The Study On The Sedimentary Characteristics And Favorable Sandbody Prediction Of The Lower Youshashan Formation In Wunan Oilfield, Qaidam Basin

In order to expand the exploration in the southwest area of Qaidam Basin further, theauthor has chosen Wunan oilfield in the southwest area of Qaidam Basin as research areaand studied sedimentary facies and reservoir characteristics of the five oil groups of thelower Youshashan formation of Neogene period.By means of core data, grainsize analysis, thin section identification and analysis ofseismicfacies and electrofacies, combining with the correlations of single well facies andprofile facies, the systemic studies of sedimentary characterization, spacial spreads of sandbodies and controlling factors of the lower Youshashan formation of Wunan Oilfield aredone. The result indicates that delta front develops during the period of depositon of thefourth and fifth oil formation; delta front subfacies and delta plain subfacies are alternateddeposits during the period of depositon of the third oil formation. During the period of thefirst and second oil formation, flood overlake develops.On the other hand, the author finds a new reservoir sand body that is subaqueouscrevasse splay. In the deposit period of fourth and fifth oil formation, region of interest is ablanket shallow environment, geomorphic slope is flatter.Because of the affection of flood,floss water bodies energy changes, subaqueous distributary channel burst easily anddevelop subaqueous crevasse splay. Fan deposit area is small and imbedded in grayargillaceous sediment of subaqueous distributary interchannel. The discovery of foundsubaqueous crevasse splay is supplement of reservoir sand body genetic type.Based on studies of regional tectonic setting, thin section identification andobservation of drilling cores, neogene soft sediment deformation structures related toearthquake are recognized in Wunan oilfield during the lower Youshashan formation andresearch their contributing factor. The soft sediment deformation structures of Neocene inthe Wunan oilfield consist of liquefied mudstone vein,water escape structures, load casts,flame structures, Seismites can be subdivided into many types, such as seismic corrugatedrocks, shattered rocks, seismic collapse rocks. The discovery demonstrates that themovement of earthquake is violent on the time of Neogene in the north of Kunlun Orogenic Belt from sedimentology, and it provides a theoretical foundation for the tectonicevolution studies of the Qaidam Basin.When earthquake was active,a great amount ofmicro fractures formed and connected with original pore. the permeability of reservoirs canbe improved obviously.Seismites are potential reservoirs and will become new explorationareas in the near future.The author thinks that typical alkaline diagenesis process undergoes in Wunan oilfieldduring the lower Youshashan formation. The secondary emposieu is the most importantreservoir space. The author thinks the third oil formation is the best reservoir.In general, during the period of depositon of the lower Youshashan formation inWunan oilfield, aquatic distributary channel of the third, fourth and fifth oil formation aswell as subaqueous distributary channel, channel mouth bar and flood channel of the firstand second oil formation as well as sand flat sedimentary physical properties develop well,and favor oil gas accumulation, they are the next focal point of exploration.