Nanopore structure characteristics and evolution of type III kerogen in marine-continental transitional shales from the Qinshui basin, northern China

Abstract

Marine-continental transitional (MCT) shale gas, owing to its great potentials in China, has become a new field of exploration and development following marine shale gas. However, the formation and enrichment mechanism of MCT shale gas is still unclear, especially there is a lack of understanding on the nanopore evolution of organic matter in this type of shale reservoirs. Based on a set of MCT natural shale samples with variable maturities from the Qinshui Basin, northern China, and their isolated kerogen, the characteristics and evolution of organic matter (OM) nanopore in this type of shales were investigated. The results from organic petrological and elemental analysis show that the shales have a Ro range of 1.25–3.90%, and their OM is composed of amorphinite, inertinite and vitrinite, belonging to type III kerogen. Scanning electron microscope observations indicate that interparticle pores related to minerals, especially clay minerals develop well, while organic-hosted pores are underdevelopment. There is no obvious secondary pore in inertinite, and secondary pores occur in vitrinite, but with a low abundance. Low-pressure N2 and CO2 adsorption experiments further reveal that OM in the shales (represented by their kerogen samples) develops micropores, and the contribution of their micropores to their total surface areas and pore volumes can reach averagely 92% and 39%, respectively. Micropores have a main control on the nanopore structure evolution of OM in the shales, and the specific surface areas and pore volumes of micropores and total pores continue to increase with increasing maturity from 1.25% Ro up to 3.90% Ro, without a decline when Ro > 3.0–3.5% as revealed in marine shales The results indicate that the exploration of MCT shale gas in the Qinshui and other basins could be extended to a higher maturity (possibly up to a Ro of 3.9–4.5%).