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The Characteristics of Storage Space of Tight Shelly Limestone Oil: A Case Study of Daanzhai Member of Sichuan Basin

Abstract

There's a kind of special tight oil in Lower Jurassic in Sichuan Basin, tight oil in shelly limestone of Daanzhai Member. It is characterized by two aspects. The first one is that the limestone is composed of shells. And the second one is the extremely low porosity and permeability. The porosity is 0.3%-3.0% (mostly <2.0%), and the permeability is 0.0001mD-1.0mD. Aiming to understand why tight oil can accumulate in such a tight reservoir, casting and fluorescence thin section, mercury injection, ESEM, FESEM, energy spectrum analysis, and nano-CT were utilized to research the characteristics of storage space of this limestone. The study was conducted in three scales, including macro-scale, micron-scale, and nano-scale. In each scale, the types of storage space were defined. In macro-scale(>1.0mm), fractures resulted from tectonic movements are the main storage space, and the vug occasionally develops. In micron-scale (1.0μm-1.0mm), micro fractures caused by the breaking and recrystallization of shells and interstitial material are the main storage space, pores occasionally develop. In nano-scale (1.0nm-1.0μm), though different kinds of fractures are still numerous, the number of dissolution pores and intracrystalline pores within shells and interstitial material increase dramatically. Based on the quantitative characterization, the porosity of macro fractures is about 0.25%. And in samples without macro fractures, the percentage of micron-scale storage space in total storage space is 8.72%, while the nano-scale is 91.28%. These results indicate that the nano-scale storage space is the majority, which is also the immediate cause of the low porosity and permeability. On the other hand, due to the large number of fractures of all scales, a network of fractures formed, which even possesses the fractal structure. Such a unique fracture network not only serves as the dominate pathway for the matrix pores, but also provides a considerable scale of storage space. Due to these two roles of the fractures, oil can reserve in the tight shelly limestone with such a low porosity and permeability.