Sedimentary Facies Research Of Chang3 Oil Formation In Zhenbei And Yanwu Area,Erdos Basin

Yanchang Formation in Zhenbei-Yanwu area is one of the most important production layer of low permeability sandstone reservoir in the OrdosBasin. Oil and gas resouces are abundant in Chang3 oil formation. However, the single well productivity is poor and the reserve recovery is low. In particular, a series of problems arouse in later development and production period, suchasthe great variation of sandbodies and the contradiction of theinteriordisconnectivity.The multiple supply sources are confused and the genetic types and distribution features of the sandstone reservoir are not clear.The strong heterogeneity of low permeability sandstonereservoirleads to many contradictions about rapid production decline, high water content and stable production stage shorten.Therefore, the refinedsedimentary geology study of the reservoirs in Chang3 oil formation in Zhenbei-Yanwu study area is carried out. Through the following research, like analyzing the genetic types and distribution features of the sandstone reservoir under the high resolutionsequenceframework, revealing the relationship between theverticaldistributionof sandstone and the base levelcycle, understanding main controlling factors of sandbody distribution, the premiumreservoir is expected to be found.The paper is based on plentiful information of drilling, core, logging data and analysis of laboratory. Based on the theory of high resolution sequence stratigraphy,through identifying the unconformity, rejuvenation surface, lithology or lithofacies transition surface, flooding surface and local marker-bed system,the relationship between the metaphase, short-term and super-short-term base level cycle and stratigraphic unit is found. It is considered that Chang3 oil formation is a middle-term regressive cycle, and it is further divided into three short-term cycles and six super-short term cycles. Among it, SC1,SC2,SC3 are corresponding to three sand groups of Chang33, Chang32, Chang31. The stratum of Chang3 formation is stable,strata lack only occure in local area where suffer from uplift erosion and river erosion. Based on the framework of high resolution sequence stratigraphy, by analyzing regional sedimentary background, usingthe facies indicators of lithology,sedimentary structures, paleontology and log facies, it is considered to be a steep slope braided river delta which has the sedimentary characteristics of near provenance,short fluid distance and large bed gradient in the western research area. Flat slope braided river delta is developed in the southern area in the same time. The sedimentary model of brachy prograding steep slope-pattern braided river delta and macro prograding gentle slope-pattern delta are built. Delta plain, delta front,shallowlake and semi-deep lake faciesare developed indepositedareafrom southwest to northeast. The delta sandbody from the west and south converge along the belt ofWuchengzi and Taibailiang. The distributary channels of delta front are interwoven to a net and interdistributary bay become blocky among the channels. Finally, the main controlling factors of thedevelopmentanddistributionof sandbody are deeply explored through the following aspects, such assedimentaryfacies,ancientlandform,tinystructure and sequence cycle. It is considered that the braided channel with interdistributary trough and underwaterdistributarychannel with interdistributary bay are the best sandbody combinations. The river vally and slope are good places for sandbody accumulation. The matching of low amplitude structureand channel sand body are easy to form traps of structureandlithology type. Favorable sandbody is developed in the middle lower and top part of the sequence cycle,especially the sandbody disposited on sequence transformation surface, which is most suitable forhydrocarbonmigrationandaccumulation. By the above study, the premiumreservoir is to be found.