Research On Reservoir Characteristics And Oil And Gas Distribution Regularity Of The Chang 6 Formation In The Xiaojiahe Area,Ordos Basin

The Chang 6 oil reservoir of the Yanchang Formation belongs to the low-permeability and ultra-low permeability lithologic reservoir in the Xiaojiahe area of the Ordos Basin.At present,it faces problems such as poor reservoir properties,insufficient formation energy,high water content,lack of understanding of the distribution of sand and the distribution of oil-water.Based on the data of drilling,logging,core and oil production in the study area,this paper applies sedimentary and reservoir geology and other multidisciplinary theories to the formation features,structural feature,sedimentary evolution characteristics,reservoir development characteristics,the regularity of oil and gas distribution and enrichment is being studied.The aim is to improve the understanding of reservoir bed development degree and the regularity of oil reservoir distribution,and lay a foundation for the subsequent development of oil fields.Through the formation division and comparison,the Chang 6 oil layer group is divided into four subgroups of C6₁ to C6₄,of which C61 is subdivided into C6₁¹,C6₁² and C6₁³,and C6₂ is subdivided into C6₂¹,C6₂²,C6 The thickness of the formation is about 120m¹40m,and the formation of each small layer is little undulation.The structure altitude difference between the east and west is about 90m and developing two groups of NE-SW to small nose-like ridges.Sedimentary characteristics studies show that the Chang 6 sediment source mainly comes from the northeast direction,developing the delta plain subfacies and the three microfacies of the distributary channel,the diversion bay and the natural embankment.The sedimentary evolution characteristics show the river sand body gradually increases from NE-SW to the development scale in the C6₄-C6₂ sedimentary period,the development scale reaches the maximum in the C6₂ period.and the development scale of the C6₁ period gradually declined.The C6₂¹ and the C6₂² diversion channel sand bodies are the most favorable reservoirs in the study area.The characteristics of C6 reservoir in Xiaojiahe area were studied by means of the slice identification,core physical property test,mercury intrusion analysis,cathodoluminescence and SEM.The results show that the C6 reservoir is dominated by feldspar sandstone,and the lithology is mainly fine sandstone.The pore type is mainly composed of intergranular pores and feldspar dissolved pores,and three main pore throat structures are identified,among which class I?small pore fine throat type?and the type II?small pore microthroat type?are relatively developed,and the type III?fine pore microthroat type?is weakly developed;the diagenesis of the Chang 6 reservoir is various in the area,which has a differential effect on the reservoir physical properties.The diagenetic stage is mainly in the A-phase of the Mid-stage diagenesis.The physical property analysis shows that the C6 oil formation belongs to the low-porosity,low-permeability,extra-low-permeability reservoir.Based on the multiple influencing factors,the classification criteria of C6 reservoirs are established.The C6reservoirs in Xiaojiahe area are mainly divided into three categories,of which class II?62.5%?is widely developed and has good reservoir performance,class I?22.5%?reservoirs have the best reservoir performance,and class III?15%?is relatively poor.The analysis of reservoir characteristics and oil layer distribution shows that the C6reservoir in the study area is a normal crude oil with light oil,low viscosity,low freezing point,low sulfur content and medium wax,the formation water is CaCl₂ type,and the reservoir drive type is elastic-dissolved gas composite flooding.The average oil layer thickness of each of the small C6 layers is 4.6m⁵.5m,on the flat surface,the oil area of the C6₂¹ is the largest?26.89km²?;on the profile,the oil layer in the direction of the river is extended laterally,and the continuity of the oil layer in the direction of the vertical channel is poor.The C6-segment developed the underside source rock and upper reservoir-type accumulation model of local lateral quasi-continuous-vertical lithologic lens.The oil and gas distribution and enrichment are mainly affected by the favorable zone of the diversion channel sand body,reservoir physical property,nose-like uplift tectonic belt,degree of crack development,in which the development of fractures makes the distribution of oil and water layers more complicated,which increases the difficulty coefficient of later development of oil fields.