Hydrocarbon Accumulation Processes Of The Lower Paleozoic Deep Reservoirs In The Central And Western Part Of Tabei Uplift,Tarim Basin

Tarim Basin is a hot spot for deep hydrocarbon exploration in China.It has experienced multi-stage tectonic superimposition and transformation,and the process and distribution of hydrocarbon accumulation are very complex.Affected by the particularity of deep hydrocarbon accumulation process and complex dynamic mechanism behind it,the migration and accumulation mechanism of deep hydrocarbon is one of the core scientific problems of oil and gas exploration in Tarim Basin.Tabei uplift is one of the most important oil and gas exploration and development target areas in Tarim Basin.There are many oil and gas reservoirs,among which the Ordovician intervals hold the largest oil reserves and the hydrocarbon accumulation mechanism of Ordovician reservoirs is the most complex.This study discusses the deep hydrocarbon accumulation processes and establishes the deep hydrocarbon accumulation models through detailed analysis of the accumulation histories of the lower Paleozoic Ordovician reservoirs in three typical areas in the central and western part of Tabei uplift,namely the Yuqi-Aiding-Tuofutai area,Halahatang area and the Shunbei area.Based on the comprehensive application of numerical simulation,organic geochemistry,quantitative fluorescence spectrum of inclusions,homogenization-temperature measurement and isotope geochemistry,the main controlling factors and accumulation models of Ordovician reservoirs in the central and western Tabei uplift are summarized through the analysis of hydrocarbon generation history of source rocks,spatial variation of crude oil properties and oil charging history.The conclusions are as follows:(1)A thermal anomaly event caused by Permian Magmatism in Tabei uplift and Shuntuogole low uplift were identified.In this study,the thermal history of Ordovician deeply buried carbonate strata in Tabei uplift and Shuntuogole low uplift was simulated by combining carbonate clumped isotope(?₄₇)with calcite U-Pb dating,using the existing clumped isotope solid-state reordering kinetic models,and combining with the results of Easy%Ro simulation of bitumen equivalent vitrinite reflectance.The results reveal that there used to be a short background thermal anomaly event during the Permian in Tabei uplift and Shuntuogole low uplift.Based on the calculated erosion thickness and geothermal gradients during the thermal anomaly,it is considered that the activity of the Permian mantle plume may be the main cause of the background thermal anomaly event.Due to the complex tectonic evolution history,the actual temperature range of the peak burial temperature during the thermal anomaly caused by the Permian magmatic activity still needs to be confirmed with more methods and evidence.However,the results of this study show that the thermal effect of Permian magmatism should be paid attention to when conducting thermal history reconstruction and source rock generation history of Tarim Basin.(2)One dimensional and two-dimensional thermal histories and source rock generation histories of three typical areas in the central and western part of Tabei uplift were simulated,namely the Yuqi-Aiding-Tuofutai area,Halahatang area and the Shunbei area.The results show that there are three main hydrocarbon generation periods in the central and western part of Tabei uplift,including late Caledonian-early Hercynian period,late Hercynian period and Himalayan period.The late Hercynian period is the main hydrocarbon generation period,with the largest hydrocarbon generation amount and in most extensive spatial range.In general,the maturity of source rocks in the northern areas is lower than that in the south areas.(3)The main controlling factors and process of hydrocarbon accumulation in Yuqi-Aiding-Tuofutai area were analyzed.The physical properties and maturity of crude oil display gradual changing patterns from the north to the south,and these variation trends are closely related to the differential biodegradation levels caused by differences in burial depth during the late Hercynian period and the differences in the maturity levels of in-situ source rocks.At the same time,the differences in vertical petroleum charging periods and intensity caused by different fault re-activation histories affects the change of crude oil properties in some areas,for example in the YQX1 well.There are three to four stages of petroleum charging,including during the early Hercynian,late Hercynian,middle-late Yanshanian and the Himalayan.Among the Ordovician reservoirs in the studied areas,the late Hercynian charging event is the most widely distributed,the Early Hercynian and middle-late Yanshanian charging event is mainly distributed in Aiding area,and Himalayan charging event is recognized in Aiding and Yuqi areas.(4)The main controlling factors and process of hydrocarbon accumulation in Halahatang area were analyzed.The pattern of hydrocarbon accumulation in Halahatang oilfield can be summarized as"fault-controlled enrichment,in-situ hydrocarbon generation,vertical migration and multi-stage charging".The maturity of crude oil shows a decreasing trend from the north to the south on the whole,and regional fluctuation can be observed.The results show that the late Hercynian period is the main hydrocarbon generation period of source rock and also the main petroleum charging period.The differences in the maturity of in-situ source rocks may have led to the spatial changes in the maturity of crude oil in Halahatang area.Petroleum charging periods includes early Hercynian,late Hercynian,middle-late Yanshanian and the Himalayan,among which the late Hercynian charging episode is the main charging period,with the widest charging range and biggest intensity.The segmentation of faults and the differential re-activation histories of fault segments may have controlled the episodes and intensity of petroleum charging in different wells,resulting in the regional fluctuations in the properties of crude oil.(5)The main controlling factors and process of hydrocarbon accumulation in Shunbei area were analyzed.The hydrocarbon accumulation mechanism of Shunbei oilfield can be summarized as"early oil generation,early charging,maturation during deep burial and late-stage adjustment".Current drilling wells in Shunbei area are mainly distributed around the NNE trending SB 1 and NNW trending SB 5 strike-slip fault zones.The density,viscosity,sulfur content,resin and asphaltene content of crude oil in SB1fault zone are lower than those in the SB5 fault zone,and the maturity of crude oil is higher in SB1 fault zone.The present-day crude oil in SB5 fault zone was charged during the late Caledonian period.The oil charging period of SB1 fault zone started from the late Hercynian period and continued to the Himalayan period,in which the late Hercynian period is the main charging period.The differential re-activation of the two fault zones caused by the regional stress field is the main reason for the differential charging of crude oil,which directly determines the difference of crude oil properties in the two fault zones.(6)Based on the oil and gas accumulation history of three typical areas,the oil and gas accumulation model in the central and western part of Tabei uplift can be summarized as"source rock in-situ hydrocarbon supply,crude oil vertical charging,multi-stage charging and accumulation,strike slip fault wealth control".The source rocks of the lower Cambrian yuertus formation generate hydrocarbon in situ.Due to the different burial depth,the maturity of source rocks is different in the north and the south,and the maturity of source rocks in the south is higher than that in the north.There are three hydrocarbon generation episodes of the source rocks,i.e.late Caledonian-early Hercynian,late Hercynian and Yanshan-Himalayan.The late Hercynian is the main hydrocarbon generation episode,with the largest amount of hydrocarbon generation.Due to the spatial difference in maturity of source rocks,even during the same charging period,the maturity of crude oil in different regions is different,with the maturity of crude oil charged in the southern region higher than that in the northern region.The maturity of present-day crude oil shows an overall decreasing trend from the south to the north.There are three oil charging episodes,i.e.late Caledonian-Early Hercynian,late Hercynian and Yanshanian-Himalayan.The exact charging episodes and charging intensity in different regions are different.The differential re-activation history of strike-slip faults controls the vertical migration of crude oil,which ultimately determines the differences in crude oil properties in different areas.