The Tarim basin experienced a complex tectonic evolutionary history from Sinian to Cenozoic.Eight large-scale and more than 20 subordinate unconformities defining tectonosequences of different protobasins formed in various tectonic settings have been identified within the Phanerozoic in the Tarim basin,their distribution determining the general characteristics of sequence stratigraphic framework of the basin.Tectonostratigraphic unit I(magasequence) consists mainly of the Sinian System,which formed in a rift or aulacogen setting and can been subdivided into two subordinate stratigraphic units(supersequences).Unconformity(T g9) between Sinian and Cambrian with surface karstification is regarded as a post-rift unconformity.Tectonostratigraphic unit II comprises the Cambrian and the Ordovician and can be divided into six subordinate tectonostratigraphic units,recording the tectonogeographic evolution of the prototype basins from Cambrian to Early Ordovician passive carbonate continental margin or cratonic depression and the Late Ordovician submarine to neritic retroarc foreland and cratonic depressions.The tectonic uplift related to the formation of the unconformity T g5-2 resulted in the remarkable change in basin tectonic setting from a passive divergent to an active convergent,with the development of the Tazhong(塔中) uplift,the Tangguzibasi(塘古孜巴斯),and the northern depression at the end of the Middle to the early Late Ordovician.The widespread angular unconformity T g5 formed by a relatively strong compressive deformation,which caused an abrupt tectonogeographic change of the basin from abyssal to a neritic setting in response to the collision and associated tectonic deformation of the North Kunlun(昆仑) orogenesis during the Late Ordovician to the Early Silurian.Tectonostratigraphic unit III is composed of the Silurian and the Lower to Middle Devonian and characterized by the development of fluvial or deltaic and clastic littoral and neritic deposits.Large-scale terrigenous clastic depositional wed
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.
The Ordovician of the Tazhong area in the Tarim Basin has suffered multi-cyclic hydrocarbon charging, making Tazhong a typical condensate gas district. In this paper, production and test data were gathered and a detailed comparison was conducted on the geology and the fluid distribution and characteristics between the eastern and western Tazhong area. Eastern and western regions exhibit significant differences in tectonic structure, fluid distribution, and physical-chemical properties of oil and gas. Compared with the eastern region, the western part has a greater development of discordogenic gas associated with strike-slip faults which, combined with the Tazhong No. 1 fault zone, control the fluid distribution. The eastern region is mainly controlled by the Tazhong No. 1 fault zone. Fluid have markedly homogeneous properties in the east, but are heterogeneous in the west. The origins of oil and gas are different between the east and the west. In the east, hydrocarbons are mainly from Ordovician source rocks and natural gas is mostly derived from kerogen pyrolysis. In the west, the hydrocarbons mainly originated from Cambrian source rocks, and the gas was mostly generated by crude oil cracking. In sum, the east region is dominated by primary condensate gas reservoirs, and the western region is dominated by secondary condensate gas reservoirs. Because of the different geological settings and fluid physical properties, differences in the condensate gas reservoirs in the eastern and the western Tazhong area have been analyzed, and appropriate formation mechanisms for condensate gas origins are established.
