Tectonic Feature And Reservoir Accumulation In The West Of Tabei Uplift

Tabei uplift, which is a typical buried uplift characterized by superimposition ofPaleozoic Cratonic platform and the front part of Meso-Cenozoic foreland basin, hasexperienced multi-period complex tectonic evolution and formed a combination ofMeso-Cenozoic gently tilting structural layer in the upper and Paleozoic foldedstructural layer in the bottom. Luntai Salient, located in the north of Tabei uplift, is thefront uplift of Kuqa depression as well as the top of lower Paleozoic part of Tabei uplift.There is great difference in layer and structure characters between the eastern sectionand the western section of Tabei uplift. The eastern section is unified uplift structure;yet the western section develops multiple independent small-scale salient. ThePaleozoic in the west is comparatively complete with obvious reduction of thethickness of the overlaid Jurassic-Cretaceous layer and hereby it is the proof that thedevelopment of uplift structure is in the pre-Jurassic.Tabei uplift in Tarim Basin has the richest oil and gas, yet its western section is themain oil-gas enrichment area and oil-gas reservoirs have been found in nine series ofstrata and highly favorable hydrocarbon accumulation conditions are characteristic of agreat many exploration targets, plenty oil bearing formation, rich hydrocarbon reservoirtypes, and so on in this section, hereby this section is indeed a realistic black-oilreservoir of increasing reserves and production. Because of fierce tectonic movementand serious strata denudation in this section, current oil&gas exploration is facing thefollowing challenges:①the understanding of hydrocarbon distribution regulation andcomplex reservoir-formation model in the west;②the research on effectiveidentification of formation and reservoir prediction technologies aiming at complexburied hill;③the study of the relationship between superimposed reformation ofmultiple tectonic movement and hydrocarbon accumulation. Hence,on the combination of the structural characteristics and tectonic evolution in the west, the analyses on maincontrolling factors of carbonate reservoir development and the research on reservoirseismic prediction technologies, it is of great value on theory and practical meaning toanalyze carbonate reservoir-formation model and distribution regulation and determineexploration direction and favorable zone in the west of Tabei uplift.The analyses of structural features indicate that the western section of Tabei uplifthas experienced four periods of tectonic evolution: slope stage during theSinian-middle Ordovician, thrust stage during the end of late Ordovician to Triassic,inversion stage during the Jurassic-Paleogene and reforming stage during theNeogene-Quaternary. Tectonism in different evolution period has a close relationshipwith reservoir development, trap formation, hydrocarbon migration and accumulation.On the whole there are two major structural layers including the Paleozoic and theMesozoic-Cenozoic. Regional unconformities of the bottom of Jurassic or Cretaceousformation can be found between the two layers. Tectonic feature of the Paleozoic issouth-inclined single-slope configuration but that of the Mesozoic-Cenozoicnorth-inclined single-slope configuration, thus the two layers form a “入”-shapedstratigraphic framework which lays the foundation for oil gas accumulation in bothnorth and south sides; the formation of carbonate buried hill creates the condition forweathering curst karstification due to strata denudation during the uplifting of the northsection of the Paleozoic; multiple stratigraphic unconformity and continued activitiesof fault not only cause the development of traps of different types which are superposedvertically in different tectonic zone but also provide transporting pathway for lateraland vertical hydrocarbon migration.The analyses of reservoir characteristics indicate that the Cambrian-Ordoviciancarbonate of the western Tabei uplift is mainly composed of dolomite and limestoneand develops weathering crust reservoir in burried hill and inner buried hill reservoirs.The development of weathering crust reservoir occurs in the middle Cambrian, upperCambrian and lower-middle Ordovician; most inner buried hill reservoirs distribute inYijianfang Group and Yingshan Group of the Ordovician. The type of reservoir spacemainly includes solution pore and fracture of which the seismic reflection featureappears as the shape of “string beads” in the inner slope area. Reservoir developmentprincipally depends on lithofacies and lithology thus it is buried hill weatheredcrustkarstification and late structural fracture that greatly improves carbonate reservoirproperty.The analyses of reservoir prediction technique indicate that complicated buried hill structure is highly complicated in the west of Tabei uplift. By opening time windowdownward from the overlying mudstone of Cretaceous Kapushaliang Group, withseismic facies and seismic multi-attribute clustering analysis technique, identificationof and effective decision on various lithology of buried hill clarifies the distributionrange of Cretaceous underlying formation and its importance, especially it is shownthat buried hill hydrocarbon leakage is resulted directly by the distribution of theJurassic. Hence, the distribution of favorable reservoir mostly appears in the upperCambrian dolomite buried hill, the lower Ordovician marlite dotomites on the north ofYm7well and bitumen-bearing sandstone member in Kepingtage Group of Silurian.By all there analysis results above, the western Tabei uplift is a compound oil andgas accumulation zone which is divided into buried hill weathered crust oil-containingsystem in the north and inner buried hill oil-containing system in the south. Owing tothe difference of denudation degree and vertical fault activities in various tectoniczones, the step migration-accumulation of oil and gas occurs along paleostructures,unconformities and faults; various oil and gas bearing series groups develop verticallywith superimposed distribution. Overall the reservoir formation in the west ischaracteristic of hydrocarbon supplying mode of bi-direction oil source from thePaleozoic Manjiaer marine sag in the south and Mesozoic and Cenozoic Kuqadepression in the north, fluid passage utilizing many ways of unconformities and faults,hydrocarbon enrichment in multiple vertical overlap layers.The distribution of oil and gas is generally in the high position of each structuralbelt and slope, but mainly in the paleo-uplift and the high position of slope which isclose to reservoir growing area of hydrocarbon transportation system and frequentlyhas regional caps overlying carbonate rocks. The distribution of favorable carbonateexploration area is mostly in Ym7-32structural zone of the northern buried hill, Yahafault structural zone, and Ym2structural belt and Halahatang area of the southerninside region. In the north, the Ordovician of Yudong lower uplift is the target forfurther exploration however the Cambrian of buried hill northern slope in Yaha faultstructural zone also deserves attention; in the south, bounded by the Ordovicianpinch-out boundary of Sangtamu Group, the segment of Yijianfang Group andYingshan Group around the north and south sides is the target for further exploration ofthe Ordovician inside region.