Reservoir and development characteristics of the Da'anzhai tight oil in Sichuan Basin, SW China

Abstract

The oil in the Jurassic Da'anzhai reservoirs in the Sichuan Basin is unconventional tight oil, which accumulated in or near source rocks, and did not experience extensive migration in a large-scale long distance. The first submember, second submember and third submember of Da'anzhai Member are dominated by shell limestone which is widely and continuously distributed, and are typical near-source lacustrine shell limestone tight reservoirs. Complex lithology, multiple types of reservoir space and complicated pore structure are developed in these reservoirs. The effective reservoir space mainly includes micro-pores and micro-fractures with strong fabric selectivity. The petrophysics experiment reveals that the average connected matrix porosity of tight oil reservoir in Jurassic Da'anzhai Member is about 2.13%, lower than that of other tight oil reservoirs but higher than the average effective porosity (0.97%) from previous single alcohol-saturated method. According to production performance data, the Da'anzhai shell limestone reservoir is not a simple porous or fractured reservoir, but has complex pore-throat-fracture association or storage-seepage mode. Because the development of fossil shells controls the development of micro-fractures, fluids are difficult to enter into but easy to escape from the reservoirs. Although the pore-throat is fine, the sorting is poor and the displacement pressure is high, the movable fluid saturation and mercury ejection efficiency of the reservoir in the Da'anzhai Member is only slightly lower than that of some storage-seepage modes, and higher than that of Oil-bearing Group 7 of Yanchang Formation in the Ordos Basin. The reservoir in the Da'anzhai Member is one of the few tight oil reservoirs with high natural productivity. The tight oil in the shell limestone of the Da'anzhai Member has great development potential, but its extensive and effective development also has some challenges, such as high seepage resistance of matrix and ineffective single development mode. The development mode of the Da'anzhai tight oil should draw lessons from the Bakken Formation in North America and Oil-bearing Gourp 7 of Yanchang Formation in the Ordos Basin, and thus, effective development technologies based on volume fracturing and fine operation for shell limestone tight oil in the Da'anzhai Member in Sichuan Basin are developed to realize the development of profit and scale. Keywords: Tight oil, Storage space type, Pore structure, Pore throat, Effective porosity, Movable fluid saturation, Development potential, Effective development mode, Shell limestone, Da'anzhai member, Sichuan basin