Pore structure, hydrocarbon occurrence and their relationship with shale oil production in Lucaogou Formation of Jimsar Sag, Junggar Basin

In order to study the relationship between shale oil production and reservoir porosity or the oil content, experimental approaches including FE-SEM, LSCM, nano CT, high pressure mercury injection and nitrogen adsorption combined analysis, NMR analysis and molecular simulation were used to quantitatively analyze the full-scale distribution and occurrence characteristics of shale oil in Permian Lucaogou Formation of the Jimsar Sag, Junggar Basin. There are significant differences in the pore-size distribution of various lithologies in shale oil reservoir of the Jimsar Sag. The dominant lithology are arenaceous dolomite, feldspar lithic siltstone and dolomitic siltstone, and the best one is feldspar lithic fine sand rock, with pores larger than 300 nm accounting for 74.1%, and the main body is intergranular (dissolved) pores and intergranular dissolved pores. Fluid occurs with large heterogeneous in micro-nano scale. Heavy components with fluorescence wavelength between 600 and 800 nm attached to mineral pore surface as thin film in pores with a radius above 300 nm, and filled in pores with a radius below 300 nm. The medium components with fluorescence wavelength between 490 and 600 nm occur in the center of pores above 300 nm. The water content is low, and occurs in the center of the pores above 300 nm wrapped by the medium component. The lower limit of pore throat production of shale oil in the Lucaogou Formation is 50 nm. Above 300 nm, the hydrocarbon in pore throat is easy to be produced and is the main contribution system of current productivity. The recovered crude oil with medium density is mainly from large pore above 300 nm. Pore-throats distributed between 50 to 300 nm are difficult for the shale oil producing, which is the key to enhance oil recovery. Negative pressure and temperature rise can effectively improve the mobility of hydrocarbons in nano-scale pores.