Origin of organic matter pore heterogeneity in oil mature Triassic Chang-7 mudstones, Ordos Basin, China

Abstract

Organic matter (OM)-hosted pores play a crucial role in hydrocarbon storage and migration in unconventional shale oil and gas systems. An accurate identification of different OM compositions is critical to understanding the origin of heterogeneity in OM-hosted pore development, which remains challenging due to the lack of apparent intrinsic connection between them. This study correlates light-electron-Raman spectroscopy to investigate various OM maceral types and their molecular structures with respect to the pore characteristics in an oil mature Triassic Chang-7 lacustrine mudstone from the Ordos Basin, China. Moreover, the occurrence and distribution of soluble bitumen and its effect on OM-hosted pore development were directly characterized by comparison of a stitched large SEM image mosaic (∼1 mm2) before and after organic solvent extraction. Results show that terrestrial OM including inertinite and vitrinite fragments display well-preserved biological structure and high reflectance intensity in light microscopy. They often are nonporous under SEM, and have no change before and after solvent extraction. Solid bitumen is the predominant maceral of the examined mudstone, and shows two general populations: 1) nonporous or weakly porous solid bitumen (SB1) that often occurs in large-sized and structured accumulation; 2) porous solid bitumen (SB2) that is filled or dispersed in mineral matrix. After extraction, SB2 was completely removed and have a noticeable recovery of pores, while SB1 changed slightly. The heterogenous pore characteristics can be related to the varying OM molecular structure which shows a decreasing trend of aromaticity in the order of terrestrial macerals, SB1, and SB2 as indicated by Raman parameters. A relatively higher pore abundance in SB2 may be attributed to chromatographic fractionation which led to more mobile and compliant components of SB2.