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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 established 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.

Identification of sweet spot is of great significance in confirming shale gas prospects to realize large-scale economic shale gas development. In this paper, geological characteristics of shale gas reservoirs were compared and analyzed based on abundant data of domestic and foreign shale gas reservoirs. Key elements of sweet spots were illustrated, including net thickness of gas shale, total organic carbon (TOC) content, types and maturity (Ro) of organic matters, rock matrix and its physical properties (porosity and permeability), and development characteristics of natural fractures. After the data in Changning and Weiyuan blocks, the Sichuan Basin, were analyzed, the geologic laws of shale gas enrichment were summarized based on the economic exploitation characteristics of shale gas and the correlation between the elements. The elements of favorable “sweet spots” of marine shale gas reservoirs in the Changning block and their distribution characteristics were confirmed. Firstly, the quality of gas source rocks is ensured with the continuous thickness of effective gas shale larger than 30 m, TOC > 2.0% and Ro = 2.4–3.5%. Secondly, the quality of reservoir is ensured with the brittle minerals content being 30–69%, the clay mineral content lower than 30% and a single lamination thickness being 0.1–1.0 m. And thirdly, the porosity is higher than 2.0%, the permeability is larger than 50 nD, gas content is higher than 1.45 m3/t, and formation is under normal pressure–overpressure system, which ensures the production modes and capacities. Finally, the primary and secondary elements that control the “sweet spots” of shale gas reservoirs were further analyzed and their restrictive relationships with each other were also discussed.