The microdistribution of fluid and its effect on pore structure in shale: A case study of lacustrine Chang 7 shale, Ordos Basin

Abstract

The pore fluid distribution in nanosized pores in shale cannot be observed directly with the current observation equipment which makes it difficult to study the fluid behavior in the shale pore system. To better understand pore fluid evolution and distribution in shale and its effect on pore structure with increasing maturity from immature to oil windows, a serial of samples with different maturity have been collected from the Lower Triassic lacustrine clay-rich Chang 7 shale in the Ordos Basin and treated with oven drying and sequential extraction to remove water, mobile oil, and asphaltene. We have measured the pore-size distributions in the presence of different pore fluids by using the mercury intrusion and N2 adsorption and desorption testing. The results indicate that (1) the pores in the clay-rich samples are mostly slit-shaped mineral pores, consist of a small number of macropores (pore size >50 nm) and a large number of mesopores (pore sizes between 7 and 36 nm), and also including a small amount of poorly developed organic matter-hosted pores. (2) Different types of pore fluids appear to occupy different pore-size ranges. Water prefers to occupy the mineral mesopores. Mobile oil (extracted by n-hexane) is mostly distributed in 2–10 nm-sized organic-matter-hosted mesopores. Asphaltenes are mostly distributed in mesopores in the range of 25–36 nm. (3) The presence of pore fluid significantly impacts the pore structures of shale. The pore volume and specific surface area decrease due to pore fluid occupation, even for low-maturity samples in which the organic matter pores also are developed but cannot be observed under a scanning electron microscope. (4) Organic matter abundance and thermal maturity have significant effects on the pore fluid composition and distribution. With increasing maturity, water is gradually displaced by oil due to hydrocarbon generation and accumulation in large mineral pores.