A Study On Low Permeability Reservoir Characteristics And Controling Factors Of Member Quan-4 In Gudian Oilfield,Songliao Basin

Member Quan-4 in Gudian oil-field of Southern Songliao Basin,is an ultra low permeability reservoir with the porosity of 5%-12.5%and permeability of 0.05mD-0.5mD.In the reservoir,the oil saturation is lower in overall,but it is fact that some areas and layers are developed with higher oil and gas production.It means that there exist potential zones in the formation.But the distribution of potential zones is not clear.It appears that those productive zones are less controlled by structure location.At the same time,the reservoir hydrocarbon accumulation changes rapidly.These phenomenons imply that the distribution of oil and gas is influenced by the reservoir factors to a large extent.By choosing Qian 227 area as the typical target,the dissertation focuses on the reservoir characteristics and controlling factors to study the formation mechanism and distribute law of favorable zones.By analysis the information of core,thin section,mercury-injection and so on,some new understandings about petrology,pore-throat structure,reservoir property and hydrocarbon accumulation are obtained.First,the target area can be divided into four sub-areas due to their difference on sedimentary resource,petrologic characteristics and digenesis environment.A series microcosmic study is carried out,including petrologic characteristics,diagenesis and pore-throat structure.The results show that sedimentary resources control the difference of four sub-areas in mineral composition.Then the diagenesis type and speed in the sub-areas are influenced,further leading to difference between sub-areas on pore-throat structure,reservoir property and hydrocarbon accumulation.Second,previous understanding that fracture is key favorable factor in member Quan-4 formation is challenged.Study results in the dissertation show that the dissolution pore is the main reason for reservoir heterogeneity and hydrocarbon accumulation in part of areas.But inter-granular pores are widely compressed;the long distance flowing of organic acidic fluid is limited so the dissolution can not occurre widely,leading to the rapid changes of reservoir properties and hydrocarbon accumulation between wells.Third,the study on relationship between depositional environment,diagenesis and pore-thoat structure shows that reservoir quality of member Quan-4 is controlled by sedimentation and transformed by diagenesis.Favorable micro-facies is distributary channel sandstones.These multi-branches channels are staggered and stacked,resulting in the versatile reservoir.Continuous compaction and wide cementation diminishes most of primary inter-granular pore.Dissolution to K-feldspar and volcanic debris can generate more inner-granular pore to improve permeability.Therefore,favorable conditions for higher quality reservoirs in the study area can be summarized.Rim cementation such as early quartz overgrowth and thin clay around grain is benefit to preserve part of primary pore and provide path for later acidic solvent migration.Direct touching between source rocks and favorable reservoir is key factor because organic acidic solvent just need to migrate shorter distance and can keep stronger dissolution ability.Widely Spread High-angle faults in top of member Quan-4 provide more chances for the direct touching.