Paleogene Oil And Gas Accumulation Stages And Reconstruction In Mahaidong Area,Qaidam Basin

The Qaidam Basin is a Mesozoic-Cenozoic poly-deformed hydrocarbon-bearing sedimentary basin.The northern margin of the basin is adjacent to the Gaxunnuoer Jurassic hydrocarbon-bearing depression,exhibiting typical characteristics of exogenous source accumulation.Previous studies have indicated that this region has undergone multiple adjustments and transformations,indicating enormous potential for hydrocarbon exploration.The Mahaidong area contains multiple phases of composite reservoirs(E₃,N_1,and N₂).Currently,the timing and mode of hydrocarbon accumulation in the Jurassic Lulehe Formation reservoirs in the Mahaidong tectonic zone remain unclear,and the understanding of the relationship between pressure evolution and accumulation law is insufficient.To address the problems of overpressure genesis and evolution,as well as the unclear relationship between hydrocarbon accumulation and evolution,this study focused on the Lulehe Formation reservoirs in the Jurassic strata,using a combination of overpressure logging curve analysis,loading-unloading curves,and analysis of the relationship between acoustic velocity and density to analyze the overpressure genesis.Comprehensive analysis of logging,stratigraphic testing,and geochemical data was conducted to investigate the pore evolution process in the Jurassic Lulehe Formation reservoirs.Through fluid inclusion petrography analysis,fluorescence spectral analysis,and the determination of the timing and duration of reservoir preservation under different conditions,the hydrocarbon accumulation process in the Lulehe Formation reservoirs was determined.The reservoir of the Lulehe Formation is mainly composed of fine sandstones,which are moderately to poorly sorted with sub-angular to angular clasts supported by either grain or matrix.Mineral contacts are mainly characterized by point,point-line,and line contacts,and the pores are cemented by both pressure solution and matrix.The matrix content is less than 20%in the former and 25-35%in the latter,exhibiting a basal-style cementation.The sandstone filling material is mainly composed of clay minerals with a high content and some authigenic minerals such as calcite,siderite,ferroan dolomite,rhodochrosite,and pyrite with a low content.In addition to high clay mineral content,the siltstone also contains high amounts of carbonate minerals such as calcite and dolomite.The three types of hydrocarbon inclusions with different fluorescence colors exhibit main peak wavelengths of 528-550 nm（bright yellow oil inclusions）,500-528nm（pale yellow oil inclusions）,and 477-495 nm（blue oil inclusions）,respectively.The homogeneous temperature ranges of the pale yellow fluorescence oil inclusions in the samples from the Lulehe Formation reservoir in Well Shangu 1 are 62.0-69.0℃,and those of the natural gas inclusions are 130-138℃.The homogeneous temperature ranges of the pale yellow to light blue fluorescence oil inclusions are 71.3-78.7℃.In Well Shangu 101,the homogeneous temperature of the bright yellow fluorescence oil inclusions is 61.5℃.In the newly drilled lateral Well Shan 3,pale blue fluorescence oil inclusions are developed in the Lulehe Formation reservoir,with homogeneous temperature ranges of 60.3-73.5℃,and corresponding same-period brine inclusions have homogeneous temperature ranges of 103.0-109.0℃.The Jurassic source rocks in the Gaxi Depression entered the hydrocarbon generation and expulsion period during the Oligocene sedimentary period.At the end of the Oligocene,the Jurassic source rocks in the Gaxi Depression entered the low-mature to mature thermal evolution stage.The three high-temperature peak distribution intervals of the oil and gas in the Lulehe Formation in the Shangu1 and Shangu101wells in the Mahaidong area correspond to the late Oligocenδe and late Miocene periods.The first period of oil and gas accumulation was related to the second period of structural movement,while the second period of oil and gas accumulation and filling was controlled by structural adjustment.The Lulehe Formation reservoir in the Shang2well in the Mahai Dong area was formed in the late Oligocene period,and the migration of oil and gas was mainly controlled by the third period of structural movement.The period of Lulehe Formation reservoir formation in the Shang3 well was in the late Miocene.In typical wells in the Mahaidong area,the mudstone section is obviously affected by under-compaction and overpressure,but the reservoir and caprock in the Shang3 well are not well-matched.The trapping pressure of the Lulehe Formation reservoir increases with the increase of the uniform temperature,indicating that as the burial depth increases,hydrocarbons gradually enter the reservoir,the geothermal gradient gradually increases,and weak overpressure is mainly developed in such reservoirs.After the crude oil is first transported out of the source rock under the drive of overpressure,it enters nearby reservoirs through fault zones and forms overpressure oil reservoirs.A small amount of reservoirs are weakly overpressured due to the influx of overpressure hydrocarbon fluids.The Mahai Dong area mainly exhibits a two-stage pressure increase characteristic,which is caused by the oil and gas charging during the Oligocene period and shows moderate overpressure.The pressure in the reservoir further increased due to the oil and gas charging during the late Miocene period,but the remaining pressure decreased.Due to uplift and erosion,the reservoir shows pressure depletion characteristics.The migration of oil and gas entered the Ma-Bei overthrust belt along the Gaxi Depression,with the early main direction of charging towards the southeast of the Ma-Bei overthrust belt.During the Middle-Late Miocene sedimentary period,the Jurassic source rocks in the Gaxi Depression entered the main oil generation period,corresponding to the main hydrocarbon generation period.At this time,the trap inheritance development was finalized,and mature oil and gas migrated vertically along the Ma-Xian and Lvnan fault systems,forming structural and structural-stratigraphic oil and gas reservoirs in favorable traps.After the oil and gas were accumulated,the Pt-E_1-2l abnormally high pressure system was formed,which was conducive to the preservation of oil and gas reservoirs.This period was also the main period of oil accumulation in the study area.Currently discovered oil and gas reservoirs in the Gujin Formation of the Shangu 1 and Shangu 101 wells in the Ma-Hai Dong structural belt,and the lower section of the Xiaoganchaigou Formation in the Ma-Bei oilfield were all accumulated during this period.In the Late Miocene,due to strong structural compression,the Green Liangshan fault was broken and the ancient structural high point migrated to one side of the Green Liangshan.The primary oil reservoirs of the Pt and E_1-2l units were adjusted and reformed,and oil and gas continued to charge into the reservoirs.