Genetic Mechanism Of Favorable Sandstone Of The Middle Jurassic Toutunhe Formation Of The Block 4 In The Central Junggar Basin

In recent years,the substantial hydrocarbon findings in the Lower-Middle Jurassic sandstones showed that the block 4 of Central Junggar Basin had large potentials for exploration.In general,there area has a relatively low degree of exploration.Owing to the affection of multi-source supply and abrupt changes in sedimentary environment,the genesis of sandstone was unclear which constricted the further exploration.This thesis took the origin of favorable sandstone reservoir as the research object,studied the sequence stratigraphy framework,source system,evolution of sedimentary environment and characteristics of favorable sandstone reservoirs,which based on the comprehensively cores,thin sections,logging,logging,seismic date,heavy mineral analysis and fluid inclusion testing.The conclusions were showed as follows:(1)Based on the classical sequence stratigraphic theory,four third-order sequences were identified in Toutunhe Formation of Jurassic,which including a lowstand systems tract,a transgressive systems tract and a highstand systems tract.The thickness of each sequence was stable and decreased from the lake center to the shore.The content of sandstone reservoir were gradually decreased from bottom to top of Toutunhe Formation,and the sandstone-riched intervals mainly corresponded to the early stage of the transgressive systems tract and the late stage of the highstand systems tract in the vertical.(2)There were three relatively independent provenances from the northeast,northwest and the south in the research area during the middle Jurassic Toutunhe Period.The parent rock of northeast provenance was Kelameili mountain volcanic rock,whereas south provenance was sourced from the Bogeda mountain metamorphic rock.The influence area of each provenance is different times:The lower unit(J₁t₁)was affected by the southern source and the northeastern source.The northeastern source has a certain impact in the northern part of Dong 2,and the southern provenance mainly affects the area south of Dong 6.The middle(J₁t₂)unit's deposits are mainly from the Tianshan area,affecting Dong 2 and its east area,and the supply power of the Kelameili mountain in the north is weakened;the southern provenance has been further strengthened,with a clear control over the area south of Dong 6.The upper unit(J₁t₃)was jointly controlled by the southern and northern sources,and the northern source is strengthened,extending to most of the areas north of Dong 2 and Dong 8.(3)In the vertical,the lithologic association had a transition from“gray mud interbedded with gray sand”to“red mud interbedded gray sand”and“red mud interbedded red sand”from the bottom to the top,which indicated the Toutunhe Formation palaeoclimate changed gradually from the hot humid to arid.The research area mainly developed lake and delta facies during the early stage,whereas developed fluvial fan facies during the late stage.The sedimentary facies of favorable sandstone in the Block 4 area was fluvial fan,which was formed by seasonal flooding and consisted of river channel and terminal fan.(4)The reservoir of the Middle Jurassic is low to ultra-low poroperm.The porosity type is mainly composed of intergranular residual pores and intergranular dissolved pores,but the fractures are not well developed.The reservoir is low maturity feldspar lithic and lithic sandstone.The type of fragment was volcanic and metamorphic debris.Diagenesis mainly included quartz overgrowth,chlorite cementation,early calcite and analcime cementation,feldspar overgrowth,anhydrite and barite cementation,chlorite cementation and quartz overgrowth,late calcite cementation.The dissolution mainly occurred after early calcite and analcime cementation.The high content of soft fragment in reservoir caused the decreased of porosity;dissolution improved the quality of porosity.The sedimentary facies control the diagenesis difference,which is the main controlling factor of the favorable sandstone distribution.