Research On The Main Fault Systems And Their Implication To Petroleum Geology In Tazhong Uplift Area, Tarim Basin

Since the oil and gas exploration implementing comprehensive in the Tarim Basin, Tazhong Uplift had been the key region of exploration. Fault system is a basic structural elements of Tazhong Uplift, and predecessors have made some extensive and deep research, but there are still much controversy on faults staging and matching, dynamic background, tectonic evolution of fault system and its controlling of the tectonic framework of Tazhong uplift, and some studies are still weak such as the oil and gas geological significance of Tazhong fault system, particularly the control action of faults to the development of the carbonate platform and karst process. For solving these problems existing in researching Fault system of Tazhong Uplift, this paper carried out comprehensive and systematic study by using seismic, drilling and regional geological data.The fault system of Tazhong uplift can be divided into three groups:the northwestward of Tazhong1 fault, Tazhong22 fault, the Tazhong2 fault and Tazhong10; The northeastward faults contain Tazhong 8 well fault, zhong 3 well fault, Tazhong 3 well fault, Madong-tangbei fault and faults system in Tangguzibasi depression, and a group of NE strike-slip faults which pentrate into the basement; the nearly east-westward of Tazhong 5 fault. Through the accuratly interpretation of seismic profiles, this paper described the geometry and kinematics characters of the fault system in Tazhong uplift detailly, and divided the structural styles into compresso-shear tectonic structures, extensional tectonic structures and strike-slip tectonic structures according to the different of deformation mechanisms, and the compresso-shear tectonic structures is the most common.According to the degree of faults development, assemble patterns and complexity of geometric relationship, compresso-shear structures can be divided into multiple sub-types.This paper improved palae-throw method, proposed an new method which based on horizon flattening and selecting the marker bed to compute the vertical fault flow of main faults in the different active epoch.Result showed that the northwestward fault system is mainly actived from the end of Early Ordovician to the early of the Middle-Late Ordovician, and weakened after the Late Ordovician, however, the northeastward or eastwestward fault systems are mainly actives in the end of Late Ordovician, after Silurian succession actived weakly.On the basis of the systematic analysis of the structure style and the tectonic activity of the main fault belt, the coordination of the main fault systems was studied in the Tazhong Areat. The result shows that the fault system striking NW was formed during the Episodeâ… in the Caledonian movement at the last period of the early Ordovician. On the map, that is the broom-lie fault system which disperse westward and convergent eastward. The fault Tazhongâ… is the main fault and its west branch was more active. The fault Tazhong 22, recoiled to the Tazhongâ… , and the faults at the main horst zone consist the pop-up structure. The east branch of the Tazhongâ… was weaker with litter ramp and shows as the gentle anticline characterized by the faulted-bend fold. Therefore, the structure style of the Tazhong uplift is very different between the east and west branch. The faults striking NE or nearly EW were formed during the Episodeâ…¡in the Caledonian movement, located at the southern margin of the Tazhong and the Tagubasi depression. These faults consist of a northwestward arcuate zone and overlap on the fault system formed at the Episodeâ… of the Caledonian movement. The difference between the earlier tectonic framework directly lead to the different overlap style. The Yutangbei-Tangnan faults belt and the Zhong-3-Well fault in the western Tanggubasi depression, and the Madong-Tangbei faults belt and the Tazhong-8-well fault consist of the opposite thrust structure and formed the compressional opposite thrust depression. The east branch of the Tazhong 5 fault directly thrust up to earlier formed gentle anticline and composed the duplex thrust structure. Meanwhile, a serial NE fault, deep into the basement, cut the earlier NW faults system and some earlier NW faults showed as continued strike slip at later. After the Episodeâ…¡of the middle Caledonian movement, the activity of the faults in Tazhong became weaker and only a few faults at the southern margin and the Tazhong 2 and 10 fault showed litter compression at the main horst zone.The activity of fault system controlled the tectonic framework about Tazhong unlift. By using the geological extrapolation method to recover the erosion thickness of each period, combining with tectonic evolution sections technology, this paper compilated three tectonic evolution profiles that crossed the uplift and subsidence history analysis of imitation wells that distributed in key position of the uplit, and researched the evolution of tectonic framework and settling process which controlled by the fault system. Meanwhile, according to summarize the results of previous studies, combine with stratigraphic correlation