Evaluation Of Chang 4+5 And Chang 6 Reservoirs In Triassic Yanchang Formation In Wan33 Well Area Of Laoshan Oilfield

The study area is located in the northern part of Ganquan County,Shaanxi Province and the southern part of Baota District,Yan’an City,with an area of about 51km².In this paper,based on the use of logging data,core data,thin section identification,scanning electron microscopy and other testing and analysis methods,guided by the disciplines of oil and gas reservoir geology,structural geology,sedimentary petrology,etc.Fine description and classification evaluation of Chang 4+5²,Chang 6¹ and Chang 6² reservoirs.It provides a certain theoretical basis for the future development of this area.The study area is located in the central part of the Ordos Basin.The overall structural form is a west-dipping monocline,with a relatively gentle stratum and some low and gentle nose-shaped uplifts locally is the main sedimentary microfacies,which controls the development of reservoirs in the study area;the lithology of the target layer in the study area is mainly gray fine-grained feldspar sandstone,followed by medium-fine grained and silty-fine-grained feldspar sandstone,with similar petrology Features: low composition maturity and high structural maturity;diagenesis is dominated by compaction,cementation and dissolution.The compaction in this area causes the reservoir to lose porosity of 20.03%,and cementation loses porosity.The porosity value is 25%,and the porosity value increased by dissolution is 5.17%.It is now in the middle diagenetic stage A;the pore types are mainly primary intergranular pores,turbidite dissolved pores,and feldspar dissolved pores,and the throat type is thin throat.The reservoir properties are characterized by low porosity and ultra-low permeability.The porosity of Chang 4+5 reservoir ranges from 3.03% to 19.5%,and the average porosity is 10.98%;The porosity ranges from 1.75% to 18.7%,and the average porosity is 10.49%;the permeability of the Chang 4+5 reservoir ranges from 0.04×10^-3 μm² to 6.54×10^-3 μm²,with an average value of 1.42×10^-3μm²,the permeability of Chang 6 ranges from 0.04×10^-3μm² to 8.01×10^-3μm²,with an average value of 1.27×10^-3μm².Based on the characteristics of sedimentary facies,physical properties,pore structure and other characteristics of this area,combined with the classification and evaluation criteria of Mesozoic sandstone reservoirs in the Ordos Basin,this paper formulates a classification suitable for Chang 4+5 and Chang 6 reservoirs in this area.standard.The target reservoirs in the study area are divided into four types of reservoirs,namely type I good reservoirs,type Ⅱ good reservoirs,type Ⅲ poor reservoirs,and type Ⅳ non-reservoirs.The reservoirs in Chang 4+5 and Chang 6 in the Wan33 well area of Laoshan are classified and evaluated,and it is concluded that the better type Ⅱ reservoirs and the Ⅲ type poor reservoirs are widely distributed in the study area,and the distribution area is large,and the type Ⅰ is good.Reservoirs and Class Ⅳ non-reservoirs are less distributed,and Chang 4+5 and Chang 6reservoirs are further considered to be low porosity and ultra-low permeability reservoirs.