Karst Reservoir Characteristics Of Ordovician Weathered Crust In Fuxian Region,Ordos Basin

The Fuxian area is located in the southeast of Ordos basin and is a karst crust gas field which formed by denudation when the strata was uplifted during caledonian movement.The 5th member of Majiagou formation is the main production reservoir.Although some gas production were discovered,the reservoir has a strong anisotropy which lead to the differential entrapment in different area.And the reservoir formation mechanism and distribution pattern is not clear which restrict the further exploration.Thus the study of the reservoir is the highest priority.On the basis of core observation and experiment analysis and combine the drill,well logging and logging data,the palaeogeomorphology and reservoir characteristics of the karst reservoir in Fuxian area,Ordos basin was studied in this article.The residual thickness of the weathered crust is study through the stratigraphic correlation.The palaeogeomorphology was analysis by the impression method and residual thickness method.The karst facies were divided by the different characteristic.The karst discipline was clear.Based on the above analysis the reservoir physical property,reservoir space type,digenesis,and the reservoir controlling factors is investigated.The thesis mainly have the following research results:(1)Through the paleogeomorphologic restoration in the study area,has been clear about the paleogeomorphologic that it includes karst highland and karst slope,Paleogeomorphologic has the characteristics that west high and east low.The most important Geomorphic unit is karst slope,it has the characteristics that zonal distribution form north to south.(2)There are five karst lithofacies in study area,different karst lithofacies have different electrical.Crackle breccias lithofacie and mosaic breccisa lithofacie have the characteristics that low gamma,low neutron value,medium-high resistivity.Clast-supportde disorders breccia lithofcie,matrix-supported disorders breccia lithofcie and cave-sediment filling facies have characteristics that medium-high gamma and medium-low resistivith.(3)Karst reservoir has a strong heterogeneity,it suffered from strong ancient karstification,forming the reservoir space of dissolution pores and cracks.After a deep burial stage,the pores destructed and reduced after compaction,filling function and cementation in the later.Reservoir has the characteristics that low porosity and permeability(4)The main controlling factors of study area are paleogeomorphologic and karst lithofacies.Crackle breccias lithofacie and mosaic breccisa lithofacie which are located in the karst slopes are advantage area.This study from the large amounts datas about core,log and based on the research methods on above,investigating the karst reservoir characteristics and controlling factors of Majiagou formation.It will be significance of reference to the further natural gas exploration.