Thin section and X-ray computed tomography for characterizing volcanic reservoirs in Chagan Sag, Yin-E Basin, China

Abstract

We have synthesized macroscopic data (cores) and microscopic data (thin sections and computed tomography images) to characterize volcanic oil and gas reservoir spaces in the Chagan Sag, Yin-E Basin, China. We also have assessed the controlling factors affecting the formation of these reservoirs. Primary pores significantly improve the reservoir quality because they enable the formation of secondary pores (dissolution pores) by allowing migration, alteration, and filling by inorganic and organic fluids. Consequently, the development of secondary pores modifies the permeability of the volcanic rocks. The formation of volcanic reservoirs is influenced by lithology, diagenic minerals, faults, and fractures. The size and abundance of pores, especially gas pores, varies with the lithology. Gas pores are consistently larger and more abundant in basalts than in andesites and tuffs. Moreover, the carbonates and chlorites readily dissolve in the vesicles and fractures, thus resulting in the formation of secondary pores. However, quartzes are not susceptible to dissolution in this study area. Further, faults and fractures are necessary for the formation of reservoir spaces and for oil and gas migration. The tomographic images provide 3D distributions of fractures, vesicles, and matrix dissolution pores. The results suggest that fractures connecting isolated gas pores provide favorable spaces for oil accumulation.