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The Main Controlling Factors and Characteristics of Cambrian Pre-salt Dolomite Reservoirs in Tazhong Area, Tarim Basin, North-West China

Abstract

Cambrian pre-salt dolomite reservoirs with limited understanding in the Tarim Basin, China, have significant potential for future discoveries of petroleum, although the exploration about the pre-salt dolomite of Cambrian has made significant discovery in such ancient rocks buried to nearly 7500m at eastern central uplift of Tarim Basin. The study herein accessed new drill core samples which provide an opportunity to understand the dolomitization process in deep basins and its impact on Cambrian pre-salt dolomite reservoirs. This study documents the origin, features, main controlling factors and distributions of the dolomite reservoirs on the basis of the only 3 drilling, 57.72m cores, 636 conventional and casting thin sections, SEPM, cathodoluminescence, conventional well logging, image logging and 3-D seismic data. Based on the study, grain dolomite reservoir of beach facies and gypsodolomite reservoir of tidal flat facies are the two main reservoirs in the pre-salt dolomite of Cambrian in Tazhong area of the Tarim Basin. In addition, grain dolomite reservoir of beach facies exists in Xiaoerbulake Formation of the lower Cambrian and Upper Shayilike Formation of the middle Cambrain. The lithology include algae granular dolomite from shoals in platform inside, algae dolomite and silty crystalline dolomite, and it's mainly controlled by petrographic. It generated at Penecontemporaneous period and the porosity was improved by the later interlayer Karst processes. It's also widely distributed. And gypsodolomite reservoir of tidal flat facies mainly exists in Awatage Formation of the middle Cambrian and lower Shayilike Formation of the middle Cambrain. The lithology include gypsiferous dolostone, silty crystalline dolomite, gypsodolomite breccia and it's controlled by both petrographic and fractures. For its microporous reservoir and its limited distribution, it generated at Penecontemporaneous period and was subjected to later organic acid dissolution. Nowadays, it is most likely made significant discovery at grain dolomite reservoirs of beach facies in Xiaoerbulake Formation of the lower Cambrian. Currently, it isn't a good choice to exploit the gypsodolomite reservoirs of tidal flat facies for the problem of its quality improving. And this work also has shown that old grain dolomite with associated anhydrite, buried to as much as 7500m or more, can still have a high potential for hosting substantial hydrocarbon resources and should be globally targeted for future exploration.