Research On Distribution Law For Changxing Formation Reef Facies Reservoirs In Yuanba

Bioherm reservoirs are excellent carbonate reservoirs.In recent years,since the rapid development of the geophysical exploration technology,the technology of reef identification has become more and more mature.There are breakthroughs in exploration and development for reef reservoirs both at home and aboard.And a large number of bioherm oil and gas fields with high-yield production have been found.The well YB1C1 produced a high-yield industrial gas in 2007,representing the discovery of the high-H₂S gas field that is bioherm reservoir of Changxing Formation in the Yuanba.Furthermore,it reveals the great potential for exploration&development and development prospects.However,this gas reservoir is characterized by deep buried,complex distribution,poor reservoir physical property and strong heterogeneity,low well-controlled reserves,extremely complicated gas-water relationship,high H₂S,high cost,large investment,etc.,which brought difficulty and investment risk to exploit.Therefore,based on geological genesis,this paper has further researched the reef facies reservoirs of the Changxing Formation and cleared the controlling factors and distribution of the reservoir development which have significance in theory and practice.Based on the previous research results,this paper have carried out a series of studies,such as sequence stratigraphic classification and correlation,sedimentary facies features,reservoir characteristics,comprehensive logging evaluation,controlling factors and distribution of the reservoir,etc.The results have showed that this area has a weak tectonic deformation and undeveloped fault.Formation thickness of the Changxing Formation platform-margin is not stable?general:200³50m,average:260m?,totally it is thinner in south?shoal?and thicker in north?reefs?.And it has the features that formation thickness is thinner in west and thicker in east of northern reefs,however,thickness is steady in southern bioclastic beach with EW trending.In a word,formation thickness was controlled by sedimentary facies belt and the Changxing Formation reservoir is reef and beach facies deposits.In the longitudinal direction,multi-stage reef-shoal superimposition is characterized by“shoal in early stage and reefs in late stage”.In a plane,reef is developed in the north and shoal in the south.The late reef is distributed in the margin of platform and has formed 4 reef belts.There are locally developed point bars in platform and patch reefs in back reef.The important rock types of the high-quality reservoirs are soluble crystalline dolostone,residual bioclastic dolomite,bioherm dolomite.Main type of the reservoir is pore,partly is fracture-pore type.Furthermore,this reservoir belongs to low porosity and medium-low permeability reservoir and type?and type?high-quality reservoirs are developed.The reservoir space is dominated by intercrystalline pores and intercrystalline dissolved pores,the next are intergranular dissolved pores and intragranular dissolved pores,moldic pores,karst cave,and developed a little micro fracture.High-quality reservoirs are mainly developed vertically in reef cap and horizontally in reef crest followed by back reef.Fore reef was seldom developed.During the early stage,the favorable sedimentary microfacies were the bases of the high quality reef reservoirs development.High-frequency cycle controlled the development position during the depositional period.Dolomitization is the material source of the high-quality reservoirs,and the key factor of its formation and improvement is dissolution.The tectonic activities formed multiphase fractures,which can improve the flowing performance of the reservoirs.Under many synthetical factors and superposition of evolution,high-quality reef dolomite reservoirs of Changxing Formation have been formed.The paper has made the essential factor clear in reef reservoirs development of Changxing Formation,and has established the formation-evolution model of reef reservoirs.