| Combining with the fundamental geological theories including the sedimentology.sequence stratigraphy,organic petrology and organic geochemistry,this study was focused on the types of the coal-accumulation environment,coal-accumulation models,characteristics of the organic petrology and organic geochemistry.Five types of coal-forming environments,four types of sedimentary organic facies,three types of coal accumulation models and one kind of sedimentary organic facies model were summarized.Finally,main favorable coal-bearing centers and hydrocarbon-generating areas were proposed in the Pinghu and Huagang formations of the Xihu Depression.Based on the drilling log and paleontological data,the sandstone-dominated lithofacies were identified from core and include mud-clast conglomeratic sandstone,structureless sandstone,cross-bedded sandstone,faint cross-bedded sandstone,ripple cross-laminated sandstone,faint ripple cross-laminated sandstone,planar-laminated sandstone,faint planar-laminated sandstone,wavy bedded sandstone,faint wavy bedded sandstone,bioturbated muddy sandstone,and contorted sandstone.Eight mudstone-dominated lithofacies were identified from core and include lenticular bedded mudstone,laminated mudstone,mottled mudstone,structureless mudstone,crumbly mudstone,carbonaceous mudstone,and coal.Five facies associations were defined:(1)tidal flats deposits;(2)tidal-influenced delta deposits;(3)braided river delta deposits;(4)meandering river deltadeposits;and(5)lacustrine deposits.Combining the results of seismic sequence division and the identification of key sequence stratigraphic surfaces by the drilling logging data,the Pinghu Formation and Huagang Formation in the Xihu Depression were subdivided into six third-order sequences.The sequence I corresponds to the 5th Member of the Pinghu Formation.sequence II corresponds to the 4th Member of the Pinghu Formation;sequence ?corresponds to the 3rd member of the Pinghu Formation;sequence IV corresponds to the 1st and 2nd members of the Pinghu Formation;sequence V corresponds to the Lower Member of the Huagang Formation;sequence VI corresponds to the Upper Member of the Huagang Formation.and the analysis of the distribution characteristics of the depositional facies in the sequence framework.The distribution characteristics of sedimentary facies and sequence stratigraphy showed that the tidal flats and tide-influenced delta were developed in the Pinghu Formation,delta and lacustrine were mainly developed in the Huagang Formation.During the sequence ? and the sequence ?,the major coal-bearing palaeogeography units were the supratidal and intertidal.The coal seam was characterized by the thin layer and a large accumulated thickness.The distribution of the coal seam showed inhomogeneous,and the cumulative thickness were presented an increasing trend.During the depositional period of sequence ? and sequence ?,the major coal-bearing palaeogeography units were tidal-influenced deltas,tidal and intertidal.The coal seam also showed the same characteristics with sequence ? and ?,and the cumulative thickness showed a decreasing trend.The distribution range showed an expanding trend.In the Huagang Formation,the distribution scope of coal seams was further expanded,and the coal accumulation center showed a tendency to migrate to the south of the central depression area.Based on the wavelet analysis of typical wells in different tectonic belts of the Xihu Depression,the long period of eccentricity was identified,and the Pinghu Formation was subdivided into 24 medium-term cycles(fourth-order sequence).The development position of the coal seam was usually located near the maximum value of the long-period cyclic curve of eccentricity.Because the maximum amount of solar radiation is near this position,the corresponding climate is relatively warm,which is conducive to the growth of coal-forming plants and the accumulation of peat.At the late stage of base level decline,sandstone-dominated reservoirs are mainly developed.By the analysis of coal accumulation control factors,it is believed that the paleoclimate was changed from the "greenhouse" to the "icehouse" during the Eocene-Oligocene.The changing of palaeoclimate resulted that the development of coal seam in Pinghu Formation was superior to Huagang Formation.By analyzing the tectonic evolution process of normal faults and combining with the characteristics of space-time changes of accommodation,the uneven distribution of coal seam in the Pinghu Formation and universal distribution of coal seam in the Huagang Formation were dominated by the different tectonic evolution stages.Based on the results of medium-term cycles subdivided by wavelet analysis in the typical well,the causes of thin coal seams were discussed from the perspective of the rate of increase in accommodation and the ratio of peat accumulation(AR/PPR).Under the high-frequency cycle background of sea level,the frequent migration of sedimentary facies belts is the main reason for the development of thin coal seams.Based on the analysis results of comprehensive depositional