The Research On Reservoir Characteristics Of Yan'an Formation In Mahuangshan Area Of Ordos Basin

Ordos basin is the second largest basin in China,and the largest basin containing three energy sources of Petroleum,gas and coal,and one of the backbone resource bases for "West to East Gas Pipeline Project".The Yan' an formation of Middle Jurassic is the most important Oil&Gas-bearing series in Mahuangshan area of Ordos basin.Which was formed after the steady sedimentation period of late Triassic indo-China movement,the fluvial-delta sedimentary system with steady thickness and distinct characteristics was formed.The aim of this thesis is to research on reservoir characteristics of Yan' an formation in Mahuangshan area,and to forecast where favorable reservoir zones developed.By applying of basic principles of sedimentary petrology and petroleum geology,based on well drilling,well logging data,combined with the ways of the rock types,sedimentary cycle,depositional texture,etc.The Yan' an formation strata was divived and correlated.Based on the subtly stratigraphic correlation,by ways of combination of litho-facies and logging facies,the sedimentary systems of Yan' an formation had two types:the fluvial system and the delta system.The Yan 10^thwas typical braided river channels-filling type deposition,before the paleo-landform was deplanated and evolved into paludification plain environment in Yan 9^th.Yan 9^th～Yan 6^thwere delta plain facies,till the end of Yan 6^th,meandering river deposition began developing.From paleo-structure pattern,depositional structure and texture,composition characteristics of debris etc.Sediment source of the study area was discussed,which indicated that there were three sediment sources,and the debris material of Yan' an formation in Mahuangshan area of Ordos basin came from the north,the west and the southwest margin uplift area,but the sediment mainly came from the north and the west.The mainstream direction of paleo-river channel in pre-Jurassic was from northwest to southeast.By analyzing a large quantity of rock cores and observing a large mount of rock thin sections,we found the lithology of Yan^10-9is feldspathic lithic sandstone,debris feldspar sandstone with particle size mainly coarse to medium,and the lithology of Yan⁸,yan⁶,Yan⁵ is mainly arkose with particle size mainly fine,medium particle being in the second place.From lower to upper,The particle size gets finer.The compositional maturity of sandstone is mainly low while the textural maturity is mainly high.The primary diageneses of the research area were compaction,pressure solution,cementation,dissolution and secondary enlargement.And the dissolution had made constructive impact on the physical properties of the reservoir.There were many kinds of pore types in the research area,such as primary inter-granular pore,intra-granular dissolved pore,mould pore and dissolution fracture etc.The physical properties of Yan' an formation were poor,the porosity was mainly 10%～15%while the permeability of most reservoirs were below 10×10^-3μm².The average value of porosity in Yan⁸ was 14.12%while the average value of permeability was 80×10^-3μm²,and the proportion of samples with permeability more than 10×10^-3μm² exceeded 60%.The average values of porosity in Yan⁹ and Yan¹⁰both were about 12%,the average value of permeability of Yan⁹ was 20.94×10^-3μm² while in Yan¹⁰it was just 1.7×10^-3μm²,poorest among Yan⁸～Yan¹⁰.So the physical properties of Yan⁸ were best among Yan⁸～Yan¹⁰,and there were apparent linear relationships between porosity,permeability and pore structure parameters,it should be a layer of porous reservoir in which inter-granular pores developed.By analyzing the sedimentary setting' control over reservoir,it indicated that the most favorable reservoir of Mahuangshan area was medium sandstone,or say parallel bedding medium sandstone facies.Gray-white parallel bedding coarse sandstone facies was in the second place while parallel bedding fine sandstone facies being poorest.According to physical properties,mercury injection data etc,the reservoir of the study area could be classified into four types as follow:â… :Good quality reservoir;â…¡:Medium quality reservoir;â…¢:Poor quality reservoir;andâ…£:Non-reservoir. According to this classification standard,the Yan⁸ was considered asâ… :Good quality reservoir while Yan⁹ and Yan¹⁰both were considered to beâ…¡:Medium quality reservoir.By comprehensive analyses of multi-factors like sedimentary facies zone and sandstone body distribution and reservoir properties etc.Reservoir was evaluated. Finally,the forecast of the favorable reservoir prospecting zones of Yan' an formation was made.