| The sedimentary facies,sequence stratigraphy and reservoir features of the Pinghu Formation-Huagang Formation in the Xihu Sag,the East China Sea Shelf Basin,are studied,including sedimentary facies division,sequence framework establishment,sedimentary facies distribution,reservoir petrology,reservoir diagenesis and reservoir physical property.Besides,the influence of sedimentary sequence on low porosity and low permeability reservoirs is discussed.This paper presents the following conclusions.(1)Five types of sedimentary facies are identified in the study area,including braided river delta,shoreland with barrier,shoreland lake,nearshore subaqueous fan and fan delta.The last two types of sedimentary facies are identified and recognized based on seismic reflection features.During Eocene,intensive transgression occurred,Pinghu Formation was deposited under lagoon background and shoreland lake was not present.During Oligocene,the entire sedimentary water level declined,Huagang Formation was deposited under shallow lake background and shoreland facies with barrier was not present.(2)Instead of tectonic movement,sea level change is the major contributor to the formation of regional unconformity surfaces at the top of Pinghu Formation and Huagang Formation.Yuquan Movement(the depositional break surface at the top of Pinghu Formation)and Huagang Movement(the depositional break surface at the top of Huagang Formation)exactly correspond to the twice global cooling events in time.(3)The provenance provided by Haijiao Uplift and Yushan Uplift in the west part has influence on the western slope belt,central inversion zone and eastern sub-sag.The provenance provided by the Diaoyu Island in the east part only has influence on the eastern sub-sag,where the actual subsidence center of Xihu Sag is located.The Cenozoic strata can be divided into seven second-order sequences(SSQ1-SSQ7).Particularly,Oujiang Formation-Pinghu Formation and Huagang Formation belong to two second-order sequences(SSQ3 and SSQ4),respectively.Pinghu Formation includes three third-order sequences(SQ1-SQ3),and Huagang Formation includes three third-order sequences(SQ4-SQ6).LST,TST and HST are identified in each third-order sequence-TST and HST identified in the western slop belt–central inversion belt,while LST,TST and HST identified in the eastern fault terrace zone.(4)During LST depositional period,the western slope belt and central inversion belt were filled by HST sandy deposits,which were outcropped,eroded and reformed in later stage,and incised valley gravel fillings occurred.The eastern sub-sag was filled by the deposits from lowstand wedge-slop belt,which were sourced from the western provenance,and the deposits from fan delta-nearshore subaqueous fan,which were sourced from the eastern provenance.During TST depositional period,the western slope belt and central inversion belt were filled by the retrograding deltaic deposits sourced from the western provenance.However,due to the erosion occurred during late HST depositional period and LST depositional period,the argillaceous deposits of prodelta-lagoon(or deep lake)were still remained at present.The eastern sub-sag was filled by the argillaceous deposits of lagoon(or deep lake).During HST depositional period,the western slope belt and central inversion belt were filled by the deltic prograding(foreset)body,when braided delta advanced eastwards as a form of foreset oblique zone,underlying the maximum flooding surfaces as a form of downlap,while the eastern sub-sag was filled by the argillaceous deposits of prodelta-lagoon(or deep lake)and the deposits of fan delta-nearshore subaqueous fan sourced from the eastern provenance.(5)The reservoirs can be divided into conventional reservoirs,low porosity and low permeability reservoirs and tight reservoirs.The porosity and permeability relations of the reservoir sandbodies which are developed in each third-order sequence in different tectonic units are calculated.The three types of reservoirs in Pingbei area and Pinghu area have a strong correlation between porosity and permeability.The tight reservoirs developed in the middle horizon of the western sub-sag have a close correlation between porosity and permeability.The reservoirs of the southern-central part as well as Chunxiao-Tianwaitian area have an ordinary correlation between porosity and permeability due to their strong stress reformation.(6)Compaction is intense in the study area,but the compaction strength is different in the reservoirs developed in different tectonic units and different sedimentary facies due to their different framework structures.Compaction is the most intense in the western sub-sag,followed by Yuquan area-southern-central part-Chunxiao-Tianwaitian area,Pingbei area and Pinghu area.Sedimentary differentiation resulted in the difference in framework structures of sandbodies with different sedimentary facies.The compaction resistance of delta plain sandbodies is stronger than that of delta front sandbodies,the compaction resistance of delta plain channel sandbodies is stronger than that of delta plain natural levee sandbodies and the compaction resistance of delta plain mouth bar and underwater distributary channel sandbodies is stronger than that of delta front sheet sandbodies.(7)Through calculation of porosity of the sandbodies at different depths in sequences and with different sedimentary facies,the following conclusions are made:(1)fresh water erosion is common in the study area,which is more severer in the upper strata close to sequence boundary than that in the lower strata far away from sequence boundary;(2)for the deltic sandbodis in the same sequence,the sandbodies of delta plain facies have higher porosity;and(3)in the delta front sedimentary belt in the same sequence,the area with high sand/formation ratio also reveals high porosity. |
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