Comparison Of Reservoir Geological Conditions Of Ordovician Carbonatite Between Tazhong And Tabei Palaeohighs

The Tarim basin was rich in hydrocarbon resource. In the Tazhong and Tabei area,Wehad made important progress in the exploration of oil-gas in Ordovician carbonate reservoir.This thesis from the view of the key element of hydrocarbon reservoir, combing withstructural geology, sedimentary petrology, organic geochemistry, etc. On the basis ofwell-understanding on the accumulation process of petroleum in Ordovician carbonatereservoir in Tazhong and Tabei area, to comparison the petroleum accumulationconditions.Through our research, we found there were some differences and similarities in thepetroleum accumulation conditions between Tazhong and Tabei area in Ordoviciancarbonate reservoir, such as tectonic movements, hydrocarbon source rocks, reservoircharacteristics, cover characteristics, migration pathway, pool-forrning stages, and soon. Research shows some points are as follows. First of all, In the tectonic, theTazhong paleo-uplift was built in middle Caledonian Orogeny and became into a hugeanticline structure due to the different periods of tectonic movement. The activitiesbecame weaker in the late Caledonian tectonic movement, but the paleo-uplift wasstill risen. The Tabei paleo-uplift was formed in the early Hercynian, it was a hugenose structure at the beginning of the tectonic movement with a feature of lying northto south. But this structure was turn into dome structure with lying south to northunder the influence of the late tectonic movement. Second, In terms of hydrocarbonsource rocks，there are two sets of hydrocarbon rock developed in middle-lowerCambrian and middle-upper Ordovician in Tazhong and Tabei area. The middle-lowerCambrian source rocks have a high organic matter abundance and thermal evolutiondegree, reach to a mature to over-mature period. But there are difference betweenTazhong and Tabei area in the development phase of middle-lower Cambrian’shydrocarbon source rocks. In Tazhong area it was mainly developed hydrocarbonsource rocks in lagoon facies and which was mainly gypsum rock. Tabei area’shydrocarbon source rocks were mainly developed in platform margin facies andwhich were mainly grey dolarenite and dark grey mudstone in the sourthernzone.Tabei area’s middle-upper Ordovician hydrocarbon source rocks were Lianglitage formation deposition in platform margin and which was mainly dark mudrock. The abundance of organic matter is high in Tazhong area,containing averageTOC of0.5%to5.54%and type Ⅰ andⅡ－Ⅲ organic matter. The abundance oforganic matter is higher in Tabei area. Third, in terms of reservoir, the matrix ofOrdovician carbonate reservoir has the characteristics of low porosity and lowpermeability in Tazhong and Tabei areas,but the ordovician carbonate reservoir stillhas high reservoir performance because of the transformation of karst and tectonicfractures on reservoir space in varying degrees of diagenesis. But the interval in thesetwo kinds of reservoir were different.Tazhong area were mainly deposition calcitedolomite、dolomite in lower and middle ordovician Yingshan formation which weredeveloped in the local plateau facies and bioclastic limestone,bioclastic limestone、micrite and biohermal limestone in the upper ordovician Lianglitagezu formationwhich were developed in the high energy reef flat facies. Tabei area were mainlydeposition biomicrite and calcarenite in middle Ordovician Yijianfang formationwhich were developed in the open platform facies. It should be pointed out that thereis difference between Tazhong and Tabei area in the dimension and issue ofweathering crust karst reservoir. In Tabei area, the ordovician carbonate rocks’karstification mainly in early hercynian and late hercynian, the second is in the middleCaledonian. In Tazhong area, the karstification was strongly in the middle caledonianact late, late caledonian and early hercynian. Fourth, in terms of cover distribution, inTazhong and Tabei area developed a large set of mudstone, marl and lime mudstone inthe middle and upper Ordovician, and offered a good condition for the karst limestonereservoir which was reconstruction by karstification. Fifth, Under the influence of thetectonic evolution and sedimentary diagenesis, Ordovician trap types are mainly reefand Karst-Vuggy lithologic traps, and fault block-denudation monadnock-vugcomposite type traps in Tazhong and Tabei areas. Sixth, Hydrocarbon migrationpathway system contain three kinds of categories, such as unconformity type,fracture-crevice type, and transporting layer type. Tazhong and Tabei areas weregenerally migrated in vertically, supplemented by lateral migration. When the faultcutting hydrocarbon source rocks, the pathway system and configured to form aneffective source rocks, the oil and gas was migration to the effective traps through thenetwork pathway system, eventually forming reservoirs. Seventh, because of theinfluence of multiphase tectonic movement in Tazhong and Tabei area, oil and gasreservoirs both experienced impact reconstruction or destruction, and superimposedeffect. In Tazhong area, hydrocarbon reservoir in the Early and late Hercynian was damaged by igneous rock and fracture. Finally, after the light hydrocarbon filling intothe Cambrian and lower Ordovician source rock in Yanshan period, the hydrocarbonreservoir was renovated. Dry gas, crude oil cracked into gas in the lower HimalayanCambrian and condensate gas reservoir, the upper Ordovician condensate oil and gasfilling, accumulation reservoir in the late Himalayan. Tabei area’s hydrocarbonreservoir was damaged in early hercynian, after the filling of mature oil and gas forma type of heavy oil reservoir which was mainly ordovician karst reservoir in the latehercynian-indosinian period. And the heavy oil reservoirs in Himalayan shapeinversion locally with the high overripe filling of oil and gas accumulation andadjustment.Generally speaking, the existing exploration achievement has already confirmedthat the carbonatite of Ordovician in Tazhong and Tabei area both have good oil-gasexploration potential. They have the advantages in made oil-gas accumulation, such asthese areas were next to the hydrocarbon soure rock, developed a mount of faults anduncomformity surface, with the stronger karstification renovation and developed ahuge regional-local seal. But comparatively speaking, the Tabei area were sufferdmuch more than Tazhong, with much more paleo-karstification stages,longerexplosion and wider distribution area, the late dome structure was beneficial topreserve the petroleum and natural gas.