aeromagnetic and rock dating data, This paper analysed the basal structure of the Tarim Basin and its affections to the later deformation, researched the tectonic evolution of the three orogenic zone around the basin, studied the genetic connection between the evolution of orogenic zone and Tazhong fault system, based on, This paper determined the tectonic evolution and the uplift-depression framework processing changes about the uplift as follows:Sinian-early Ordovician period, the basin is surrounding with paleo-oceans and Tazhong area is extensional setting, commonly developing stretching structure. The basin is revealed it is a north dipping slop; At stageâ… of Caledonian movement of the end of the early Ordovician, large-scale Kunlun paleo-ocean was subducted and shortened, generating Taxinan peripheral foreland basin. In addition, stresses of Tazhong were transformed from stretching to compression. Thrust zone of foreland basin is characterized by front-edge northwest dipping faults and fractures system. The difference of faulting activity controls different characteristics of north-east trending tectonic framework, i.e. pop-up structure are developed to the west, and to the east it is low amplitude anticline, dipping toward north and deformation zone decreasing The uplift divided originally unity of depression into two relatively small-scale depressions, Tanggulazi depression and Manjiaer depression. Tectonic framework of Tazhong area inherited style of early and middle Ordovician period. Northwest trending front-edge faults systems were further developed and uplift of Tazhong area is ongoing, resulting to persistent subsidence of Tangguolasi and Manjiaer depressions. In all, uplift is flanked by two depression gradually shown there. The end of the Late Ordovician is Caledonian stageâ…¡movement, southeastern basin margin Aerjin paleo-ocean is closed and orogeny is commencing. Northeast or East-west trending arc-shaped faults system are appeared. This system consisted with Gucheercheng-Tangguolabasi piedmont tectonic belt, Yutianbei-Tangnan faults and Zhong well 3-Tazhong No. 5 faults front edge foreland faults system. Tanggubasi depression was broke up and north segment of Tazhong uplift rose dramatically, giving rise to tectonic framework transformation from west-high-east-low to east-high-west-low. Meanwhile, subsidence rates of Manjiaer depression, lie to northern Tarim basin, are reduced. In addition, Gunlun ocean was closed leading to development of Taxinan peripheral foreland basin. Compression tectonic stress from the southwest and southeast led to strike-slip in Tazhong area and Northeast trending strike-slip system are appeared there, the previous Northwest trending faults also acted as inherited strike-slip fault. Uplift is stable; Silurian- late Devonian is Late Caledonian-Hercynian tectonic movement, subduction of South Tianshan ocean, located in northern basin margin, is beginning. Whereas East Kunlun ocean, located in southeast basin margin, begin to close with orogenic collision. Faults in Tazhong area are weak due to weak distance effect. Only a few faults in the South edge of Tazhong, northeast trending strike-slip faults, as well as faults on the main horsts experience twisting stress, uplifts continue to rise and eventually uplifting; After late Devonian, paleo-ocean surrounding Tarim basin were gradually closed and at early Triassic period Tarim block experience intro-continent orogeny. Faults in Tazhong area stop development and occasionally strike-slip is generated. Uplift and subsidence of Tarim basin results to disappear of uplift.The karstification of carbonatite is an important backgroud for reservoir development in Tazhong, the development of fault system and the evolution of uplift which were controled by the fault system was one of the important factors that controedl the development of carbonate platform and its karstification. An more in-depth analysis have been taken in the final of this paper, the models of carbonate platform and its karstification development have been taken in this area. The Tazhong uplift Area Carbonate platform had been experienced three different evolution models in construction period from Cambrian to the early of Middle-Late Ordovician: Early-Middle Cambrian, ramp carbonate platform developed which trend to northeast; Late Cambrian to Early Ordovician, rimmed shelf carbonate platform developed which was controlled by hydrodynamic conditions and sea level changes; The end of Early Ordovician to the eraly of Middle-Late Ordovician, controlled by the northwestward thrust fault system which associated with the West Kunlun orogenic belt, developed isolated reef platform in Tazhong uplift Area. The end of Middle-Late Ordovician, controlled by the northeastward thrust fault system associated with Arkin orogenic belt which led the platform and depression lifting,disintegration and destruction.In the process of evolution mentioned above, three major tectonic movements had been controled the horizon and the scope of karstification, and showed different characters in the different tectonic unit. Comprehensive analysed all factors, this paper established three different karst model.