The Characteristics of Storage Space of Tight Oil in Shelly Limestone of Da'An'Zhai Member in Sichuan Basin, China

Abstract

Shelly limestone is a kind of special limestone that is mostly composed of shell fossils, which is rare as oil reservoir in the world. The limestone of Da'An'Zhai(DAZ) Member of Lower Jurassic in Sichuan Basin is this type. This limestone possesses 3 characteristics, including its continuous distribution and widespread area (over 3×104km²), extremely low porosity and permeability(ø: 0.3%–3.0%, mostly <2.0%, K:0.0001mD–1.0mD), and long history of oil production(more than 50 years). According to these features, oil in shelly limestone of DAZ Member is no doubt a kind of tight oil, a new type which is different from Bakken and Eagle Ford. The most noticeable uniqueness of DAZ Tight Oil is the contradiction between the long history of oil production and the extremely low ø and K of the reservoir. According to the study, such a contradiction is the result of the special storage space structure of shelly limestone. In the research, casting and fluorescence thin section, mercury injection, ESEM, FESEM, energy spectrum analysis, and nano-CT were utilized to analyze the characteristics of storage space of this limestone. Through the research, 3 scales of storage space were found, including macro-scale(>1.0mm), micro-scale(1.0μm–1.0mm), and nano-scale (1.0nm–1.0μm). In macro-scale, fractures resulted from tectonic movements are the main storage space, and vugs occasionally develop. In micro-scale, micro fractures caused by the breaking and recrystallization of shells and interstitial material are the main storage space, dissolution pores occasionally develop. In nano-scale, not only different kinds of fractures, but also pores in shells and interstitial material increase dramatically. Based on the quantitative characterization, the porosity of macro fractures is about 0.25%. In samples without macro fractures, micro-scale storage space accounts for 8.72%, while the nano-scale is 91.28%. These results indicate that the nano-scale storage space is the majority. 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, the smaller of the scale, the more of the fracture quantity. Such a unique fracture network together with the tiny scale of majority storage spaces, cause the contradiction of DAZ Tight Oil.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015