The Cretaceous Sandstone Pore Structure Characteristic And Reservoir Evaluation Of Keshen Area In Kuqa Depression

Kuqa cretaceous reservoir is a set of typical tight sandstone.Poroperm relationship in this area is very complexed and can not be evaluated by conventional petrophysical analysis accurately.Instead,pore structure,controlling the permeability,can well reflect such tight sandstone reservoir quality.By means of polarizing microscope,confocal microscopy and scanning electron microscopy(SEM),high-pressure mercury penetration and image analysis technology,the petrology,reservoir space,diagenesis of Bashijiqike Formation in Keshen area,Kuqa Depression have been studied,on the basis of petrology,this study focus on the pore structure characteristics and its controlling factors and types,formed the thinking and technique of this kind of reservoir evaluation,achieved the following results:(1)Mainly including lithic feldspar sandstone and feldspar lithic sandstone,with a small amount of feldspar sandstone;Mainly containing carbonate cementation,with low content of matrix;Rock structure characteristics:the particles contact relationship is always point-line,the sorting is mediocre-good,the particles roundness is always subangular,angular-subangular.(2)The main body of the reservoir nowadays is in the middle diagenetic stage A,it can be divided into two diagenesis stages and four diagenesis periods.The burial diagenesis stage has experienced three periods:(1)The weak compation,weak quartz and feldspar cementation and heavy calcitic cementation.(2)The dissolution of feldspar,carbonate,analcime cements,etc.(3)The weak gypsum and stronger calcitic cementation;The structural diagenesis stage: the heavy structural pressure,the growth of fissure,the late dissolution.(3)there are seven types of reservoir space,including residual inter-granular pore,residual inter-granular dissolution pores,the late inter-granular dissolution pores,intra-granular dissolution pores,late enhanced inter-granular dissolution pores,micro pores and fissure.(4)he pore morphology is elongated,length-width ratio almost between 2 ~ 3,pore distribution and sorting,part of the early dissolution pores present directional arrangement after strong tectonic compression.Throat type is relatively single,The throat type mainly contains the laminated throat and the control shape throat,the throat radius is narrow and concentrate.(5)The larger pore throats control the permeability,The smaller pore throat offer little permeability contribution value or only reservoir function.Some pore structure characteristic parameters have good correlation with permeability,such as Expulsion pressure,maximum pore throat radius.With the better throat sorting and the more concentrated the radius size,the permeability becoming worse,it is consistent with the typical tight sandstone pore throat characteristics.(6)Pore structure mainly affected by diagenesis,the main controlling factors of the pore structure in the study area is compaction.Under the burial compaction and structural compaction,the porosity decreases,throats narrow down,particles line contact-concave convex contact,the reservoir quality sharply decrease,but the rock cracks and produce many fractures with the strong tectonic compression,reservoir property is improved.Early strong carbonate cementation block pores and throats,especially clay or some authigenic minerals,divide the pore into many small units,the throats changed into control shape throats,the reservoir permeability decrease sharply.But the pore space is preserved because of the existence of the early cements,it provides the material basis for later dissolution.Dissolution has greatly increased the reservoir connectivity and the dissolution along the rock cracks not only make the crack width wider,but also make the surrounding pore corrosion expand,reservoir quality become better,but in the place with poor corrosion,dissolution is characterized by damage to the reservoir,so in the low permeability sandstone reservoir,the dissolution is not necessarily benign transformation.(7)pore structure in study area can be divided into four types and has the characteristics of partition distribution: Keshen 6 well area mainly develop the?type(high pore-mid throat),it is the best pore structure type,Keshen 8 and 9 wekk areas mainly develop the ?type(mid pore-small throat),it is the better pore structure type,Keshen 2 and 5 well areas mainly develop the ? type(low pore-micro throat),it is the worst pore structure type and the ? type(low pore-fracture)is one special pore structure system,it develop in areas with the strong tectonic compression,due to crack as connected throat of reservoir,so it is of high permeability,belongs to the good pore structure types.