Reservoir micro structure of Da'anzhai Member of Jurassic and its petroleum significance in Central Sichuan Basin, SW China

Abstract

Based on the qualitative study of microscopic reservoir features using core analysis, cast and fluorescence thin sections inspection, scanning electron microscope (SEM) and field emission scanning electron microscope (FESEM) and quantitative examination of pore size and geometry using mercury injection, nano-CT and nitrogen adsorption, reservoir rock of Da'anzhai Member were divided into 9 types, while storage spaces were divided into 4 types and 14 sub-types. The study shows that sparry coquina is the most promising reservoir type. Pores that smaller than 1 μm in diameter contribute 91.27% of storage space volume. Most of them exhibit slot-like geometry with good connectivity. By building up storage space models, it was revealed that micron scale storage spaces mainly composed of fractures and nanometer scale pores and fractures form multi-scale dual porosity system. Low resource abundance, small single well controlled reserve, and low production are related to the nano-scale pore space in Da'anzhai Memer, whereas the dual-porosity system composed of pores and fractures makes for long-term oil yield. Due to the existence of abundant slot-like pore space and fractures, economic tight oil production was achieved without stimulations. Key words: Central Sichuan Basin, Da'anzhai Member, coquina storage space, structure characteristics, multi-scale fractures, isothermal adsorption, micro-nano pore