Quantitative characterization of fractures and pores in shale beds of the Lower Silurian, Longmaxi Formation, Sichuan Basin

Abstract

Fractures and pores are important storage and percolation spaces in tight reservoirs, and the identification, characterization and quantitative evaluation on them are the key aspects and difficulties in shale gas reservoir evaluation. In view of this, quantitative evaluation was performed on the fracture porosity of organic-rich shale intervals of Longmaxi Fm, Lower Silurian, Sichuan Basin (Wufeng Fm, Upper Ordovician included), after a dual-porosity medium porosity interpretation model was built on the basis of drilling data of Fuling Gasfield and Changning gas block in the Sichuan Basin. And then, the following conclusions are reached. First, shale fracture porosity interpretation by using dual-porosity medium model is the effective method to evaluate quantitatively the fracture porosity of shale reservoirs, and the development of quantitative characterization techniques of marine shale reservoir spaces. Second, the matrix pore volume of the principal pay zones in this area and its constitution regions are stably distributed with matrix porosity generally in the range of 4.6%–5.4%. And third, the development characteristics of fracture porosity vary largely in different tectonic regions and indifferent wellblocks and intervals even in the same tectonic region, presenting strong heterogeneity in terms of shale reservoir storage and percolation properties. It is indicated by quantitative characterization of fractures and pores that there are two types of shale gas reservoirs in Wufeng Fm – Longmaxi Fm, Sichuan Basin, including matrix porosity + fracture type and matrix porosity type. The former are mainly developed in the areas with special structure settings and they are characterized by developed fracture pores, high gas content, high free gas content, thick pay zones and high single-well production rate. And in the Sichuan Basin, its distribution is possibly in a restricted range. The latter are characterized by high matrix porosity, undeveloped fracture porosity and medium–high single-well production rate. And it is predicted that marine shale gas is predominant in this basin.