Microscopic Characteristics And Densification Of Tight Reservoirs

The Permian Lucaogou Formation in Jimusaer sag is a set of mixed fine-grained sediments composed of organic rich shale,siltstone and carbonate under the sedimentary background of saline lake basin.Under the microscope,it can be observed that the argillaceous,silty fine sand and carbonate have the characteristics of massive or laminar enrichment,and its reservoir microscopic characteristics and densification process need to be further revealed.Based on a series of test and analysis work,such as field core observation,rock fluorescence thin section,cast thin section identification,scanning electron microscope analysis and whole rock diffraction analysis,the petrological characteristics,physical properties,diagenesis and densification process of Lucaogou Formation tight reservoir were systematically studied and analyzed.The main achievements are as follows:(1)The mineral composition of rock in Lucaogou Formation in is complex,with quartz,feldspar,(iron)dolomite and clay minerals as its main mineral components.According to the content of mineral components,the rock can be divided into four types:terrigenous clastic rock,pyroclastic rock,carbonate rock and mixed rock.According to the test results and the previous division scheme,these four types of lithology can be further divided into 16 types,such as carbonate rock and tuffaceous carbonate rock.(2)The results of physical properties test show that the reservoir of Lucaogou Formation is characterized by low porosity and permeability.And the dissolution pore is a relatively developed pore type in the reservoir space.The primary pores include a small amount of primary intergranular pores which are not filled by interstitial materials,residual intergranular pores formed by compaction of buried depth and intergranular pores of dolomite / calcite minerals.(3)The distribution of sedimentary microfacies determines the type and distribution of reservoir rocks,while diagenesis affects the development characteristics of reservoir rocks.Both compaction and cementation affect the reservoir during the process of diagenesis,destroy,plug and fill the pores,reduce the porosity and permeability of the reservoir.Dissolution can improve the porosity and permeability of the reservoir to a certain extent through a series of dissolution reactions.The cementation types of Lucaogou tight reservoir include(iron bearing)carbonate cementation,clay mineral cementation such as illite,mixed-layer illite-smectite,and siliceous cementation in the form of quartz secondary enlargement.Microscopic observation and analysis indicate that carbonate cementation plays the most important role in the diagenesis of Lucaogou tight reservoir,while clay mineral cementation and siliceous cementation are relatively less developed.(4)It is recognized that the Lucaogou Formation tight reservoir in Jimusaer sag has experienced three densification stages.The early compaction makes the sediment compacted rapidly.Continuous compaction,carbonate cementation and siliceous cementation play a leading role in the middle densification stage,and the late compaction continues.At the same time,carbonate cementation and clay mineral cementation work together on the reservoir,which makes the reservoir more compact.In the whole process of reservoir densification,compaction and cementation significantly reduce the porosity and permeability of rock,which is the fundamental reason for the reservoir densification of Lucaogou Formation in Jimusaer sag.