The Feature Of The Major Ordovician Unconformities And The Evolution Of The Palaeogeomorphy In Tazhong Uplift Of Tarim Basin

Further study on the geological conditions and main controlling factors of hydrocarbon accumulation of marine carbonate, especially the study of the feature of the major Ordovician unconformities and denudation thickness recovery, paleotectonics, paleoenvironment and their evolution, are critical to find and explore prospective petroliferous regions. There are several significant aspects in this paper included as follows:1.This article summarizes the identifications features and the distribution style of the major Ordovician unconformities and preliminary discusses the cause of the unconformities. Five large unconformities are identified in ordovician strata of Tazhong area,namely the bottom of Penglaiba formation, the bottom of Yingshan formation, the bottom of Tumuxiuke formation and the bottom of Sangtamu formation. They corresponds to the seismic boundary of Tg6,Tg53,Tg52 and Tg51. The development degree of those unconformities has significant differences in different parts of Tazhong uplift. They can transfer from high angle unconformity to parallel unconformity and conformity.2. Aiming at the drowned unconformity(T_g5~1), this paper restores the paleogeomorphy of last stage of Lianglitage formation sedimentary period by using the method of filling and completing. In combination with single well facies analysis, the rules that paleogeomorphy effects on the carbonate sedimentary are as follows:the places where the strata is thicker represents the reef flat complex development position; the places with a lower terrain in the platform interior represents the depressions that develop in the platform; the grain banks mainly develop at the place with higher terrain in the platform interior.3. For the Tg52 unconformity, denudation is estimated by using the method of stratum structure related extension. The calculation results shows that the maximum denudation belt is controlled by the distribution of fault-uplift zone. In this period, the uplift amplitude of northwest part is higher than the southeast part of Tazhong uplift, and the uplift amplitude of the central fault horst zone is higher than the north and south sides of the slope.4.The tectonic evolution of Tazhong uplift can be divided into five stages, respectively corresponding to the sedimentary period of the late Cambrian, the late Cambrian Qiulitage formation and early Ordovician Penglaiba formation, the early Ordovician Yingshan formation, the early and middle Ordovician Yijiangfang formation and the late Ordovician Sangtamu formation. The Ordovician sedimentary evolution can be divided into four period correspondently.5.Based on the analysis of typical oil reservoir profile, this paper summarizes reservoir forming conditions and also illustrates the types and features of traps related to unconformities in Tazhong area. The distribution range of Reef flat facies reservoirs of Lianglitage formation and three favorable exploration targets of middle Caledonian karst reservoir are predicted.