Multi-scale characterization of organic matter pore space in deep marine shale combined with mathematical morphology

Organic matter (OM)-hosted pores are the primary pore type in the deep shale reservoirs of the Longmaxi Formation, in the Sichuan Basin, China, as well as the main locations for shale gas enrichment. However, the quantitative characteristics of OM pore space are not clear, and there is a lack of characterization methods. In this study, scanning electron microscopy (SEM), mercury intrusion porosimetry, low temperature gas adsorption, and logging data are all used to carry out multi-scale quantitative characterization of OM-hosted pores. The shape factor (F) model, fractal dimension model, and the OM porosity model are improved by combining them with mathematical morphology. The study primarily examines the effect of heterogeneity and the development of full scale pores in the OM pore space of the Luzhou area. The results show that (1) the ideal shape factor (F*) reflects the compaction degree of OM-hosted pores. From macropores to micropores, F* ranges from 0.61 to 0.93 as the shape gradually changes from flattened to round. (2) Shape factor and fractal dimension analyses revealed that the heterogeneity strength of the nanopore structure is mainly controlled by macropores. At the micro level and macro levels, the OM space shows lateral homogeneity and vertical heterogeneity. (3) Compared to the estab-lished method, the relative error of the improved model used in this study for predicting OM porosity was reduced by 19%. (4) Compaction and pore structure heterogeneity influence the formation of nanoscale pores in the Luzhou region and these factors also promote the development of mesopores and micropores, respectively. Furthermore, the OM porosities of the dominant shale reservoirs in Luzhou account for more than 50% of the overall porosity. This understanding will be of considerable benefit for evaluating deep shale reservoirs.

Automated summary

In this study, multiple methods such as scanning electron microscopy, mercury intrusion porosimetry, low temperature gas adsorption, and logging data were used to quantitatively characterize the organic matter-hosted pores in the deep shale reservoirs of the Longmaxi Formation. The study focused on the impact of heterogeneity and deposition on the organic matter pore space, and obtained key findings.