Genesis Of Mixed Sedimentation And Densification Mechanism Of Tight Reservoir In Terrestrial Lacustrine Basin

The tight oil in Permian Lucaogou Formation,Jimsar sag is the important development target of tight oil in China.The extremely complex lithology in Lucaogou Formation created many contradictions in the development of tight oil,in which the sedimentary characteristics and pore-throat structure of complex lithologies,densification mechanism are the key problems to be solved urgently.Guided by the sedimentary theory of modern saline lake,based on the certain data as cores,drilling data,logging data,high precision 3D seismic data and laboratory analysis test data,we subdivided the lithofacies types of Lucaogou Formation,and the mixed sedimentary characteristics,microscopic pore-throat characteristics and diagenesis are revealed.The mixed sedimentary origin and the reservoir densification mechanism of Lucaogou tight reservoirs are clarified.This study can not only provide important geological basis in the development of the oilfield,but also enrich the theoretical knowledge of tight oil reservoirs.Based on core observation as well as thin section,SEM(scanning electron microscopy)and X-ray diffraction(XRD)analyses,we found that the saline lacustrine mixed sedimentation were extensively developed in the Permian Lucaogou Formation,Jimsar sag,which contains mixed sedimentary rocks and mixed sedimentary sequence.On the macro-scale,the mixed sedimentary sequence are characterized by interactive depositition of terrigenous clastic rocks,carbonate rocks and volcaniclastic rock,while the mixed sedimentary rocks are consist of terrigenous detrital component,carbonate component and pyroclastic component in the micro.Based on the analysis of the source of mineral components in different lithologies,more than 50 kinds of lithologies in Lucaogou Formation is classified into 8 lithologies.There are three genetic types of mixed sedimentation in this area include source mixing,edge facies mixing and mutational in-situ mixing.Then the mechanism of “multi-sources in corresponding period” mixed sedimentary for the saline lacustrine sedimentary environment was clarified.The results of scanning electron microscopy(SEM),three-dimensional CT imaging,pressure-controlled mercury injection(PMI)and rate-controlled mercury injection(RMI)revealed the pore-throat characteristics of 8 lithologies and their contribution on physical property of tight reservoirs.The results showed that the reservoir space of Lucaogou Formation is mainly consisted of many kinds of dissolution porosity and a few micro-fractures.Micro/nano pore take the dominant position,which radius range from hundreds of micrometer to tens of nanometer.The tuffaceous siltstone,dolomitic siltstone and silty dolomite are the dominant lithology in the sweet spot,which has the best physical property.The permeability and oiliness are controlled by the pore-throat structure,the dominant lithology have a bigger pore throat and wider distribution.Throat structure,mainstream throat radius and volume of effective throat are the key control factors to permeability.Through the stable carbon and oxygen isotopes analysis and the fluid inclusions homogenization temperature,the Lucaogou tight reservoir underwent various diagenetic alterations including mechanical compaction,quartz cementation,carbonate cementation(mainly calcite,dolomite,and ankerite),feldspar and tuffaceous component dissolution,and clay mineral alteration.The major authigenic mineral cements derived from the original formation water,clay mineral alteration,dissolution of acid-soluble components,and the compaction and drainage of the interbedded mudstone is also believed to be another important source of ions.Two stages of carbonate cementation,feldspar dissolution and clay mineral alternation occurred in the Lucaogou Formation.In addition,one quartz cementation stage occurred.It is likely that oil emplacement occurred after the quartz cementation,the first stage of carbonate cementation,and feldspar dissolution,but before the second stage of calcite cementation and feldspar dissolution.The organic acid from hydrocarbon generation was involved in the dissolution of the feldspar and tuffaceous components,locally preventing the porosity from decreasing due to the cementation and compaction that occurred at this time.Carbonate cementation reduced the porosity and permeability significantly,whereas the cementation of clay minerals does not consistently correlate with the physical properties.Lithologic differences,compaction,and cementation were earlier and stronger than that of dissolution,which was the primary reason the formation of tight reservoirs in Lucaogou Formation.Four densification types of tight reservoir in Lucaogou Formation are summarized,including the weak dissolution of carbonate rocks,the strong compaction of siltstone rich in debris,the normal compaction and calcite strong cementation,and the carbonate cementation overlaied by clay minerals alternation.The three genetic types of physical sweet spots were also summarized,including local overpressure and early chlorite protection,strong dissolution in phase I and dissolution superposition in two phases.