environment,coal accumulation conditions,and coal seam distribution,coal accumulation model of the multi-depressions basin dominated by the east inclined fault array,coal accumulation model of the multi-depressions basin dominated by graben and coal accumulation model of the multi-depressions basin dominated by horst were proposed in the Xihu Depression.From the results of the macerals of the Paleogene coal-bearing source rock,the results showed that the vitrinite is the dominant component with an average content of 88%.Followed by the liptinite,the average content is up to 10.6%.The average content of the inertinite is only 1.4%.The most typical features of coal-bearing source rock in the Xihu Depression are the rich-liptinite which was dominated by the resin.The coal-bearing source had a higher hydrocarbon generation potential.Through the analysis of various organic geochemical parameters,the evaluation index system for coal-bearing source rocks in Xihu depression was based on the hydrocarbon generation potential(S1+S2),hydrogen index,total organic carbon and chloroform asphalt "A".According to the evaluation index,the source rocks of coal-bearing source rocks in Xihu Sag are evaluated.The results showed that the range of hydrocarbon generation in the Upper and Lower Members of the Huagang Formation varied largely.The hydrocarbon generation potential in the second and third Member of the Pinghu Formation was the best,followed by a fifth and fourth Member.The molecular geochemistry of coal-bearing rocks in the Xihu Depression indicates that the source rocks are dominated by higher plants under oxidative conditions.The organic matter maturity is basically in the mature stage,only a small part is in the low-ripening and high-maturity stage.Based on the results of sedimentology,organic petrology,and organic geochemistry,four types of sedimentary organic facies of coal-bearing source rocks of the Pinghu Formation and Huagang Formation in Xihu Depression were proposed and include the dry-swamp sedimentary organic facies,the forest-swamp sedimentary organic facies,the living-water swamp sedimentary organic facies and the broad-water sedimentary organic facies.The results of Rock-Eval showed that the dry-swamp sedimentary facies had the highest hydrocarbon generation potential,followed by the forest-swamp and living water-swamp sedimentary organic facies,and the broad-water swamp sedimentary organic facies is the worst.According to this,the favorable hydrocarbon generation regions within each sequence are pointed out.The most favorable hydrocarbon generation region in sequence I was located in the Kongqueting and Baoyunting area,which were dominated by the dry-swamp and forest-swamp sedimentary organic facies.During the sequence ?,the most favorable hydrocarbon generation zone was mainly located in the Pinghu area,and the dry-swamp and forest swamp sedimentary organic facies were developed in this area.During the sequence ? the northern part of Hangzhou tectonic belt,Kongqueting and Baoyunting areas were the most favorable hydrocarbon generation area,the dry-swamp sedimentary organic facies and forest-swamp sedimentary facies are dominated in the areas.During the sequence ? sequence,the most favorable hydrocarbon generation zone was located in the Pinghu Tectonic Belt,the northern part of Hangzhou Tectonic Belt and the northern part of the Tanita Tectonic Belt,which were dominated by the dry-swampy sedimentary organic facies.During the sequence V,the most favorable hydrocarbon-generating area were the Canxue and Huangyan area.The sedimentary organic facies belt is dominated by the living-water swamps sedimentary organic facies;During the sequence VI,the favorable hydrocarbon generation zone is mainly located in the Pinghu Tectonic Belt,which was mainly developed the forest-swamps and living-water swamp sedimentary organic faciesBased on subdivision of the four-order sequence,the distribution model of the sedimentary organic facies of coal-bearing source rocks was proposed under the low-accommodation background.The swamps were gradually developed with the increase of the AR/PPR ratio.As the AR/PPR ratio increasing,the swamps were slowly developed and entered the peat window which was located above the paludification surface,and the peat begins to accumulate and be preserved.The dry-swamp deposits were accumulated at first.In the deepening-up successions,the forest-swamps sedimentary organic facies,the living-water swamps facies and broad-water sedimentary organic are developed in the proper order.The distribution model of sedimentary organic facies was summarized in the coal-bearing source rock of the Xihu Depression,which was characterized by the dry-swamps,forest-swamps,living-water swamps and broad-water sedimentary organic facies successively on the plane.The corresponding sedimentary environments are the upper delta plain,upper tidal zone;the top of lower delta plain,upper part of intertidal,bottom of the lower delta plain,lower part of intertidal,subtidal zone or lake,respectively. |
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