Study On The Sedimentary Characteristics And The Controlling Factors Of The Beach-bar Of The Second Member Of The Paleogene E₁f₃ Formation In The Gaoyou Sag

The third member of the Paleogene Funing Formation in Gaoyou Sag is one of the main oil production layers in the Gaoyou Sag.This paper selects the third member of the E₁f₃ Formation as the research object,and studies the core,logging,seismic and various analytical test data.Based on the results of the work,a detailed description of the sedimentary characteristics,depositional model,and genesis mechanism of the thin interbedded beach-bar sand body in the study area was carried out.At the same time,aiming at the development characteristics of beach-bar sand bodies in the middle-third section of the Paleozoic,the quantitative analysis of the controlling parameters was performed on key sand layers in the study area,and a quantitative prediction model for beach-bar sand bodies was established for future prediction.This study provides new theoretical methods for favorable areas.The main findings are as follows:?1?On the basis of the isochronal sequence stratigraphic framework in the whole region,through the analysis of basic geological data,the third member of the E₁f₃Formation is divided into four sand groups,each sand group representing a small-scale anti-recession,A large-scale thin interbed beach-bar sand body is developed in each sand group,and the north slope-shore shallow lake region is dominated by the developmental wave-controlled delta-beach-bar system.The development scale of the delta is limited.The observation of the core shows that the lithology of beach-bar sand body is mainly fine-grained sandstone,and the thickness of single layer generally does not exceed 5m.The sedimentary sequence has both positive and negative sequence.The sedimentary structure formed by the wave action is very significant.Bioturbation structure,flushing,bedding,parallel bedding,wave marks,etc.are common.Through the analysis of single well phase and connecting well phase,the sedimentary system and sand body distribution in the sequence grid were clearly defined.Combined with the Percentage contour map of sandstone and the sandstone thickness map,the beach-bar was divided into four types of microfacies,namely the bar body,bar side margins,outer margins,beach sands,and established corresponding deposition patterns.?2?Based on a clear understanding of the sedimentary microfacies in the study area,summarizing the characteristics of beach-bar in terms of lithology,logging and seismicity,respectively.Bar sand monolayer thickness is relatively large,generally greater than 4m,log curves are mostly box-shaped,funnel-shaped;beach sand thickness is generally less than 4m,log curves are mostly sharp knife-shaped,finger-shaped.In the seismic profile,the thin layer beach bar sand body is mainly reflected by the weak amplitude,and only in the place where the bar sand is concentrated,it shows a strong reflection,and presents a“hill-shaped”reflection feature,with poor lateral continuity,and the overall is“weak in sight.The reflection characteristics.Taking into account the geomorphic features,source systems,and ancient water depths,the in-depth study of the origin of the beach-bar was conducted to find out the mechanism of the development of beach-bar sand bodies.According to the different formation mechanisms of sand bodies,beach-bar was divided into three genetic types:storm beach-bar,normal windsurf beach-bar and composite beach-bar.?3?The development of the thin-bed beach bar sand body is controlled by multiple factors.These control factors mainly include ancient material source,ancient landform,ancient water depth,ancient wave conditions,ancient wind direction,and ancient wind force.The source material in the deposition period of the third member of the E₁f₃ Formation came from the Jianhu uplift.The source supply was sufficient.The third sand group was selected,and the“phase sequence method”was used to recover the ancient water depth of this period from approximately 3m to 16m.The paleogeomorphology of this period was restored accordingly and it was found that the beach sand body was mostly developed in the underwater slope,fold zone with gentle slope area and low uplift perimeter area.Combined with the flat distribution pattern of the beach-bar,it is believed that the beach-bar sand bodies in this period were controlled by the hydrodynamic belts such as breaker zone and surf zone from the lake shore to the inner part of the lake basin,forming outer bar and inner bar.Based on the"wave breaking model"of the cause of the outer bar,according to the corresponding relationship between the thickness of the sand body and breaking wave parameters,it is calculated that the main wind force forming the bar in this period is between grade 4 to 9.Through controlling factors restoration,it provides a new theoretical guidance method for quantitatively predicting beach-bar sand bodies.