--> Diagenesis and Origin of Porosity Formation of Upper Ordovician Carbonate Reservoir in Tazhong No.1 Slope Break, Tarim Basin

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Diagenesis and Origin of Porosity Formation of Upper Ordovician Carbonate Reservoir in Tazhong No.1 Slope Break, Tarim Basin

Abstract

A spectrum of condensate gas fields had been exploited from the reef carbonate reservoirs in Lianglitage Formatin of Upper Ordovician in Tazhong No.1 Slope Break since 2005. Additionally, the early paradigm for porosity formation in this Fm. stressed subaerial exposure and attendant shallow meteoric diagenesis in syngenetic period (Xinyuan Zhou, 2006; Zhenyu Wang, 2007; Zhaoming Wang, 2007). However, this theory above had been proved to be invalid during the exploration of oil and gas when it was applied to the northwestern No.1 Slope Break. Subsequently, numerous authors (Yixiong Qian, 2007; Yushan Sun, 2007; Hongqiang Ma, 2010; Bo Zhou, 2013) have interpreted deep-burial dissolution in carbonate reservoirs, and some have proposed that primary volumes of porosity were created in this manner. As Wayne M. Ahr (2008) and other argued, carbonate pore types are formed by depositional, diagenetic, or fracture processes such that the spatial distribution of porosity may or may not conform to depositional facies boundaries and pores may be formed or altered by diagenesis and brittle fracture. Therefore, a comprehensive origin of reservoir should be taken into consideration. Cores, thin sections, cathodoluminescence and geochemical analysis technique are utilized to understand the categories of porosity and cementation and analyze the origin of porosity formation from broader perspective. The result of this study demonstrates that pore network in northwestern Tazhong is mainly controlled by intracrystal microporosity, enlarged corrosion pores and fissures. Furthermore, 5 phases' differential cementation and 5 phases' dissolution of aragonitic skeletal allochems are identified in the diagenetic process. In contrast to the porosity mechanism of Lianglitage Fm. in central or eastsouthern Tazhong, tectonic movement and meteoric dissolution processes in 2nd phase of Middle Caledonian are the origin of porosity formation.