| Tazhong area in Tarim basin, with both oil and gas, contains over 5 billion tons of hydrocarbon resources, and Ordovician carbonate reservoir is the main target for exploration in this area. Both reef flat complexes in Lianglitage Formation and karst weathering crust reservoir in Yingshan Formation of Upper Ordovician are distributed in wide layered manner and contain hydrocarbons on the whole, although the heterogeneity of reservoir is intensive. Tazhong area shows obvious multiple hydrocarbon accumulation characteristics, such as multiple source rocks, multiphase accumulations, and multilayer oil-bearing, with complex reservoir–forming process. Therefore, it is of great significance to study the hydrocarbon accumulation characteristics and reservoir forming process for guiding the exploration in this area.It is indicated by study and analysis of a large amount of seismic, logging, drilling, core and geologic data that:①Ordovician oil and gas present different physicochemical properties depending on areas: the natural gas production is high in reservoirs near to Fault Belt I, and decreases gradually towards internal belt.②Various geochemical parameters indicate that, natural gas in Tazhong area belongs to pyrolysis gas of Cambrian crude oil, and crude oil is generated from Cambrian and Ordovician source rocks. At the early stage, natural gas was originated from Middle-Lower Cambrian high maturity crude oil pyrolysis gas. Condensate gas reservoir went through three periods of generating process: Middle-Late Caledonian and late Hercynian crude oil filling, and Himalayan gas filling. The large-scale ingression of Crude oil pyrolysis gas sourced from Cambrian into early stage residual oil reservoir in Himalayan period resulted in Ordovician condensate gas reservoir. Multi-source and multi-phase reservoir forming, multi-stage adjustment and oil-gas migration pathway sharing are the root causes of source mixing for crude oil. Two sets of source rocks and three-stage hydrocarbon expulsion provide abundant resource potential for Ordovician oil-gas accumulation in Tazhong area.③Tectonism controls the karst developing, fracture forming, hydrocarbon generating and migration, and damage to reservoir at later stage in Tazhong area fundamentally. It is considered that the factors for forming of Ordovician carbonate karst reservoir in Tazhong I slope break belt include paleoclimate, palaeogeomorphology, sedimentary facies, lithology and diagenesis, other than Tectonism. The mesh oil/gas transportation system formed by fractures, unconformity and seam-cavern development belt results in the universality of multiple migration and accumulation of palaeohigh in Tazhong area, and the palaeohigh background with long term stable development is the foundation of large area hydrocarbon accumulation of Ordovician in Tazhong area.This article discusses the four key factors that determine the Ordovician oil/gas accumulation in Tazhong-I slope belt break: Cambrian-Ordovician source rocks provide sufficient oil/gas resources; carbonate seam-cavern system controls the distribution of oil/gas; fractures, unconformity and seam-cavern system form the transportation system for oil/gas migration; palaeohigh background developed stably in long term provides favorable conditions for oil/gas accumulation. Ordovician reel flat complexes and Lower Ordovician Yingshan Formation karst weathering crust are two recent realistic exploration fields with quite abundant oil/gas resource potential.
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