Geochemical Characteristics And Hydrocarbon Accumulation Model Of The Ordovician Reservoirs In Tazhong Area

During the geological period,the deep reservoirs in the basin have experienced multi-period of oil and gas filling and transformation.How to track the process of oil and gas migration and accumulation is the frontier research direction of petroleum geology,and it is also an important problem to be solved in deep oil and gas exploration.Based on comprehensive collection and summary the test data of source rock biomarkers from open source,this paper studies the Ordovician carbonate reservoirs in the Tazhong area of the Tarim Basin.Representative oil samples were tested by means of chromatography-mass spectrometry and carbon isotope and crudes oil genetic type by means of cluster analysis.At the same time,it combines the anatomy of developmental characteristics of fault system and its matching relationship with hydrocarbon accumulation period,attempts to use effective geochemical indicators to trace the dynamic processes of migration and accumulation of oil and gas along fault systems and carbonate fracture systems.We have mainly achieved the following understanding:Ordovician oil in Tazhong area is generally mixed source oil,which is mainly divided into two categories:(1)The biomarkers of class?oil are characterized by high C₂₁/C₂₃TT,high C₂₈/(C₂₇+C₂₉)???₂₀R regular sterane,high?waxane/C₃₁R hopane,low C₂₄Te T/C₂₆TT,low C₃₁R hopane/C₃₀hopane,low rearrangement decane/regular decane and low homohopane/C₃₀hopane.Class?oil may be derived from the Cambrian-Lower Ordovician source rock.(2)The biomarkers of the Class II oils have opposite characteristics to the biomarkers of the Class?oils.Class II oils may be all mixed source oil,probably from the middle-upper Ordovician mature oils and from the Cambrian Lower Ordovician high-over mature oil.The high maturity caused the abundance of steroids and terpenes to decrease,resulting the biomarkers of the mixed oil exhibiting similar characteristics to the biomarkers of the Middle-Upper Ordovician source rocks.Aiming at the characteristics of“multi-source and multi-period”oil and gas mixing in deep reservoirs,this paper attempts to propose an operational scheme for trace oil and gas migration in deep and complex geological conditions:(1)Judging oil source by effective source parameters.(2)Use the stable maturity parameters to judge the filling period of oil and gas.(3)Use the oil physical properties and gas-oil ratio parameters to judge the late gas intrusion.(4)Finally,combined with the geological characteristics of the transport framework to trace the hydrocarbon migration process.The organic geochemical tracer indicates that the oil and gas in the Tazhong area has undergone three stages of migration along the fault transport conductor:(1)In the late Caledonian-Early Hercynian period,the early low-mature oil of the Cambrian-Lower Ordovician source rocks migrated vertically in the faults of the Cambrian and the Penglaiba Formation.A small amount of oil and gas migrated vertically along the active faults to the reservoirs of the Yingshan Formation and the Lianglitage Formation.In the Devonian,uplifting and denudation caused adjustment and destruction of early oil and gas reservoirs,leaving a few ancient reservoirs.(2)In the Late Hercynian period,mature oil derived from the Middle-Upper Ordovician source rocks migrated along the fault transport conductor to the Ordovician reservoir and mixed with the oil of the early reservoir to form a mixed source oil and gas reservoirs.(3)During the Himalayan period,high-over-mature oil and cracked gas from the Cambrian-Lower Ordovician migrated along the fault to the paleo-oil reservoir to form a condensate gas reservoir.The condensate gas may be further transported to the high structural part,hydrocarbon reaccumulation or lead to further mixing and transformation of ancient oil reservoirs.