Features And Model Of Mixed Sediments In The 1st Member Of Shahejie Formation In The Paleogene Of Lixian Slope,Raoyang Sag

Not only do mixed rocks distribute in marine basins,but they also distribute in a large number of hanging wall slope areas in rift sags.Lixian Slope of Raoyang Sag is a northeastward inherited large sedimentary slope.It has a large amount of residual hydrocarbon resources.It is the main battlefield for increasing reserves and production in Huabei Oilfield.In the lower section of the First Member of Shahejie Formation,the Raoyang Sag experienced rapid flooding,resulting in a great increase in the lake basin area and accommodation space.The siliciclastic influx from the river-delta systems was transferred in the lake basin by waves and coastal currents and deposited with carbonate sediments to form a set of coastal shallow-lake and beach-bar facies,which was distributed throughout the area.In order to clarify the characteristics and sedimentary model of the mixed rocks in the hanging wall slope areas of the rift sags,by using the theories of sequence stratigraphy and sedimentary petrology,under the control of the stratigraphic framework,this work is mainly based on the core data and logging data of the Lixian slope.Through the core and slice observation,grain size analysis,log facies analysis,we studied the sedimentary characteristics and distribution of mixed beach-bars in the work area.Seismic data,pollen data,and trace element data were analyzed to restored the depositional environment controlling factors such as paleoclimate,paleo-provenance,paleogeomorphology,and paleo-water characteristics in the First Member of Shahejie Formation.On these bases,the sedimentary model of the mixed beach-bars in the Lixian Slope was systematically summarized.The results show that the stratigraphic framework of study section is clear.The 1^stand 2^nd member of Shahejie Formation is divided into two third-order sequences.The main reservoirs is the 1^st member of the Shahejie Formation.It is furtherly divided into five sand groups:SQw,SQt,SQs III,SQs II and SQs I.And the beach-bar facies developed in the hanging wall slope area of Raoyang Sag can be divided into siliciclastic beach-bars,carbonate beach-bars,and mixed beach-bars according to petrologic characteristics.Moreover,mixed beach-bars can be divided into component-mixed beach-bars and stratified-mixed beach-bars.The lithology of component-mixed beach-bar was mainly calcareous sandstone,sandy oolitic limestone,and bioclastic limestone;Stratified-mixed beach-bars were mainly composed of biological limestone or dolomite interbed with thin layers of sandstone and siltstone.The distribution of mixed beach-bar was controlled by factors such as paleoclimate,paleo-provenance,paleogeomorphology,and paleo-water characteristics.Therefore,different sedimentary patterns developed in different positions and different sedimentary stages on the slope.Based on the differences of the paleo-sedimentary environment in different areas of the slope,the hanging wall slope of the rift sag can be further divided into a near-shore ramp,a far-shore ramp,and a far-shore low uplift.In the early stage of flooding,the water depth of the lake basin was large.Stratified-mixed beach-bars and siliciclastic beach-bars were deposited in the near-shore ramp,and component-mixed beach-bars were deposited in the far-shore ramp.Carbonate beach-bars were mainly distributed on the far-shore low uplift;For the late flooding stage,the water gradually became shallower.Consequently,siliciclastic beach-bars were mainly developed in the near-shore ramp.Component-mixed beach-bars and stratified-mixed beach-bars were formed in the far-shore ramp.This research can provide new ideas for not only the study of mixed rocks sedimentary systems but also oilfield exploration.