Micropore structure characteristics and quantitative characterization methods of lacustrine shale-A case study from the member 2 of Kongdian Formation, Cangdong sag, Bohai Bay Basin

Abstract To better understand the micropore characteristics of lacustrine shale and develop quantitative methods for characterizing lacustrine shale, the reservoir space types, structures and spatial distribution patterns of effective pores in the shale of Member 2 of Kongdian Formation in Cangdong sag are studied using cores, thin sections, scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), nuclear magnetic resonance (NMR), whole-rock X-ray diffraction (XRD) data, etc. Various shale porosity calculation methods are evaluated. The study results show that the reservoir spaces of the shale mainly include three types, i.e., matrix pores, organic pores and fractures. Flaky pore throats formed by dissolution-induced pores and mould pores can improve the pore connectivity. NMR effective porosity of the shale varies from 0.59% to 4.42% with an average of 2.38%, accounting for 49.54% of the total NMR porosity and 34.53% of the gas porosity. The shale is divided into the unimodal-type felsic shale, bimodal-type felsic shale, carbonate shale and mixed shale. The different lithologies exhibit linear correlations among NMR, gas and effective porosities. The shale has effective porosity of 0.56%–4.53% with an average of 2.12%. Furthermore, the shale reservoir is divided into four classes: Class I reservoir with effective porosities of more than 2.5%; Class II1 reservoir with effective porosity of 2.0%–2.5%; Class II2 reservoir with effective porosity of 1.5%–2%; and Class III reservoir with effective porosity of less than 1.5%.