Preliminary study on the pore characterization of lacustrine shale reservoirs using low pressure nitrogen adsorption and field emission scanning electron microscopy methods: a case study of the Upper Jurassic Emuerhe Formation, Mohe basin, northeastern China

In this study, we investigate the pore systems in lacustrine shales from the Upper Jurassic Emuerhe Formation, Mohe basin, northeastern China using organic geochemistry analysis, X-ray diffraction, field emission scanning electron microscopy, and low pressure nitrogen adsorption analysis. Because of the large amount of terrestrial input, the organic matter in these lacustrine shales are dominated by type III-kerogen, which have moderate total organic carbon contents ranging between 0.37 wt.% and 4.13 wt.% and thermal maturity ranging from 0.6 % Ro to 1.15 % Ro. Brittle minerals including quartz and feldspar in the Emuerhe Formation comprise >50% of the samples by weight and illite dominates the clay minerals with the relative content ranging from 45% to 95%. The dominant pore types in the Emuerhe Formation shale reservoir are interparticle pores between clay mineral crystals and intraparticle pores within feldspar grains. Pores that are larger than 20 nm primarily contribute to the total pore volume, whereas pores that are less than 10 nm primarily contribute to the specific surface area. The major storage and flow space for hydrocarbon in these lacustrine shales largely resides in inorganic matter porosity and partially in microfacture porosity. Organic matter pores are rare in the lacustrine shales of the Emuerhe Formation, attributed to the humic organic matter and the relatively low thermal maturity. The negative correlation between the total organic carbon and nitrogen BET surface area also indicate that the organic fractions have no significant influence to the petroleum storage potential of these shales. In this case, the clay minerals, specifically illite, could be the major medium for gas adsorption.