Research On Sedimentary Facies And Evolution Of Jiufotang Formation In Qianhouhe Area,Ludong Depression

After more than 30 years of exploration,Ludong sag has achieved certain results,but the quality of seismic data is low,the geological conditions are complex,and the understanding of the sedimentary system of Jiufotang Formation is still insufficient,and the understanding of hydrocarbon accumulation law is still unclear.Therefore,further research on sedimentary facies is of great significance to guide hydrocarbon exploration.Based on the regional geological research in Qianhouhe area of Ludong sag,combined with logging and 3D seismic data,the 4th-order sequence in study area is divided by using INPEFA.On this basis,combined with the core data,the sedimentary facies,subfacies and microfacies developed in the study area are determined,and the single well facies and connected well facies are made,and the development law of sedimentary facies in the vertical direction is clarified.Then use machine learning and principal component analysis to process seismic data,and combine seismic attribute slice and logging data to clarify the laws of plane distribution and rules of vertical evolution of sand body in each period.The research results show that:(1)The Jiufotang Formation in the Qianhouhe area of the Ludong Depression can be divided into three third-order sequences and further divided into eight fourth-order sequences.(2)The Jiufotang Formation in Qianhouhe area of Ludong sag mainly develops fan delta,nearshore underwater fan and lacustrine facies deposits,and braided river delta deposits are developed in the north of the study area.The fan delta mainly develops the fan delta front and the front fan delta subfacies,and the front subfacies mainly develops the underwater distributary channel,underwater distributary channel,estuary bar and sheet sand microfacies;The nearshore underwater fan mainly develops fan middle and fan end subfacies,and the fan subfacies mainly develops braided channel,braided channel inter-flow and sheet lobule microfacies;The lacustrine facies develops deep lacustrine and semi-deep lacustrine subfacies,mainly developing turbidite thin sand and deep lacustrine and semi-deep lacustrine mud microfacies.The study area basically does not develop fan delta plain subfacies and nearshore underwater fan root subfacies.(3)Vertically,the sq1-1,sq3-1 to sq3-3 period in the study area mainly develop nearshore underwater fan and deep lake-semi-deep lake deposits,while the sq1-2 to sq2-2 period mainly develop fan delta and deep lake-semi-deep lake deposits.(4)The nearshore underwater fan sand body has a close advance distance,a narro2 w distribution range,a small thickness of sand body,and the lithology is mainly siltstone.The fan delta sand body has a long advance distance,a wide distribution range,a large thickness of sand body,and the lithology is mainly fine sandstone and siltstone.The distribution range of sand bodies in each period is related to the rise and fall of the lake level.The distribution of sand bodies reaches the maximum in sq2-1 period,and contacts with the northern braided river delta sand bodies in Guangfa area.The above research results have important reference value for guiding oil and gas exploration in the study area.