Pore characteristics of lacustrine shale within the oil window in the Upper Triassic Yanchang Formation, southeastern Ordos Basin, China

Abstract

Organic matter (OM)-rich shale in the Yanchang Formation is recognized as a promising hybrid shale oil/shale gas system in lacustrine strata in China. Being mainly in the oil window, both the type and distribution of the pores in the shale have been affected by several factors, including petroleum expulsion and retention, mineral and organic composition, and compaction. To obtain a better understanding of the factors controlling OM pore development, 10 core samples were selected with various OM content from the Chang 7 and Chang 9 members of the Yanchang Formation for pore characterization. The investigation combined microscopic observation, both of thin sections and ion milled surfaces of samples with low-pressure carbon dioxide and nitrogen adsorption and bulk porosity measurements, with the main emphasis being on OM-associated pores.The selected samples generally have relatively low total porosity, with the pores being poorly connected. Most pore types found in marine shales were present. The size of porous OM is usually small, mostly measuring several hundreds of nanometers to a few micrometers, which suggest a general compaction effect on pore development. The pore development is related to different petroleum expulsion processes. Shale with very porous OM was a very important feature. The OM coexisting with fluorescent lipitinites was present in a cross-linked nanofiber structure, possibly related to altered extracellular polymeric substances (EPS) by compaction and maturation. Various OM types that varied in both size and shape did not contain any visible pores (e.g. woody relics, migrated solid bitumen and OM laminae in close association with clay minerals). Unusually low meso- and macropore volume in the sample with the highest total organic carbon (TOC) content was confirmed by the predominance of OM laminae without visible pores; however, abundant micropores were indicated by CO2 adsorption analysis. Meso- and macropores in this sample had probably not developed due to a significant compaction effect. Due to the small numbers of studied samples, these OM pore characteristics need to be viewed with caution.