Classification and controlling factors of organic pores in continental shale gas reservoirs based on laboratory experimental results

Abstract

This study aims to document the classification and controlling factors of organic pores distributed in the continental shale reservoir of the Chang-7 Member of the Yanchang Formation in the Ordos Basin. More than 105 sample blocks were collected from the continental shale gas reservoirs of the Chang-7 Member from 18 separate wells located in central and southern Ordos Basin. Samples were etched via argon ion polishing in preparation for imaging using core plugs and drill cuttings. Next, an environmental scanning electron microscope Quanta 250 FEG was used to scan the micropores; these results are presented in the Samples section. The Quanta 250 FEG can also be used to analyze the media of micropores via energy dispersive spectroscopy. Detailed pore structure parameters were calculated by analyzing the adsorption and desorption isotherms of the samples. Specimen results indicate that all organic pores developed in type-II kerogen and combined with different thermal evolution stages of organic hydrocarbon expulsion can be divided into either 1) oil outlet pores, 2) gas outlet pores, or 3) gas pore groups. Most organic pores are irregular, bubble-like, elliptical, and elongated. Pore diameters are primarily less than 1 μm, with median values ranging from 0.1 μm to 0.2 μm. Relatively small oil outlet pores, ranging from 10 nm to 150 nm, are always concave or elliptical when isolated. Gas outlet pores with diameters ranging from several tens of nanometers to hundreds of nanometers are elliptical, spherical, or ellipsoidal. Numerous randomly aggregated and dispersed gas outlet pores that form gas pore groups emerge and are partially connected to a certain extent.The controlling factors of organic pores in the study area are vitrinite reflectance and the rock brittleness index. Particularly, thermal evolution controls the development of organic pores through different vitrinite reflectances (Ro) with specific total organic carbon (%) of shale. In addition, relative brittleness indexes combined with the degree of diagenesis and evolution are calculated using a mineral constituent method. The results show a positive correlation between the gas pore groups distribution and the rock brittleness index based on the relationship between relative brittleness indexes and the number of organic pores in the samples.