Evaluation of pore structure characteristics of four types of continental shales with the aid of low-pressure nitrogen adsorption and an improved FE-SEM technique in Ordos Basin, China

Abstract

The upper Triassic continental shale of Yan-Chang #7 Formation, in Ordos Basin, China, exhibits complex geological conditions that lead to a complicated pore structure of the targeted formation. To reveal the pore structure and understand its influential factors, we first classify the shale samples into four categories according to the organic and clay contents (type #1: clay content > 52.0% and TOC > 3.0%, type #2: clay content >52% and TOC < 3.0%, type #3: clay content < 52.0% and TOC > 3.0%, type #4: clay content < 52.0% and TOC < 3.0%). Then, the pore structure, total pore volume, and specific surface area are investigated by the low-pressure nitrogen adsorption analysis for each category. Next, the field-emission scanning electron microscopy analysis combined with the image processing is used to quantify the surface porosities of different types of pores and pore-size distributions of the samples. Results demonstrate that total pore volume and specific surface area are positively correlated with TOC and clay minerals (i.e., illite) but negatively correlated with brittle minerals (i.e., quartz and feldspar). Clay mineral is a major contributor to interparticle pores and micro-fractures. Higher TOC, illite, and chlorite contents lead to more pores/fractures in organic matter, while higher quartz content results in more intraparticle pores. The obtained results help to characterize the pore structure of Yan-Chang #7 Formation accurately as well as explore and exploit shale resources in this area.