Diagenetic Fluid Evolution And Dynamic Hydrocarbon Accumulation Process In The Strike Slip Fault Zone Of Northern Shuntuoguole Low Uplift

The Shuntuoguole Low Uplift is one of the 12 first-order structural units of the Tarim Basin.It is sandwiched between the two paleouplifts of the Tabei and the Tazhong in the north-south direction.It is located in the low uplift between the Manjiaer depression and the Awati depression in the east-west direction and extends to the Guchengxu uplift in the southeast.It is a relatively stable structural unit with an area of about 3.2×10⁴km².In 2016,Sinopec discovered the world’s first ultra-deep fault solution reservoir of marine carbonate in the Middle and Lower Ordovician of its northern region,with a buried depth of more than 7,000 m and distributed along a small-scale strike slip fault zone in the Tarim Basin,exhibiting unique characteristics and high research values.Significant oil and gas fields have been discovered in the Shunbei 1,3,5,and 7 strike-slip fault zones developed in the north of the Shuntuoguole Low Uplift,with an annual production of one million tons of light oil.The carbonate reservoirs in the main strike-slip fault zones of the Shuntuoguole Low Uplift are petroleum-bearing as a whole,but the reservoir properties and enrichment levels are quite different,thus the accumulation process is very complicated.Previous studies focused mainly on the seismic identification of fault solution,the structure and activity characteristics of strike slip fault zone,the relationship between fault and reservoir configuration,and oil and gas reservoir characteristics and distribution differences from a perspective of production demand.However,the relationship among multi-stage fault system,fluid activity,reservoir formation,and hydrocarbon accumulation mechanism in the strike slip fault zone is not clear,the transformation mechanism of fluid to fault-controlled reservoir is not deeply studied,and the control mechanism of faults on hydrocarbon accumulation is not well understood.Taking the dynamic reservoir-forming and hydrocarbon accumulation of the strike slip fault zone in the Shuntuoguole Low Uplift as the main line,this thesis focuses on two key scientific issues,including the relationship among diagenesis evolution,fluid activity,and hydrocarbon accumulation of carbonate reservoirs and the determination of oil charge timings in multi-stage hydrocarbon accumulation.Structural analysis,oil and gas property analysis,thermal and hydrocarbon generation history analysis of source rocks,evolution and source analysis of diagenetic fluids,absolute dating of calcite and oil charge,and correlation analysis between fault activity and transport system were carried out,the dynamic hydrocarbon accumulation process were reconstructed,and the accumulation models under different strike slip structural styles were established in the main strike slip fault zones（Shunbei 1,5,and 7）of the northern part of the Shuntuoguole Low Uplift.The structural deformation patterns and activity of three main strike-slip faults in the study area were analyzed by using the fine analysis technology of 3D seismic small-scale slip distance strike-slip faults,and the structural deformation style of“plane segmentation and vertical stratification”of strike slip faults was defined.Vertically,high-steep strike-slip faults are developed on the carbonate strata below the T₇⁴interface,and the overlying clastic strata develope en echelon normal faults Horizontally,the main strike-slip fault zone develope three different segmented splicing styles,including superimposed pull-apart style,superimposed compression style,and strike-slip translation style in the Middle and Lower Ordovician.There are obvious differences in the reservoir properties and enrichment degree of different strike-slip fault zones and different sections of the same strike-slip fault.The physical property statistics and geochemical characteristics analyses of crude oils in the fault zone show that the gas oil ratio,H₂S content,natural gas drying coefficient,and thermal maturity increase,whereas the crude oil density decreases from the north section of the Shunbei 5 fault zone to the middle section of the Shunbei 5 fault zone,and then to the Shunbei 1 fault zone.By means of core observation,cast thin section identification,and cathodoluminescence observation,calcite,bitumen,and silica were observed in the Middle and Lower Ordovician carbonate reservoir of the Shunbei Low Uplift,with calcite widely distributed.Four stages of calcite veins including C1,C2,C3,and C4were identified in this study.The timings of calcite veins C1,C2,and C3 formation obtained by laser in-situ calcite U-Pb isotope dating were 460～446Ma,440～436Ma,and421～420Ma,respectively,all of which were formed in the Caledonian.According to tracer mineral identification,fluid inclusion analysis,isotope geochemistry（carbon,oxygen,strontium,silicon,etc.）,and rare earth element analysis,multiple types of diagenetic fluids,such as calcium-rich fluids,silicon-rich fluids,sealed seawater in burial period,hydrothermal fluids,and hydrocarbon fluids were identified in the Middle-Lower Ordovician reservoirs.Combined with the results of diagenetic sequence and mineral chronology,the sources of different types of diagenetic fluids were comprehensively determined.It is believed that the diagenetic fluids of the third episode of Middle Caledonian came from the marine stratum water and the silicon-rich fluid of