Characteristics And Origin Mechanism Of The Fifth Member Of Upper Triassic Xujiahe Formation Tight Gas Reservoir In West Sichuan Depression

According to cores and outcrops observation,logs and seimic data interpretation,sequence framework of the fifth member of upper Triasic Xujiahe Formation(T3x5)was established and its sedimentary facies distribution within the freamwork was analysed.By means of experiments such as thin section,X-ray diffraction analysis,FE-SEM,high presure mecury injection,nitrogen adsorbtion,NMR and mesurements of prostity and permeability,characteristics of tight gas reservoir of T3x5 were studied and its origin mechanism was generated.The following conclusions could be drawn from the study aboved:According to onlap and truncated reflector of Xujiahe Formation in regional seismic section,it was defined as a megacycle.According to unconformities formed by marine regression after T2 x and Anxian movement after T3x3,Xujiahe Formation was divided into three super long term cycles,and T3x4 to T3x5 was one of them.According to lithological change surface,T3x5 was defined as a long term cycle.According to lithological associations and maximum flood surfaces,T3x5 was divided into three middle term cycles.Research area was considered as the distant end of delta front and shallow lake environments,with weak stream energy and inadequate sand provision.It was dominated by shales and silty mudstones,including fine sandstones and siltstones with high matrix content.Through thin-section ovbservation,almost no pores could be recognized.Through FE-SEM observation,clay intercrystalline pore,organic pore,intraparticle dissolved pore,residual interparticle pore could be identified.There were more but smaller pores in shales,yet less but larger pores in sandstones.According to high pressure mercury injection and nitrogen adsorption experiments,pore-throat diameters distribution was related to lithology: 5~30nm in shales,10~50nm in siltstones,30~100nm in fine sandstones.Be aimed at T3x5 tight reservoir with nanoscale pore-throats,helium gas porosity is more accurate: 3.89% in shales,3.13% in siltstones,2.44% in fine sandstones,and pulse-decay permeability is more accurate: 0.0651 md in shales,0.0021 md in siltstones,0.0082 md in fine sandstones.Shales had higher porosity was because there were more clay intercrystalline pores and organic pores,which were much less in siltstones and sandstones that were also lack of intraparticle dissolved pores and residual interparticle pores.Shales had higher permeability was because there were widespread laminar fractures in shales.Low energy coal-forming envoirments was considered as the primary controlling factor of genetic mechnisim of tight gas reservoir of T3x5.Low energy envoirments formed sediments with fine grain size and high matrix content,and coal-forming envoirments harmed early carbonate cementation.Thus,original porosity decreased rapidly and throats reduced sharply because of intense compaction,with only clay intercrystalline pores preserved.Due to lack of migration pathway,acid liquid could not enter reservoir and form dissolved pores.Therefore,reservoir quality was much worse in siltstones and sandstones than in shales.