The Coupling Relationship Between Evolution Of Diagenetic Fluid And Hydrocarbon Accumulation Of Ordovician In Tahe Area, Northern Of Tarim Basin

The Tarim Basin is located in the Xinjiang Uygur Autonomous Region,NW China and is a multi-cycle superimposed basin,which is of great potential of hydrocarbon resources.Tahe area locates at Akekule uplift which is in the middle of Shaya uplift,northern of Tarim Basin.The palaeostructure high part of Akekule uplift experienced multi-stage migration,resulting in the diversity of the development of karstic reservoir,the distribution of cap rock and the development of faults of Ordovician in different areas in Tahe area.Multiple hydrocarbon charging history and adjustment occurred,meanwhile the plane distribution of characteristics of crude oil and period of hydrocarbon accumulation has strong heterogeneity in study area.Based on diagenesis,isotopic geochemistry and fluid inclusion studies,we try to identify the type and the origin of diagenetic fluid in Yuqixi area,Aiding area,Tahe main area and Tuoputai area,aiming to discuss the mechanism of carbonate reservoir reconstruction.According to geochemical characteristics of crude oil,the period of hydrocarbon accumulation and fault activities,the factors of hydrocarbon accumulation are summarized using to discuss the controlling factors and models of hydrocarbon accumulation.Base on the petrologic characteristics of core samples and thin sections,several diagenetic events are identified in the Ordovician of Yuqixi area,including dissolved fractures?DFC?filled with calcite cement and bitumen,earlier fractures filled with calcite cement?FC1?,stylolites?Sty?,earlier vugs filled with calcite cement?VC1?,dolomite?Dol?,calcite cement filling in collapse breccia?CC3?,dedolomitization?Dedol?,quartz from underground river?Q?,calcite cement precipitated from mixture of underground river and formation water?CC4?,later fractures filled with calcite cement and bitumen.In Aiding area,several diagenetic events include earlier vugs filled with calcite cement?VC1?showing geopetal structure,earlier fractures filled with calcite cement?FC1?,later fractures filled with calcite cement?FC2?cutting VC1 and FC1,stylolites?Sty?,later vugs filled with calcite cement?FC2?along Sty.In Tahe main area,several diagenetic events include earlier vugs filled with calcite cement?VC1?,the second generation vugs filled with calcite cement?VC2?cutting the first generation fractures filled with calcite cement?FC1?,FC1 and VC2 cut by stylolites?Sty?,dolomite?Dol?along Sty,the second generation fractures filled with calcite cement?FC2?cutting Sty and Dol,the third generation fractures filled with calcite cement?FC3?,the latest generation fractures filled with calcite cement?FC4?in the center of FC3.The diagenetic events in the Ordovician of Tuoputai area include earlier fractures filled with calcite cement?FC1?cut by stylolites?Sty?,dolomite?Dol?along Sty,silicification,vugs filled with calcite cement?VC1?in the center of siliceous limestone,the second generation fractures filled with calcite cement?FC2?and the third generation fractures filled with calcite cement?FC3?in dolostone,barite in the center of FC3.By means of diagenesis,colour of minerals in the cathode luminescence,fluid inclusion and isotope geochemistry,the diagenetic fluid in Tahe area include seawater,fresh water,formation water and hydrothermal fluid.Fresh water made the most contribution to carbonate reservoir reconstruction of Ordovician in Yuqixi area,while Fresh water and formation water made the most contribution in Tahe main area.In tuoputai area fresh water and hydrothermal fluid reconstructed the carbonate reservoir.In this study,diagenetic sequence,fluid inclusion petrography,micro fluorescence observation were employed to obtain homogenization temperatures?T_h?of oil inclusions and their coeval aqueous inclusions,and used the method of T_h of fluid inclusion projecting on burial history diagram with thermal curve to obtain hydrocarbon charging times.The results show that three events of hydrocarbon charging occurred in the Ordovician of Yuqixi area:the first event took place from 452.5Ma to 447.8Ma?during the Late Caledonian?;the secondary event from 150.2Ma to 100.2Ma?during the Indosinian?;the tertiary event from 19.8Ma to 1.4Ma?during the Late Himalaya?.The oil charging events occurred at 435.2-426.6Ma and 289.5Ma in Aiding area,and 454.8Ma,259-95Ma and 20.4-5.3Ma in Tahe main area,and 433.7-420.5Ma,249.4-110Ma and20.1-12.1Ma in Tuoputai area.Gas chromatography?GC?of crude oil from well YQX1 and well YQX101 shows complete series of n-alkanes and UCM,indicating two or more hydrocarbon charging events occurred in Yuqixi area,which is consistent with the results of fluid inclusion data.Although the tertiary hydrocarbon charging event occurred in well YQX2,the oil is still heavy because of oil mass.However,medium-light oil pools formed in well YQX1because of late light oil migration.Therefore,migration pathways and cap rocks are controlling factors for hydrocarbon accumulation in Yuqixi area.The effective migration pathways ensure hydrocarbon charges into the reservoir,meanwhile effective cap rocks ensure that hydrocarbon accumulates in the reservoir.The first and secondary events of hydrocarbon charging were controlled by paleouplift-paleoslope,paleostructure ridge and paleokarst reservoir.Diamondoid compounds and GC of crude oil shows that biodegradation of crude oil occurred resulting from tectonic uplift in Yuqixi area,Aiding area and Tahe main area,but cap rocks in Tuoputai area protected oil from biodegradation.The tertiary hydrocarbon charging event took place resulting in oil mixed with heavy oil.The plane distribution of T₇⁴,T₅⁰ and T₂⁴interfacial faults indicates that faults activities occurred during the Late Caledonian,the Late Hercynian and the Late Himalaya.Coherent properties of T₈¹ interface shows that NE strike-slip faults is important pathway linking source and reservoir.The complex hydrocarbon accumulation rule includes discontinuous migration pathway,marine source rock and late light oil migrated vertically along NW?NNE strike-slip faults.