Micro-pore Structure Characterization And Classification Of Xujiahe Formation Tight Sandstone Reservoirs In The Central Sichuan Basin

Previous studies on tight sandstone reservoirs of the Xu2 Member of the Xujiahe Formation in the central Sichuan Basin mostly focused on fracture development characteristics and high-quality reservoir prediction,with few studies on the characterization of its micropore structure and relatively simple means.It is urgent to deepen the quantitative characterization of pore space on a full scale to provide support for high-quality reservoir evaluation.This study takes the tight sandstone reservoir in the second member of the Xujiang Formation of Anyue Gas Field as the research object,and comprehensively utilizes technical means such as physical property testing,XRD,cast thin sections,SEM,high-pressure mercury injection,constant velocity mercury injection,and fractal analysis to carry out researches on petrological characteristics,diagenesis,and characterization of micropore structure.Based on this,an improved full-scaled pore characterization method for tight sandstone is proposed.The intelligent extraction and quantitative characterization technology of pore structure based on casting thin sections has been developed,and the classification and evaluation of reservoirs have been realized based on the characteristics of pore structure.The tight sandstone reservoir in Xu2 Member of the Xujiang Formation in the study area is mainly feldspathic lithic sandstone and lithic feldspathic sandstone,with high compositional maturity,fine to medium grain size,porous cementation,and medium sorting.It is an ultra-low porosity and ultra-low permeability reservoir,which is now in the middle diagenetic stage B.High quality reservoirs have the characteristics of a large number of rigid particles,relatively low rock debris content,strong compaction resistance,and being subjected to strong dissolution and fracturing in the later stage.There are a certain number of micro sized primary intergranular pores and dissolution pores,and microcracks are developed.The poor reservoir has a high degree of plastic mineral filling and weak compaction resistance,and is later subjected to low dissolution and weak fracturing.Under strong compaction,there are almost no primary pores,and the presence of nanoscale scattered dissolution pores is badly conducive to gas seepage inside the reservoir.Invalid fractures further reduce the reservoir’s physical properties,resulting in complex pore structures and strong heterogeneity.The characteristics of medium reservoirs lie between the two.Overall,small throats mainly control reservoir seepage capacities.The permeability prediction model established based on porosity and HPM parameter r₃₅ can be well applied to tight sandstone reservoirs in Xu2 Member,with high correlation between the predicted values and the measured values..The fractal curve obtained by the box-counting method for the Xu2 tight sandstone presents a one-step pattern with good correlation,with an average fractal dimension of 1.585.Fractal analysis of HPM and RCM shows that the pore space presents a four-segment fractal feature with good consistency.The full-scaled fractal curve established by combining the two can overcome the technical flaws of HPM and the impact of microfracture on pore space heterogeneity.Each fractal segment has more fitting points,higher fitting accuracy,and better correlation.The four fractal dimensions are all between 2 and 3,demonstrating fractal characteristics.The dissolution pores corresponding to the middle pore segments and the nano-scale small pore throats corresponding to the small pore segments are the main factors controlling reservoir physical properties.Based on RQI and FZI of macroscopic physical properties,the reservoir is classified and evaluated using high-pressure mercury injection,rate-controlled mercury penetration,and combined mercury injection fractal parameters based on microscopic pore structure characteristics.The results of different reservoir classification methods are highly consistent,but there are still differences in local details due to different principles of characterization methods.The comprehensive multi-aspect classification accuracy of tight sandstone reservoirs is higher and more comprehensive,providing a certain guiding significance for the exploration and development in the later stage of the study area.