Research On Sedimentary Facies Types And Development Laws Of Second Member Of Yinggehai Formation,Dongfang Gas Field,Yinggehai Basin

The main gas-producing strata of the second member of the Pliocene Yinggehai Formation?Y₂ member?in the Yinggehai Basin are Y₂I,upper Y₂II?Y₂IIU?,lower Y₂II?Y₂IIL?and upper Y₂III?Y₂IIIU?gas formations.The Dongfang 1-1 gas field has entered the late stage of development,the production decline is rapid,so there is an urgent need to tap potential adjustment and delay production decline.However,at present,the sedimentary system of the study area has been controversial.Predecessors have studied the sedimentary system of the second member of Yinggehai Formation in the study area,but there is a lack of clear understanding of sedimentary models and development laws.Therefore,combined with investigation and analogy,synthesize a variety of methods,clarify the sedimentary models and development laws of the study area,and on this basis,carry out the study of fine sedimentary facies for the purpose of tapping the potential in the later stage of development,which is of guiding significance for the adjustment of the development scheme.On the basis of the previous research results,this paper combines the geological,seismic,well logging,geochemical test and analysis data,and comprehensively uses the multi-disciplinary methods of geophysics,geochemistry,paleontology and so on,the following work have been completed:?1?the sequence stratigraphic framework of I,IIU,IIL and IIIU strata of the Pliocene Y₂ member in Dongfang 1-1 gas field was established,and the variation and distribution characteristics of well-connected strata thickness in well control areas were clarified;?2?the main provenance directions of the study area were analyzed by zircon U-Pb dating method,and the paleoenvironment and paleo-water depth during the target strata deposition period in the study area were clarified by the geochemical tests of main and trace elements,the discrimination method of grain size analysis and the percentage content calculation method of planktonic foraminifera;?3?the main types and characteristics of sedimentary facies in the study area were summarized by identifying the core facies marks,logging facies marks and seismic facies types of typical well coring sections;?4?based on the distribution characteristics of seismic facies and modified by referring to seismic attributes and strata thickness,the development laws of Y₂ member sedimentary facies were analyzed on the principle of unifying point?single well facies?,line ?connected well facies?and plane?seismic facies and sedimentary facies plane distribution?.The properties of sedimentary facies were determined by logging facies and core facies?sedimentary structures?,and the types of sedimentary facies were defined as rich or poor sand facies by sand-strata ratios.Finally,the comprehensive sedimentary model was established.The overall stratigraphic thickness of Y₂ member decreases gradually from northwest to southeast,which has an obvious step-like decline law.The whole 0 stratum is thick in the northwest and central part and thin in the southeast,with an overall average thickness of about61.3m.The thickness of I stratum is characterized by thicker in the northwest and southwest,and thinner in the central,east and south-central part,and the thinning rate in the west to central is greater than that in the central to east.The IIU stratum is characterized by thicker in the southwest,obviously decreasing in the direction from the southwest to the central and east central part.The IIL stratum is widely distributed in the study area,whose distribution is smooth and thickness is relatively stable,showing the characteristics of thick in the west and thin in the east,thick in the south and thin in the north.The distribution of IIIU stratum as a whole shows that the edge of the study area is the thickest and the stratum getting thinner as going to the center of the basin.Detrital zircon dating results show that the study area as a whole exhibits the age characteristics of the northern and western provenance.The main and trace elements analysis indicates that the paleo water environment during the sedimentary periods of I and IIL gas formations in the study area is oxygen enrichment and sub-oxygen-rich environment,belonging to the shallow sea environment with shallow paleo water depth.The paleo water depth and paleosalinity of the IIL gas formation are greater than those of the I gas formation.During the sedimentary period of I gas formation,the paleo water depth is shallow in the west part and deep in the east part.Sahu function discriminant method indicates that the sedimentary periods of IIU and IIL gas formations are shallow sea environment.Based on the quantitative analysis method of planktonic foraminifera,the sedimentary periods of IIIU?I gas formations in well DF1-1-11 belong to the inland shelf sedimentary environment.The lithology of the study area is mainly fine clastic.The sedimentary structural types reflecting the shallow water environment of coastal and shallow sea deltas were mainly developed.The quasi-sequence funnel type with continuous reverse cycles,strong toothed bell type and toothed low funnel type logging facies were mainly developed.Eight types of seismic facies were identified:medium amplitude parallel sheet reflection,strong amplitude wedge reflection,medium amplitude retrogradation reflection,low frequency parallel-subparallel blank reflection,extremely low frequency mound blank reflection,low angle forward prodgradation,high angle lateral accretion,and channel fil ing seismic facies.There are various sedimentary types of Y₂ member in Dongfang area.The coastal and shallow sea slope break delta deposits,shallow sea sandy debris flow deposits and littoral sand bar deposits of north and west provenance were developed under the coastal and shallow sea background,as well as high curvature gravity flow channels and mud flow channels in the north-south direction.During the sedimentary period of IIIU to IIU strata,delta deposits were mainly developed in the west part of the study area,while shallow sea sandy debris flow deposits were mainly developed in the east part.The high-curved gravity flow channels and mud flow channels were distributed in a north-south cutting pattern,which had strongly eroded and reformed the underlying sediments.The I stratum is relatively gentle,which more reflects the sedimentary characteristics of coastal and shallow sea sand bars.