Characterization of shale pore heterogeneity and its controlling factors: A case study of the Longmaxi Formation in Western Hubei, China

Abstract

To quantitatively characterize the complexity of shale pore structures and their controlling factors in the Longmaxi Formation of Western Hubei, our study focused on the organic-rich shale outcrops of the Longmaxi Formation in the Yidu-Hefeng compound anticline. We conducted tests for shale organic content, maturity, and whole-rock mineral composition, along with employing high-pressure mercury injection and low-temperature gas adsorption experiments. Utilizing the V-S, FHH, and sponge models, we calculated the fractal dimensions of micro-, meso-, and macropores. In the Yidu-Hefeng region, the Longmaxi Formation is characterized by calcium-rich shales that are abundant in organic matter. Our analysis of samples revealed a total organic carbon (TOC) ranging between 1.04% and 4.24%, with an average of 2.5%. The Ro values fluctuate between 2.98% and 3.57%, with a mean value of 2.845%, indicating an over-mature stage from early to late thermogenesis. Constituents such as quartz span from 39.8% to 51.3%, with a median of 44.3%, while feldspar oscillates between 3.8% and 12.4%, averaging at 8.48%. Clay minerals constitute 24.3% to 41.7% of the samples, with a mean of 34.16%. Shale porosity exhibits a segmented fractal nature. For instance, D1 varies from 2.1278 to 2.4056, with a mean of 2.2767; D2 fluctuates between 2.4995 and 2.7492, averaging at 2.6309; and D3 ranges from 2.6835 to 2.9427, centering around 2.8111. These variations indicate the intricacies of the macropore structure. Positive correlations between TOC and maturity with D1 and D2 are evident, whereas a negative association is observed with D3. The collaborative interplay between siliceous minerals and organics mirrors the relationship between the siliceous mineral content and its fractal dimensions, akin to TOC. Clay mineral transformations, due to accumulation and dehydration, predominantly contribute to macro-porosity, weakly aligning negatively with D1 and D2 but positively with D3. Variations in carbonate and siliceous minerals and their role in primarily yielding dissolution macropores manifest a subtle negative link with D1 and D2 while enhancing D3. Pore volume correlates positively with D1 and D2, exhibits no conspicuous association with D3, and trends negatively. The compaction and transformation processes of clay minerals seem to favor the generation of macropores, mildly aligning positively with D3.