Development Characteristics And Influence On Reservoirs Of Fractures In Third Member Of Shahejie Formation In North Area Of Dongpu Depression

Fractures developed commonly in the Es3 sandstone reservoirs of Dongpu Depression.Various techniques such as core overview,thin sections analysis,scanning electron microscopy and physical property analysis were applied to study the fractures of Es3 sandstone reservoirs,and statistical data of parameters of the various fracture types in the study area are collected.On the basis of fundamental researches,further insight into the fracture characteristics,the origin mechanism of fractures and its influence on the reservoirs are proposed.The average porosity of Es3 Sandstone Reservoirs is 12.6%,and average permeability is 38.2 ×10-3?m2.The Reservoir belongs to low porosity and especially low permeability kind,whose diagenetic environment is complicated,namely acid-base alternating environment.Reservoir properties on the longitudinal can be divided into three anomalous high porosity zones.Distribution of reservoir properties on the plane also has partitions,and high porosity regions are mainly in Huzhuangji,Xulou,Pucheng.Fractures can be divided in two categories according to their size,which are the macro-fractures and micro-fractures.They also can be divided in three types,tectonic fractures,fractures related to overpressure and diagenetic fractures.Meanwhile the micro-fractures could be classified into transgranular micro-fractures,intragranular micro-fractures and grain-edge micro-fractures.Fractures are formed mostly towards NNE and NE.There is a certain correlation between opening,density,length of the fracture's parameters.Most macro-fractures are half filled by gypsum and carbonaceous.The micro-fractures are of comparatively good effectiveness.The extent of the fractures is mainly controlled by sedimentation,tectonics,overpressure and diagenesis.The higher brittle particle content,more coarse granularity and thinner thickness,the more fractures developed;The development of tectonic fractures are controlled by faults,the closer the distance to fault,the more fractures developed,and the broken ratio of 10 is the lower limit of tectonic fractures.The overpressure often formed between salt or under salt,and the pressure coefficient of 1.2 is the lower limit of overpressure fractures.Constructed by two tectonic movements as NW-SE tensile stress and ES compressive stress in the Himalayan,the tectonic fractures formed in two stages.The first stage was formed in late of Es3,the second stage was formed in the late of Ed.The formation of Overpressure fractures is related to hydrocarbon filling and dehydration of salt and gypsum and its formation period is the late of Es2,which is consistent with hydrocarbon filling period.The formation of diagenetic fractures is related to compaction,dissolution and so on.The formation sequence of fractures is: early tectonic fractures-pressure fractures-dissolving fractures-overpressure fractures-contraction fractures-late tectonic fractures-dissolving fractures-pressure fractures.Fractures are important reservoir types of this area.The layer that contain fractures is generally oily,and the production capacity of single well increases with an increase of fracture in the formation.The macro-fractures mainly act as flow paths,which can greatly improve the reservoir permeability.The micro-fractures mainly act as reservoir space,which not only increase the reservoir porosity,but also increase the pore connectivity.Flowing fluid in the fracture also can form dissolution pores,improving the reservoir property.