Abstract:

Hangjinqi area has great potential for tight gas exploration in Ordos Basin and the Lower Shihezi Formation is the major gas pay zone in the area.The reservoir rocks are mainly composed of lithic sandstones and lithic quartz sandstones.Based on the observation of cast thin section and the analyses of the SEM images,high pressure mercury injection,X-ray diffraction of clay minerals and cathodoluminescence,and combining with the relevant porosity and permeability data,we studied the petrological characteristics and physical properties of reservoirs in the Lower Shihezi Formation in Hangjinqi area of northern Ordos Basin and identified the developmental features of detrital grains and their influence on reservoir physical properties.The following results were obtained.The Lower Shihezi Formation reservoir is mainly composed of medium-coarse sandstones characterized by coarse grain size,with an average content of detritus as high as 40%.The reservoir of the study area is deeply buried,and experienced strong diagenesis and compaction later,leading to the filling of most primary pores and the secondary pores acting as the main reservoir space.The reservoir detritus is diversified in types.According to the influence of detritus on reservoir physical properties,the detritus as a whole can be divided into three types,namelypore reducing,pore increasing and pore retaining.The growth of pore reducing detritus leads to massive reduction of intergranular secondary solution pores and it is uneasy for these detritus to dissolve,resulting in the destruction of reservoir physical properties.Among other,a high content of phyllite detritus plays the most significant role in destroying the reservoir quality.While the growth of pore increasing detritus can function to reserve part of the intergranular pores and it is easy for the detritus to dissolve inside,so being of great help for improving the reservoir physical properties.A high content of metaquartzites is the most important factor for this improvement.Besides,the pore retaining detritus usually develops synchronously with the pore increasing one;it's strong in nature in resisting compaction and most of them is difficult to dissolve.Therefore,this detritus can protect the original pores,but could not improve the reservoir physical properties by self-dissolution,and is in indirect positive correlation with physical properties.

Key words: category of detritus, pore-forming mechanism, physical property, tight sandstone, Lower Shihezi Formation, Hangjinqi area, Ordos Basin