Sedimentary Microfacies Research Of Chang4+5Reservoir Of Yanchang Formation In Youfangzhuang Area, Ordos Basin

Youfangzhuang area is a westward inclined monocline which located in themiddle of Shanbei Slope of Ordos Basin. It has been found that the reservoirs arelocated in Chang4+5, Chang7, Chang9of Yanchang Formation of Triassic and Yan`anFormation of Jurassic, it is necessary to study further for Chang4+5reservoir becauseof the less research. Based on the sequence stratigraphy, reservoir sedimentation,petroleum geology, with all kinds of drilling cores, logging and dynamic and staticanalytical data, the sequence stratus frames of the study area is established, and thetypes, distribution and evolution of sedimentary microfacies is researched inChang4+5reservoir. Finally, the depositional models is established for the furtherexploitation and development.Combined with the regional geological background and tectono-sedimentaryevolution, applying the principle of “sedimentary-cycle and same-thickness”, theChang4+5oil layer group is divided into two4th-order and six5th-order with attributeanalysis of maximum entropy spectrum. The high-precision isochronic sequencestratus frames of the Chang4+5oil layer group in Youfangzhuang area is establishedeventually.Through the sandstone rock types, textural maturity and compositional maturityof clasolite, and heavy mineral analysis, it shows that the sandstone rock type ofChang4+5oil layer group of Youfangzhuang area is mainly lithic sandstone with highquartz component, the sandstone with finer granularity has good sorting and texturematurity. The compositional maturity is low in the northeast of the area with aincreasing trend toward southwest, and the ZTR index is gradually increasing fromnortheast to southwest in Youfangzhuang area, these thesis determine sediments in theChang4+5oil layer group come from the northeast of the study area. Using probability cumulative curve and C-M chart, the grain-size distributionand sorting characteristics are studied as well as the palaeosedimentary environmentof the Chang4+5oil layer group. It turned out that the study area was in shallow-deeplake which was controlled by tractive current with suspend fine-grained sediments,they showed characteristics of delta-front facies.Based on the research of high-precision sequence stratus and sedimentaryenvironment, combined with drilling cores, logging and a great quantity of laboratorydata, delta depositional system, delta-front facies especially, is recognized which lackof delta-plain facies and prodelta facies. The sedimentary microfacies are mainlyunderwater distributary channel, mouth bar, overbank, distributary and sheet. Thedistribution of sandstone is controlled by delta depositional system, sand bodies ofboth underwater distributary channel and mouth bar are good reservoirs of Chang4+5oil layer group in Youfangzhuang area.