| Deep-ultra-deep for oil and gas exploration,and also a research hotspot in recent years.The Shunbei area is located in the northern slope of the Tazhong Uplift,Tarim Basin,where the Middle-Lower Ordovician carbonate reservoir presents great potential for hydrocarbon accumulations,with an annual output of more than one million tons of crude oil,which has broad exploration prospects.The average burial depth of the target reservoir intervals in this area is greater than 7500 m,and the oil and gas enrichment zones are mainly distributed along the strikeslip fault zones.Multi-stage tectonic movements have occurred in the Shunbei area,which promoted fluid activity and hydrocarbon migration,and determined the complex history of hydrocarbon accumulation in the Shunbei area.Therefore,it is of great significance to determine the time,temperature and pressure conditions of the hydrocarbon charging at different periods in the Shunbei area for the reconstruction of the dynamic process of oil and gas accumulation and the study of the formation and distribution pattern of oil and gas reservoirs.This paper takes the Middle-Lower Ordovician Yijianfang Formation(O2yj)and Yingshan Formation(O1-2y)of the SB1,SB5 and SB7 strike-slip fault zones in the Shunbei area as the main research objects.Based on the previous researches on the characteristics of Ordovician reservoir and hydrocarbon charging history,the sources of diagenetic fluid in different periods and the mechanism of reservoir transformation in the Shunbei area are identified from the observation of diagenesis,carbon and oxygen isotopes of carbonate minerals,rare earth elements and fluid inclusions analysis.The geochemical characteristics of oil and gas are studied by using oil and gas physical properties,light hydrocarbons,saturated hydrocarbons,aromatics,carbon isotopes and other parameters to determine the source of oil and gas.On this basis,the calcite U-Pb dating method combined with fluid inclusion homogenization temperature(Th)dating method was used to determine the hydrocarbon accumulation history,discuss the accumulation controlling factors in different fault zones,and restore the dynamic hydrocarbon accumulation process.On the basis of thin sections microscope observation,it is identified that the Ordovician reservoir in the Shunbei area has undergone micritization,dissolution,calcite cementation,recrystallization,cracking,pyrite mineralization,dolomitization,silicification,compaction and pressure dissolution.Compaction and cementation reduce reservoir pores significantly and reduce reservoir physical properties.Dissolution and cracking are the main factors to improve reservoir physical properties.Dolomitization also plays a constructive role in improving reservoir physical properties,while silicification has a double impact on reservoir physical properties.According to the cathodoluminescence color and vein cutting relationship,at least four stages of fractures and four generations of cal cite cementation can be identified in SB1 fault zone,and at least three stages of fractures and three generations of calcite cementation can be identified in SB5 fault zone.On the whole,the cathodoluminescence color is dark(dark brown)→dark orange-red(dark purple)→orange-red→orange-yellow.On the basis of diagenetic sequence,based on the analysis of carbon and oxygen isotopes of calcite,rare earth elements,major and trace elements and fluid inclusion temperature and salt measurement,it is concluded that the diagenetic fluids of the MiddleLower Ordovician reservoir in the Shunbei area mainly include atmospheric fresh water,syngenetic seawater,high salinity formation water of Cambrian and hydrothermal fluids.The reconstruction of atmospheric freshwater includes syngenetic karstification and interlayer karstification,in which the pores formed by the reconstruction of atmospheric freshwater in syngenetic period have been seriously reduced,and it is difficult to have a constructive effect on reservoir physical properties,while the interlayer karstification can form a certain scale of dissolution pores under the T74 interface in the Shunbei area.The third episode of middle Caledonian-late Caledonian stage is the main formation period of the "fault-karst" reservoir in the Shunbei area,and the fluid activity is most active in this period.The diagenetic fluids include atmospheric fresh water,syngenic seawater and high salinity formation water from deep Cambrian strata.In the late Hercynian,the diagenetic fluids include syngenic seawater,high salinity formation water from deep Cambrian strata and silica-rich hydrothermal fluids,while the diagenetic fluids of middle Yanshanian and Himalayan period are mainly mixed fluids of syngenic seawater and deep hydrothermal fluids.Based on the organic geochemical parameters,combined with the trace element data of crude oil determined by synchrotron radiation X-ray fluorescence analysis technology(SRXRF),the geochemical analysis of reservoir crude oil and natural gas and oil source comparison in the study area were carried out.The results show that there are certain differences in the properties of crude oil and natural gas in different fault zones in the study area.From the SB7 fault zone to the SB1 fault zone,gasoline ratio increases,and the density and viscosity of crude oil in the SB7 fault zone are obviously higher than other fault zones.The crude oil in the Shunbei area is mature to high mature oil as a whole,and its maturity increases gradually from the SB7 fault zone to the SB1 fault zone.The reservoir oils in the Shunbei area come from the same set of source rocks,all of which come from the black shale of Lower Cambrian Yuertusi Formation,which deposited under anoxic bottom water environment,with the organic matters being mainly derived from algae and belonging to marine sapropelic organic matter.However,the formation environment of source rocks in the SB7 fault zone is different from other fault zones,and they are deposited in a relatively oxidized bottom water environment,and mainly from marine argillaceous rocks containing calcium.The CO2 in the natural gas in the Shunbei area is mainly from inorganic origin,and the natural gas is a single source.On the whole,it is mainly from kerogen cracking gas,and only the SB1 fault zone may have a small contribution of crude oil cracking gas.Based on the fluorescence spectrum data of oil inclusions and the microthermometric data of fluid inclusions,combined with the U-Pb dating data of calcite,the hydrocarbon charging history was reconstructed,and it was concluded that four stages of hydrocarbon accumulation occurred in the Shunbei area as a whole,with the first stage occurred in the late Caledonian(437.2～420.0 Ma),the second stage in the late Hercynian-Indosinian(325.2～216.5 Ma),the third stage in the middle Yanshanian(161.0 Ma)and the fourth stage in the Himalayan(46.8～2.2 Ma),and there is a good coupling relationship between tectonic activity,hydrocarbon charging and fluid activity.Four stages of hydrocarbon charging occurred in the SB1 fault zone and three stages in the SB5 fault zone.However,only two stages of hydrocarbon charging occurred in the SB7 fault zone.Confocal laser scanning microscopy(CLSM)and VTflinc software were used to recover the pressure during fluid inclusion capture.The results showed that the pressure during hydrocarbon charging gradually changed from medium-low overpressure to normal pressure from late Caledonian to late Himalayan.Laser Raman analysis of hydrocarbon inclusions shows that the crude oil charged in the early stage experienced thermal evolution in the reservoir and retained bitumen.Based on the relationship between the fluorescence spectrum parameters QF-535 of crude oil and oil inclusions and the API gravity,the contribution of ancient oil charging in different fault zones at different stages can be quantitatively calculated.The results show that the SB1 fault zone is dominated by the hydrocarbon charging of late Himalayan,and its contribution is 86.58%.The hydrocarbon charging of late Himalayan and late Hercynian in the SB5 fault zone has similar contribution to the present oil and gas reservoirs.The SB7 fault zone is dominated by late Caledonian hydrocarbon charging,and its contribution is 81.71%.Overall,the eastern Shunbei area is dominated by the later episodes of hydrocarbon charging,while the western Shunbei area is dominated by the early episode of hydrocarbon charging The difference of hydrocarbon charging stages,the different activities of strike-slip faults and the evolution of source rocks are the main factors leading to the differential enrichment of oil and gas in different fault zones. |
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