Pore evolution characteristic of shale in the Longmaxi Formation, Sichuan Basin

Abstract

Through the field emission scanning electronic microscope (FESEM) and the nitrogen adsorption test, pore type and structure of shale reservoir in the Longmaxi Formation in the Sichuan Basin were well studied. Result showed that the pore type includes organic pore, intercrystalline pore, dissolution intracrystalline pore and interparticle pore, and the organic pore was one of major pore types; among the organic pore, the micropore had large pore volume and specific surface area, and was the main storage space of shale gas. Through study on effect of total organic carbon (TOC), organic matter maturity (Ro), diagenesis and tectonism on shale porosity, influence of TOC on porosity could be divided into four stages: the rapid increasing stage (TOC from 0 to 2%), the slow decreasing stage (TOC from 2 to 3%), the rapid increasing stage (TOC from 3 to 4% or 6%) and the rapid decreasing stage (TOC > 4% or 6%); influence of the maturity on porosity of shale could be divided into three stages: the rapid decreasing stage (Ro from 1.5 to 2.2%), the rapid increasing stage (Ro from 2.2 to 2.7%) and the rapid decreasing stage (Ro > 2.7%); during the high thermal evolution stage, the organic diagenesis was stronger than the inorganic diagenesis; the tectonism had a great impact on porosity, and the more intense the tectonism was, the smaller the porosity would be. The evolution of shale porosity of the Longmaxi Formation underwent five stages: the immature rapid compaction stage (Ro<0.7%), the mature hydrocarbon generation and dissolution stage (Ro from 0.7 to 1.3%), the high mature pore closed stage (Ro from 1.3 to 2.2%), the overmature secondary pyrolysis stage (Ro from 2.2 to 2.7%) and the overmature slow compaction stage (Ro>2.7%); among which the mature hydrocarbon generation and dissolution stage and the overmature secondary pyrolysis stage were the most favorable shale pore development stages.