The albitization of K-feldspar is a common diagenetic process that has thus far received little attention and is not fully understood in fine-grained sedimentary rocks. To better understand the albitization of K-feldspar, studies in organic- and silt-rich fine-grained rocks of the lower Cambrian Qiongzhusi Formation in the southwestern Upper Yangtze region, China, were carried out via X-ray diffractometry (XRD) and field emission scanning electron microscopy (FE-SEM). The results show that five types of albitized K-feldspar textures have developed: microcrystal albite replacement, irregular blocky replacement along margins, cleavage planes or microcracks of K-feldspars, complete pseudomorphic replacement, albite overgrowth, and albite pore filling. Organic- and silt-rich fine-grained rocks differ from sandstones and mudstones in terms of the rock structure and mineral assemblage, which results in differences in the textures and degree of albitization of K-feldspar. Illitization of clay has an impact on the albitization of K-feldspar. In provenance analyses using feldspar, fine-grained rocks, especially those that underwent mesogenesis, should be treated with caution because detrital feldspars have been destroyed. Theoretically, the albitization of K-feldspar could increase the porosity of reservoirs, although, from our observations, most of the related secondary pores are cancelled out or became isolated pores due to other diagenetic processes (compaction, cementation, etc.) in organic- and silt-rich fine-grained rocks.

This study uses logging data, mineral component content, total organic carbon (TOC) content, and microscopic characteristics of the organic-rich shales in the Wufeng and Longmaxi Formations, as well as data reported by other researchers, to demonstrate that upwelling has played an important role in the organic matter enrichment. The results show that (1) the organic-rich shales of Well N211 in the Upper Yangtze region are located in the Wufeng Formation and the lower Longmaxi Formation, with a burial depth between 2308–2357 m. (2) The organic-rich shales are enriched in biogenic silica. (3) Based on paleogeographic location and characteristics of organisms, this study determines that upwelling occurred during the deposition of the organic-rich shales in the Wufeng and Longmaxi Formations, promoting the enrichment of organic matter in the shales. (4) The upwelling intensity gradually increased from the sedimentary period of the organic-rich shales in the mid-lower Wufeng Formation to the sedimentary period of the Guanyinqiao Member, and then decreased gradually from the sedimentary period of the Guanyinqiao Member to the sedimentary period of the organic-rich shales in the Longmaxi Formation, and leads to the different enrichment of organic matter in the vertical direction. The different developments of upwelling led to the coexistence of both high and low TOC contents in the Guanyinqiao Member along the vertical direction.

Considerable debate remains regarding the mechanisms responsible for the reduction in organic matter (OM) pores in post-mature shales. To address this issue, complementary techniques including scanning electron microscopy (SEM), total organic carbon (TOC) analysis, and helium porosity measurement were employed to characterize the microstructure and porosity of post-mature shales from the Qiongzhusi Formation in the southwestern Upper Yangtze region, China. The results show that OM pores in these shales are poorly developed and exhibit highly irregular morphologies. Notably, the degree of OM pore development is negatively correlated with TOC. Interestingly, in samples with TOC < 2.5 wt.%, well-preserved spongy migrated OM is still observable under SEM. The average porosity of Qiongzhusi mudstones is 1.8%; siltstone samples with TOC < 2 wt.% yield an average porosity of 3.5%, whereas samples with TOC > 4 wt.% have an average porosity of only 1.9%. These findings do not support the hypothesis that graphitization causes the significant destruction of OM pores in post-mature shales. Instead, we propose that compaction has been the dominant factor controlling OM pore destruction. Accordingly, we introduce a “depth window” for the development of high-quality shale gas reservoirs: Beyond a certain maximum paleoburial depth, compaction leads to extensive OM pore destruction and a marked decline in reservoir quality. This study advances our understanding of pore evolution in post-mature shales and provides practical guidance for shale gas exploration.