposits were carried,modified and deposited,forming sand banks and other sedimentary bodies.In the early period of Pinghu,the sea level was relatively high,Xihu depression was in the fault-depression stage,and the basin was sagged or uplifted.During the late period of Pinghu formation,the sea level was relatively low,the depression deposited sag in the period of fault depression,and the terrain was relatively flat,which leads to early obvious increase in the scale of the late Pinghu Formation delta and relatively developed beach bar. |
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