THE EFFECT OF PORE FLUID ON PORE STRUCTURE IN SHALE: A CASE STUDY OF LACUSTRINE CHANG 7 SHALE , ORDOS BASIN

With increasing thermal maturity organic matter generates hydrocarbon products, which vary in molecular composition and distribution, accompanied by variation in shale pore system. Aim to better understand of pore fluid distribution and their effect on pore structure, increasingly mature shale samples from marginal mature to oil peak were collected from the Lower Triassic lacustrine Chang 7 member in Ordos Basin, and they were treated with oven drying and sequential extraction, the pore size distribution of shale samples with the presence of different pore fluids were measured with mercury injection, N₂ adsorption and desorption testing.

The results show that the pore in the clay mineral-rich Chang 7 shale, mostly in slit shape, composed by a small number of macropore and mostly mesopore between 7 nm and 36 nm, with the pore volume of mesopore up to 0.023 cm³/g. The pore water is distributed in mesopore, with pore size in a range of 10-25 nm. Light hydrocarbon is mostly distributed in mesopores with a pore size of 2-10 nm. Asphaltenes are mostly distributed in the mesopores in a range of 25-36 nm. The presence of water and hydrocarbon reduces the pore volume and the specific surface area of shale. Total organic matter content (TOC) is directly proportional to the pore volume and specific surface area of mesopores. The pore structure of lacustrine shale is significantly affected by clay minerals, kerogen and asphaltenes themselves, presence of pore fluids make the pore interconnection and shale penetration capacity complicated.