Characteristics Of Tight Glutenite Reservoirs And Controlling Factors Of Quality Reservoirs In The Southern Trough Slope Of Bayindulan Sag

The Member 4 of the Arshan Formation（K₁ba₄）in Bayindulan Sag,Erlian Basin,known to be characterized by tight glutenite reservoirs with complex lithology and different particle sizes,indicating good hydrocarbon potentials in other sags there,has,so far,witnessed no great achievements.The current work is thus fixed on such tight glutenite reservoirs in Member 4 of the Lower Cretaceous Arshan Formation in the Bayindulan Sag,to investigate their reservoir characteristics and controlling factors of quality reservoirs.Together with previous relevant findings,coring,and other geological data,this study conducted fine core description,conventional-unconventional reservoir quantitative characterization experiment,and comprehensive comparison and theoretical analysis of logging-seismic data,completing all-round qualitative-quantitative analysis of the distinctions in reservoir formation of different lithofacies,caused by differed fluid hydrodynamic lithofacies,sedimentary facies,and diagenesis features of tight glutenite reservoirs.The current work has thrown light on the classification and recognition mode for different types of lithofacies.This mode,an attempt to rationalize the complicated lithofacies genesis,facilitates the multi-dimensional description and precise quantitative characterization of the pore-throat structure of different lithofacies reservoirs,indicating qualitative features of differed pore space and storage that serves as a summary of the sedimentary characteristics and diagenesis types of dominant reservoirs.Additionally,this study has established a reasonable evaluation scheme for quality tight glutenite reservoirs,taking the sedimentation,diagenesis,pore-throat characteristics,connectivity,and reservoir properties into consideration.By comparing samples from different lithofacies and wells,the type and scale of pore-throat space in quality reservoirs can be recognized which expounds the influence of“sedimentation and diagenesis”on the development of tight glutenite reservoir space,revealing the differences in the origin of pore space developed by different lithofacies.The research results indicate:Member 4 in the target area enjoys multiple provenances and short-term rapid deposition,leading to the development of a fan delta tight glutenite sedimentary system,characterized by fast facies changes,strong heterogeneity,and uneven hydrocarbon distribution.By analyzing different fluid hydrodynamic features of sedimentary microfacies,three major types of hydrodynamic lithofacies,including debris flow deposition,traction current deposition,and turbidity current deposition,were identified,which specifically can be subdivided into the gravel debris flow,sandy debris flow,muddy debris flow,gravel traction flow,sandy traction flow,high-density turbidity flow,low-density turbidity flow,and plume,totaling 8 types.Besides,22 lithofacies（conglomerate facies B1-B8,sandstone facies S1-11,mudstone facies L1-L3）and their further 19 combinations（conglomerate facies A-C,conglomerate-sandstone facies D-G,sandstone facies H-L,sandstone-mudstone facies M-N and mudstone facies P-S）were identified as well,based on different hydrodynamic diagenesis.The bed of interest in the target area has experienced multiple stages of compaction,the water environment changing from alkaline to acidic,and carbonate minerals dissolution after extensive cementation.Consequently,the diagenetic sequence of Member 4 of the Arshan Formation is expected to be restored as follows:mild compaction→dissolution of quartz in an alkaline environment and cementation of carbonate minerals→strong compaction→discharge of organic acids from hydrocarbon generation→acidification of water and recrystallization of quartz→calcite cementation and feldspar corrosion.With the application of coring,core thin section casting,high-pressure mercury intrusion,nano-CT,and scanning electron microscope,the observation of pore space at the millimeter-micron-nano scale occurs.Hence given the scale,spatial distribution,and morphology of pore space,as well as the impact of sedimentation-lithofacies development-diagenesis,the identification mode of different lithofacies pore space is hoped to be established.Moreover,cross observations and tests help understand the development of pore-throat structure and their two-dimensional distribution and three-dimensional spatial connectivity,and 8 types of pore space,including intergranular dissolved pores,intragranular dissolved pores,grain boundary micro-fractures,and structural micro-fractures.At the same time,a classification standard for quality tight glutenite reservoirs in the target area was set up,and lithofacies were divided into three types:Type Ⅰ,Type Ⅱ,and Type Ⅲ,with Type Ⅰones being quality reservoirs characterized by super large dissolved pores,perfect spatial connectivity,and gravel traction current in larger throats.By comparing the sedimentary environment,lithofacies type,diagenesis,pore-throat type,and pore-throat scale,the controlling factors and characteristics of Type I reservoir pore formation were found,revealing the sedimentary features of quality reservoirs,the impact of diagenesis,the lower limit of thickness and physical properties of quality reservoirs.The research results show that quality reservoirs are supposed to assess by imbricate gravel traction current lithofacies developed in high calcium-subaqueous deposition-sand bodies with a thickness greater than 7m,a porosity greater than 8.2%,and a permeability greater than 1.06m D.The quality reservoirs in Member 4 of the Arshan Formation are affected by sedimentation and complicated diagenesis.In other words,differences in the rock structure,material composition,particle sorting,and pore structure lie in fluid hydrodynamic depositions–appropriate fluid hydrodynamic forces act as good supports for microconglomerate sedimentation where there is more primary porosity in good condition because the grain supporting has stronger compaction resistance.On the other hand,different depositional environments dominate the post-reformation of reservoir pore space.The gravel debris flow（B1-B2）developed in the overwater environment is mostly filled by matrix-mud with more argillaceous cements,while in the underwater environment with more calcareous cements.In addition,different lithofacies are controlled by factors such as sediment particle size and sorting properties,resulting in different impacts of formation compaction on it.This study scrutinizes the characteristics of tight glutenite reservoirs developed in Member 4 of Bayindulan Sag and the controlling factors for the quality reservoirs formation,completing the fine quantitative characterization of the pores of tight glutenite reservoirs on multiple scales,the identification mode of complex tight glutenite facies,the multi-method cross-comparison and quantification of pores developed in different lithofacies,and the determination of controlling factors and evaluation of quality reservoir formation,paving the way for better understanding of tight glutenite reservoirs in Erlian Basin.