Types And Characteristics Of Sedimentary Facies In Nantun Formation Of Lower Creteaous, Ta'nan Depression

Ta'nan Depression is a secondary tectonic unit in Tamuchage basin,and is complex fault depression that fault in the east and overlap in the west.It is composed of west slope belt, west secondary depression,western buried-hill-fault structural belt,center secondary depression,central buried-hill-fault structural belt,east secondary depression and fault-nose belt.Lower Cretaceous mainly developed Tunbomiao Formation,Nantun Formation, Damoguaihe Formation and Yimin Formation.This paper mainly focuses on Nantun Formation.By comprehensive research on drilling well and seismic explanation,Nantun Formation is divided into five layers,and the types of sedimentary facies,distribution regularity and evolution characteristics of each layers within substratified framework have been discussed.Seen from the tectonic evolution,Nantun Formation is at fault-depressed diversionary period.So,Nantun Formation has the double characteristics of fault and depression.According to careful observation and description of rock types,particle size characteristics,primary sedimentary structures,sedimentary rhythm,biological fossil assemblages and clay mineral characteristics of the drilling cores in three-dimensional seismics in Ta'nan depression,and by comprehensive analysis of logging data of 105 wells combined with the identification of rock thin sections and particle size analysis,three sedimentary facies such as delta,submarine fan and lacustrine facies have been identified,and further been divided into 8 kinds of sedimentary subfacies and 15 kinds of sedimentary microfacies.The basin area of Ta'nan Depression is big,and the evolution time is very long,and the sedimentary facies changed very quickly in space and time.The establishment of subsequence stratigraphic framework and the discussion of the paleostructure framework evolution have laid a foundation for the research on the distribution characteristic of sedimentary facies.Single-well analyses of sedimentary facies give the information of point azimuth,the analyses of the connecting-well sections show the series and distribution regularity of sedimentary combinations in vertical and transverse.Thus,the distribution characteristics of sedimentary facies during the period of Nantun Formation in Ta'nan Depression have been objectively summarized in detail.Large-scale submarine Fans mainly developed by the side of controlling faults in the East during the early period of Nantun Formation,the fan bodies gradually dwindled and moved back by the time evolution,and small-scale delta prograduted to the end stage of Nantun Formation.Delta facies mainly developed in the North,and the fan bodies gradually enlarged from the early to the end period of Nantun Formation.Shore-shallow lacustrine facies developed in the gentle slope belt in the West during the whole sedimentary period of Nantun Formation.Small-scale turbidite fans developed in semi-deep lake facies in the South.Semideep-deep lake facies developed in the center of the basin during the whole period of Nantun Formation.The sedimentary evolution of Nantun Formation in Ta'nan Depression was mainly controlled by tectonic subsidence,provenance supply,lake-level fluctuation and climate factor.By studying the growth indexs of main controlling faults during every period,the intensity changes of fault activities during different periods and their control actions on sedimentary systems are discussed.Climate is an important factor that influences the lake water quantity and water-level change,which is shown to control the lake-level variation, determine the size of accommodation space,influence the types,supplies and distribution ranges of sediments,and play an important role in controlling the sedimentary fill and evolution of lake basin.Climate was warm and humid during the period of Nantun Formation.By calculating the sedimentary rate,the intensity of sediment supply in Ta'nan Depression is analyzed.Three direction of provenances developed in this basin,which was in Northern,Eastern and Southern part.There were two high rate points of provenance supply in the North,and its supply rate was totally very high.The rate of sediment supply in Northeast part during the early stage of Nantun Formation was very small,but it was up to the highest during the period from T222 to T23,then the rate gradually decreased.The intensity of provenance supply in Southeast part was biggest during the early stage of Nantun Formation,and then it gradually decreased by the time evolution,and was up to smallest at the late period of Nantun Formation.The intensity of provenance supply in Southern part was relatively high during the period of T3-Sq1-2 and T23-T221,and it was relatively small during other stages.Lake-level fluctuation influences the overlay manners of parasequence sets(progradation or retrogradation),and controls the evolution of sedimentary systems together with tectonic subsidence,climate and the intensity of provenance supply. This paper takes the T3 period as the starting point,and discusses the lake-level fluctuation of Nantun Formation.As a whole,the lake-level during the period of Nantun Formation was rising.Seen from the sedimentary system,the delta in the North of Nantun Formation was gradually bigger and bigger by the time evolution.Submarine fan mainly developed in the East during the early stage of Nantun Formation,and then the provenance gradually migrated to the northeast direction,and the delta developed at the late stage.Subaqueous Fan in the South of the basin was gradually to move back by the time evolution.