According to the different geometries and reflected characteristics in the seismic sections,the carbonate platform margin of the northern slope can be summarized as three basic depositional architectures in the Late Ordovician Lianglitage(良里塔格) Formation of the Tazhong(塔中) uplift.The type one mainly located in the west of the carbonate platform margin,and it showed obvious imbricate progradation from the interior to the margin of the platform.The type two was in the middle of the carbonate platform margin,which showed retrogradational stacking pattern in the same transgressive systems tract period,and the slope strata of the platform margin showed progradational sequence in the highstand systems tract period.The type three located in the east of the carbonate platform margin,and it showed the parallel aggradational architecture.The crossing well section along the northern slope of the Tazhong carbonate platform showed that the depositional thickness became thinner from the east to the west.The thickest belt located in the east of the platform margin,and became thinner rapidly towards the basin and the platform interior.These indicated that the paleogeomorphology of the Tazhong uplift was probably high in the west and low in the east during the period of the Late Ordovician Lianglitage Formation.According to the interpretation of seismic profiles and the computer modelling result,the depositional architectures of sequence O 3 l-2 showed aggradation,retrogradation and progradation from the east to the west of the carbonate platform margin during the transgression period.This meant that the accommodation became smaller gradually from the east to the west along the northern carbonate platform margin of the Tazhong uplift.The difference of the accommodation was probably caused by the difference of tectonic subsidence.Also,computer-aided modelling can be used to deeply understand the importance of various control parameters on the carbonate platform depositional architectures and processes.
Twenty unconformities,primarily superimposed types,were identified based on interpretation of a 46 000 km seismic profile combined with data from over 40 drilling wells.These respectively correspond to the main tectonic evolution stages and the boundaries between those stages.Reconstruction of the original depths of eroded strata was conducted for the Middle Caledonian,Early Hercynian,Late Hercynian,Indosinian,Early Yanshanian and Late Yanshanian unconformities using the virtual extrapolation of seismic reflection.Eroded strata thicknesses were also calculated for individual periods and intervals.Based on the reconstructed data,in combination with data from research on sedimentary facies,a paleogeomorphological profile was constructed for different tectonic evolution stages of the basin during the Early Paleozoic.The profile indicates the presence of obvious regularity in the temporal and spatial evolution of these unconformities.Based on the characteristics of paleo-uplift evolution and post-layering reconstruction,the paleo-uplifts were divided into inherited(e.g.,Tazhong(塔中)),residual(e.g.,North Tarim) and active(e.g.,Southwest Tarim and Bachu(巴楚)) types.The huge North Tarim uplift represents a typical form of residual paleo-uplift.The Paleozoic strata in the upper layers of the uplift is in poor condition for reservoir accumulation and preservation;however,the Upper Mesozoic-Cenozoic structural layer can form a secondary reservoir that is relatively rich in oil and gas.Furthermore,the flank slope area of the uplift is always a key source for oil and gas collection and the most advantageous position for the formation of the original reservoir.The Tazhong paleo-uplift has been stable since its formation in the Late Ordovician,where petroleum accumulation has been distributed not only in the uplift,but also in the deep and slope belts of the uplift.Important breakthroughs in petroleum exploration of the slope break in the North Tazhong area dating back to the Paleozoic have further confirmed the enrichme
