Hydrocarbon Accumulation Period Of Triassic Reservoirs In Lunnan And Surrounding Area Of Tarim Basin

Lunnan and surrounding area is a rich composite hydrocarbon accumulation region with multi-layers of oil and gas reservoirs in the Ordovician,Carboniferous and Triassic strata.There are some disputes about hydrocarbon accumulation periods of Triassic reservoirs in the study area.In this study,based on the collected data and sampling tests from Lunnan and Sangtamu Oilfield,the burial history and thermal evolution history were simulated with basin modeling technique.Hydrocarbon accumulation period and time of Triassic reservoirs in the study area were discussed with geological comprehensive methods macroscopically and qualitatively,including studies of reservoir saturation pressure,oil/gas water contact retrospecting,and hydrocarbon generation history etc.Microscopically and quantitatively study hydrocarbon accumulation period and time includes fluid inclusion analysis and K-Ar dating of authigenic illite.Hydrocarbon accumulation process of Triassic reservoirs was remodeled based on tectonic evolution,fault activity history and hydrocarbon migration analysis.Simulation of the burial history and thermal evolution history indicate that Lunnan and surrounding area experienced several times of tectonic uplift with varied degree of extensive strata erosion,and organic matter of the studied area shows lower degrees of thermal evolution than that of depression,due to the higher structural position The major hydrocarbon generation and expulsion phase of middle-upper Ordovician source rock is Late-Hercynian epoch in the Manjiaer depression where most amount of hydrocarbon were generated.Based on former study of trap formation history,reservoir saturation pressure and oil/gas water contact retrospecting,hydrocarbon accumulation period of Triassic reservoirs is qualitatively identified as no earlier than Late-Yanshan to Early Himalayan and no later than Late-Miocene,which is much later than hydrocarbon generation period of source rock,indicating secondary characteristics of hydrocarbon pools.Fluid inclusions of Triassic reservoirs in the studied area mainly occur in the micro-fissures of quartz.The hydrocarbon inclusions are bright yellow to yellowish,which indicates a medium maturity of hydrocarbon.Homogenization temperatures of associated saline inclusions show a unimodal distribution characteristic with the peak values.It is considered that one period of hydrocarbon charging occurred in the Triassic reservoirs in the Miocene.Combining with data analysis of K-Ar dating of authigenic illite,hydrocarbon accumulation period of Triassic reservoirs is quantitatively identified as Late-Paleogene to Miocene Kangcun period.Hydrocarbon accumulation periods of the tectonic units are similar,and those of Lunan and Sangtamu fault horst zones are a little earlier than those of other tectonic units in the studied area.Combining studies of fault activity history,oil neutral nitrogen compounds,and sealing ability evolution,hydrocarbon accumulation process of Triassic reservoirs is divided into three stages.During Early-Himalayan,oil was migrated vertically through reactived deep fracture from Ordovician paleo-oil reservoirs to Triassic strata,while no accumulation occurred due to poor sealing ability of Triassic strata.During Late-Paleogene to Miocene Kangcun period,secondary hydrocarbon pools were begin to formed due to better sealing ability.During Late-Himalayan,internally lateral and vertical migration of hydrocarbon occurred in the Triassic reservoirs due to conversion of dipping direction of strata from south to north,and dry gas charged into Triassic reservoirs in Jilake area only.