The Major Unconformities And Structure Style And Evolution Of Tazhong Uplift In Tarim Basin

Tarim basin is a large oil and gas bearing basin in the western China, witha low prospecting degree and rich hydrocarbon resources. Tazhong uplift is a largeinherited uplift,which intersects the center of Tarim basin. This paper isattempted to investigate the development of Tazhong uplift, reveal its structuralfeature and evolutionary process, and to provide benefit guidance for hydrocarbonexploration. Study on the unconformities, denudation and uplift is a hot shot inthe analysis of oil and gas bearing basins. The study acquired some major development as flows:(1) Basing on the geologic information of seismic,well logs and crops, weanalyzed the features and patterns of the major unconformities. 3 first orderstructure sequence and 7 second structure sequence were distributed, whichrevealed the distribution feature of the high angle and superimpose unconformitiesand the low angle and truncated three-angle unconformities. The superimposeunconformities lay in the uplift zone and the truncated three-angle unconformitieslay on the lope on the fringe of the uplift.(2) By the geologic extrapolation methods, the denudation of the majorunconformities was estimated and the feature of the major structure phase wasilluminated. In Middle-Late Ordovician era, the maximum denudation about 1000mlay at the center fault zone, and the denudation decreased along the surrounding.In Silurian era, the maximum denudation about 1200m lay at the guchengxu nose atthe eastern tazhong uplift. The denudation decreased from the east to the westand from the center fault zone to the limb. In Late Devonian era, the maximumdenudation about 1500m lay at the guchengxu nose yet. The denudation decreasedfrom the eastern to the western. (3) Basing on the seismic, the feature and the patterns of the fault systemwas revealed. The patterns of flower structure, Y-fault, thrust block, back toback thrust fault, and ramp were developed in this zone. In the layout, the twoorientation faults, NW and NE, were converged as penicillate from the western tothe eastern. Moreover, the features of the faults were different in the sectionfrom the western to the eastern. In the western, the major pattern of the faultswas the flower structure. In the middle, the major pattern of the faults was thrustfault with high angles and involved base. Near the convergence of the faults, themajor pattern of the faults was ramp thrust. In the eastern and near the fifthfault, the major pattern of the faults was back to back thrust fault.(4) Basing on the analysis of the unconformities, the denudation and the faultsystem features, the evolutionary phase were distributed and the shape featureof the uplift in the different era was illustrated. In the Middle-Late Ordovicianera, many thrust faults and onlap unconformities developed. The center fault zoneupthrew and suffered denudation, forming the embryonic tectonics of the tazhonguplift. From Silurian to Early-Middle Devonian era, the faults converged towardthe eastern forming the brush structure. Many superimpose unconformities and onlapunconformities are developed. The eastern of Tzhong uplift upthrew and sufferedintensive denudation, tazhong uplift continue to develop. In Late Devonian era,the faults thrust forming deep and large faults. Many superimpose unconformitiesand onlap unconformities are developed. The eastern Tazhong further upthrew andthe denudation increased, Tazhong uplift falling into a pattern. In Carboniferousera, the tectonic was sustained and the sediments draped on the uplift formingthe composite anticline.(5) With the modeling program, the fractures in the Ordovician carbonaterock were simulated. The dip fracture was the main pattern of the fracturesdeveloped in the Ordovician carbonate rock in Tazhong. The orientation of thefractures was different form the north to the south. On the north of the 1st fault,the orientation of the fractures was submeridional. Among the 1st fault, the 7-8well fault and the 5th fault, the orientation of the fractures was NE. On the southof the 5th fault, the orientation of the fractures was NEE. The fracture wascompresso-shear.(6)By analyzing the conditions of the oil and gas accumulation and thefeatures of the unconformities, the ramp type and features in connection with theunconformities were revealed. There were 5 types of the trap related tounconformities developed in tazhong, which were the ovelap unconformity trap ofthe donghe sandrock, the trap of the Bachu claystone and the Ordovician ooliticlimestone on the superimpose unconformities, the trap of the bachu claystone andthe Lower Ordovician dolomitite on the superimpose unconformities, the truncatedunconformities trap of the Silurian red claystone and the underlay argulite, thetrap of the Silurian red claystone and the Lower Ordovician dolomitite on thesuperimpose unconformities.7 benefit traps were predicted.