| The oil-bearing series of lower Jurassic Da'anzhai member in the Langzhong-Nanbu area of northeastern Sichuan province was selected as the research object to describe geological characteristics of tight oil reservoir and evaluate its development potential,and also to provide a subtle oil pool geological model for tight oil exploration and development in the area.Through investigating a large number of relevant literature,combining with field outcrop,core observation,drilling and logging material,testing and analysis and seismic data,etc.,based on petroleum geology,sedimentology,sequence stratigraphy and carbonate reservoir geology theory,using the comprehensive research methods of trinity with geological,logging and seismic,we carried out the comprehensive research work on the tight reservoir of Da'anzhai member in Langzhong-Nanbu area of northeastern Sichuan province,evaluated the geological characteristics and development potential of reservoir in each sub-member,and gave suggestions to well deployment.The main work and realization achieved in the article have been present as following.According to the interpretation and reprocessing of the old well logging data,the lithology identification and division from Da'anzhai member is carried out by applying comprehensive analysis and processing technology of well logging data,the support vector machines methods,the intersection graph recognition methods and the rose graph recognition methods to mutually complement and verify.It is concluded that the lithology identification and division technology from lacustrine carbonate thin reservoirs are based on the old logging data.For the first time in the study area,the high frequency sequence stratigraphic division and correlation of the Da'anzhai member were carried out.The results show that Da'anzhai member can be classified into 10 fifth-order sequences(SS1-SS10)and 2 fourth-order sequences(S1,S2).The fourth-order sequences consists of two semi cycles(S1-1,S1-2,S2-1,S2-2),among them,S1-1 is composed of SS1,SS2,SS3 while S1-2 is composed of SS4 SS5,S2-1 is composed of SS6 and S2-2 is composed of SS7,SS8,SS9,SS10.The third sub-member(Dasan sub-member,J1Z4-3)contains SS1,SS2;the second sub-member(Da'er sub-member,J1Z4-2)contains SS3,SS4,SS5,SS6;the first sub-member(Dayi sub-member,J1Z4-1)contains SS7,SS8,SS9,SS10.Through sedimentary facies analysis,it presents that sediments of Da'anzhai member forms in lacustrine facies and develops two sedimentary sub-facies of shallow lake and semi-deep lake facies as well as the microfacies such as bio-fragmental beach core,sublittoral beach,shallow lake mud,foreslope of beach.The favorable reservoir in Dayi sub-member and Dasan sub-member mainly develops in the microfacies of beach core and edge while it mainly develops in beach of lake slope microfacies in Da'er sub-member.The fracture distribution was predicted with fracture modeling technology,combining with the research on reservoir characteristics and logging evaluation.The reservoir lithology is mainly shell limestone,sparry shell limestone and argillaceous clastic shell limestone.The analysis results show that reservoir space consists of dissolved pores,intergranular pore and fracture,and these reservoirs are of low porosity and low permeability.The reservoir's matrix porosity is 0.19%?3.22%,with average at 1.082;the permeability is from<0.1 to 159.5(10-3?m2).The pore structure is characterized by micro pore throat and fine throat type,and the pore structure of the reservoir is generally poor.The electrical characteristics are:the thick massive shell limestone with low natural gamma value(generally between 14?30API),the time difference of acoustic wave is mainly low value(generally between 46?69?s/ft),the depth bilateral is to the high value based(generally between 500?8000?.m),the thin thick layer of shell limestone with low natural gamma value(generally between 30?60API),the acoustic time is higher difference(generally between 50?65?s/ft),the resistivity value is low(generally between 35?750?.m).The types of reservoir is mainly pore(hole)-fracture type,(micro-)pore-fracture type,fracture type;developing compaction pressure solution,cementation,recrystallization,mud crystallization and dissolution diagenetic types,among them,the process of compaction,cemented filling and recrystallization are the main factors to cause the formation of the reservoir;fracture types are mainly formed by the fracture and structural fracture,the main development area of fracture is structural stress concentration zone and ancient landform uplift zone,the property of fracture is mainly low angle seam,horizontal and vertical seam.There are five fracture systems in the reservoir of study area.Particularly the fracture develops best in Chuanshi-43 system,secondary in Shilong-2 system.According to drilling,well logging,testing logging and production dynamic and the reservoir characteristics,the comprehensive evaluation criteria of lacustrine carbonate reservoir in the study area is put forward exploratory and the comprehensive evaluation of the each sub-member is carried out.It is resulted that Da'anzhai reservoirs are mainly based on the III reservoir,accounting for 66.5%;the ? reservoir accounts for 29.5%;the I reservoir is few,accounting for 4%.Using log information to calculate the matrix porosity and fracture porosity of each sub-member,for the first time in the study area,the determination of the lower limit value of the porosity of all kinds of reservoir types was carried out in the study area,and the classification criteria of reservoir type is proposed.Based on the fracture characteristics,the Coulomb failure criterion and the shear ultimate fracture strength criterion,fractures prediction in the study area was carried out by using modeling and simulation of structural deformation reverse engineering system based on MVE Company.The results show that the fractures are mainly developed in the Chuanshi 47 well-Chuanshi 55 well-Chuanshi 7 well block,Chuanshi 59 well block,Shilong 7 well block in the south to Shilong 15 well block.Research on characteristics of hydrocarbon accumulation has been developed in Da'anzhai member.The reservoir contains oil in large area without edge and with bottom water,however,the enrichment degree of which is heterogeneity.Production is mainly controlled by reservoir thickness,lithology and fracture development degree but the thickness of the reservoir and the fractures play a decisive role.The fracture development degree is the main reason for the difference in the production well.Based on the study of the sedimentary facies,reservoir characteristics,classification statistics and fracture prediction distribution,combining with the drilling,display of wells logging,interpretation,test results and development characteristics,it is the first time to put forward the comprehensive evaluation standard of oil and gas-bearing area,determined the distribution of hydrocarbon zones in each sub-member of Da'anzhai member,and with reference to industry standards,the volume method was adopted respectively to the favorable and sub favorable areas reservoirs of Dayi,Da'er and Dasan sub-member for reserves calculation.The total original geological reserves were 1398.26×104t,and the current recovery degree of geological reserves account for 1.03%.On the basis of extensive research of exploitation potential technology in neighbor region of Da'anzhai reservoir,water-flooding couldn't be used because of various uncertain factors(e.g:strong reservoir heterogeneity,lower matrix porosity)and poor long-term economic benefit.Rolling extension wells,development infill wells were better to seek new reserves.It is suggested that fractured reservoir be mined by injecting gas. |
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