Origin and Distribution of Medium-Macro Crystalline Dolomite in Lower Permian Carbonate of Central Sichuan Basin, China

Five years ago, it is found that the lower Permian dolomite with medium-macro crystalline widespread develops in northwestern Sichuan basin near Longmen Mountains. Through studying origin of the dolomite by many specialists,... [ view full abstract ]

Five years ago, it is found that the lower Permian dolomite with medium-macro crystalline widespread develops in northwestern Sichuan basin near Longmen Mountains. Through studying origin of the dolomite by many specialists, it was considered that the dolomite is formed by hydrothermal dolomitization. And more and more oil and gas have been producing in the dolomite reservoir. In the past few years, it is found that the lower Permian dolomite with medium-macro crystalline also widespread develops in central Sichuan basin. And some wells also got much higher production of oil and gas in the dolomite. For predicting the distribution of the dolomite reservoir, it is necessary to study the origin of the dolomite. Through analyses of fluid-inclusions in the dolomite, cathodoluminescence and plane-light petrography, stable isotopic analyses and so on, it is considered that the medium-macro crystalline dolomite in central Sichuan basin is formed with limestone replaced by hydrothermal fluid along faults of Dongwu movement, which is same period to the faults in the northwestern Sichuan basin.
The origin evidences of hydrothermal dolomitization include the following four aspects. ①Homogenization temperatures of fluid inclusions in the dolomite are normally higher than 120 centigrade degree ②The salinity of bi-phase fluid inclusions of macro-crystalline dolomite is the twice times of normal sea water of early Permian. They normally range between 12 and 22wt% NaCL equivalent. It shows that diagenetic fluid of dolomitization is with high salinity. ③The value of oxygen isotope of medium-macro crystalline dolomite is much low. They are often less than -15‰. The value of oxygen isotope in some saddle-dolomite is even lower than -25‰. ④The value of Sr isotope of medium-macro crystalline dolomite is same to the value of mantle-derived rock. It is suggested that dolomitization fluid comes from deep thermal water along the faults of Dongwu movement. Moreover the thickness of dolomite horizontally changes quickly, and the dolomite normally distributes along the deep faults and volcanic rock areas forming during Dongwu movements