the Middle-Upper Cambrian,that of the late Caledonian-early Hercynian was sourced from marine stratum water of the Middle-Lower Ordovician,hydrocarbon fluids supplied by source rocks at the bottom of Cambrian,and the deep hydrothermal fluids in the Late Hercynian.Combined with reservoir petrography and the development and evolution characteristics of strike-slip faults,the evolution process and model of deep diagenetic fluids in the main fault zone of the study area were further revealed.Using the comprehensive identification technology of fluid inclusion accumulation period constrained by U-Pb isotopic age of hydrocarbon inclusion host mineral（calcite vein）,it is determined that the Ordovician reservoir in the study area has experienced three stages of crude oil charging process,and the accumulation time is determined.The timing of the first stage crude oil charge was about 436～421Ma,i.e.late Caledonian,which was damaged and transformed by the Early Hercynian tectonic movement to form residual reservoir bitumen;the timing of phase 2 crude oil charge was about385～363Ma,that is,the Early Hercynian period;the timing of phase 3 crude oil charge was about 258～235Ma,i.e.the Late Hercynian Indosinian period.The comprehensive analysis of fault activity,hydrocarbon accumulation period,and diagenesis fluid evolution in the main fault zone has determined the diagenetic-fluid-accumulation coupling relationship of the Yijianfang and Yingshan Formation under the strike slip fault controlled environment in the study area,which is clarified as argillization→cementation/recrystallization→dolomitization→structural rupture→C1 calcite filling（Episode III in the Mid-Caledonian）→silicification（cryptocrystalline silicon filling）→tectonic rupture→C2 calcite filling（the Late Caledonian）→phase 1 crude oil filling（the Late Caledonian）→C3 calcite filling（the Late Caledonian）→reservoir densification→paleoreservoir destruction and asphalt filling（the Early Hercynian）→tectonic rupture→phase 2 crude oil l charge（the Early Hercynian）→compaction and pressure solution→hydrothermal dissolution→C4 calcite Pyrite filling→phase 3crude oil charge（the Late Hercynian-Indosinian）→continuous burial evolution（Yanshan-Himalayan）.The Middle and Lower Ordovician reservoirs in the study area are ultra-deep carbonate fault solution light-volatile reservoirs with a buried depth of more than 7,000m,showing the obvious characteristics of strike slip faults controlling reservoir,storage and enrichment.As a whole,the reservoir forming mode of“Cambrian hydrocarbon supply,fault solution forming reservoir,vertical transportation,multi-stage accumulation,and strike slip controlling enrichment”was proposed.At present,the industrial crude oil production is mainly charged into reservoirs during the second and third stages.Different fault zones and different positions of the same fault zone have experienced different hydrocarbon charging and mixing processes,resulting in the current complex distribution of oil and gas plays,which is mainly controlled by the activity of strike-slip faults and their temporal-spatial allocation with the hydrocarbon generation stages of source rocks.The reservoirs of the Shunbei 7 fault zone were mainly accumulated in the Early Hercynian,filled with yellow fluorescent crude oil with relatively low maturity.The northern Shunbei 5 fault zone experienced two hydrocarbon accumulation processes in Early Hercynian and Late Hercynian,and was mainly filled the yellow fluorescent crude oil with relatively low maturity in the Early Hercynian;the middle Shunbei 5 fault zone,and the branch and the secondary fault of Shunbei 1 fault also experienced twohydrocarbon accumulation processes in the Early Hercynian and Late Hercynian,and was mainly filled with blue-green fluorescent crude oil with relatively high maturity in the Late Hercynian-Indosinian;The crude oil in the Shunbei 1 main fault zone was mainly accumulated in the Late Hercynian-Indosinian,and was filled with relatively high maturity crude oil that emits blue-green fluorescence.There are also great differences in oil and gas enrichment degrees in different fault zones and different locations of the same fault zone,which is mainly controlled by the continuity of strike slip faults,the degree of fault evolution,and segmented deformation style.The main strike slip faults with good longitudinal penetration tend to have high oil and gas filling intensity,high degree of evolution,and large scales of reservoir development.The pull section of the same strike-slip fault zone has large fault width and large longitudinal reservoir depth,and the oil and gas enrichment degree is the highest,followed by the compression section and translation section.This research provides some new ideas for revealing the fault-storage-reservoir relationship of ultra-deep carbonate rocks,forms a comprehensive identification technology of reservoir-forming period based on U-Pb isotope dating,determines the timing of oil and gas charge,clarifies the coupling relationship of diagenesis-fluids-reservoir forming under the deep fault controlling environment,and restores the dynamic process of reservoir forming in different strike slip fault zones.It plays an important role in promoting the development of carbonate reservoir-forming dating technology in marine superimposed basins and deepening the understanding of the accumulation mechanism and enrichment law of fault solution oil and gas reservoirs in the Shuntuoguole area,Tarim Basin.