Progradational Sequence And Reservoir Prediction Of Yanchang Formation In Longdong Area

In recent years,with the development of the tight oil in the center of Yanchang Formation in Ordos Basin,we have found thickness massive sandstone in the upper of Chang6 and Chang7 oil layer of the Yanchang Formation.It has the promising prospects for exploration and development.So,it is one of great significance to discuss and study the characteristics of the sedimentary,seismic and basin evolution which relate to tight reservoir of the basin center of Yanchang Formation in Ordos Basin.This paper comprehensively based on the seismic data and drilling data,and combined with the previous researth results to analysis the progradation sequence and seismic facies and sedimentary facies,and to discuss the Geological significance of the progradation sequence,basin evolution,ancient water depth,ancient landform,reservoir distribution,and to reveal the relationship between the thick sand body source and progradation sequence with tight reservoir in the lake basin,discussing the seismic response characteristics and the method of seismic prediction of the gravity flow in the center of the lake basin.Therefore,it can provide reference to understand the formation law and improve development efficiency in the large oil field of the Yanchang Formation.In this study,the main conclusions are as follows:(1)According to a large number of multi-direction and multi-stage predepositional sequences of the development in the periphery of the lake margin of the Yanchang Formation,the evolution sequence and sedimentary characteristics of the lacustrine basin after Chang7 largest lake intrusion have been defined.Research shows that the filling structure of lake basin is mainly composed of oblique,Rather than the parallel layered filling structure;The relations between zhangjiatan shale of the Chang7 and the so-called Chang 9 lijiapan shale section are inverted V-shaped skew,which are not parallel;In the period of Chang 4 + 5,there is no developed and large scale unified transgressive phenomenon,but the sedimentation response of diversion and replacement of the progradational sequence in the top of the super-surface which is near the muddy delta plain or semi-deep lake.(2)Based on the study of sedimentary microfacies of progradation sequence,the heterogeneous relation between deep-water gravity flowsand bodies and delta sand bodies in the center of lake basin are defined,where the corresponding relationship were formed in progradational sequence structure,rock stratigraphic units and sedimentary microfacies characteristics.The study shows that the upper section of the S sequence of the Yanchang Formation is mainly corresponding to the shallow lacustrine deposits and delta plain deposits of the traditional drilling stratified Chang3-Chang4+5 period,with vertical aggradation dominated;the middle section mainly corresponds to the Chang 4+5-Chang 6 sedimentary period of traditional drilling layering,which consists of delta front subfacies and front delta subfacies,with lateral progradation as the main component;the lower section mainly corresponds to the Chang 6 and Chang 7 sedimentary period of traditional drilling layering,which is mainly composed of half deep lake and deep lake facies and deep water gravity flow,with vertical aggradation mainly.(3)The seismic forward models in connection with the various thickness and structures of sandstone have been built on the basis of the structural characteristic of gravity-flow sands in lower member of forest sequence.The result demonstrates that the superimposed structures of gravity-flow sands in lower member of forest sequence including three types,individual layer,mult-layer overlaps and vertical division overlaps.Seismic forward models reveal that sandstone can be identified on seismic data when its thickness exceeds turning thickness(30m).(4)Based on the study of reservoir distribution characteristics of the progradation sequence,it is indicated that the tight reservoirs are mainly developed in the lower member of the progradation sequence,and the distribution characteristics of the tight reservoirs are affected by the bedform of Lake Basin.In general,lower steep ramp owns more sandstones than upper steep ramp,however,gentle ramp showing transitional distribution instead of obvious zoning features.Thus,sand bodies are well developed at lower ramp.In the southwest area,tight reservoirs in Chang7 member are particularly developed,and tight reservoirs in Chang72,Chang71,Chang63,Chang62 and Chang6,are more and more undeveloped,and oil-bearing is worse and worse.