| Shilijiahan Area is located in north of Ordos Basin,and south of Porjianghazi fault.The study area belongs to Yishan Slope which is the secondary tectonic units.The study area is rich of natural gas which is mainly storied in He1 sublayer.In recent years,some breakthroughs have been made in exploration and development.However,there are still many problems which restricting further exploration and development.The drilling result shows that every well produces waters,and the Gas saturation is generally low relative to other areas of the basin.The relationship between gas and water is complex.And the gas reservoir is characterized by low porocity,low permeability and low pressure.And the reservoir has strong heterogeneity and poor continuity.Therefore,the main controlling factors of gas and water distribution and the natural gas accumulation are still insufficient.This thesis is based on the previous research and using sedimentary theory,petroleum geology,accumulation dynamics and other related theoretical knowledge,to study the characteristics of reservoir,the types and distribution of tight sandstone gas reservoir of He1 sublayer,formation pressure characteristics and preservation conditions.Based on these investigations,the main controlling factors of differential enrichment in tight sandstone gas reservoirs are analyzed.And,the geological model of sweet zone is also established at the end.The results and achievemetns are orgnised as follow:?1?In the He1 sublayer,there are glutenite sandstone,pebbly sandstone,coarse sandstone,medium sandstone and fine sandstone.Based on the analysis of the thic section,the detrital fraction of sandstone is dominated by quartz,followed by lithic and less feldspar which are mostly eroded.Therefore,the sandstone is mainly lithic sandstone,less feldspar,lithic sandstone and lithic quartz sandstone.In the study area,the amount of debris increased and the content of quartz decreased from west to East.There are mud fillings,siliceous and calcareous cement,mainly illite,chlorite,kaolinite,calcite,authigenic clay film,dolomite,quartz secondary enlargement,and authigenic quartz and pyrite cementation,which mainly argillaceous shale cementation cementation.Because of the high content of debris,early rapid deposition results in strong compaction and a large reduction of primary pores.With cementation,the sandstone particles are mostly filled with mud and the primary pores are basically lost.Through the analysis of cast thin sections and scanning electron microscopy,the present sandstone pores are mainly intergranular dissolution pores,intergranular micropores?mainly kaolinite micropores?,and partly developed intragranular dissolved pores and microcracks.The physical properties of sandstone are obviously affected by grain size,interstitial material,especially clay mineral content.The coarser the particle size is,the less clay mineral content is,the better the sandstone physical property is.According to the data of sandstone physical properties and mercury intrusion data,the reservoirs are divided into four types,of which type I and II are high quality reservoirs.?2?According to the Vitrinite reflectance?Ro?value,smectite layer?I/S?,combined with the results of the analysis of previous pore types of fluid inclusions,the study area for the diagenetic stage of middle diagenetic stage A-B.The original porosity of the He1 sublayer is 31%-34%.After compaction,the original pore is reduced to12%-15%,and the cementation results in the loss of the remaining primary porosity.According to diagenetic evolution analysis,it is inferred that the reservoir has been tight in the early Jurassic.?3?The gas and water relationships of gas reservoirs in He1 sublayer of the upper Paleozoic in the study area are complex,and the gas and water types are various.There are gas zones,gas bearing zones,water bearing gas reservoirs,gas and water layers,water layers and dry layers.The vertical gas water types can be divided into high saturation zone?pebbled sandstone?upper dense aquifer?fine sandstone?,weak differentiation zone?gravelly coarse sandstone in upper part of high saturation layer,lower layer?,gas water two-phase?gas and water mixed layer pebbly coarse sandstone or coarse sand rock water and gas?,layered overlay?different size of sandstone superimposed?,weak differentiation containing gas and water?gas water gravel grit on the part of the low saturation layer with lower water layer?,isolation water layer.On the plane,the gas reservoirs in the He1 sublayer of the study area are relatively large and relatively continuous in the West.The gas saturation is higher;the isolated water layer is more in the central part;the gas and water is the main layer in the East;the gas reservoir is few and the other is discontinuous.?4?The coal seams of Taiyuan formation and Shanxi formation in the study area are the main source rocks of Upper Paleozoic gas reservoir.Based on thermal evolution analysis of hydrocarbon source rocks,burial history and thermal history analysis,two accumulation stages of the study area are at the end of Jurassic and early cretaceous.The characteristics of the source rocks,reservoir characteristics,preservation conditions and distribution of gas reservoirs are analyzed.It is considered that the distribution of the source rocks and the dominant reservoirs controls the distribution of gas reservoirs macroscopically.Through analysis of formation water,the type of water in gas reservoir is CaCl2,indicating that the storage conditions are pretty good.The gas reservoir of He1sublayer is low pressure,but the Shiqianfeng Formation and Shihezi Formation of thick mudstone still exist abnormal pressure,which are the regional cap rocks.Based on the analysis of mudstone under compaction,abnormal pressure condition of gas reservoir directly overlying mudstone section of He1 sublayer has important influence on gas accumulation.The greater the abnormal pressure of overlying mudstone section,the higher the gas saturation of gas reservoir under the condition of sufficient gas supply.In addition,the influence of regional tectonic characteristics on gas accumulation is mainly characterized by upper reservoir depletion and lateral sealing of faults.In Western China,the reservoir is over dip,point and plug,while the fault of the fault of the Haizi fault in the middle part of the central sea is poor,and the natural gas leaks northward through the fault,and the sealing of the eastern fault is better.?5?The gas and water types of tight sandstone gas reservoirs are mainly influenced by the reservoir faccumulation dynamics and the capillary force of the reservoir.After accumulation,if gas column buoyancy is greater than the reservoir reached 20%in gas saturation capillary pressure,natural gas began to gather,gas and water produced weak difference,the top of the gas saturation is larger than the bottom,but not enough to form a pure gas.When the height of gas column is larger than that of reservoir,the gas water is vertically differentiated in the reservoir and the formation of pure gas reservoir at the top of the reservoir when the gas saturation reaches 50%capillary resistance.The west of the study area of hydrocarbon source rock thickness and hydrocarbon potential,natural gas filling large,gas filling into the reservoir,the formation of gas column is larger,can produce larger buoyancy,the gas and the water vertical variation of reservoir at the top of gas reservoir with high degree of saturation.Therefore,the West has more gas layer.?6?The seet zone in He1 sublayer is defined as the gas saturation is more than 50percent.The area where has better preservation condition,developing tuype?and type?reservoirs which the porocity is more than 5%.Also,the thickness is more than 3m that have enough power or the reservoir is continuness.Three the geological model of sweet zone in He1 sublayer are established based on the analysis of controlling factors and the sealing of the predecessors of the Porjianghaizi fault.They are updip wedge-out sweet zone,anticline sweet zone,and fault plugging sweet zone. |
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